Southern Hemisphere cyclones and anticyclones: Recent trends and links with decadal variability in the Pacific Ocean

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 7: (007) Published online 6 February 007 in Wiley InterScience ( DOI: 0.00/joc.477 Southern Hemisphere cyclones and anticyclones: Recent trends and links with decadal variability in the Pacific Ocean Alexandre Bernardes Pezza, a, * Ian Simmonds a and James A. Renwick b a School of Earth Sciences, The University of Melbourne, Victoria 300, Australia b National Institute for Water & Atmospheric Research Limited, Private Bag 4-90, Kilbirnie, Wellington 6003, New Zealand Abstract: The aim of this paper is to study the association between the extratropical Southern Hemisphere and the decadal variability in the Pacific Ocean (PO). We discuss a pattern of coherent large-scale anomalies and trends in cyclone and anticyclone behaviour in light of the climate variability in the PO over the ERA40 reanalysis period (957 00). The two representative PO indices are the Pacific Decadal and Interdecadal Oscillations (PDO and IPO), and here the PDO is chosen owing to it being less associated with the southern oscillation index (SOI). Composites of the indicators of the density and intensity of cyclones/anticyclones given by an automatic tracking scheme were calculated for the years when the PDOI was more than one standard deviation above or below its mean. Although the ERA40 is not free from noise and assimilation changes, the results show a large-scale feature, which seems to be robust and agrees with earlier studies using different data sets. The sea-level pressure shows a strong annular structure related to the PDO, which is not seen for the SOI, with lower pressure around Antarctica during the positive phase and vice versa. More intense (and fewer) cyclones and anticyclones were observed during the positive PDO. This is less consistent for the SOI, particularly during the summer when a different PDO/SOI pattern arises at high latitudes. The trends project a pattern coincident with the positive PDO phase and seem to be linked with the main climate shift in the late seventies. Trends observed over the Tasman Sea are consistent with declining winter rainfall over southeastern Australia. Most patterns are statistically significant and seem robust, but random changes in ENSO may play a part, to a certain degree, in modulating the results, and a physical mechanism of causality has not been demonstrated. Although global warming and related changes in the Southern Annular Mode (SAM) may also help explain the observed behaviour, the large-scale response presented here provides a new insight and would be of considerable interest for further modelling studies. Copyright 007 Royal Meteorological Society KEY WORDS cyclones; anticyclones; Southern Hemisphere climate; climate variability; Pacific Decadal Oscillation Received 7 April 006; Revised 7 November 006; Accepted 9 November 006 INTRODUCTION Synoptic activity in the Southern Hemisphere, particularly extratropical cyclone and anticyclone behaviour, is strongly associated with climate variability in the Pacific Ocean (PO) basin. The most commonly known example of this association is the El Niño/Southern Oscillation (ENSO), which is one of the most studied interannual modes of variability in the global climate system. There is extensive discussion in the literature about ENSO predictability and teleconnection effects on temperature and precipitation (Rasmusson and Carpenter, 98; Philander, 985; Cane et al., 986; Karoly, 989; Ropelewski and Halpert, 989; Smith and Stearns, 993; Hoerling et al., 997; Folland et al., 00; Guo et al., 004) and synoptic * Correspondence to: Dr Alexandre Bernardes Pezza, School of Earth Sciences, The University of Melbourne, Victoria, 300, Australia. apezza@unimelb.edu.au weather systems (Sinclair et al., 997; Rusticucci and Vargas, 00). The terms Pacific Decadal Oscillation (PDO) (e.g. Mantua et al., 997; IPCC, 00) andinterdecadalpacific oscillation (IPO) (e.g. Power et al., 999a; Salinger et al., 00; IPCC, 00) have been used to describe ENSO-like decadal patterns of variability. The term PDO was first introduced in a multidisciplinary study of the association between low frequency SST and pressure anomalies in the North Pacific and salmon production in different locations off North America (Mantua et al., 997). The PDO/IPO was later shown to be one of the main modes of decadal to interdecadal variability in low tropospheric fields and SSTs for the whole globe (Tomita et al., 00). The impacts of this oscillation on Southern Hemisphere climate patterns have been acknowledged but are not fully comprehended (Sinclair et al., 997; Zhang et al., 997; Folland et al., 00 and references therein). The recent PDO time variability is marked by an abrupt change towards a warmer tropical eastern Pacific and a Copyright 007 Royal Meteorological Society

