The Global 3-Dimensional Structure of the Pacific Decadal Oscillation. A Thesis. Presented in Partial Fulfillment of the Requirements for

Transcription

1 The Global 3-Dimensional Structure of the Pacific Decadal Oscillation A Thesis Presented in Partial Fulfillment of the Requirements for The Degree Masters of Science in the Graduate School of The Ohio State University By Erik Alan Fraza, B.S. ***** The Ohio State University 2010 Master s Examination Committee: Dr. Jialin Lin, Advisor Dr. Jay S. Hobgood Dr. Jeffrey C. Rogers Approved by Advisor Graduate Program in Atmospheric Science

2 Abstract The Pacific Decadal Oscillation (PDO) is a dominant mode of decadal variability in the global climate system. Decadal variability is important in terms of global warming projections as it can mix with human-induced climate change signals. Therefore, getting a better description and understanding of decadal variability can help to separate humaninduced signals from natural variability. Most of the previous studies of PDO focused on its two-dimensional structure at the surface. This study examines the global threedimensional structure of the PDO using 62 years ( ) of the NCEP/NCAR reanalysis data. The warm phase of the PDO is characterized by tropospheric warming around the world, especially between 30 N and 90 S. There is high pressure anomaly throughout the troposphere in the tropics and midlatitudes, but low pressure anomaly in the two polar regions, leading to enhanced polar jet streams in both hemispheres. The Hadley circulation, Ferrel circulation and polar circulation are all intensified. The Walker circulation, on the other hand, is weakened. The wintertime polar vortex is intensified from the surface all the way up to 10mb in the northern hemisphere, but only up to 100mb in the southern hemisphere. Most of the previous studies of PDO focused on its extreme phases (i.e. the warm phase and cold phase), but not the transition phases in between. This study further ii

3 examines the transition phases of the PDO using the 62 year dataset. The cold to warm transition phase of the PDO is characterized by a cold SST anomaly in the North Atlantic subtropics, which is associated with a significantly high SLP anomaly and weakened precipitation. iii

4 Acknowledgements I would like to greatly thank Jialin Lin for helping learn how to become a researcher and (hopefully) scholarly author. Without him, I am not sure where my academic career would be today. I would also like to thank my committee members Jay S. Hobgood and Jeffrey C. Rogers for their support and counsel. A special thanks to my family, especially my parents, and also my friends, for their support through this arduous journey over these past two years. I cannot begin to imagine where I would be without their support. Finally, a special thanks to Michael Davis and Meng-Pai Hung for their guidance through the graduate program, and especially in preparing my thesis, and its defense. Further, another special thanks to Scott Melaragno for proofreading my thesis and general discussions concerning this whole process. iv

5 VITA September 27, 1984 Born Grosse Pointe, Michigan May 2008 B.S. Meteorology, Central Michigan University Mount Pleasant December 2008 B.S. Computer Science, Central Michigan University Mount Pleasant Major: Atmospheric Science FIELD OF STUDY v

6 TABLE OF CONTENTS ABSTRACT ii ACKNOWLEDGEMENTS iv VITA v LIST OF FIGURES. vii Chapters: 1. Introduction 1 2. Data and Methods Global 3-Dimensional Structure of the Extreme Phase Global 3-Dimensional Structure of the Transition Phase Summary and Discussions List of References vi

7 Figure LIST OF FIGURES Page 1.1 (from Mantua and Hare, 2000) The PDO index (PDOI) with the positive figures indicating the warm phase and negative values indicating the cold phase General graph explaining the difference between the extreme phase (smoothed index) and the transition phase (transition index) Linear correlation of annual mean zonal air temperature with smoothed index for the extreme phase Same as Fig 3.1 except for geopotential heights Same as Fig 3.1 except for zonal winds Same as Fig 3.1 except for Omega Same as Fig 3.1 except for meridional winds Same as Fig. 3.5 but for horizontal map of extreme phase SST Same as Fig. 3.6 except for precipitation rates Same as Fig. 3.6 except for SLP Same as Fig 3.6 except for 1000mb zonal winds Same as Fig. 3.6 except for 1000mb meridional winds Same as Fig. 3.6 except for 200mb geopotential heights Same as Fig. 3.6 except for 200mb zonal winds Same as Fig. 3.6 except for 200mb meridional heights Same as Fig 3.1 but for horizontal map of northern hemisphere DJF mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb vii

8 3.15 Same as Fig except but for southern hemisphere JJA mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb Schematic depiction of the global structure of PDO s warm phase at 200mb (upper) and 1000mb (lower) Linear correlation of annual mean zonal air temperature with the transition index for the transition phase Same as Fig. 4.1 but for geopotential heights Same as Fig. 4.1 but for zonal winds Same as Fig. 4.1 but for Omega Same as Fig 4.1 but for meridional winds Same as Fig. 4.1 but for horizontal map of transition phase SST Same as Fig. 4.6 but for precipitation rates Same as Fig. 4.6 but for SLP Same as Fig. 4.6 but for 1000mb zonal winds Same as Fig. 4.6 but for 1000mb meridional winds Same as Fig. 4.6 but for 200mb geopotential heights Same as Fig 4.6 but for 200mb zonal winds Same as Fig 4.6 but for 200mb meridional winds Same as Fig. 4.6 but for horizontal map of northern hemisphere DJF mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb 117 viii

9 4.15 Same as Fig but for southern hemisphere JJA mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb Schematic depiction of the global structure of PDO s transition phase at 200mb (above) and 1000mb (below) ix

10 CHAPTER 1 INTRODUCTION The Pacific Decadal Oscillation (PDO) is a dominant mode of decadal variability in the Pacific climate system as well as the global climate system (e.g. Mantua et al. 1997, Zhang et al. 1997). Decadal variability is important in terms of global warming because it can mix with the human-induced climate change signals; therefore getting a better understanding of decadal variability is beneficial to separate human-induced signals from natural variability. Like El Nino, the PDO plays an important role in the climate of North America. Research has been done concerning different regions within North America and the effects that the PDO can have (Goodrich, 2004; Goodrich, 2006; Pavia et al. 2006). However, possible forecasters that might use the PDO as a guide for seasonal forecasts must also consider ENSO, as the interactions between the PDO and ENSO can cause even further changes associated with the climate of North America. Gershunov and Barnett (1998) were the first to note the constructive and destructive phases of the PDO-ENSO interactions, which they called the modulation effect. During the constructive phase (warm PDO and El Nino or cold PDO and La Nina), we see a stronger, more stable ENSO climate signal associated with winter precipitation in the western United States. On the other hand, in a destructive phase (warm PDO and La Nina or cold PDO and El Nino), the ENSO climate signal is weakened, making seasonal 1

11 forecasting more challenging. This would indicate that a better knowledge of the PDO would aide in seasonal forecasting, especially for western North America. Over the past 30 years or so, plenty of research has been done in order to better understand the El-Nino Southern Oscillation (ENSO). One of the main reasons for this research stems from the effect ENSO has on the climate of North America. Over the past 15 years, though, some researchers have been turning their attention to a phenomenon that appears like a long-lived El-Nino type pattern of Pacific climate variability (Zhang et al. 1997). Formally named in 1997 by Mantua et al, the PDO is the leading mode principal component (PC) from an unrotated empirical orthogonal function (EOF) analysis of monthly residual North Pacific sea surface temperatures (SST) anomalies north of 20 (Hare, 1996; Zhang, 1996; Mantua et al. 1997). Along with the climate of North America, the PDO also has an effect on the ecosystems contained within the Pacific Rim. A prime example of this is what led to the formal naming of the PDO. While researching Salmon production in the Pacific Northwest and Alaska, Mantua et al. (1997) noted changes in Salmon catches in these basins, which coincide with phase changes of the PDO. Further noted by Mantua and Hare (2002) is the change of 40 climatic and biological changes, mostly biota in the North Pacific, associated with the 1977 phase shift of the PDO. Changes can even be seen in coral samples taken from the south Pacific (Linsley et al. 2000). All of these changes aide in the prediction of phase changes of the PDO, but can also cause false 2

12 reports of a phase change. It is possible for the main characteristics of the PDO to change phase for a few years before returning to its previous phase. In this case, it is not considered a phase change, but instead an anomaly within the regime. An example of this is in Hare and Mantua (2000), in which it is noted that some characteristics show a possible phase change in Further research, however, has shown that this was not a phase change, but an anomaly. Therefore, when we do see changes in the North Pacific that may lead to the belief of a phase change of the PDO has occurred, we must wait a few years to confirm this belief. As more research has been done concerning the PDO, more characteristics have been noted. For instance, a standard PDO regime lasts about years (Mantua et al. 1997, Minobe 1997). Therefore, a complete cycle of the PDO should last between years, though many researchers usually state a complete cycle to be years (Minobe 1997, Mantua and Hare 2002). When looking into possible changes, one variable that researchers analyze is temperature. When looking for PDO modifications, the temperature changes are best seen at the mid-latitudes. More specifically, the SST s off of the west coast of North America and also the central North Pacific play a vital role in tracking the possible changes in the PDO. There is a definite inverse relationship between the SST s between these two regions (Graham 1994, Miller et al. 1995, Zhang et al. 1997, Mantua et al. 1997) that will be fully discussed later. Researchers also look at the Sea Level Pressure (SLP) in the Pacific region of the Northern Hemisphere. Namely, 3

13 attention is paid to changes in the Aleutian Low, which has an inverse relationship with pressure systems over western North America and the Subtropical Pacific (Trenberth and Hurrel 1994, Graham 1994). Using both these SST s and SLP s, indices were developed by projecting the observed monthly patterns of North Pacific SST and SLP anomalies onto the characteristic SST and SLP patterns (Trenberth 1990, Trenberth and Hurrel 1994, Zhang et al. 1997, Mantua et al. 1997). Positive indices indicate the following conditions: anomalously cool SST s in the central North Pacific, anomalously warm SST s along the Pacific coast and below average SLP s over the North Pacific (Mantua). As for negative indices, all of the conditions are reversed. When both indices are negative (SST and SLP), this indicates a cool PDO regime. Further, for a warm PDO regime, both indices will be positive. With this information, the current regime of the PDO can be tracked by researchers, namely with the PDO index (PDOI). Developed by Mantua and Hare, the PDO index can be used in order to determine what phase the PDO is currently in. As stated before, one must be careful and wait a few years before declaring a phase shift. A lot of anomalies are shown in Fig. 1.1, and it is a mistake that is made often. 4

14 Fig. 1.1 (from Mantua and Hare, 2000) The PDO index (PDOI) with the positive figures indicating the warm phase and negative values indicating the cold phase. 5

15 Although described as a long-lived El-Nino like pattern of Pacific climate variability (Zhang et al. 1997), there are some key features that differentiate the PDO from ENSO. The temporal difference between the two is the first difference that jumps out. As stated earlier, a single phase of the PDO lasts approximately years. On other hand, ENSO has a much shorter time span of around 18 months. Further, where these phenomena have the greatest impact also differs. For instance, it has been shown that the main features associated with the PDO are found in the North Pacific/North America area (Mantua) and secondary characteristics are found in the tropical Pacific. When looking at ENSO, the opposite is true: the main features are found in the tropical Pacific and secondary features in the North Pacific/North American region. However, with these major differences, research has been done concerning a possible relationship between PDO and ENSO. As stated earlier, the constructive or destructive relationship between ENSO and the PDO play a key factor in the North American climate. However, the breadth of the relationship between the PDO and ENSO has not been fully analyzed to this point, and we cannot say for sure that one of these phenomena is not just a feature of another phenomenon. Since the mid 1990 s, PDO research has been done in earnest. However, while the research done in identifying the phase changes or possible climate effects on North America, there is less work done in understating the physical structure of the PDO. Some of the beginning works concerning on the PDO offered jumping off points concerning the 6

16 physical structure, but little more. For instance, Minobe (1997) mentions that the two possible causes could be either external oscillatory forcings or internal oscillations of the atmosphere-ocean system. It is finally stated that the PDO is likely a combination of these two characteristics. Given that the PDO was beginning to come to the forefront for research, this was a good start. And although many papers have been done since Minobe (1997), few have really delved into understanding the structure of the PDO quite like Miller and Schneider (2000) did. In this paper, Miller and Schneider look at previous works, postulate whether or not they could be valid, and also compare papers to one another that are contradictory. They also present some of their own work and ideas. They identify a dominant (primary) pattern and a secondary pattern associated with decadal variability in the North Pacific Ocean. Miller and Schneider (2000) state that the dominant pattern is the canonical SST pattern driven and maintained by an equal anomalous patterns of surface heat fluxes, Ekman advection of mean SST gradients and turbulent vertical ocean mixing organized by the large-scale structure of the Aleutian Low. This leads to the secondary pattern in which large scale Ekman pumping resulting from changes in the Aleutian Low also forces western-intensified thermocline depth anomalies. Miller and Schneider also point out that this may play a role in affecting the Kuroshio-Oyashio Extension (KOE), but that the possible associated feedbacks are not well understood. To put it simply, the two 7

17 physical components that Miller and Schneider believe are associated with decadal changes in the North Pacific are wind-stress curl and subduction. In their paper, Miller and Schneider also reference a paper by Gu and Philander (1997), in which they propose a mechanism that atmospheric teleconnections from warm tropical SST drive cool anomalous SST in the central North Pacific or South Pacific. This then changes the temperature of water that is subducted into the thermocline and follows the mean circulation to the tropics. Once the anomalously cool subducted water reaches the tropical strip many years later, it is presumed to upwell to the surface and cause the SST to cool along the equator, resulting in oppositely signed teleconnections. Since the Miller and Schneider paper and the Gu and Philander paper, when most authors reference the PDO and attempt to explain the structure of the PDO, these two papers are referenced with little else. However, some authors do state a line or two about the structure that may not be from either of the above papers. For instance, Linsley et al (2000) states that the PDO is a recurring pattern of ocean-atmosphere variability in which the central gyre cools at the same time as the eastern margin warms, or vice versa. Further, Goodrich (2004) notes that both Gedalof et al (2002) and Newman et al (2003) suggested that ENSO may be the forcing mechanism for the PDO. Even though many researchers compare the PDO to ENSO, far less is understood about the physical structure of the PDO compared to ENSO, even today. 8

18 In researching the PDO, a few unanswered questions or problems come up that need to be addressed. As stated previously, the dynamics of the PDO is a bit of a mystery. In order to fully understand the PDO, researchers need to further study this aspect of the PDO so that we can fully understand its impact on climate. Another question that remains unanswered is the PDO s interaction with ENSO. Now while we understand the constructive and destructive phases of the PDO/ENSO interaction and the effects they have on North American climate (Gershunov and Barnett 1999), we do not know if the PDO is perhaps a part of the ENSO phenomena, or vice versa. Although plenty of research has been done concerning ENSO and it is rather well understood, by the time all the pieces of ENSO were put together, the PDO was just being formally named. Therefore, it is plausible that a piece could have been left out because researchers did not know much, if anything at all, concerning this decadal variability. In studying a phenomenon like the PDO with multidecadal variability, a few centuries of data would be optimal in order to begin to piece together the components. However, with the PDO seen in the Central North Pacific for the most part, data does not exist much before This presents quite an issue in researching the PDO and its phase changes. Thus, researchers began to piece together previous climates in order to better understand the PDO. One of the most popular ways involves tree rings. Proxy climate records from tree rings have provided high-resolution, accurately dated information on North Pacific climate at scales much longer than those attained by 9

19 instrumental data (D Arrigo et al 1999; Cook et al 2000). For example, Biondi et al (2001) used a sample of tree rings from Southern California and Baja California to recover the timing, amplitude and frequency of decadal-scale climate oscillation in the Pacific basin. They were able to indentify PDO/ENSO swings, or PDO phase changes that coincided with ENSO changes in 1750, 1905 and Further, the tree data research provided proxy data back to Without instrumental data, this provides valuable information in piecing together the history of the PDO. Other researchers have used tree ring data to gain valuable information concerning the PDO: Gedalof and Smith (2001) looked at high elevation trees from Oregon to southern Alaska to piece together a mean spring PDO index; Evans et al (2000) used tree rings from various trees throughout the Pacific influenced regions of extratropical North and South America to reconstruct one pattern in SST that is structurally similar to the PDO. Once this data is acquired, researchers will then do hindcasting to test its accuracy before predicting data prior to instrumental records. This is to make sure that data is within a certain realm of accuracy and can be trusted. Although not as popular for climate proxies concerning the PDO, coral samples have also been used in order to construct past climates. For example, Urban et al (2000) used 18 O in coral from Maiana to reconstruct regional SST back to Another example is Linsley et al (2000) in which they use a Sr/Ca record back to 1726 from coral in Rarotonga to piece together a monthly SST for the region. Although coral samples are 10

20 helpful, they are assumed to be a secondary climate proxy data type when researching for the PDO. One reason is the fact that coral data is usually concerning an area within the tropics, whereas tree rings are usually from the mid-latitudes. Since the mid-latitudes are considered the primary area for PDO influence, mid-latitude proxies would work best. Further, tree ring time series typically provide an annually resolved record of growing season conditions, whereas coral continuously integrate their immediate environment, and the resolution is limited by sampling frequency and data control (Gedalof et al 2002). From this, it appears that tree ring data is by far the better proxy for studying the PDO, but coral samples would be better than nothing, and can also be used to understand what, if any, role the tropics plays in influencing the PDO. While doing research concerning the PDO, however, researchers have glanced over a very important feature. When analyzing the shifts from the warm phase to the cold phase and vice versa, researchers have presumed that the same processes are operating in both cases. Although a simple presumption, this could very well be a false one. In this paper, we will analyze the differences between the climatic shifts between the warm phase to cold phase and the cold phase to the warm phase, compare results of plots and data, and show that this transition phase is actually two separate phases that cannot be combined together. Along with this, the study will look at the 3-dimensional structure of the PDO. Most previous works concerning the structure of the PDO have concentrated on the 11

21 surface structure only (i.e. Mantua et al 1997). With more comprehension of the entire physical structure of the PDO, a better understanding can be gained on the phenomenon. Further, previous studies have only concerned themselves with extreme phases of the PDO (i.e. Goodrich 2004). The transition phase needs to be taken into account as the different characteristics associated with the transition phase that are different from those of the extreme phase cause a muddled understanding of the effects the PDO can have on climate. 12

22 CHAPTER 2 DATA AND METHODS The dataset used in this study is the NCEP/NCAR reanalysis (Kalnay et al. 1996). The datasets are monthly data for 62 years ( ), and the spatial resolution is 2.5 longitude by 2.5 latitude. There are 18 levels in the vertical direction. The variables we used include SST, precipitation rates, SLP, geopotential height, zonal wind, meridional wind, and vertical velocity for the whole globe. The NCEP/NCAR reanalysis was started in the 1990 s in order to help researchers have a single place they can access global data. Prior to this, data for the world was scattered about and if one wanted this data, they would have to go to multiple sources. With the concern of global warming truly starting in the 1990 s, it became easier to get funding to do this type of work. Also, it was a bit easier to get other nations (i.e. China) to share their data in order to have it all compiled in one place. The reanalysis entered the processing phase in May 1994, with an NCEP supercomputer processing one month s worth of reanalysis data and forecasts in a day (Kalnay et al. 1996). At this pace, 13 years of data was processed between May 1994 and September Prior to the processing, there was a pre-processing application done in order to remove any erroneous data or to find any holes in data. For instance, the significant level winds were low from August 1989 to September 1991 in NCEP s 13

23 database; but with the addition of the European Center for Medium-range Weather Forecasting (ECMWF), this area that was lacking data was resolved. Initially, the reanalysis was supposed to be from 1957 and on. However, an extra portion was added to this project when data from between was also added to the database. The PDO index was constructed by Zhang et al. (1997) and Mantua et al. (1997) and is available at From this raw index two indexes are developed: the smoothed index and the transition index. An 85 month running mean is applied to the raw index to get the smoothed index. The transition index is simply defined as the time derivative of the smoothed index. The time derivative is calculated using the central difference scheme except for the first and last data points, for which forward and backward difference schemes are used, respectively. Please see Fig. 2.1 for the definition of different phases of PDO. The composite structures of PDO's extreme phases and transition phase are constructed by calculating the linear correlation between the NCEP/NCAR reanalysis data and the PDO smoothed index and transition index. We looked at the zonal mean cross-sections, global horizontal maps and polar stereographic maps of all the different variables. Correlation with the smooth index represents the warm phase of the PDO, and if multiplied by -1, the cold phase of the PDO. Correlation with the transition index represents the cold to warm transition phase, and if multiply by -1, the warm to cold transition phase. For correlation with 62 years of data, the 95% confidence level is

24 Fig. 2.1 General graph explaining the difference between the extreme phase (smoothed index) and the transition phase (transition index). 15

25 CHAPTER 3 GLOBAL 3-DIMENSIONAL STRUCTURE OF THE EXTREME PHASE The first part of the analysis is concerning the extreme phase. Correlation with the smooth index represents the warm phase of the PDO, and if multiplied by -1, the cold phase of the PDO. We will look at the zonal mean cross-sections, global horizontal maps and polar stereographic maps of all the variables. The first set of analysis is concerning the zonal mean cross section of selected variables. For these selected variables, a cross section of the entire globe will be analyzed in order to better understand the effects of the PDO. The first variable to analyze for the zonal mean cross section is the air temperature anomaly, as seen in Fig The main point of Fig. 3.1 is that the warm phase of the PDO is characterized by tropospheric warming around the world, especially between 30 N-90 S. Starting in the northern Polar cell (60 N-90 N), it is, for the most part, dominated by a fairly strong positive air temperature anomaly, namely in the lower and mid-troposphere. The strongest part of this positive air temperature anomaly is located north of 75 N, around 600mb vertically. In the upper troposphere, along with a strong positive air temperature anomaly center, there is also a negative air temperature anomaly. This negative anomaly is located between 60 N-70 N and between mb vertically. The negative 16

26 Fig 3.1 Linear correlation of annual mean zonal air temperature with smoothed index for the extreme phase. 17

27 anomaly appears to be just a branch of an even stronger negative air temperature anomaly. However, the majority of this negative air temperature anomaly is located in the stratosphere. Moving south into the northern Ferrel cell (30 N-60 N), there are a few air temperature anomalies of note. Starting at the surface, there is a fairly small, but strong, negative air temperature anomaly present around 40 N between the surface and 975mb vertically. A little north of this and a little higher in the atmosphere is a positive air temperature anomaly. The center of this anomaly is around 55 N and is between mb vertically. More towards the top of the atmosphere, a portion of the negative anomaly of the northern Polar cell does reach into the northern Ferrel cell, but the main effects of the anomaly are contained within the northern Polar cell. The other anomaly in the upper atmosphere is a very strong positive air temperature anomaly. This anomaly, found in the northern Ferrel cell between mb vertically, is just a branch of the main positive air temperature anomaly. This positive air temperature anomaly completely dominates the southern Ferrel cell (30 S-60 S) at all heights, and is still very dominant in the northern Hadley cell (0-30 N) and the southern Hadley cell (0-30 S). In both Hadley cells, between mb vertically, there is an area within the positive air temperature anomaly that is not as strong as the surrounding anomaly. However, this is still very much a positive air temperature anomaly. Finally, concerning the southern Polar cell (60 S-90 S), the strong positive air temperature anomaly dominates most of the area north of 75 S. However, south of 75 S there is a weakening of the positive air 18

28 temperature anomaly and even some small regions of a weak negative air temperature anomaly. The centers of the weak negative air temperature anomaly are located around 600mb, and although not very strong, they are in sharp contrast to the very strong positive air temperature anomaly that dominates almost all of the Southern Hemisphere, along with parts of the Northern Hemisphere as well. The next variable to look at is the geopotential height, which is displayed in Fig 3.2. The key point of Fig. 3.2 is that there are high pressure anomalies throughout the troposphere in the tropics and midlatitudes, but low pressure anomalies in both of the Polar Regions. Unlike the air temperature anomaly figure, the geopotential height anomaly figure is much less busy. Starting in the northern Polar cell, the area below 700mb is dominated by negative geopotential height anomalies. The strongest part of the anomaly is located between 65 N-70 N, stretching from the surface to about 950mb. There is another negative geopotential height anomaly in the northern Polar cell. However, most of the anomaly, including its center, is located in the stratosphere. In between this, in the area of mb, is a somewhat strong positive geopotential height anomaly. The center of this anomaly is north of 75 N, ranging from mb vertically. This anomaly is just a branch of a very strong positive geopotential height anomaly that dominates a good portion of the overall atmosphere. This positive anomaly is strongest in both the northern and southern Hadley cells, but is also prevalent throughout the northern Ferrel cell and a good portion of the southern Ferrel cell. Further, 19

29 Fig. 3.2 Same as Fig 3.1 except for geopotential heights. 20

30 this positive geopotential height anomaly extends from the surface into the stratosphere. In the southern Ferrel cell, there is a very strong geopotential height gradient from the surface in the southern Ferrel cell, upward in the atmosphere into the stratosphere of the southern Polar cell. This gradient, with positive geopotential height anomalies north of the gradient and negative geopotential height anomalies to the south of it, starts at the surface at about 45 N. As the gradient goes up in the atmosphere, it begins to move south, and the gradient crosses into the southern Polar cell at around 300mb. The gradient continues up in the atmosphere as it moves south, reaching about 100mb at 90 S. The area to the south of this gradient is dominated by a very strong negative geopotential height anomaly, with the strongest portion of this anomaly going from the surface to about mb throughout the whole southern Polar cell. The cross section of the zonal wind is the next analysis to be performed, and is displayed in Fig The key point seen in Fig. 3.3 is that the pressure anomalies stated earlier lead to enhanced polar jet streams in both hemispheres. In this analysis, positive anomalies indicate a westerly wind anomaly, and vice versa for negative anomalies. Starting in the northern Polar cell, there are two main anomalies of note. Closest to the North Pole is a westerly wind anomaly, which is found north of 75 N and its center stretches from the surface to 600mb. A bit to the south is an easterly wind anomaly. This anomaly is between 65 N-75 N, with two strong centers between the surface and 900mb, along with mb. The upper center also stretches into the stratosphere, but that is 21

31 Fig. 3.3 Same as Fig 3.1 except for zonal winds. 22

32 outside the scope of this study. Moving into the northern Ferrel cell, there are two more anomalies of note. The smaller and weaker of the two is an easterly wind anomaly, which is found between 30 N-40 N, and although the center is barely above the surface, parts of this anomaly reach 500mb. The other anomaly is a westerly wind anomaly that appears to be a part of a larger system. Overall, this anomaly has three primary centers: one in the southern Hadley cell between 0-10 S and from the surface to 550mb vertically. The next center is in the upper atmosphere in both of the Hadley cells, and although the center does appear to be in the stratosphere, some of the strongest portions of this center extend down as far as 300mb. The final center is also the smallest and weakest, and is located in the northern Hadley cell, between 15 N-30 N and between mb vertically. In this large expanse of westerly anomalies is a small, but very strong easterly wind anomaly. This anomaly is located in the northern Hadley cell around 15 N, and although the center is very small, strong portions of this easterly wind anomaly reach from the surface to about 820mb. Now in the southern half of the southern Hadley cell and also in the Ferrel cell is a very strong easterly wind anomaly. The main center of this anomaly is located further up in the atmosphere in both the southern Hadley and Ferrel cells, between about 20 S-45 S and between 500mb upward into the stratosphere. There is a second center likely associated with the main easterly wind anomaly located in the southern Hadley cell. The center of this anomaly is located around 15 S and is between mb. This second center is much weaker than the main easterly 23

33 wind anomaly field, but they are definitely associated with one another. Moving further south, there is one more very strong westerly wind anomaly that covers half of the southern Ferrel and Polar cells. The strongest area of this anomaly starts at the surface between 45 S-65 S, and continues up almost vertically until about 200mb, when the anomaly begins turning slightly to the south. A portion of this westerly wind anomaly does cover the whole surface/lower atmosphere area of the southern Ferrel cell and even reaches just a bit into the southern Hadley cell. This area of the anomaly is not as strong as the main portion, but is still fairly strong in its own right. The final anomaly of note is located in the southern Polar cell, south of 70 S. This strong easterly wind anomaly goes from the surface to a just above 300mb, and is strongest between the surface and 625mb. The next analysis is the cross section for omega, as shown in Fig 3.4. For Fig 3.4, the key point is that the Hadley, Ferrel and Polar circulations are all intensified. In this figure, positive anomalies would indicate sinking motion, which would likely signal the presence of a High Pressure system, or at least air that is more stable. On the other hand, negative values would indicate rising motion, meaning a Low Pressure system is likely, or at least less stable air. Starting in the northern Polar cell, there is a small negative omega anomaly, found mainly north of 80 N. This anomaly is strongest at the surface to about 900mb, though the column does go above 100mb. A bit to the south of this is a very strong column of positive omega anomalies with two centers present, both of which are found between 65 N-80 N. The weaker of the centers is found closer to the surface, 24

34 Fig. 3.4 Same as Fig 3.1 except for Omega. 25

35 with its center between mb. The larger center is located further up in the atmosphere, with its center found between mb. These two centers are definitely connected by a weaker, though still fairly strong column of positive omega anomaly values. To the south of this, in both the southern portions of the northern Polar cell and the northern portions of the northern Ferrel cell is a very large and strong area of negative omega values. The anomaly field is located between 55 N-65 N, with the center located between 850mb to above 100mb. This column, however, does stretch from the surface to above 100mb, though it is not especially strong right at the surface. As for the rest of the northern Ferrel cell, it is dominated by a very large area of positive omega anomalies that also branches into the northern Hadley cell. The main branch of the positive omega values is found between 20 N-55 N, and the center goes from the surface to above 100mb. Above 150mb is where the branching occurs to the other portion of the positive omega anomaly field. This branch is found between 0-15 N, with its center located between 900mb upwards to above 100mb. The column of this branch does reach the surface with some fairly strong omega anomalies. Within this large field of positive omega values, there are two small areas of negative omega values within the northern Hadley cell. The first is the weaker of the two, with its center found between 25 N-30 N and between mb vertically. The other negative omega anomaly field is found between 15 N-20 N with its center found between the surface and 875mb. These two 26