2 404 A. B. PEZZA, I. SIMMONDS AND J. A. RENWICK colder extratropical central North Pacific in (Trenberth and Hurrell, 994; Mantua et al., 997; Zhang et al., 997), and the reasons for this are still unclear. The PDO/IPO has also been linked to changes in the decadal climate variability over Australia (Power et al., 999b), in the impact that ENSO has on Australia (Power et al., 999a) and in the position of the South Pacific Convergence Zone (Folland et al., 00). It has been proposed that decadal large-scale midlatitude anomalies in the atmospheric circulation over the Pacific basin may cause decadal modulation of the ENSO through a teleconnection mechanism based on the projection onto the wind field overlying the equatorial regions (Barnett et al., 999). Such a mechanism would suggest that the PDO/IPO is not just a result of the random processes in the ENSO, although it has been demonstrated that El Niño is also a leading mode for interdecadal variability (Karoly et al., 996). The causes of the PDO/IPO have been investigated by Power and Colman (006). They showed that even random changes in ENSO activity on decadal and longer timescales will, in general, give rise to ENSO-like decadal patterns, even in the complete absence of predictability beyond interannual timescales. They noted, however, that the broader meridional structure of the decadal patterns is rather different from the interannual patterns, and showed that this could not be explained by randomness alone. Power et al., (006) noted that interdecadal changes in ENSO teleconnections to Australia were not predictable in their CGCM, again suggesting that the bulk of the decadal variability is driven by random changes in ENSO activity. The PDO/IPO versus ENSO issue could also be seen as part of a debate on the real nature of lower frequency oscillations in the atmosphere, and whether they represent a real climate behaviour that would, in principle, be at least partially predictable, or whether they are just noise due to stochastic mechanisms (e.g. Meinke et al., 005; Power and Colman, 006). Yasunaka and Hanawa (005) argue that some of the important regime shifts that occurred in global SSTs during the last century have happened concurrently with ENSO events, and suggest that decadal-scale variations and the ENSO events are not independent of each other. However, the PDO/IPO appears to have a much stronger signal in the extratropics than the typical El Niño teleconnection mechanisms (Latif and Barnett, 994, 996; Mantua et al., 997; Zhang et al., 997), and not much is known about its links with cyclone or anticyclone behaviour. Meinke et al. (005) made an assessment on the global rainfall variability on a range of timescales and stressed its importance at the decadal and interdecadal timescales in the Southern Hemisphere, particularly over the Australian region, with enormous implications for agricultural management. A recent paleo-climate reconstruction of the PDO over the past millennium suggested that although a year periodicity was seen for the past 00 years, it was not strongly evident during preceding periods (MacDonald and Case, 005). Therefore, caution should be exercised when using the present interdecadal behaviour to estimate the temporal spectrum of the PDO in terms of climate predictions (Power and Colman, 006). Recent studies using automatic tracking schemes to address cyclone and anticyclone behaviour in the Southern Hemisphere have pointed out emerging trends in some of the indicators associated with these systems, such as total number of systems, system density (SD), radius, depth (DP) and other factors. In particular, an increase in the intensity of cyclones accompanied by a decrease in their number has been suggested in a number of studies (Simmonds and Keay, 000; Pezza and Ambrizzi, 003 and references therein), although the significance of the results may be partially dependent on the type of data set and period used. In this article, we study the association between the decadal variability in the PO and cyclones and anticyclones in the Southern Hemisphere. This is presented in light of the recent trends observed in the indicators of density and intensity of cyclones and anticyclones over the last 50 years, as given by the Melbourne University automatic tracking scheme (Murray and Simmonds, 99). In the section Data and Methods, we describe the automatic tracking scheme and data set used. In the section Results, we present the results for the mean sea-level pressure (MSLP) patterns obtained for the PDO and southern oscillation index (SOI) composites (3.), the indicators of density and intensity of cyclones (3.) and anticyclones (3.3) for the PDO behaviour during winter and highlight some of the differences between PDO and SOI during the summer (3.4), followed by a discussion of recent large-scale trends (3.5). A brief discussion in terms of possible links between the anomalies and trends over key regions and important features of the Southern Hemisphere synoptic climatology, such as polar air outbreaks in South America, is presented in the section Localized Trends and Possible Links with Key Synoptic Features. DATA