36 centers of negative omega anomalies, although close to one another, do not appear to be associated with one another as they are separated by an area of positive omega anomalies. Moving into the southern Hadley cell, it is mostly dominated by a very large and strong negative omega anomaly field. This field, which is found between 10 S-20 S expands to take up the region between the equator and 20 S above 600mb. The center of this anomaly is also very strong, as it starts from about 950mb and goes to above 100mb. In the southern portions of the southern Hadley cell is part of a positive omega anomaly that is located in the southern Ferrel cell. This positive omega anomaly is associated with another positive omega anomaly found in the central and southern portions of the southern Ferrel cell. The northern center, which is located between 28 S-35 S has its center going from the surface to about 940mb, though a fairly strong portion of this center goes as high as 475mb. In the central and southern portions of the southern Ferrel cell is the main portion of the positive omega anomaly. Found between 40 S-60 S, the center starts at about 950mb and reaches above 100mb. Portions of this anomaly do reach the surface, though not everywhere. The last anomaly of note in the southern Ferrel cell is a negative omega anomaly in the upper portions of the atmosphere. This negative omega anomaly is centered between 30 S-35 S with its center between mb, though fairly strong portions of this anomaly go as far down as 315mb. In the southern Polar cell, there are a few more anomalies of note. The first is a strong negative omega anomaly. This anomaly is found between 60 S-65 S between the 27

37 surface and 600mb, whereas above 60mb the anomaly expands to be found between 60 S-75 S. The center of this anomaly is found between the surface and 180mb, and overall this negative omega anomaly column goes from the surface to above 100mb. South of this are three centers of positive omega anomalies. The main center is located between 75 S-85 S and the center of the anomaly goes from the surface to about 650mb. One of the branches of the main positive omega anomaly is centered between 65 S-72 S, and its center is located between the surface and about 700mb. The other branch of the positive omega anomaly is located further up in the atmosphere, with its center between mb vertically and between 75 S-78 S. The final anomaly of note is a rather small but fairly strong negative omega anomaly with two centers. Located south of 85 S, both of the centers are located further up in the atmosphere. One of the centers is located between mb and the other between 250mb to above 100mb, though the overall column goes down in the atmosphere as far as almost 725mb. The final cross section to look at is the meridional wind anomalies (Fig. 3.5). Like Fig. 3.4, the key point of Fig. 3.5 is that the Hadley, Ferrel and Polar circulations are all intensified. In the meridional wind anomaly case, a positive anomaly refers to a northerly wind anomaly, and vice versa for negative anomalies. Like the zonal wind anomaly graph, the meridional wind anomaly graph is also very busy. Starting with the northern Polar cell, at the surface there is a weak northerly wind anomaly present. This anomaly is found north of 75 N, and portions of it go from the surface to about 875mb. 28

38 Fig. 3.5 Same as Fig 3.1 except for meridional winds. 29

39 Above this weak anomaly are two small but very strong southerly wind anomalies. The center of one of the anomalies is at 85 N and the other at 75 N. Both centers are located at around 700mb, with portions that go from 900mb to about 400mb, which would likely indicate that these two centers are related. The final anomaly of note in the northern Polar cell is a very strong northerly wind anomaly. Although the center of this anomaly is located between mb, strong portions of this anomaly go down in the atmosphere as far as 350mb. Moving into the northern Ferrel cell, there are two strong wind anomalies near the surface. The first is a northerly wind anomaly, located between 40 N- 60 N, with its center between mb vertically. A portion of this anomaly field does reach a bit into the northern Polar cell, and although very strong, it is rather small. These northerly wind anomalies appear to be associated with a larger scale northerly wind anomaly field that go upward in the atmosphere of the northern Ferrel cell, crossing into the northern Hadley cell between mb, and connects with another very strong northerly wind anomaly center. This anomaly center extends into the southern Hadley cell, as far south as 15 S, and although the center of this anomaly field goes into the stratosphere, the strongest points of this center of the anomaly reach as far down in the atmosphere as 250mb. Moving further south, there is a very large southerly wind anomaly field present. Like the anomaly field just discussed, this one is also an expansive field stretching into the stratosphere and through multiple circulation cells. This southerly wind anomaly comes into contact with the surface at two locations: about 30

40 35 N in the northern Ferrel cell and between 5 N-10 N in both the Hadley cells. The anomaly field in the northern Ferrel cell has a maximum anomaly center between mb, and this center connects with the largest center. The largest southerly wind anomaly center is located in both the Hadley cells, between 15 N-10 N and from mb vertically. The anomaly field begins to move south between mb, where it encounters another southerly wind anomaly center. This center is almost exclusively in the southern Ferrel cell, between mb vertically. Finally, the southerly wind anomalies continue up in the atmosphere and move further south to the final southerly wind anomaly center. This center is in both the southern Hadley and Ferrel cells, and although a majority of it is in the stratosphere, parts of it reach down in the atmosphere as far as 225mb. Within the very large southerly wind anomaly field are two small northerly wind anomaly fields. Once is in the northern Hadley cell, centered between 10 N-20 N and between mb vertically. The other cell is much stronger, and is located in the southern Hadley cell. Its center is between 0-15 S and is located around 700mb. Continuing to move south, there is another large anomaly field, with this one being another northerly wind anomaly field with multiple centers of maximum. This northerly wind anomaly field starts at the surface in the southern Hadley cell between 10 S-30 S, and moves slightly south as it goes up in the atmosphere. Another portion of this anomaly field that starts at the surface is in the southern Polar cell, south of 70 S, with 31

41 one portion branching up in the atmosphere into the stratosphere and another portion going a bit north. These two portions of the anomaly field do appear to connect at about 63 S and 850mb, though it is difficult to say with certainty. The main center of maximum of this northerly wind anomaly field is located in the southern Ferrel and Polar cells. The field is found between 35 S-80 S, and can be found between 850mb to the top of the atmosphere. There are two other anomalies of note, both of which are southerly wind anomalies. The first is located near the surface in both the southern Ferrel and Polar cells. The center of this anomaly field is found between 45 S-65 S, and stretches from the surface to about 900mb. The other anomaly field has multiple centers of maximum, but only in the southern Polar cell. The three centers of note are located at 65 S, 70 S and 75 S, and are vertically at 700mb, 600mb and 150mb respectively. They all appear to be associated with one another, though some of the connections are somewhat weak. The next analysis technique to be used is the global horizontal analysis. With this method, each variable is analyzed one at a time. First a global scale analysis is performed before a synoptic scale analysis is performed. The first variable to be analyzed is the Sea Surface Temperature (SST), as shown in Fig 3.6. Starting with the global analysis, there is a fairly strong positive SST anomaly present north of 60 N. The strongest part of this anomaly is located in the Arctic Ocean, just off the coast of the Canadian Arctic region. Moving south, between 30 N-60 N, this cell of SST does not fall into a positive or 32

42 Fig. 3.6 Same as Fig. 3.5 but for horizontal map of extreme phase SST. 33

43 negative anomaly realm. For instance, there is a strong negative SST anomaly present in a good portion of the Pacific Ocean. On the other hand, there is a small, but strong, positive SST anomaly field present in the western half of the Atlantic Ocean. However, between 30 N-30 S, these cells do fall into a SST anomaly category. This region is dominated by positive SST anomalies, especially in the Indian Ocean. There are a few small patches of negative SST anomalies found in this region. However, the overwhelming majority of SST anomalies in this area are the positive SST anomalies, so this region will be defined that way. The cell south of 60 S can also be characterized as a positive SST anomaly cell, with the strongest anomaly in the Weddell Sea. As for the 30 S-60 S cell, this region is quite peculiar. Between 30 S-40 S, this area has mainly positive SST anomalies. However, between 40 S-60 S, there is a clear belt of negative SST anomalies present that are quite strong. Moving on now to the synoptic analysis, starting in the North Pacific, or more specifically, near where one would find the Aleutian Low, there is not anything of note in the Aleutian Sea. However, to the south in the central North Pacific, there is a very large cold anomaly area. This pool of cool water reaches from the Asian continent to almost the North American continent. There appears to be a few centers of especially cool water found in the central North Pacific, in the Sea of Japan, the Sea of Okhotsk, and a small area at around 8 N, 165 W. With the exception of this area, most of the rest of the North Pacific appears to be above average when it comes to the SST. There are a few areas that 34

44 are much warmer than expected. One such area originates of the southwest coast of North America, around Baja California, and reaches down to near the equator. Another area appears to be in the very southern portions of the Gulf of Alaska, just off the west coast of Canada. Finally, there is a very large warm SST anomaly in the East Indies, extending out around the equator, almost reaching the International Date Line. Moving into the South Pacific, there are a few more SST anomalies of note. First, there is a very large warm SST anomaly found off the west coast of South America. This area extends into the equatorial regions of the central portions of the South Pacific, where the anomaly covers a much larger area. Moving to the west, there is an area of cool SST anomaly present. This anomaly, however, seems to be associated with a cool SST anomaly that circumnavigates the globe. Between about 35 S-60 S, part of which is roughly defined as the Southern Ocean, there is an area of almost of cold SST anomaly present. While some warm SST anomalies are found in this belt, the region is dominated by what looks like a single cold SST anomaly. Only one portion of this cold belt extends outside of the area defined, which is the area found just NE of New Zealand. This is merely an extension of the cold anomaly belt, but this extension is by far the closest this anomaly field gets to the equator. Moving into the South Atlantic, there is a very large area of warm SST anomaly present. From just north of the cold SST belt through the Southern Ocean, all the way into the southern portions of the North Atlantic, this whole area is a warm SST anomaly. 35

45 Some portions of this anomaly are weaker than others, but basically all of the South Atlantic is above what is to be expected for SST. Moving into the North Atlantic, there are a few more regions of anomalous SST s besides the warm anomaly from the South Atlantic that reaches up a bit into the North Atlantic. This first area of note is in the Caribbean Sea, where there is a cold SST anomaly present. This anomaly extends down to the waters off the NE coast of South America, and a few centers of especially cool water are present. These areas include off the Gulf coast and the Atlantic coast of Florida, and off the NE coast of South America. East of the cold anomaly off of Florida there is an area of warm SST anomaly present. This area, in the central and western portions of the Atlantic, is where the development of many of the hurricanes found in the North Atlantic occurs. To the North/NNW of this, we see another area of warm SST anomaly. This area, however, is much smaller, as it reaches from the coast of the Mid-Atlantic eastward, moving slightly north into the central North Atlantic. Although small, this warm anomaly is quite strong, with what appears to be at least two areas very warm water present. To the north of this, there is a very strong cold SST anomaly. This anomaly reaches from Baffin Bay, across the Atlantic and into the North Sea and Norwegian Sea. This cold SST anomaly is quite strong, with a primary belt of very cold SST reaching from the coast of mainland Canada to just north of the British Isles. Finally, north of Iceland and to the east of Greenland, there is a concentrated area of a warm SST anomaly. Although this warm anomaly 36

46 appears to be throughout the Arctic Ocean, and reaching down slightly in certain places, there is at least one center located in the North Atlantic with an especially strong warm SST anomaly. Some other SST anomalies have not been mentioned, due to their locations and the unlikelihood that they affect the PDO in any way. However, these areas are still taken into account when the PDO structure was determined. This is also true for all the other variables discussed in this paper. Next is an analysis of the precipitation rates during the extreme phase, as shown in Fig This will aid in determining the position of pressure systems at the surface, as one would expect to see decreased precipitation in areas of High Pressure, and vice versa for Low Pressure systems. Starting first with the global analysis, looking at the area north of 60 N, it would appear that this cell cannot be characterized one way or the other. For instance, there is a strong negative precipitation anomaly present in the Arctic Ocean, just north of the Aleutian Sea. On the other hand, just to the west of this, there is a strong positive precipitation anomaly just off the Siberian coast, also in the Arctic Ocean. Along with this cell, the areas between 30 N-60 N, and also south of 60 S 37

47 Fig. 3.7 Same as Fig. 3.6 except for precipitation rates. 38

48 cannot be categorized either. The cell between 0-30 S can be categorized, however. This cell is the only cell that has above average precipitation anomalies present. The strongest area of this anomaly is located over most of the northern half of South America. This leaves two cells left, 0-30 N and 30 S-60 S, to fall into the negative precipitation anomaly category. Both of these cells have very strong anomalies, with the strongest found throughout the central Pacific and in the southern Indian Ocean, respectively. Now for the synoptic scale features, starting in the North Pacific, and more specifically, the Aleutian Sea, there is a small, weaker area of decreased precipitation anomaly. This anomaly extends slightly south into the North Pacific, and extends northward into the Arctic Ocean. There are some areas with a strong decrease in precipitation, mainly in the Arctic Ocean. However, this temperature anomaly is not as strong in the Aleutian Sea as other portions of the anomaly are. Moving into the eastern portions of the North Pacific, there is a very large area of increased precipitation anomaly, starting from the Aleutian Islands and extends all the way to southern Central America. This anomaly has a few very strong positive anomalies located in the southern Gulf of Alaska, off the California coast, and off the coast of southern Mexico. Further, this anomaly has an arm that reaches into the central North Pacific, all the way to northern Japan, and slightly into the Sea of Okhotsk. The only other positive precipitation anomaly found in the North Pacific is found in the East Indies. There is a very strong positive precipitation anomaly, which appears to be centered in the eastern 39

49 portions of the Indian Ocean. As for the decreased precipitation anomalies in the North Pacific, there is one more of note. This anomaly appears to be centered along the equator in the central and eastern portions of the Pacific Ocean. However, portions of the negative precipitation anomaly extend to just north of Hawaii, portions of Indonesia, and also up to Japan. Further, this anomaly is likely associated with the decreased precipitation anomaly found in the Aleutian Sea and the Arctic Ocean. However, the strongest areas of decreased precipitation are found along the equator, and also up and around Japan. Moving into the South Pacific, the first anomaly of note is a strong positive precipitation anomaly just south of the equator. This anomaly does not extend far north to south, but it does appear to traverse the Pacific Ocean. There are a few centers in this positive precipitation anomaly, but by far the largest is found from around 150 W to around 105 W. South of this, there is another positive precipitation anomaly, though this one is not as strong and is much smaller. This anomaly is centered just ENE of New Zealand, but does not appear to be associated with the positive precipitation anomaly found covering all of Australia. Finally, the last positive precipitation anomaly of note is found in the just off the coast of Antarctica. Further, it appears that this anomaly circumnavigates the globe through the Southern Ocean. This anomaly never extends all that far from the coast of Antarctica, but still has plenty of centers. As for the negative precipitation anomalies, there is only one of note. This anomaly, however, is quite large 40

50 as well. This negative precipitation anomaly also circumnavigates the globe, much like the negative SST anomaly noted earlier. Although not as well defined as the SST anomaly, there are a few centers found in most of the Pacific Ocean, over the southern tip of South America, through the Atlantic Ocean and in the Indian Ocean. This anomaly also wanders a bit as it is found mostly between 20 S-60 S. Moving over to the South Atlantic, the only anomaly of note that has not been discussed yet is found around the equator. The center of this anomaly appears to be positioned over Brazil, and may also be connected to the narrow positive precipitation anomaly near the equator in the South Pacific. Portions of this anomaly almost reach Africa, and other portions extend down to around 45 S. Moving north into the North Atlantic, there are a few more anomalies of note. First off, there is a small positive precipitation anomaly found in the middle of the North Atlantic at around 50 W. This anomaly is not very strong, but it does appear to have a small center in the southern portion of the anomaly. Completely surrounding this warm precipitation anomaly is a large, strong negative precipitation anomaly. This anomaly appears to have two centers found in the eastern Caribbean Sea and off the east coast of Florida. This anomaly, however, appears to be connected to the negative precipitation anomaly centered over the Sahara and Sub-Saharan regions in Africa. In the northern portions of the North Atlantic, there is a fairly strong positive precipitation anomaly. There appears to be a few centers located just south of Greenland and Iceland, and also off the west coast of Great Britain. 41

51 Finally, there is a strong negative precipitation anomaly with a center located just north of Iceland. This center appears to be associated with a larger region of negative precipitation anomaly that reaches into the Barents Sea and the portions of the Arctic Ocean. The next feature to be analyzed is the surface Sea Level Pressure (SLP), as shown in Fig This figure will aid in determining the placement of pressure systems at the surface, but the other factors do play a role in determining the structure of the extreme phase of the PDO. First looking at the global scale features of SLP anomalies, there are strong negative SLP anomalies found at both the poles (north of 60 N, south of 60 S). The strongest anomaly in the Arctic region is found in the eastern portions of Russia. As for the Antarctic region, almost all of the area south of 60 S is at the maximum negative SLP anomaly. In both of the temperate region cells (30 N-60 N, 30 S-60 S), neither of these areas can be categorized by one SLP anomaly or the other. For instance, a fairly strong negative SLP anomaly is found over most of the Pacific Ocean, whereas most of the rest of the 30 N-60 N cell is dominated by a positive SLP anomaly. As for the tropical regions (30 N-30 S), these cells are dominated by very strong, positive SLP anomalies. There is no specific area where the strongest SLP anomaly center is located. The anomaly instead covers most of the tropical region with about the same strength. Now looking at the synoptic scale features, starting in the Aleutian Sea of the North Pacific, there is a very strong negative pressure anomaly. This anomaly covers 42

52 Fig 3.8 Same as Fig. 3.6 except for SLP. 43

53 most of the North Pacific above the Tropic of Cancer. The center of this negative pressure anomaly appears to be located about half way between the Hawaiian Islands and mainland North America. There is also a smaller, weaker negative pressure anomaly that appears to be associated with this larger anomaly. The center of this anomaly is a little ENE of the Philippines, and although one could make the argument that this is a separate anomaly, given the large, expansive structure covering most of the North Pacific, this smaller anomaly is most likely a part of the larger anomaly. The only other pressure anomaly in the North Pacific is a positive pressure anomaly found along the equator, and also extending northward along the east coast of Asia into the Sea of Okhotsk. This anomaly appears to be a part of one large positive pressure anomaly that circumnavigates the globe at the equator. This positive pressure anomaly is most prevalent between the Tropic of Cancer and the Tropic of Capricorn. However, portions of this anomaly reach northward into the Arctic Circle through the North American and Asian continents. Further, this positive pressure anomaly reaches southward of Africa and Australia. The strongest portions of this anomaly are found around the equatorial regions, but these strongest areas can be found between 55 N-50 S. Moving southward into the South Pacific, the only anomaly found here that has not been discussed is a negative pressure anomaly. This anomaly also circumnavigates the globe, but is mostly found south of 30 S, with the strongest areas being found south of 40 S. There are a few places where this negative pressure anomaly reaches northward 44

54 of 30 S, such as just off the west coast of South America. This arm of the anomaly is not as strong as the main portions of the negative pressure anomaly, but it is still quite negative nonetheless. Going into the South Atlantic, there is only one anomaly of note present that has not been discussed to this point. This is a small, but strong negative pressure anomaly found over Brazil and portions of the South Atlantic near the Equator. This anomaly is centered over Brazil and its strength appears to be quite centralized. Finally, in the North Atlantic, the only anomaly present that has not been discussed is found in the northern portions of the North Atlantic. With strong negative pressure anomaly centers found over Hudson Bay and Greenland, this anomaly also circumnavigates the globe. However, this anomaly does not extend further south than 75 N at some portions. Further, there only appears to be one more negative pressure anomaly center of note other than the two already mentioned, which is located in northeastern Siberia. The final figures to be analyzed are the 1000mb zonal and meridional wind anomalies, as shown in Fig. 3.9 and Fig. 3.10, respectively. From these figures, one would be able to find areas that appear to have rotation, which would likely indicate the presence of a pressure system. For Fig. 3.9, the key feature is that unlike what is seen in the circulation cells in the Omega and meridional wind cross sections, the Walker circulation is shown to be weakened in the zonal wind anomaly graph. First looking at the global circulation patterns, the zonal wind anomalies are very variant. For example, 45

55 Fig. 3.9 Same as Fig 3.6 except for 1000mb zonal winds. 46

56 Fig. 3.10: Same as Fig. 3.6 except for 1000mb meridional winds. 47

57 the strong easterly wind anomaly located over Greenland, along with a strong westerly wind anomaly over the Arctic Ocean, just north of the eastern portions of Russia. Along with the North Hemisphere, the 0-30 S and south of 60 S cells do not fall into any category either. The only cell that can be categorized is the 30 S-60 S cell. The strong westerly wind anomaly shows its greatest concentration between 50 S-60 S. As for the meridional wind anomalies, there is also a lot of variance present when looking at the global circulation patterns. North of 30 S, none of the cells can be categorized as one type of anomaly or the other. An example of this is found between 0-30 N, where throughout the Eastern Hemisphere is dominated by southerly wind anomalies. However, throughout most of the Western Hemisphere, especially over the Pacific Ocean, this region is dominated by northerly wind anomalies. South of this, between 30 S-60 S, this cell is dominated by southerly wind anomalies. The strongest of these anomalies is located off the east coast of South America. Finally, the cell south of 60 S has primarily a northerly wind anomaly pattern, with the strongest anomaly located over the Ross Sea. Now concentrating on synoptic scale features, around the Aleutian Sea in the North Pacific, there is some cyclonic rotation directly over the Aleutian Islands. There are strong wind gradients in both the zonal wind and meridional wind anomaly graphs. Moving SE of this, about half way between the Hawaiian Islands and mainland North America, there is another area that appears to have cyclonic rotation. This rotation is 48

58 shown both in the zonal and meridional wind anomaly graphs, which both have strong wind gradients present. Moving almost directly west into the western North Pacific, there is one more area of cyclonic rotation south of Japan. Both the zonal wind and meridional wind anomaly graphs show fairly strong wind gradients present, likely indicating the presence of a Low Pressure system. Finally, the last area of interest in the North Pacific is just SE of the previous area of interest, just west of the International Date Line. This area, however, is exhibiting anti-cyclonic rotation. Although not especially strong in either the zonal wind or meridional wind anomaly graphs, there is a definite clockwise rotation present. Moving into the South Pacific, there are a few more areas of possible rotation to note. The first area is found just north of New Zealand. This area of anti-cyclonic rotation does not show up that well when looking at the meridional wind anomaly graph, but there is definitely a strong wind gradient present in the zonal wind anomaly figure, likely indicating weak counter-clockwise rotation. Moving directly east, there is another area of anti-cyclonic rotation. This area of rotation is just like the previous area of rotation in that the rotation is not that evident in the meridional wind anomaly figure, but is displayed very clearly in the zonal wind anomaly figure. The final anomaly of note in the South Pacific appears to be the strongest of the three. This area of cyclonic rotation is found just east of the previous rotation, just off the west coast of South America. There 49

59 is a fairly strong wind gradient in the zonal wind anomaly graph, but there is definitely a very strong wind gradient in the meridional wind anomaly graph. Moving east into the South Atlantic, there are areas of circulation worthy of note. The first area of rotation is a smaller area of anti-cyclonic rotation found off the southeast coast of Argentina. There is a large, strong wind gradient in the zonal wind anomaly graph. However, the wind gradient in the meridional wind anomaly graph is much smaller, though just as strong. Moving ENE into the center of the South Atlantic, there is an area of cyclonic rotation. In this case, the opposite is true as compared to the last case: a very small, but strong wind gradient is seen in the zonal wind anomaly graph, whereas a large, strong wind gradient is seen in the meridional wind anomaly graph. North of this, over the eastern portion of Brazil, there is another area of cyclonic rotation. When looking at the zonal wind anomaly graph, although gradients are present, an east-west gradient would be most conducive for rotation. Although the gradient is not like this, and combined with the strong pressure gradient seen in the meridional wind anomaly graph, it still appears that there is at least weak cyclonic rotation. Finally, directly east of this in the middle of the Atlantic Ocean, there is an area of anti-cyclonic rotation. This area appears to have very strong rotation, as there are strong wind gradients in both the zonal wind and meridional wind anomaly graphs. Moving north into the North Atlantic, there are a few more areas of circulation to note. First, there is an area of anti-cyclonic rotation just off the west coast of northern 50

60 Africa. This is a smaller area of circulation, though it appears to be strong as there are strong wind gradients in both the zonal wind and meridional wind anomaly graphs. Further, this is where the African waves travel as they begin their journey across the Atlantic Ocean, sometimes becoming a tropical cyclone. It would seem that a High Pressure being present in this location would not be conducive to forming such storms. Moving northward, there are two areas of cyclonic rotation. The first is found over Hudson Bay. Although there appears to be almost no wind gradient near Hudson Bay in the zonal wind anomaly graph, there is definitely a wind gradient when looking at the meridional wind anomaly graph. Further, in looking at SLP in Fig 3., there is a strong, negative pressure anomaly found over Hudson Bay. This likely indicates the presence of cyclonic rotation. The final area of note is located over the southern portions of Greenland. The opposite appears to be true in this case, as there is a very strong wind gradient over southern Greenland in the zonal wind anomaly graph, but almost no wind anomaly in the meridional wind anomaly graph. Another look at the SLP in Fig 3. shows that all of Greenland has a negative pressure anomaly, thus likely indicating the presence of another cyclonic rotation. Now going up in the next graph to analyze is the 200mb geopotential heights, as shown in Fig Starting first with the global scale circulations, the northern Hadley cell, along with the southern Hadley and Ferrel cells are dominated by positive geopotential height anomalies. The strongest area appears to be the southern Ferrel cell 51

61 Fig Same as Fig. 3.6 except for 200mb geopotential heights. 52

62 that is almost exclusively dominated by the maximum positive geopotential height anomalies. As for the northern Ferrel and Polar cells, neither can be categorized one way or the other as both anomalies are present and neither is dominating. For instance, there is a very strong negative geopotential height anomaly over the eastern portions of Asia, whereas there is a very strong positive geopotential height anomaly over the western half of North America and parts of the eastern Pacific Ocean. Finally, the southern Polar cell is dominated by negative geopotential height anomalies, with the strongest parts of the anomaly over the Antarctic continent. Moving to the synoptic scale, starting with the North Pacific region, there are a few anomalies of note. The first to mention is the strong negative geopotential height anomaly that is centered over eastern Asia. This anomaly reaches out into the North Pacific past the International Date Line. Most of the rest of the North Pacific region, however, is dominated by a large positive geopotential height anomaly field. This field appears to be centered in the equatorial regions, though it could be argued that the center is around 45 S as well. The positive geopotential height anomaly covers almost all of the area west of the Hawaiian Islands, and also along the west coast of North America. There is a peculiar feature SE of the Hawaiian Islands near the equator. There is a center of weakening of the positive geopotential height anomaly. This center, located at around 140 W, is still a fairly strong positive geopotential height anomaly, is weaker than its surroundings and is worthy of note. This area of weakened positive geopotential height 53

63 anomaly extends to the southwest, where it ends at another center of weakened positive geopotential height anomaly to the north of New Zealand, near Samoa. Most of the rest of the region in the South Pacific region north of 60 S, with the exception of the trail of the weakened positive geopotential height anomaly, is dominated by an extremely positive geopotential height anomaly. Between 60 S-65 S, there is a very strong gradient of the height anomaly as it shifts to a negative geopotential height anomaly south of this gradient. Moving into the South Atlantic, this whole region is dominated by the extremely positive geopotential height anomalies, with the exception of two areas. Both of these areas are like the areas encountered in the tropical Pacific: a weakened, though still fairly strong positive geopotential height anomaly. One of the centers is located just off the coast of Brazil, at around 25 S, with the other centered over the eastern reaches of Africa. This anomaly is also found at around 25 S, but is in more of an oval shape as portions of this anomaly stretch from the eastern South Atlantic into the central reaches of the Indian Ocean, and some could even argue that it reaches Australia. However, since this area is very unlikely to affect the PDO, it will not be explored further. Finally, the North Atlantic region has a few areas of note. For the most part, the North Atlantic is dominated by the extremely positive geopotential height anomaly south of about 50 N, with one notable exception. There is another center of the weakened positive geopotential height anomalies centered over the eastern portion of Africa, at 54

64 around 15 N-30 N. This anomaly stretches across Africa and over the Atlantic Ocean, with its strongest points making it about half way across the Atlantic to North America. This is interesting because this is there area of African Wave development, a key factor for tropical cyclone development in the region. This case should be explored in a future study. The last anomaly of note in the North Atlantic region is a negative geopotential height anomaly that is centered over Greenland. Portions of this anomaly reach Canada to the west, and just past Iceland to the east. There is a fairly sharp gradient associated with the transition from the positive geopotential height anomalies that dominate the equatorial region to this negative geopotential height anomaly. This gradient area is found anywhere between 50 N-70 N depending on the longitude. Staying up in the atmosphere, an analysis of the zonal and meridional wind anomalies at 200mb follows, as shown in Fig and Fig. 3.13, respectively. The key point of Fig 3.12 is the same as Fig 3.9, with the Walker circulation weakened, unlike the Hadley, Ferrel and Polar circulations. Looking at the global scale first and more specifically, the zonal wind anomalies, there are no easterly dominated wind anomalies in any of the six global cells. However, there are a few westerly wind anomalies and a few cells that cannot be categorized. For instance, the cell north of 60 N cannot be categorized either way. There is a strong westerly wind anomaly over the Canadian Arctic and into the Arctic Ocean region. Most of the remaining portions of this region 55

65 Fig Same as Fig. 3.6 except for 200mb zonal winds. 56

66 Fig Same as Fig. 3.6 except for 200mb meridional heights. 57

67 are dominated by easterly wind anomalies, but not enough for the cell to be defined this way. Along with this cell, the 0-30 N and the 30 S-60 S cells cannot be defined either. As for the remaining cells (30 N-60 N, 0-30 S and south of 60 S), these can be categorized as westerly wind anomaly dominated. A good example of this is the strong westerly wind anomaly belt circumnavigating the globe around 60 S. As for the meridional wind anomalies, none of the cells can be categorized one way or the other. The overall picture of the anomalies is quite convoluted, but they also appear somewhat symmetric in a north-south track. Now concentrating on the synoptic scale, starting first around the Aleutian Sea in the North Pacific, there is an area of anti-cyclonic rotation to the north in the Arctic Ocean above Alaska. There is a strong wind gradient in the zonal wind anomaly graph, and also in the meridional wind anomaly graph, though the gradient is not directly northsouth as one would like to see. This would likely indicate a weaker pressure system. However, it is likely there is still one present as there is some rotation present. Moving south to just east of the Hawaiian Islands, there is another area of anti-cyclonic rotation present; there is a very strong wind gradient in the zonal wind anomaly graph. Further, there is a fairly strong wind gradient found in the meridional wind anomaly graph, thus likely indicating the presence of a fairly strong anti-cyclonic rotation. The last area of circulation of note in the North Pacific is just SE of this anti-cyclonic rotation. This-area of cyclonic rotation, found just north of the equator, shows very little wind gradient when 58