AND METHODS The monthly PDO index used in this work is that described in Mantua et al. (997), i.e. derived as the leading principal component (PC) mode of monthly SST anomalies in the North PO poleward of 0 N. Although this is the traditional definition based on SSTs over the Northern Hemisphere, there is a clear influence over the Southern Hemisphere, as shown in Figure of Mantua et al. (997). The typical SST signature during the positive phase of the alternative IPO index is seen in Figure of Power et al. (999a) and is very similar to the PDO. We performed correlation tests between the PDO and IPO and obtained a correlation coefficient of during the winter, but interestingly the correlation between the PDO and SOI is lower than that between the IPO and SOI over the last 50 years (+0.36 and +7, Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

3 SOUTHERN HEMISPHERE CYCLONE AND ANTICYCLONE LINKS WITH THE PACIFIC VARIABILITY 405 respectively). This led us to select the PDO (rather than the IPO) as the natural choice for our study so that the associations we identify are as minimally influenced as possible by the SOI. It is to be noted, however, that the main results we obtained were very similar, regardless of whether we used the IPO or the PDO. The PDO data was obtained through the Joint Institute for the Study of the Atmosphere and Oceans at the University of Washington ( The monthly mean global average SST trends were removed in an attempt to eliminate any possible linear signal associated with global warming. It is important to note, however, that some of the atmospheric features associated with the PDO may be also influenced by global warming, and this technique does not take into account such a possibility. The SOI was obtained from the Bureau of Meteorology in Australia ( The association between the PDO and the Southern Annular Mode (SAM) was also explored. Monthly data for the Antarctic oscillation index (AAOI) (Thompson and Wallace, 000) produced by NOAA ( noaa.gov) was used in order to calculate the correlations with the PDO or SOI. The AAOI is indicative of the strength of the SAM, which is the leading mode of variability in the Southern Hemisphere. This mode has recently been shown to have undergone a significant positive trend, which has been partially linked to global warming and ozone losses (Fyfe et al., 999; Thompson and Solomon, 00; Cai et al., 005; Arblaster and Meehl, 006; Cai, 006). The PDO and SOI are available for long time series, approximately covering the last century, and the AAOI is available only after 979. The standard period of analysis for all composites involving indicators of cyclone and anticyclone activity was the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis ERA 40 (Uppala et al., 005) reference period of The Melbourne University automatic tracking scheme (Murray and Simmonds, 99) was used to calculate the cyclone and anticyclone trajectories and their statistical properties. This algorithm utilizes a totally automatic approach for diagnosing low- and high-pressure centres on a sphere and calculating their tracks (Simmonds and Murray, 999). The scheme was chosen because of its proven reliability in very accurately capturing the weather patterns and synoptic climatology of the transient activity in the Southern Hemisphere (Jones and Simmonds, 993, 994; Simmonds et al., 999, Simmonds and Keay, 000; Pezza and Ambrizzi, 003, 005) and because it deals directly with sea-level pressure, giving a synoptic meaning to the analyses. In addition, the algorithm has proved to be robust even for an extremely complex hybrid cyclone that affected the Brazilian coast in March 004, which became a hurricane during the mature phase (Pezza and Simmonds, 005). The main physical principle of the tracking scheme is that the centre of a closed cyclone (anticyclone) is unequivocally identified with its point of minimum (maximum) pressure; this is normally found within one grid space of the Laplacian maximum (minimum), depending on the degree of symmetry of the system. A cyclone (anticyclone) is deemed to exist at any point at which the pressure is lower (higher) than at any of a small number of surrounding grid points. In the second stage of the scheme, the path of each system is tracked from the time of its appearance to its dissipation. To make the appropriate decisions, the procedure estimates the new position of each system, calculates the probability of associations between the predicted and realized positions and finds the appropriate match of these associations with the highest overall probability (Murray and Simmonds, 99). The statistical component of the software comprises a series of calculations based on estimated physical properties, such as radius, Laplacian of the pressure and velocity of displacement. The most important variables calculated in terms of their spatial distribution used in this work are the SD and the DP. The first is defined as the number of cyclones/anticyclones in a reference area of 0 3 (deg. lat), whereas the