68 looking at the zonal wind anomaly graph. However, there is a very strong wind gradient in the meridional wind anomaly graph, likely indicating the presence of weak cyclonic rotation. Moving southward into the South Pacific, there are a few more areas of rotation to be discussed. The first area of note is an area of cyclonic circulation found just north of New Zealand. This cyclonic rotation appears to be fairly strong as there are strong wind gradients in both the zonal wind and meridional wind anomaly graphs. To the SSE of this, and a bit ESE of New Zealand, there is an area of anti-cyclonic rotation. Although this area shows up very well in the zonal wind anomaly graph with a very strong wind gradient, the meridional wind anomaly and its wind gradient is much weaker because it is not directly north-south. However, rotation is still present, though not as strong as it could be. The final area of circulation to note is found just off the southern coast of Chile. There is very strong anti-cyclonic rotation, as indicated by the strong wind gradients exhibited in both the zonal wind and meridional wind anomaly graphs. Moving across South America into the South Atlantic, there are not many areas of circulation to note. The first is an area of cyclonic circulation found over the southern coast of Brazil. This cyclonic circulation appears to be quite strong as there are strong wind gradients present in both the zonal wind and meridional wind anomaly graphs. The other area of circulation is found just to the east, where there is an area of anti-cyclonic rotation centered over the west coast of southern Africa. Although the wind gradient is 59

69 small when looking at the meridional wind anomaly graph, it is quite strong. Further, the wind gradient in the zonal wind anomaly graph is strong, indicating that there likely is a small, but robust area of anti-cyclonic rotation. Moving into the North Atlantic, it is only slightly busier. Just north of the equator, and just off the coast of northern Africa, there is an area of cyclonic rotation. The zonal wind anomaly graph shows a very strong wind gradient present. And although the meridional wind anomaly graph also shows a strong wind gradient, it is not directly north-south, this weakening the circulation. However, the circulation still appears to be there, just not very strong. Also, this likely area of Low Pressure is also where one would expect to see African Waves passing through, which could affect possible tropical cyclones developing in the North Atlantic. Moving almost directly north of this, there is an area of anti-cyclonic rotation. This area of circulation appears much stronger, with very strong wind gradients present in both the zonal wind and meridional wind anomaly graphs. Further, the wind gradients in the zonal are almost directly east-west, and the wind gradients in the meridional are almost directly north-south, indicating that this area of circulation is quite strong. Finally, the last area of circulation in the North Atlantic is an area of cyclonic rotation over the southern portions of Greenland. This area of circulation appears to be quite strong, with strong wind gradients present in both the zonal wind and meridional wind anomaly graphs. 60

70 Now to take a look at the December, January and February geopotential height anomalies for the Northern Hemisphere, as shown in Fig The key feature of Fig is the wintertime polar vortex, which is intensified from the surface all the way up in the atmosphere to 10mb. First looking at the 1000mb geopotential heights, there are a few areas to note. For the most part, there appears to be one positive and one negative geopotential height anomaly dominating, with a few smaller anomalies also present. The positive geopotential height anomaly covers most of the tropical and temperate regions of the Northern Hemisphere. This anomaly stretches from the west coast of North America eastward to just short of the west coast of the Hawaiian Islands. The largest anomalies appear to be closer to the equator, although a few areas stretch further north, such as into the central plains of the United States and also into northern China. Besides this anomaly, there is only one more positive geopotential height anomaly present in the Northern Hemisphere that is not connected to the main anomaly. This small positive geopotential height anomaly is found in the Sea of Okhotsk. Although nowhere near as strong as the main anomaly, it is nonetheless completely 61

71 Fig Same as Fig 3.1 but for horizontal map of northern hemisphere DJF mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb. 62

72 surrounded by a strong negative geopotential height anomaly. This negative geopotential height anomaly is the other large anomaly of note in the Northern Hemisphere. This anomaly is located more in the Polar Regions, although it does cover most of the central and eastern North Pacific. The most negative heights appear to be found in the central and eastern North Pacific, over Greenland and over eastern Russia. There are three smaller negative geopotential height anomalies completely separated from the main anomaly, the largest of which is located over Central China. This anomaly is not very large or strong, and is almost completely surrounded by the main positive geopotential height anomaly. Moving up in the atmosphere to 500mb, one begins to see the geopotential height anomalies concentrate. There is only one positive geopotential height anomaly present. It circumnavigates the globe, with its strongest area again near the equator. However, there is one fairly positive geopotential height anomaly present, with its center located over western Canada and reaching northward to the Arctic Ocean. As for the negative geopotential height anomalies, there are three large, concentrated areas. The largest and most anomalous is located in the North Pacific, reaching from the edge of the Asian continent to just shy of the west coast of the United States. The center of this anomaly is located in the central portions of the North Pacific. The next negative geopotential height anomaly is located in the North Atlantic, and also over Iceland and portions of Greenland and Canada. This anomaly field is slightly smaller and weaker, with its center being 63

73 located just south of Greenland. Finally, the weakest of the three anomalies is located in northern Europe and Asia. This anomaly does not have a very well defined center, but it appears to be located somewhere over northern Russia. Moving up in the atmosphere even more, this time to 100mb, the geopotential height anomalies are consolidating even more. The positive geopotential height anomaly covers most of the tropical and mid-latitudes of the North Hemisphere, and even extends into some polar regions, such as northern Canada and Alaska. It appears that the most positive geopotential height anomalies are concentrated again near the equator, though stretching further north than seen at 100mb or 500mb. As for the negative geopotential height anomaly, it covers most of the polar region in the Northern Hemisphere. The most negative area of this anomaly appears to stretch from the northeast coast of Canada, across Europe and Asia and just reaching the Pacific Ocean. Finally, at 10mb, one begins to see the geopotential height anomalies weaken. It appears that the positive geopotential height anomaly is weakened more, but is still located in almost all of the tropic and mid-latitude regions. Its strongest areas appear around the equator in the Eastern Hemisphere. As for the negative geopotential height anomaly, it too has been weakened, though not as much as the positive geopotential height anomaly. The negative anomaly is almost exclusively found from 90 W eastward to 90 E, with the most negative area of the anomaly located over northwestern Russia. 64

74 Moving southward, the analysis will now take a look at the June, July and August geopotential height anomalies for the Southern Hemisphere, as shown in Fig The key feature of Fig is that the wintertime polar vortex is intensified from the surface to 100mb. First, in looking at the 1000mb graph, it would appear that there is one positive and one negative geopotential height anomaly present. The positive geopotential height anomaly is located mainly in the tropics and the mid-latitudes of the Southern Hemisphere. Further, the anomaly stretches from the west coast of South America, eastward through the Atlantic and Indian Oceans before dying in the central South Pacific. The most positive portions of the anomaly are located close to the equator, with the anomaly going as far south as the southern tip of South America. As for the negative geopotential height anomaly, it has three centers, with the strongest located over the South Pole. The negative anomaly reaches a bit into the South Atlantic, where the smallest of the three centers are located, and reaches a bit further north into the South Pacific. There is also a small negative anomaly over a portion of Brazil, but is likely a part of this main anomaly that is not shown as it is in the Northern Hemisphere. Moving up to 500mb, the positive geopotential anomaly is getting stronger and larger, whereas the negative geopotential height anomaly is doing just the opposite. The positive geopotential height anomaly is moving south, reaching into the Southern Ocean in most places. However, the most positive height anomalies are still located closer to the equator, with the most positive areas still reaching the tip of South America. There is 65

75 Fig. 3.15: Same as Fig except but for southern hemisphere JJA mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb. 66

76 also a very small, very weak positive geopotential height anomaly present over Antarctica worthy of note. As for the negative geopotential height anomaly, it is still located in the polar regions of the Southern Hemisphere, but is getting smaller as height increases. The two areas of most negative geopotential height are located over the Antarctic Peninsula, and also just off the coast of Antarctica south of Australia. There is also a tiny, weak area of negative geopotential height present within the positive height field. This anomaly is located just west of the International Date Line in the South Pacific. Moving up in the atmosphere again, this time to 100mb, there is the positive geopotential height anomaly taking control of the Southern Hemisphere. The positive areas cover all of the tropics and mid-latitudes and reach down onto parts of Antarctica. The most positive regions, although close to Antarctica, appear not to make it to the continent. There are also two slightly weaker locations of the positive height anomaly located over Brazil and the southern portions of Africa. The negative geopotential height anomaly only covers a small portion of the Antarctic continent, including a portion of the Ross Ice Shelf. The vast majority of the negative geopotential height anomaly is located in the Western Hemisphere. Finally, at 10mb, the geopotential height anomalies take a different shape. The stronger of the two positive geopotential height anomalies is present in the polar region, with the most positive height anomalies located over all of Antarctica, reaching 67

77 northward to the southern tip of South America. The other positive height anomaly is located in the tropics, and appears to be associated with a positive height anomaly in the Northern Hemisphere. As for the negative height anomaly, the only one present is in a semi-circle shape through the mid-latitudes of the Southern Hemisphere. The negative height anomaly field reaches from just south of the African continent westward through the Pacific Ocean, across Australia, and finally stops in the eastern portions of the Indian Ocean. The two most negative areas of the anomaly are found in the South Atlantic and also over Australia and parts of the eastern Indian Ocean. Putting all this information together, we were able to create the 3-Dimensional structure of the PDO during its extreme phase, as shown in Fig The bottom figure in Fig depicts the extreme phase of the PDO at 1000mb, whereas the top shows the extreme phase of the PDO at 200mb. As stated earlier, even though portions of the previous maps were not discussed in the paper, they were taken into account when creating the 3D structure of the PDO. 68

78 Fig Schematic depiction of the global structure of PDO s warm phase at 200mb (upper) and 1000mb (lower). 69

79 CHAPTER 4 GLOBAL 3-DIMENSIONAL STRUCTURE OF THE TRANSITION PHASE Now we will take a look at the transition phase. The transition phase is formed from the transition index which is during the transition from the cold phase to the warm phase. Different features that make up the three-dimensional structure of the PDO will be discussed. In order to create such a three-dimensional structure, we will examine different variables to create such a structure. We will look at the zonal mean crosssections, global horizontal maps and polar stereographic maps of all the different variables. The first zonal mean cross section for analysis is the air temperature, as shown in Fig For this cross section and all of the others, an analysis of the entire atmosphere (90 S-90 N, ) is done from the surface, upward in the atmosphere and well into the stratosphere. In looking at the air temperature cross section, there are some areas of note. In the northern Polar cell (60 N-90 N), there is a strong positive air temperature anomaly present in the lower and mid-atmosphere. The strongest portion of this anomaly is located north of 75 N, and between mb vertically. Moving south to look at the northern Ferrel cell (30 N-60 N), the northern Hadley cell (0-30 N), and the southern Hadley cell (0-30 S), all of these cells are dominated by negative air temperature anomaly. The center of this anomaly is located in the northern Hadley cell, 70

80 Fig. 4.1: Linear correlation of annual mean zonal air temperature with the transition index for the transition phase. 71

81 between 15 N-30 N at about mb vertically. Away from the center of the anomaly, there are many branches of this anomaly stretching into other cells, though much weaker. Almost the whole northern Ferrel cell is dominated by the weaker portion of this negative anomaly, and same for the southern Hadley cell, extending from the surface up to the stratosphere. Even parts of the weaker portion of this anomaly extend into the northern Polar cell, but only in the upper atmosphere. As for the southern Ferrel cell (30 S-60 S), there a two small, weak positive air temperature anomalies present. The first is the smaller of the two, located between 35 S-40 S, stretching from the surface to about 885mb vertically. The other anomaly is located in the mid-atmosphere, between 30 S-45 S, and found between mb vertically. Finally, in the southern Polar cell (south of 60 S), there are two air temperature anomalies of note. The first is a negative air temperature anomaly located at the surface. This anomaly appears to have two centers: one located between 65 S-70 S, stretching from the surface to about 975mb, and the other anomaly is found south of 85 S and between mb vertically. Higher up in the atmosphere, there is a strong positive air temperature anomaly present. Within the troposphere, the center of this anomaly is located south of 85 S, and between mb vertically. However, this anomaly appears to be a part of a larger positive air temperature anomaly that is found throughout the stratosphere. Next is to analyze the zonal mean cross section of the geopotential height (Fig. 4.2). The geopotential height could indicate areas where High or Low pressure systems 72

82 Fig. 4.2 Same as Fig. 4.1 but for geopotential heights. 73

83 would be present. Unlike the air temperature cross section, the geopotential height cross section is much less busy. Right along 60 N, there is a negative geopotential height anomaly present, almost equally between the northern Polar cell and the northern Ferrel cell. The center of this anomaly is located between 55 N-65 N, and between about mb. This negative anomaly appears to be associated with another negative geopotential height anomaly present in the northern Hadley cell. The center of this negative height anomaly is found between 10 N-25 N and between mb vertically. These two centers appear to be connected together by a weak negative anomaly in the upper troposphere and lower stratosphere. As for positive geopotential height anomalies, the only one of note in the Northern Hemisphere is located in the Ferrel cell. The center of this positive anomaly is located between 35 N-40 N, and stretches from the surface to about 950mb. Moving into the Southern Hemisphere, the only anomaly of note in the southern Hadley cell is a portion of the negative height anomaly found in the northern Hadley cell. It is not especially strong, nor does it have a center, but it appears to cover approximately the whole southern Hadley cell. In the southern Ferrel cell is a small, but rather strong positive geopotential height anomaly. This anomaly is found between 50 S-60 S, and stretches from the surface to about 350mb. This positive anomaly appears to be associated with a stronger positive geopotential height anomaly located in the stratosphere of the southern Polar cell. Finally, below this center in the mid-atmosphere is a negative geopotential height anomaly in the southern 74

84 Polar cell. The strongest portion of this anomaly is located south of about 82 S, and is found between mb vertically. Now for the zonal cross section of the zonal wind anomalies (Fig. 4.3) in which the westerly wind anomaly will be identified by positive values, and vice versa for the negative values. Starting in the northern Polar cell, there are two anomalies to note. The first is a westerly wind anomaly located near the surface. The strongest portion of this anomaly is located between 75 N-85 N, and stretches from the surface to about 875mb. The other anomaly of note is an easterly wind anomaly located in the middle of the atmosphere, with its thin center between 65 N-70 N, between mb vertically. Moving into the northern Ferrel cell, the only westerly wind anomaly of note is centered between 45 N-50 N, and is between the surface and 280mb vertically. In the very southern portion of the northern Ferrel cell is part of an interesting easterly wind anomaly structure. This easterly wind anomaly is found between 25 N-35 N, and stretches from the surface and into the stratosphere. From there, this anomaly curves south and connects with another easterly wind anomaly, with this portion located in the southern portions of the southern Ferrel cell. The arm in the Southern Hemisphere has the stronger anomaly and also appears to be larger than the other arm in the Northern Hemisphere. Along with this is a smaller, but still fairly strong easterly wind anomaly located in the northern Hadley cell, found between the equator and 15 N. This extension connects to the northern arm of this anomaly field between mb, but is not connected to the 75

85 Fig. 4.3 Same as Fig. 4.1 but for zonal winds. 76

86 southern arm. As for the southern Hadley cell, the only westerly wind anomaly of note is a fairly weak one, located between the equator and 10 S, and is between mb vertically. Portions of this anomaly do stretch into the northern Hadley cell, but this westerly wind anomaly is the weakest of all the anomalies present and is overshadowed by the easterly wind anomaly that surrounds it. The only other anomaly of note is a very strong westerly wind anomaly that dominates the southern Polar cell. This anomaly covers basically the whole Polar cell and stretches from the surface all the way to the stratosphere. The next cross section to analyze is the Omega cross section, as shown in Fig In this figure, positive anomalies would indicate sinking motion, which would likely signal the presence of a High Pressure system, or at least air that is more stable. On the other hand, negative values would indicate sinking motion, meaning a Low Pressure system is likely, or at least less stable air. Starting with the northern Polar cell, it appears to be split up by two anomalies. North of 80 N is an area of negative omega values, with the center located between mb. There is also another center located above 200mb, but this is starting to be too high in the atmosphere. Between 60 N-80 N, there is a fairly strong positive omega anomaly present, stretching throughout the entire column. The center of this anomaly field appears to be further up in the atmosphere, though rather positive values still reach as far down as about 525mb. 77

87 Fig. 4.4: Same as Fig. 4.1 but for Omega. 78

88 Moving into the northern Ferrel cell, there are two more anomalies of note, almost dividing the cell up again. A strong negative omega anomaly is found between about 50 N to 60 N and even about 62 N at some heights. The center of this anomaly is located between mb, though this entire anomaly goes from the surface to above 100mb. South of this is a fairly strong positive omega anomaly field. This field has two branches that are located in both the northern Ferrel and Hadley cells. There are a few centers of maximum positive omega anomaly of note. There is one found at about 45 N and between mb vertically. Another is located between 35 N-40 N, with its center between mb. The last anomaly center in this branch is found at about 45 N and between mb. Between about 20 N-40 N is the connecting branch to the other portion of the positive omega anomaly field. Where the two branches appear to meet is an area of increased positive omega anomaly values. This center is between 25 N-30 N and stretches from the surface to about 920mb. The final center of note for this field of positive omega anomalies is the strongest and largest. Its center is at about 15 N and is between mb, though this column does go from the surface to above 100mb. The only other anomaly of note in the northern Hadley cell is a small, though fairly strong negative omega anomaly. This anomaly is found between 0 N-15 N, with its center between mb, though the effects of this anomaly go from the surface to about 725mb. 79

89 Looking at the southern Hadley cell, there are again two anomalies of note. The first is a fairly strong negative omega anomaly found between the equator and about 20 S. There appear to be two centers of stronger negative anomalies, with one at the surface around 10 N, and the other between mb at around 15 S. However, the overall scope of this negative omega anomaly is quite large as it goes from the surface to just above 200mb. South of this is a positive omega anomaly that is found over the southern portion of the southern Hadley cell and northern portion of the southern Ferrel cell, between about 25 S-35 S. This positive omega anomaly has two centers of note. Closer to the surface is the weaker of the two, with its center found right at 30 S between mb. Further up is the stronger anomaly center, which is also centered at about 30 S, though this center is between mb. This column, overall, reaches from the surface to above 100mb. Continuing in the southern Ferrel cell, there is also a fairly strong negative omega anomaly present. This anomaly is located between 35 S-45 S, with its center found between mb. Overall, this column starts at around 925mb and goes about as high as 125mb. South of the negative omega anomaly is another positive omega anomaly. This anomaly is located between 50 S-60 S and has two centers. The first is located between the surface and 930mb, with the other center found between mb. Overall, this column is fairly strong and goes from the surface to about 175mb. 80

90 Finally, moving into the southern Polar cell, there are a few anomalies of note. There is a very strong negative omega anomaly found between 65 S-75 S. This negative omega anomaly is the strongest one in this figure, with its center found between mb. Just south of this is a strong positive omega anomaly, found between 75 S-80 S. This positive omega anomaly has its center located between about mb. Just south of this is another negative omega anomaly, this one found between 80 S-85 S, with a strong center located at between mb. The final anomaly of note is a positive omega anomaly found south of 85 S. This anomaly center is not especially strong, though its center is located between the surface and 650mb. Overall, all of the anomalies in the southern Polar cell are very prominent as all of them go from the surface to above at least 200mb. The final cross section analysis is for the meridional wind anomalies, as shown in Fig For this figure, the positive values represent northerly wind anomalies, whereas the negative values represent southerly wind anomalies. Like the zonal wind anomaly figure previously, the meridional wind anomaly figure is also quite busy. Starting in the northern Polar cell, there appears to be a southerly wind anomaly present between 60 N- 75 N and between mb. This southerly wind anomaly, however, appears to be a branch of the main southerly wind anomaly, which is located in the northern Ferrel cell. The main center for this anomaly is between 40 N-50 N, and between mb. There is one more branch to this southerly wind anomaly field, which is located between 81

91 Fig. 4.5 Same as Fig 4.1 but for meridional winds. 82

92 30 N-35 N, and stretches from the surface to about 900mb. This branch s connection to the main southerly wind anomaly is not as strong as the branch located in the northerly Polar cell, but it does appear that it is indeed a branch of the main anomaly in the northern portion of the northern Ferrel cell. Continuing with the Ferrel cell, there are two northerly wind anomalies to note. The first is located in the middle of the Ferrel cell, centered at about 45 N, with its center reaching about 900mb. The other northerly wind anomaly is centered between 30 N- 35 N at around 500mb. This anomaly is rather extensive, as it stretches throughout the middle of the atmosphere in the northern Hadley cell and reaches into part of the southern Hadley cell. The only other anomaly of note in the northern Hadley cell is found between 10 N-20 N, and stretches from the surface to about 900mb. Now in the southern Hadley cell, there is a strong northerly wind anomaly present that appears to be associated with the expansive northerly wind anomaly found in the northern Ferrel cell. This branch s center is between 20 S-30 S, and stretches from the surface to about 885mb. This branch of the anomaly is weakly connected to the main part of the anomaly, but it is definitely connected. Further up in the atmosphere of the southern Hadley cell, there is a small but fairly strong southerly wind anomaly present, with its center around 25 S at around 200mb. Although this center is starting to get close to the stratosphere, a good portion of this anomaly is located in the troposphere and is still worthy of note. Moving into the southerly Ferrel cell, there are two northerly wind anomalies that appear 83

93 to be associated with one another, along with another anomaly in the southerly Polar cell. The main portion of this anomaly is located in the southerly Ferrel cell, centered between 40 S-50 S and between mb vertically. The first branch is located in the upper troposphere of the Ferrel cell, with its center at about 35 S and between mb vertically. The other branch is located in the southern Polar cell, with its center between 60 S-65 S and stretching from mb. This branch also extends up into the stratosphere, where there is another center, but will be ignored due to its location. In the southern Polar cell, there is one other northerly wind anomaly of note, which could be related to the northerly wind anomaly in the southern Ferrel cell. The anomaly center is located between 75 S-80 S, and between mb. As for southerly wind anomalies in the southern Polar cell, there are two centers that do not appear to be associated. The stronger of the two has two centers located at about 70 S, with one at the surface and the other around 600mb. The other southerly wind anomaly of note is located at about 85 S and is also found at around 600mb vertically. Now variables of the global horizontal maps for the annual mean will be analyzed. The purpose of this is to get a global look of specific variables in order to determine what could possibly affect the PDO. The first figure is the Sea Surface Temperature (SST), as shown in Fig The key feature of Fig. 4.6 is that the cold to warm transition phase of the PDO is characterized by cold SST anomalies in the North Atlantic subtropics. First, looking at the global SST anomalies, the key features are the 84

94 Fig. 4.6: Same as Fig. 4.1 but for horizontal map of transition phase SST. 85

95 SST anomalies found in some of the latitude belts. For instance, there is a warm SST anomaly belt found north of 60 N. The strongest portion of this belt is found in the northern portions of the Norwegian Sea. Moving south, between 60 N and 30 S, there is primarily a cold SST anomaly belt present. The strongest portion associated with this belt is located in the North Atlantic. This anomaly is located in the region where many African waves develop, some of which become tropical cyclones. This cold SST anomaly could therefore affect tropical cyclone development and strengthening, but this is outside the scope of this paper. Then, between 30 S and 60 S, there is another warm SST anomaly belt present. The strongest areas of this belt are found around the southern tip of South America in both the South Atlantic Ocean and the South Pacific Ocean. Finally, south of 60 S, there is another cold SST anomaly belt present. The strongest area of this belt is found in the Ross Sea of Antarctica. However, there is not a lot of ocean area found south of 60 S to take into account. Now, moving on to the synoptic scale, there are some key points located in the North Pacific Ocean. The first key point is an area of decreased SST anomaly located just south of the Aleutian Islands. This anomaly stretches from the Pacific Northwest of the United States westward across the Pacific Ocean and into the Sea of Okhotsk. Below average SST s could indicate the presence of a Low Pressure system, although this feature alone does not mean the system is present. The second point of interest is located in the Sea of Japan, where we see an area of increased SST anomaly. Increased SST 86

96 could indicate the presence of a High Pressure system, though this feature alone does not mean the system is present. Moving to the east a bit, just off the coast of Alaska, in the Gulf of Alaska, there is a small area of above average SST. Normally, there is an inverse relationship with this area and the SST off the coast of the Pacific Northwest of North America. This is likely associated with the inverse relationship of Salmon catches found in this area (Mantua et al. 1997). The next point is located SE in the East Central Pacific, just south of the Gulf of Alaska, where there is a presence of abnormally warm SST s in the area, which could indicate the presence of a High Pressure system. Finally, the last point is SE of this, just off the coast of Baja California, where there is an area of abnormally cold SST s. Although cooler SSTs are found off the west coasts of continents, this is even colder, so this is likely a location of interest. Looking at the Western half of the North Pacific, there are a few more areas of SST anomalies. First, off the east coast of Japan, nearing the International Date Line, there is the presence of a warm SST anomaly. There is also a warm anomaly in the Sea of Japan, though due to its small size, it is unlikely this would play much of a role in controlling the PDO. On the other hand, there are a few cool anomaly pools present, mostly in the southern portion and western portions of the North Pacific. There is a large cold anomaly pool around the Marshall Islands, extending south of Papua New Guinea to the NE coast of Australia. Finally, there are two small cold anomaly pools off the coasts of Vietnam in the South China Seas and off of China in the East China Seas. Although 87

97 these are not especially deep cool anomaly pools, given the fact that warmer temperatures off the East Coast of continents are the norm, these are likely to be places of interest. There are also some anomaly pools in the South Pacific. Even though these are farther away from the areas of interest concerning the PDO, they will be noted for the sake of completeness. Other than the cold pool near NE Australia, there are two other cold pools in the South Pacific. The first one is centered at around 130 W, but is spread out over a large area over Central and Eastern Pacific. The other cold pool anomaly is found in the Ross Sea area just off the coast of Antarctica. As for the warm pool anomalies found in the South Pacific, there are also only two present. However, both of these warm pool anomalies extend into other oceans. The first one is primarily found south of Australia. This warm pool stretches from the Eastern Indian Ocean at its western boundary all the way to the Tasman Sea at its eastern boundary. Unlike the large cold pool anomaly in the South Pacific, this warm pool anomaly does not appear to have any sort of a definitive center. The other warm pool anomaly spans from the Central South Pacific, across the South Atlantic and almost into the Indian Ocean. There appears to be two centers: one found in the Eastern South Pacific, and the other in the Western South Atlantic. However, this anomaly appears to be a single anomaly with two centers versus two anomalies interacting. In the Atlantic basin, there are a few temperature anomalies of note. Even though these anomalies are in the Atlantic, they could still be affecting the PDO, and therefore will be discussed. First, 88

98 the only cold pool anomaly seen in the Southern Atlantic is found at around 15 S, off the coast of Brazil. This cold pool anomaly could be associated with, or a part of, the cold pool anomaly found primarily in the middle of the North Atlantic. There appears to be a bit of a weak connection between these two locations, although the cold pool anomaly is much deeper in the North Atlantic, just off the coast of North Africa, and extending to the eastern reaches of the Caribbean Sea. To the ENE of the cold pool anomaly in the North Atlantic is a warm pool anomaly, primarily found along the east coast of the United States, and extending a bit into the middle of the North Atlantic. These waters must be very warm as normal conditions would suggest warm waters on the East Coast of continents, but this pool of water is even warmer than is expected. Finally, the last anomaly found in the Atlantic is rather far north, where the center appears to be in the northern parts of the Norwegian Sea. This anomaly is a warm pool anomaly that extends around the north coast of Iceland and ends along the southern tip of Greenland. This could be associated with the North Atlantic current, but since there is no clear trail to the Norwegian Sea; this is merely speculation. The next variable to analyze is the precipitation rate anomalies, as shown in Fig This can be helpful in determining the locations of High Pressure systems and Low Pressure systems as one would expect to find increased precipitation anomaly associated with a Low Pressure system and a decreased precipitation anomaly associated with a High Pressure system. The key feature of Fig. 4.7 is that there is weakened precipitation 89

99 Fig. 4.7 Same as Fig. 4.6 but for precipitation rates. 90

100 in the North Atlantic subtropics that is associated with the cold SSTs found in Fig First, looking at the global set up, there are large areas of positive precipitation anomalies located north of 60 N. The strongest areas appear to be located in the Arctic Ocean, just north of Alaska, as well as just north of Iceland. The only negative precipitation anomaly band present is located between 30 S and 60 S, with the strongest anomalies located off the east coast of South America. Concerning all the other regions not mentioned (between 60 N and the equator, as well as south of 60 S), both of these regions do not have a definitive precipitation anomaly pattern present. For instance, there is a strong positive precipitation anomaly located over the northwestern portions of Africa. Just off the coast from this, there is a strong negative precipitation anomaly present. Now moving on to the synoptic scale, starting in the North Pacific, looking in the location of the Aleutian Low, there are areas of increased precipitation; however these areas are rather patchy and not well defined. Moving south, however, more defined areas of precipitation anomalies begin to appear. First, there is an area of decreased precipitation in the Gulf of Alaska, which is also seen in the Pacific Northwest region of North America that extends southward to an area just east of the Hawaiian Islands. There is also an area of increased precipitation just to the east of the decreased precipitation anomaly, which extends down from Baja California, around to Central America, and reaches just into the Gulf of Mexico and the Caribbean. 91

101 Moving a bit west, there is a small, patchy, though strong area of decreased precipitation. This area is located around the International Date Line, from about 10 N to around 40 N. To the WNW of this, there is a strong area of increased precipitation over the Japanese Islands. This area of increased precipitation covers most of the Japanese Islands, extends out a bit into the North Pacific, and is also found in the Sea of Japan and most of the East China Sea. Finally, the last anomaly found in the North Pacific that is likely to be of interest is an area of decreased precipitation, located primarily in the South China Sea. Although a small anomaly, it is also quite strong, and could possibly play a role in the PDO. Moving to the South Pacific, there are a few larger areas of precipitation anomalies. The first area is a large region of increased precipitation just off the east coast of Australia and just north of New Zealand. The area extends into the western hemisphere, to around 155 W. This region has no real definitive center, but a few regions within it of localized maximums. To the ENE of this, there is a very small, though very strong area of increased precipitation. Even though this area is rather small, due to the fact that this area has a strong increased precipitation rate, it is quite likely that this region could be associated with a Low Pressure system. Finally, to the ESE, off the coast of Chile, there is an area of decreased precipitation. This anomaly appears to have a fairly good size, with a center located around 100 W. 92