second is the pressure difference between the edge and the centre of a given system, given in hpa (Simmonds et al., 003). The radius and the DP are related by the equation: D = R (P ). This definition of DP based on the Laplacian of the pressure has the additional advantage of being relatively insensitive to artificial trends in the MSLP, which might exist in the reanalysis. In the case of anticyclones, although for reasons of simplicity we retain the use of the word DP, the variable DP turns out to be representative (in reality) of the system height (which would be interpreted as equivalent to negative DP). The tracks and statistical properties (indicators of number and intensity of cyclones and anticyclones) were calculated on the basis of the MSLP derived from the ERA40 reanalysis (Uppala et al., 005) for the period , this being the climatological period of reference for all figures presented, unless otherwise stated. The ERA40 is one of the best data sets available for long-term studies of MSLP and synoptic climatology of the Southern Hemisphere, but it is not free from noise and assimilation changes, in particular for data obtained before the satellite era (Simmons et al., 004). Another data set of similar quality is the NCEP reanalysis (Kalnay et al., 996; Kistler et al., 00), which has the advantage of presenting a longer time series ranging from 948 to the present day. Although we opted for standardizing all the cyclone and anticyclone analyses on the basis of the ERA40 data and climatology, the NCEP reanalysis was also used to calculate the annual MSLP composites for the PDO and SOI in order to take advantage of its longer period and therefore increase the significance of the anomalies. It has been recently shown that the quality of the mean sea-level pressure over the Australian region given by the NCEP reanalysis before the satellite era could be superior to that of the ERA40 (Hope et al., 006). This was done as a complementary analysis, but the main results discussed here are insensitive to the data set chosen. All composites were calculated for the extreme PDO and SOI phases Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

4 406 A. B. PEZZA, I. SIMMONDS AND J. A. RENWICK using the criterion of one standard deviation above or below the average. The analyses have been performed on a seasonal basis, and results for the summer and winter seasons are shown here after verifying that these were the periods of the year when the correlation between the PDO and the AAOI (SAM) was maximum, and therefore more suggestive of the possible links with the extratropical circulation over the Southern Ocean. Emphasis, however, is given to the winter period. Although the statistical significance is not explicitly shown (for clarity), it was calculated for all the maps and a significance of at least 90% is seen over all the anomalous regions discussed, with up to 99% coincident with the maximum intensity of SD and DP magnitude. The analyses of trends are presented in terms of SD and DP for the Southern Hemisphere and also selected for key areas of interest, such as the South Pacific High (SPH) region, defined as the area between 75 and 90 Wand 0 and 40 S, and the southern Brazilian coast, defined as the region between 30 and 60 W and 5 and 35 S. These are regions of influential and important synoptic systems affecting South America, the latter region being recently discussed in the literature after the first documented South Atlantic hurricane (Pezza and Simmonds, 005). Daily minimum temperatures of the meteorological station at the University of São Paulo (3 S, 46 W, 799 m above mean sea level) was used as a complementary analysis to provide insight into the discussions presented in the section Localized Trends and Possible Links with Key Synoptic features. RESULTS Composites of PDO and SOI anomalies and typical MSLP signatures Table I shows the years used in the composites, which were associated with the negative and positive phases of thepdoandthesoi.theplus(+) sign indicates periods when the index was above the average plus one standard deviation, and the minus ( ) represents cases when the index was below the average minus one standard deviation. Although the PDO and SOI time Table I. Years used for the PDO and SOI composites, where the plus sign (+) is for the cases in which the index was above the average plus one standard deviation and the minus sign ( ) isfor the cases below the climatological average minus one standard deviation. The PDO index from the Joint Institute for the Study of the Atmosphere and Oceans at the University of Washington and SOI from the Australian Bureau of Meteorology were used for the ERA reference period. Years of coincident phase, i.e. negative SOI with positive PDO or positive SOI with negative PDO, are indicated in bold, and the years of PDO and SOI with the same sign are underlined. See text for more details. PDO/SOI phase Years Percentage of coincident years (%) Winter PDO+ 983, 987, 99, 993, SOI 965, 97, 977, 98, 987, 993, 994, PDO 96, 96, 963, 967, 97, 973, 975, SOI+ 964, 973, 975, 98, 988, 996, Summer PDO+ 977, 984, 985, 986, 987, 988, 994, SOI 959, 973, 978, 983, 987, 99, PDO 96, 965, 969, 97, 97, 974, 976, 99, SOI+ 96, 97, 974, 976, 989, 999, 000, Autumn PDO+ 980, 98, 983, 984, 986, 987, 993, 996, SOI 983, 987, 99, 99, 993, 994, 997, PDO 96, 964, 967, 97, 97, 975, SOI+ 964, 97, 974, 975, 989, 999, Spring PDO+ 957, 976, 983, 986, 987, 99, 993, SOI 965, 97, 977, 98, 99, 994, PDO 96, 96, 970, 973, 975,994, 999, SOI+ 964, 970, 97, 973, 975, 988, 998, Annual PDO+ 98, 983, 984, 986, 987, 99, 993, SOI 965, 977, 98, 983, 987, 99, 99, 993, 994, PDO 96, 96, 964, 97, 97, 973, 975, SOI+ 964, 97, 973, 974, 975, 988, 989, 999, Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