102 Continuing to move east into the South Atlantic and the South American continent, there are a few areas of interest, though these areas all appear to be quite strong. First off, in the southern South Atlantic, there is a small, but rather strong area of increased precipitation. A rather long and narrow region of increased precipitation, the center appears to be in the eastern portion of the southern South Atlantic. Just to the north of this, in the southern portion of the tropics, there is yet another small, but very strong precipitation anomaly. This anomaly, however, is a decreased precipitation anomaly. Off of the coast of Brazil, the center appears to be in the western portions of the South Atlantic. Finally, there is a large area of increased precipitation over most of the central or northern portions of South America. A lot of this precipitation is likely associated with the Amazon jungles, and they are likely to have little to no influence on the PDO. Moving into the North Atlantic, there are only 2 areas of precipitation anomaly to note. These areas, however, are quite large and quite disparate from normal. The first one is a tongue-like shape sticking out from the Mediterranean Sea. This area of decreased precipitation is not very large, but is rather strong, and may be connected to the area of decreased precipitation in the South Atlantic. The other area of note is a very large area of increased precipitation in the northern half of the North Atlantic. This area extends from the eastern reaches of Canada, across the North Atlantic, and seems to end around the border of the Greenland Sea and the Barents Sea. This area of increased 93

103 precipitation appears to have a few centers, but given how well the system appears to be connected, this will be assumed to be one anomaly with multiple maxima. Now examining Fig. 4.8Fig, which is the yearly Sea Level Pressure (SLP) during the transition phase of the PDO. Although this figure will be extremely helpful in developing the surface structure of the PDO, this will not be the only factor analyzed. Including the precipitation anomalies and SST anomalies previously mentioned, the geopotential heights and the winds must be taken into account in order to best understand the PDO structure. The key feature of Fig. 4.8 is the significantly high SLP anomaly found in the North Atlantic subtropics that is associated with the cold SSTs that are found in Fig 4.6. The first job is to take a look at the global set up for the SLP anomalies. Starting from the north, there are both positive and negative SLP anomalies north of 60 N. Because of this, it cannot be defined as a positive or negative belt of SLP. For example, there is a strong negative SLP anomaly over the Greenland and extending into the North Atlantic Ocean, including the Norwegian Sea, and into the Arctic Sea. On the other hand, there is a strong positive SLP anomaly positioned over Siberia, approximately where the Siberian High is normally positioned. Along with this area, there is one other very large area that cannot be characterized as an above average SLP belt or below 94

104 Fig. 4.8 Same as Fig. 4.6 but for SLP. 95

105 average SLP belt. This area is located between 30 N-60 S. The only area with a negative SLP belt is found between 60 S-90 S. The strongest negative SLP anomaly in this area is found over eastern Antarctica, though it is not especially strong. Further, due to the topography of Antarctica, it is difficult to trust pressure measurements over the continental Antarctic region. Now moving on to the synoptic scale, starting in the North Pacific, primarily in the area one would expect to see the Aleutian Low. There is a negative pressure anomaly present, but it is a bit smaller than one would expect. Moving east to the Gulf of Alaska region, there is a very large positive pressure anomaly. This area covers most of the western half of Canada, most of the Gulf of Alaska, and extends down just south of Hawaii. The two regions just mentioned are related, according to previous research. During the cold phase of the PDO, the following situation is present: a weak Aleutian Low along with a High Pressure system in the southern portion of the Gulf of Alaska. During a warm PDO phase, however, there is a very strong, deep Aleutian Low and no High Pressure system in the Gulf of Alaska region. This may explain the changing Salmon catch patterns in the Alaska region and the Pacific Northwest (Mantua et al. 1997), although further research needs to be done. Off the coast of Baja California, there is an area of decreased pressure. This area is found on the leeward side of the Rocky Mountains, goes south of the Rockies, across Mexico, and extends a bit into the eastern portion of the North Pacific. West of this, on 96

106 the other side of the date line, there are a few more pressure anomalies. First off, there are two small positive pressure anomalies just past the International Date Line in the Western Pacific. Although these two areas are close together, they appear to be separate entities and will be treated as such. Finally, a little west of these positive pressure anomalies, there is an area of low pressure. This area appears to be one anomaly with two centers: one is centered just off the southern coast of Japan, and the other is found in the East Indies. Moving into the Southern Pacific, there are a few more pressure anomalies of note. First off is a strong negative pressure anomaly, centered NNE of New Zealand. The center of this negative anomaly appears to be quite strong; however, most of this anomaly weakens quickly moving away from the center. Almost directly to the east of this, there is another negative pressure anomaly. This anomaly, however, is much smaller and weaker than the one previously mentioned, but is still large enough to be worthy of note. Looking south of New Zealand, there is a very large positive pressure anomaly. This anomaly looks to be centered south of New Zealand, but overall this anomaly reaches from the far western Indian Ocean to about the central South Pacific. The other positive pressure anomaly located in the South Pacific is just off the coast of Chile. The center of this positive pressure anomaly appears to be just off the coast, but the anomaly covers a good portion of the southern half of South America. 97

107 Moving into the South Atlantic, there are a few more areas of pressure anomalies. First, there is a rather strong negative pressure anomaly off the SE coast of South America. Although the center is found just off the coast, portions of this negative pressure anomaly extend into the North Atlantic. To the SSW of the negative pressure anomaly center, there is a moderately strong positive pressure anomaly. The center appears to be just off the southern tip of South America, and although not especially strong, portions do reach into the South Pacific. Finally, there is a small, weak negative pressure anomaly near the southern African coast. Although this system is smaller and weaker to its counterparts in the South Atlantic, it is worthy of note as it will be discussed later. Moving northward into the North Atlantic, there are only two areas of pressure anomalies. However, both of these areas are very large is size and quite anomalous. First, in the middle North Atlantic, there is a very strong positive pressure anomaly. The center appears to a little distance off the coast of Spain, although there is a very strong region that traverses the Northern Atlantic. Overall, this anomaly is very large, as it reaches from the eastern portions of North America, across the Atlantic, to the western reaches of Europe and the NW portions of Africa. Finally, there is a very large negative pressure anomaly in the northern portions of the North Atlantic. Although there is no clear center, this negative pressure anomaly covers most of the Arctic Ocean, all of Greenland, and some portions of northern Europe. 98

108 Next to be examined is the zonal and meridional wind anomalies found at the surface. Both images are shown in Fig. 4.9 and Fig. 4.10, respectively. Analyzing these two features is probably the single most important analysis in the process of constructing the pressure systems as a certain level. This is because these anomalies will help show where rotation is present, thus indicating that there is likely a High or Low pressure present. Starting first with the global perspective, the analysis will be concerning the direction of the winds, rather than whether they form some sort of a rotation. For instance, there are primarily westerly wind anomalies found north of 60 N, with the strongest portion of the anomaly located in the Arctic Ocean, just north of Siberia. On top of this, a good portion of the global structure seems to be affected mainly by westerly wind anomalies, as these winds are also located between 30 N-60 N and also south of 30 S. The only easterly wind anomalies that dominate any of the global circulation cells is found between 0-30 N, with the strongest of these anomalies located in the North Atlantic Ocean. The final region, between 0-30 S, cannot be defined primarily by one anomaly or the other, as neither truly dominates the cell. For instance, there is a strong westerly wind anomaly located in the western South Pacific, whereas there is a strong easterly wind anomaly located over most of Australia. Now for the meridional wind anomalies, they only fall into two categories. The first is where there are primarily northerly winds, which are located between 0-60 S. 99

109 Fig. 4.9 Same as Fig. 4.6 but for 1000mb zonal winds. 100

110 Fig 4.10 Same as Fig. 4.6 but for 1000mb meridional winds. 101

111 The strongest of these anomalies appear to be in the central and eastern portion of the South Pacific. Now the rest of the cells (north of 0 and also south of 60 S) cannot be characterized as one type of anomaly or another; therefore, they will not be characterized as either. For instance, there is a strong northerly wind anomaly located in the central and western portions of the North Atlantic. As opposed to this, there is a strong southerly wind anomaly located over the central portions of the North American continent. Moving on to the synoptic scale in the North Pacific, and more specifically, the Bering Sea, the wind anomaly gradients are quite strong in the very south of the Bering Sea, as shown in Fig. 4.9 and Fig There is also the presence of a small negative pressure anomaly in Fig. 4.8Fig. This is to be expected, however, as the Aleutian Low is much weaker during the cold phase of the PDO, and thus should be weakened during the transition phase as well. SSE of the Aleutian Low, there is another pressure field anomaly. Although not very clear in the zonal wind graph, the anti-cyclonic rotation is quite clear in the meridional wind anomaly graph. This is confirmed in looking at the SLP figure, which indicates a positive SLP anomaly. As stated before, these two pressure anomaly fields appear to be inversely related. Continuing the SSE path from the southern Gulf of Alaska, there appears to be cyclonic rotation off the coast of southern California and Baja California. Although this rotation is not that pronounced when looking at the zonal winds, there is a more definitive wind gradient present in the meridional wind anomaly. This in confirmed when looking at the SLP anomaly graph, 102

112 which shows the Low Pressure anomaly extending into the central plains of North America. Directly west, there are two small areas of anti-cyclonic rotation, both of which reside just on the other side of the Date Line. As with the cyclonic rotation off of California, both of these rotations do not show up that well on the zonal wind anomaly map. But again, both systems do display quite nicely on the meridional wind anomaly map, and are confirmed to be present in the SLP anomaly map. Neither system is especially large, but these are the only anti-cyclonic systems on the west side of the Date Line in the North Pacific. Next to be examined is the South Pacific. The first system of note exhibits some strong cyclonic rotation. In looking at the zonal and meridional wind anomaly maps, there are wind gradients that exhibit clockwise rotation. This is confirmed in the SLP anomaly graph that shows a moderately deep negative pressure anomaly. Next, there is another anomaly of the SW coast of New Zealand. This system shows up rather clearly in the zonal wind anomaly graph, but there is not much to go off of in the meridional wind anomaly graph. However, this anti-cyclonic rotation is confirmed by the SLP anomaly graph, likely indicating the presence of the High Pressure system. Finally, in the central South Pacific, at around 130 W, there appears to be some weak rotation. Although shown in both the zonal and meridional wind anomaly graphs, the gradients are not especially strong, as compared to some of the other pressure systems. However, the 103

113 proper clockwise rotation is present, and the pressure anomaly is found on the SLP anomaly graph, though again it is not very strong. Moving into the South Atlantic, there are some areas of strong circulation off both coasts of South America. Off of the west coast, there is a very well defined anti-cyclonic rotation. This shows up very clearly in both the zonal and meridional wind anomaly graphs, both indicating counter-clockwise rotation. This is further confirmed by the positive SLP anomaly present in approximately the same area as the wind gradients are. Then looking off the east coast of South America, there is another well defined system, although this time it has strong cyclonic rotation. Again, both the zonal and meridional wind anomaly maps show the necessary wind gradients to indicate the presence of clockwise rotation. This is also displayed in the SLP anomaly graph, as this negative anomaly is the strongest negative anomaly present throughout the world for these conditions. This High/Low Pressure setup is strong enough that it could possibly play a role in the PDO, though further research is necessary to confirm such a hypothesis. There is also an area of weak cyclonic rotation in the eastern South Atlantic, just off the southern coast of Africa. Both the zonal wind and meridional wind anomalies show some cyclonic rotation and the SLP anomaly figure does show a negative pressure anomaly field present, but it does not appear to be very strong or large. Moving north into the North Atlantic, there are a few areas that may indicate rotation. First off, in the central North Atlantic, there is a large area of anti-cyclonic 104

114 rotation. This rotation is displayed very well by the zonal wind anomaly graph, but not as well by the meridional wind anomaly graph. However, by looking at the SLP anomaly graph, there is a very large positive pressure anomaly field that would likely confirm the presence of anti-cyclonic rotation. Moving into the northern portions of the North Atlantic, there is the likely presence of cyclonic rotation. As in the previous case, this rotation is very well displayed by the zonal wind anomaly graph, but not the meridional wind anomaly graph. But again, the SLP anomaly graph shows an area of negative pressure, indicating the likely presence of cyclonic rotation. Some of the other areas of rotation are not discussed here, but will be displayed in Fig. 4.16, which will display the structure of the PDO at the surface and at 200mb. The next variable to analyze is the horizontal structure of the geopotential heights at 200mb, as shown in Fig Starting with the global circulation scale, a good portion of the circulation cells are dominated by negative geopotential height anomalies. The northern Polar and Hadley cells, along with the southern Hadley cell, are all dominated by negative geopotential height anomalies. The strongest portion of the anomalies is located in the southern portion of the northern Polar cell. The center of this anomaly is located over Iceland and parts of the Atlantic Ocean, with very strong portions of this anomaly reaching the eastern coast of Greenland. The northern Ferrel cell cannot be characterized one way of the other as both positive and negative geopotential heights are present within the cell and neither appears to be dominant. For 105

115 Fig 4.11 Same as Fig. 4.6 but for 200mb geopotential heights. 106

116 instance, there is a fairly strong negative geopotential height anomaly over the center of Canada, but there are also 3 centers of positive geopotential height anomalies off both coasts of North America. As for the southern Ferrel and Polar cells, both of these can be characterized as positive geopotential height anomaly cells, even though a good portion of both of the cells have very little to no anomalies. The strongest of these cells is located south of Australia and New Zealand, and is located in both the southern Ferrel and Polar cells. Moving on to the synoptic scale, starting in the North Pacific region, there are a few anomalies of note. Starting in the region of the Aleutian Sea, there are two small centers of negative geopotential height anomalies located in the very southern portion of the Aleutian Sea and also over the Russian coast to the Aleutian Sea. Neither is especially strong nor big, but due to their location, they could very likely be indicating a deepening of the Aleutian Low. Just to the east of this are two positive geopotential height anomaly cells. The centers are a little stronger and larger than their negative counterparts to the west, but still are not very strong or very large in their own rights. One is located over part of Alaska and the Gulf of Alaska, and their other is directly south, centered at around 40 N. It is quite possible that these positive anomalies are present because of the negative height anomalies located in the Aleutian Sea region, previously suggested by Mantua et al (1997) of the opposite pressure systems during the cold phase of the PDO. The last two anomalies of note in the North Pacific region are 107

117 two more negative geopotential height anomalies, both located to the west of the Hawaiian Islands, Neither of these anomalies are especially strong, with one of them centered at about 165 W and the other at about the International Date Line. Moving south in the South Pacific region, there are a few anomaly centers of note. First are two smaller negative geopotential height anomaly cells located in the tropic regions. The first is the smaller of the two, though the stronger. This one is located over some South Pacific Islands, including Samoa. Although the stronger of the two, this anomaly is not especially strong in its own right. The other anomaly is located eastward, with the center located around 120 W. Most of this anomaly is located in the South Pacific, though a portion of it does extend into the North Pacific region. Further south of this are two positive geopotential height anomalies. The first is the smaller and weaker of the two, located just off the west coast of South America at around 33 S. This anomaly is comparable in size and strength to the negative geopotential height anomaly located over Samoa and the surrounding islands. Further south is the other positive geopotential height anomaly, which is the largest and strongest positive geopotential height anomaly in this figure. Located just south of New Zealand and Australia in the Southern Ocean, and over parts of Antarctica, it is unlikely that this large anomaly field plays much of a role in the development and progression of the PDO. Eastward into the South Atlantic region, there are only three anomalies of note. The first is a small, weak negative geopotential height anomaly located mainly over 108