5 SOUTHERN HEMISPHERE CYCLONE AND ANTICYCLONE LINKS WITH THE PACIFIC VARIABILITY 407 series extend over a period significantly longer than the ERA40 reanalysis, only the ERA40 reference period from was considered when calculating the years that appear in Table I, with all standard deviations being calculated over the same period. Cases of coincident PDO+ with SOI and PDO with SOI+ are shown in bold, and cases of PDO and SOI having the same sign are shown in underline. From this table it can be seen that, as expected, a significant percentage of El Niño-type conditions tend to occur during the positive phase of the PDO, whereas a significant percentage of La Niña years occur during the negative phase. In fact, the linear regression between both indices on a monthly basis reveals a negative association; however, this is not strong, which in turn is not surprising given that both phenomena have markedly different frequencies of variability. This can also be seen by presenting the percentage of coincident years under each category, which is shown in the right hand column of the table. On an annual basis, 50% of the El Niño and 56% of the La Niña conditions are accompanied by the coincident PDO phase. On the other hand, in terms of PDO and SOI having the same sign (opposite phases), this happened only for the spring of 994 (in underline), suggesting that such a combination is a very rare occurrence, at least in the last 50 years. Therefore, the results clearly reproduce the well-known association between the PDO index and the ENSO phase (Mantua et al., 997), but at the same time it is clear that a great variability exists, reinforcing the idea of some independence between these features. Figure shows the time series of the monthly PDO index (left hand scale) over the whole period analysed (the extreme minimum temperatures below C for São Paulo in southeastern Brazil were superimposed on the same plot and this is discussed separately in the section Large-scale trends in cyclone and anticyclone behaviour ). As discussed in the section Data and Methods, the PDO index was chosen because it had a lower correlation with the SOI, but the variability presented by the equivalent IPO index for the same period is very similar (figure not shown) and the main results discussed in the paper show little sensitivity to the index used. The time series shows the typical pattern of the interdecadal variability during the last century, with the last period being a persistent warm phase from 977 to the present. Some indications suggested that a reversion was going to happen around the year 000, but in reality the index returned to a strong positive signal after that period. The last cold phase can be regarded as having lasted between 947 and 976, preceded by a generally warm phase between 95 and 946 and finally by a somewhat mixed signal in the beginning of the last century. Figure shows a linear regression between the monthly AAOI and the PDO index for the month of January during the period From this figure, a negative relationship can be seen, in which about 30% of the variability is explained by the linear regression according to the least square method (Wilks, 995). This is suggestive of the PDO being also linked to the storm tracks around Antarctica, and such an association would be reflected in the indicators of extratropical cyclones and anticyclones. This is of fundamental importance for the precipitation and temperature regimes over the extratropical latitudes of South America, Africa and Oceania. Interestingly, January is the month with the strongest relationship between the PDO and the AAOI, and this relationship as discussed is negative. Given the negative correlation between the PDO and the SOI, this result is in agreement with what has been found in the literature in terms of the ENSO relationships with the SAM (Carvalho et al., 005; Meneghini et al., 006, and references therein). In fact, for most of the year no evident association is seen, with the exception of the summer time PDO Index Year Minimum temperature below C in Sao Paulo Figure. Monthly series of the PDO index and minimum temperatures below.0 C in São Paulo during See text for further details. This figure is available in colour online at Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