118 Argentina, but does reach over a bit of the South Atlantic Ocean. This anomaly appears to be an anomaly center associated with the strong belt of negative geopotential heights that dominate the tropical regions. Over the central portion of the South Atlantic Ocean is the second anomaly of note, which is a positive geopotential height anomaly. This anomaly is bigger in size than the previous anomaly, but is not very strong and is unlikely to have an effect on the PDO. The final anomaly of note is a part of the main tropical region negative geopotential height anomaly that extends over Africa and a bit over the eastern portions of the South Pacific Ocean. A good portion of this anomaly is over Africa, into the Indian Ocean a bit, and into the North Atlantic. As this anomaly moves into the North Atlantic region, is has two negative geopotential height anomaly centers associated with it. Both are located between 20 N- 30 N, with one centered around 60 W and the other at about 25 W. Both are approximately the same size and strength, and could possibly affect the PDO as portions of these anomalies do reach into the North Pacific region, though barely. North of these anomalies in the only positive geopotential height anomaly in the North Pacific region. Centered in the mid-atlantic at around 50 W, this fairly strong positive anomaly reaches from the Maritime Provinces of Canada most of the way to continental Europe. The final anomaly of note is the strongest anomaly present for this figure. This strong negative geopotential height anomaly is centered mainly over Iceland. Portions of this anomaly field reach from Northern Canada to about Scandinavia. Although a bit out of the way of 109

119 where PDO would be expected, due to its size and strength, it is possible that this negative geopotential height anomaly could play a role in the effects of the PDO. Next to be examined is the zonal and meridional wind anomalies (Fig and Fig. 4.13, respectively) at 200mb. This will help to determine the overall structure of the PDO during the transition phase. As with the surface wind anomalies, strong gradients in both figures will likely indicate some sort of rotation. Again, the global structure will first be analyzed. Starting with the zonal wind anomalies, there is no definitive anomaly found in the cell north of 60 N. For instance, there is a strong westerly wind anomaly present over eastern Alaska, whereas there is a strong easterly wind anomaly located over Greenland. Because of this, this cell cannot be characterized by one type of anomaly. Further, this is also true for the 30 N-60 N and 30 S-60 S cells. There are three remaining cells that do fall into one category or another. Two of these cells, located between 0-30 N and 60 S-90 S, are both dominated by westerly wind anomalies. The strongest of these anomalies are located over the central Pacific Ocean and the far western portions of Antarctica, respectively. The final cell, located between 0-30 S, is dominated by easterly wind anomalies. The strongest of this wind anomaly is located off of both coasts of South America. 110

120 Fig 4.12 Same as Fig 4.6 but for 200mb zonal winds. 111

121 Fig 4.13 Same as Fig 4.6 but for 200mb meridional winds. 112

122 As for the meridional wind anomalies at 200mb, they are much like the meridional wind anomalies found at the surface: most of the cells do not fall into one of the two wind anomaly categories. An example of this is the cell located north of 60 N. There is a very strong northerly wind anomaly located in the Atlantic Ocean, extending into the Arctic Ocean. On the other hand, there is a very strong southerly wind anomaly located over the western portions of Greenland and parts of Davis Strait. Along with this, the cells located between 30 N-60 N, 0-30 N, and south of 60 S do not fall into one wind anomaly field or the other. This leaves two cells that do fall into one of the wind anomaly fields. Between 0-30 S, this cell is dominated by southerly wind anomalies. The strongest of these anomalies are located over the southern portions of Brazil. As for the cell between 30 S-60 S, it is dominated by northerly wind anomalies, with the strongest anomaly located just off the west coast of South America. Moving on to the synoptic scale features starting in the North Pacific, and more specifically the Aleutian Sea, the presence of a Low looks in doubt when looking at the meridional wind anomaly graph. Although we see strong gradients in the zonal wind anomaly graph, there is very little gradient present in the meridional wind anomaly graph to indicate the presence of a Low pressure. Moving south, just below the southern portions of the Gulf of Alaska and just north of the Hawaiian Islands, there is an area of anti-cyclonic rotation. There are very strong gradients in both the zonal wind and meridional wind anomaly graphs. Moving a bit further east of this, there is another area 113

123 of anti-cyclonic rotation directly over the southern tip of Baja California. This rotation appears rather small when looking at the zonal wind anomaly graph, but looks quite strong in the meridional wind anomaly graph. Finally, just off the SE coast of the Hawaiian Islands, there is some cyclonic rotation. Although the rotation is present, it does not appear to be very strong in either the zonal or meridional wind anomaly graphs. Moving into the Western North Pacific, there are two more areas of interest. Although these areas appear to be on the very eastern portions of the Asian continent, they are definitely close enough to the Pacific Ocean to possibly have an effect on the PDO, and will therefore be discussed. First there is an area of anti-cyclonic rotation just north of Manchuria, centered over the Sea of Okhotsk. Although the rotation is not especially strong when looking at the zonal wind anomaly map, the rotation is definitely present, as demonstrated by the meridional wind anomaly map. Then, so the SSW of this rotation, there is an area of cyclonic rotation over eastern China. When looking at the zonal wind graph, there is not a strong gradient to indicate the possibility of rotation. However, the meridional wind anomaly graph does show a strong gradient, likely indicating weak rotation. Moving into the South Pacific, there are not as many areas of rotation as would be expected, given its size. First, there is an area of small, concentrated gradients just north of New Zealand. This cyclonic rotation is not especially large or strong, but is definitely present in both the zonal wind and meridional wind anomaly graphs. The other area of 114

124 rotation is a larger area of anti-cyclonic rotation off the west coast of South America. The wind gradients appear rather strong, and also cover a much larger area than the previous area discussed. Now in the South Atlantic, there are more areas of rotation. The first area of note is an area of anti-cyclonic rotation in the very southern portions of the South Atlantic. Neither the wind gradients nor the area they cover appear to be very strong, but given the fact that they are present in both the zonal wind and meridional wind anomaly graphs, it is very likely that there is some rotation present. NNW of this, just off the coast of Argentina, there is an area of cyclonic rotation. Although both very large and very strong pressure gradient in the meridional wind anomaly graph, the zonal wind anomaly graph is much less impressive. However, enough evidence is there to indicate the likely presence of rotation. Finally, to the NE, just off the coast of Brazil, there is another area of anticyclonic rotation. There are fairly strong pressure gradients in both the zonal wind and meridional wind anomaly graphs. However, the gradients appear to be concentrated together in a small area where the anti-cyclonic rotation is believed to be. Moving into the North Atlantic, there appears to be a few more areas of rotation to note. First, in the western Mid-Atlantic, there is a small, centralized area of cyclonic rotation. The pressure gradients appear to be rather strong in both the zonal wind and meridional wind anomaly graphs, but are concentrated together in a small area. Almost directly east, there is another area of rotation just off the coast of Africa. This anti- 115

125 cyclonic rotation does not appear to be very strong or well-defined, as indicated by both wind anomaly graphs. The rotation, however, does appear to be present. Finally, in the northern portions of the North Atlantic, between Iceland and Greenland, there is an area of rather strong cyclonic rotation. There appears to be strong pressure gradients in both the wind anomaly graphs, and appears to cover a fairly large area. Now to examine the northern polar vortex of geopotential heights during December, January and February, as shown in Fig There are a few areas of anomalous geopotential heights. Positive geopotential height anomalies indicate the possible presence of a High Pressure, and vice versa for a Low Pressure. First looking at 1000mb, there are quite a few anomalous areas. Starting first with the North Pacific region, there is a positive geopotential anomaly over the northern half of the North Pacific. This is where one would expect to find the Aleutian Low, but it is likely not showing up because it is much weaker during the cold phase of the PDO, so it is unlikely it would be seen during the transition phase. In the eastern portion of the North Pacific, there is a negative geopotential height anomaly, about half way between Hawaii and Baja California. This is expected, however, as there is cyclonic rotation present, and a negative pressure anomaly in the SLP anomaly graph. In the western portion of the North Pacific, there are two weaker areas of negative geopotential anomalies. Although 116

126 Fig 4.14 Same as Fig. 4.6 but for horizontal map of northern hemisphere DJF mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb 117

127 the one anomaly by the Asian coast seems like it is in the right area, there is nothing much to tie together with the anomaly in the western portion of the Central Pacific. The next anomaly to note appears to cover about three-quarters of the northern portion of the Northern Hemisphere. This large negative geopotential height anomaly appears to have a few centers: in the Arctic Ocean, north of Alaska; over England, Scandinavia and portions of northern Europe; eastern portion of the Mediterranean Sea; and over the Himalayas. The strongest of these centers appears to be over the Himalayas. All of these centers are connected together by weaker areas of negative geopotential anomalies. Finally, there are only two other areas of positive geopotential height anomalies. The first one appears to be centered in the western part of the Gulf of Mexico, but weaker portions of this anomaly reach all the way to the coasts of Europe and Africa. The other positive anomaly appears to be centered over Russia. This anomaly has a large center, but is not much bigger. Moving up to 500mb, there are fewer anomalies, and the ones that are present are generally becoming larger. Starting in the North Pacific, there is a large positive geopotential height anomaly present, though centered a bit SW of where the Aleutian Low would normally be found. This anomaly stretches from Japan, across the entire Pacific Ocean, through the southern United States and Mexico, and into the Gulf of Mexico. In the North Atlantic, there is another positive geopotential height anomaly field, though this one is not as large and not very strong. The center appears to be just off 118

128 the coast of Portugal, but does not have a truly definitive center. The final positive geopotential height anomaly of note is found in northern Russia, by the Arctic coast. This anomaly is a bit north of the one found at 1000mb, but still appears quite strong, if not especially large. As for the negative geopotential height anomalies, they all appear to be connected at this height. The main centers are found over eastern Canada, Iceland, the western reaches of the Himalayas, and two centers over the eastern and western North Pacific, respectively. All of these centers are connected together by weaker anomalies, but the overall structure is quite impressive. Moving further up to 100mb, there are even fewer anomalies. In taking a look at the two positive geopotential height anomalies present, there is only one over the North Pacific, a little bit SW of the Aleutian Sea. This field reaches from western border of China, all the way to central North America. The other positive geopotential height anomaly is centered off of Portugal. Although this anomaly is not as strong or as large, it still stretches from western portions of the North Atlantic into Eastern Europe and Russia. As for the negative geopotential height anomalies, there are also only two present. The larger of the two is centered in the southern portions of the North Atlantic, although this field reaches around the globe. The other negative geopotential height anomaly field is centered over the North Pole, reaching into Russia and Greenland. This anomaly field is fairly strong, and not as large as the other negative field, is still quite large in its own right. 119

129 Finally, at 10mb, there is only one positive and one negative geopotential height. When looking at the negative geopotential height anomaly, its center appears to be over the northern portions of Russia. This anomaly appears to be quite strong, and covers most of the northern portions of the North Hemisphere. As for the positive geopotential height anomaly, this field circumnavigates most of the rest of the Northern Hemisphere. There appears to be a few centers, located in the North Pacific, over North America, and over Africa. However, these centers are not as well defined as the center of the negative geopotential height anomaly. Moving south, the southern polar vortex of geopotential heights during June, July and August, as shown in Fig. 4.15, will be examined. The first height to look at is 1000mb, which, not surprising, has the most anomalies. When looking at the positive geopotential height anomalies, there is one large anomaly and two smaller ones. Starting with the large one, this anomaly appears to have three centers positioned over eastern Australia, just east of New Zealand, and the final one positioned in the eastern Indian Ocean. These are all connected together by weaker anomaly branches. Next, just off the southwestern tip of Africa, there is another strong positive geopotential height anomaly. Though not as large, this anomaly stretches from the central South Atlantic to Madagascar. Finally, the smallest and weakest positive geopotential height anomaly is found off the western coast of South America. This anomaly, the smallest of the three, does not have a clear central point. Now looking at the negative geopotential height 120

130 Fig 4.15 Same as Fig but for southern hemisphere JJA mean geopotential height at (a) 1000mb, (b) 500mb, (c) 100mb, and (d) 10mb. 121

131 anomaly field, there only appears to be one present. However, this one anomaly is quite large. This anomaly has centers located off the east coast of Australia, over Antarctica, off the southwestern tip of South America, off the northern coast of Chile, and one over Brazil that stretches into the Atlantic Ocean. This anomaly appears to be connected together by portions that reach into the northern hemisphere. The strongest center appears to be just off the northern coast of Chile. Moving up to 500mb, there are fewer anomalies present. Looking at the positive geopotential height anomaly field, there appears to be one very large field and one smaller field. The smaller field is not all that small to begin with, however, as it stretches from Brazil to South Africa. The center of this positive geopotential height anomaly field appears to be just off the western coast of South Africa. The larger positive geopotential anomaly field reaches from the central Indian Ocean, through the Pacific to the western coast of South America. There appear to be three centers present, located in the central Indian Ocean, just off the east coast of New Zealand, and the largest and strongest of the centers is located off the west coast of South America. Looking at the negative geopotential height anomaly fields, it appears that there are three present. The smallest and the weakest of the three are centered just off the coast of Argentina. The next largest, and quite possibly the strongest appears to be centered directly over the South Pole. This field covers almost of Antarctica, and stretches some of the way into the South Pacific. The final negative geopotential height anomaly field appears to be more located in the 122

132 Northern Hemisphere. However, a few areas reach into the Southern Hemisphere, namely one portion with a strong center that is position north of New Zealand, near the equator. Continuing to move up in the atmosphere, there is only one positive geopotential height anomaly field and one negative geopotential height anomaly field present at 100mb. This field is centered south of New Zealand, just north of the Antarctic coast. This field is quite large, reaching from Australia into the eastern portion of the South Pacific. When looking at the negative geopotential height anomaly field, it appears that it is primarily located in the Northern Hemisphere, with portions reaching down into the Southern Hemisphere. There are two rather strong centers, however, that are located in the Southern Hemisphere. The first is centered over Antarctica. This portion of the anomaly field covers a good portion of Antarctica, and although barely connected to the main portion of the negative anomaly, it is connected nonetheless. The other portion is not as detached, as it is centered over southern Africa, as is firmly connected to the main portion of this negative anomaly field. Last are the anomalies at 10mb. There is not much going on here, with only two positive geopotential height anomaly fields and one negative geopotential height anomaly field. The smaller and weaker of the positive geopotential height anomaly fields is located over the central Indian Ocean, though it does not have a clear center. The other positive geopotential height anomaly field is centered ESE of New Zealand. This field is 123

133 much larger, about the size of Antarctica, and is also over portions of Antarctica. As for the only negative geopotential height anomaly field, it is the largest of the three, by far. Although no definitive center, it would appear the strongest portion of this field traverses the Indian Ocean. Overall, this negative geopotential height anomaly field stretches from just off the west coast of South America, through the Atlantic and Indian Oceans, and stops at the southeastern coast of Australia. Putting this information together, Fig was created. The bottom figure in Fig shows surface conditions, including the locations of pressure systems, above and below average SSTs, and areas of above and below average precipitation. Looking in the North Pacific, there is a small area of increased precipitation found in the northern Aleutian Sea, which extends into the Arctic Ocean and onto North America. There are also areas of increased precipitation found over Japan and just off the east coast of Asia, along with another area found off the west coast of North America and Central America, and extending into the Caribbean Sea. There are three areas of decreased precipitation to note in the North Pacific. First, in the eastern North Pacific, there is a fairly large area of decreased precipitation around Hawaii that extends both East and North onto the North America continent, namely in the Pacific Northwest and the plains states. Further, there is a small area of decreased precipitation just west of the International Date Line at 124

134 = Above average SLP = Below average SLP Fig Schematic depiction of the global structure of PDO s transition phase at 200mb (above) and 1000mb (below). 125