6 408 A. B. PEZZA, I. SIMMONDS AND J. A. RENWICK y = x R = AAO PDO Figure. Linear regression between the monthly AAO and PDO indices during January This figure is available in colour online at PDO SOl 40 Variance (%) Month Figure 3. Percentage of variance (R ) explained by the least square linear fit for AAO versus PDO and versus SOI. Negatives of R are used, when R is below zero, to convey the sign of the correlation. The period is used. This figure is available in colour online at and late winter when a possible relationship comes up between the PDO and SAM and the SOI and SAM, as shown in Figure 3. Interestingly, the relationship between the SAM and PDO shown here for the summer is stronger than the relationship between the SAM and the SOI recently documented in the literature (Meneghini et al., 006). Figure 3 shows the month-by-month percentage of the variance (R ) explained by the linear regression between the AAO and PDO and AAO and SOI for the period Cases in which the linear coefficient is negative are shown as R multiplied by in order to retain the original sign. From this figure, it can be observed that the behaviour discussed earlier, in which the explained variance for the association with the SAM arises only during the summer and late winter, is actually valid for both the PDO and SOI. The opposite sign observed in the graph is due to the negative correlation between the PDO and the SOI. It is interesting to see that the PDO has a stronger correlation in January, while for the SOI this is stronger in August. Although the amount of variance explained is very low and does not achieve strong statistical significance during any month, one can argue that there is coherence when the PDO and SOI information is brought together as in Figure 3, given that they are expected to have a significant degree of independence as shown by Table I. We speculate that this might reflect a possible physical link between the PDO and the annular mode over the Southern Ocean in such a way that the relationship is Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

7 SOUTHERN HEMISPHERE CYCLONE AND ANTICYCLONE LINKS WITH THE PACIFIC VARIABILITY 409 opposite in summer and winter. If the correlation shown in Figure 3 has physical meaning, it would imply that the years when the PDO is in its cold phase in summer (winter) would have a tendency for lower (higher) pressure around Antarctica and higher (lower) pressure at midlatitudes, increasing (decreasing) the westerlies over the Southern Ocean. Figure 4 shows the annual MSLP for (a) the (PDO+) (PDO ) and (b) the (SOI ) (SOI+) composites. As discussed in the section Data and Methods, the NCEP reanalysis was used in this case to obtain a longer period of analysis ( ) and therefore increase the significance of the differences observed by means of increasing the sample (PDO + 9 years, PDO 5 years, SOI + 0 years, SOI years). However, the main pattern obtained in Figure 4 is insensitive to the choice of the pressure data set or the interdecadal index (PDO or IPO) used. It can be seen that both PDO and SOI composites in Figure 4 present similar patterns, but there are robust differences as given by the intensity of the anomalies and spatial coherence presented. The pressure tends to be higher in the Eastern Hemisphere when the PDO is in the (a) (b) Figure 4. Annual mean sea-level pressure (hpa) for (a) (PDO+) (PDO ) and (b) (SOI ) (SOI+) for the period Total number of years used in the composite: PDO + 9 years, PDO 5 years, SOI + 0 years, SOI years. NCEP reanalysis used. 4 positive phase and the ENSO corresponds to the El Niño phase, with a marked maximum over Australia, which is stronger for the SOI composite, Darwin being one of the poles of the SOI. Over the Western Hemisphere there is a broad area of below-average pressure over the PO, particularly for the SOI composite (b), indicating that the pressure found on that area during El Niño events is significantly lower. This is also seen for the PDO composite (a) to a lesser extent. In terms of the differences between the PDO and the SOI MSLP patterns, the most striking feature seen in Figure 4 is that the PDO shows a much stronger annular pattern around Antarctica, with a marked difference of the order of several hpa. This is in agreement with the previous results, proposing a physical meaning for the correlations shown in Figure 3, and is significant given the total amount of years used in the composites, i.e. 4 years for the PDO and years for the SOI. Another substantial difference concerns the well-known blocking high over the southwest of South America, which is associated with El Niño conditions (Sinclair et al., 997; Renwick, 998; Salinger et al., 00; Kidson et al., 00). This is clearly seen in the SOI composite but does not appear in the PDO case, further emphasizing that distinct processes are involved. Further modelling studies are needed to arrive at a tentative physical mechanism for the suggested association between PDO and SAM (Figure 4(a)), but the comparison between Figures 3 and 4 indicates that a PDO link with the extratropics cannot be discounted. Figure 5 shows the wintertime synoptic climatology of all (a) cyclone and (b) anticyclone tracks lasting more than 4 h superposed on the same map for both phases of the PDO. The tracks, which occurred during the PDO+ (PDO ) phase, are shown in red (blue). Regions where tracks were found during both the phases are shown in green. No pressure restriction was applied, allowing for the inclusion of weak systems over the subtropical ridge and low latitudes. Although the green area shows that, overall, the high density regions are crossed by cyclones and anticyclones during both phases of the PDO, it does not mean that significant differences are not found. This is shown in terms of the SD and DP, discussed in the following paragraphs. However, the tracks in red and blue do point out interesting regional behaviour. For example, there are more cyclones in the equatorial western Pacific and Indian Ocean (in red) during the PDO+ and more cyclones in subtropical latitudes during the PDO (in blue, Figure 5(a)), and there is also a suggestion of more anticyclones over very high latitudes during the PDO+, although they tend to prevail to the south of 45 S during the PDO (Figure 5(b)). The annular distribution of blue and red colours to the north and south of the common green area for both cyclones and anticyclones may suggest a latitudinal shift associated with the PDO phase and is in agreement with the annular-like response in the MSLP shown in Figure 4. Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

8 40 A. B. PEZZA, I. SIMMONDS AND J. A. RENWICK (a) (a).5.5 (b) (b) Figure 6. Cyclone SD (System Density) anomaly for the (a) PDO and (b) PDO+ composites during the winter season (JJA). Figure 5. Superposed (a) cyclone and (b) anticyclone tracks for JJA PDO+ and PDO composites. The tracks which occurred only during the PDO + (PDO ) phase are shown in red (blue). Regions where tracks were found during both the phases are shown in green. Tracks had to be at least 4 h long to be considered. ERA40 MSLP has been applied to the automatic tracking scheme for the reference period of in this figure and all subsequent figures. See text for more details. Winter cyclone anomalies Figure 6 shows the winter cyclone SD anomalies associated with the negative (a) and positive (b) phases of the PDO (PDO and PDO+, respectively). The climatology of the SD based on the ERA40 data set for the period shows that for the Southern Hemisphere wintertime the typical values range between SD unit in midlatitudes and up to between 6 and 9 SD units around the Antarctic coast, in the so-called storm track region (figure not shown). The general SD pattern observed over the Southern Ocean and mid-to-high latitudes in Figure 6 shows densities that are typically above average during the negative phase and below average during the positive phase of the PDO. This is in agreement with more cyclones appearing in blue over midlatitudes in Figure 5(a). Close to the Antarctic coast a different pattern is observed, with strong anomalies of opposite sign in the Ross and Weddell Seas, suggesting an intense dipole over that area. As seen in Figure 4, that region of the globe presents a marked difference in terms of the pressure pattern comparing the PDO and the SOI composites. The typical order of magnitude for the anomalies seen in Figure 6 is about SD unit for most of the midlatitudes, indicating an increase (or decrease) of up to 00% of the climatology for some cases, for example, in the case of the negative anomalies to the south of Australia during the positive phase of the PDO, and therefore is physically meaningful. The Student s t- test (Wilks, 995) is not explicitly shown as discussed in the section Data and Methods, but all anomalies presented here have significance of at least 90%, and up to 99% given by a patchy pattern following the magnitude of the anomalies over the hemisphere. The anomalies found over high latitudes around Antarctica may be less representative in terms of the percentage of the climatology, given the typical very high SD found over the region, but they still represent a significant percentage of change. The significance also tends to increase when the difference between the phases is calculated. Apart from the direct statistical significance given by the Student s t-test, the physical interpretation resultant from the coherent spatial pattern is an additional indicator that Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

9 SOUTHERN HEMISPHERE CYCLONE AND ANTICYCLONE LINKS WITH THE PACIFIC VARIABILITY 4 (a) (b) Figure 7. Cyclone DP (hpa) anomaly for the (a) PDO and (b) PDO+ composites during the winter season (JJA). the anomalies do not simply arise by chance, as discussed throughout the text. The corresponding cyclone DP anomalies are shown in Figure 7, where a marked pattern can be observed, with below average DP during the negative phase and above average values during the positive PDO composite. This is seen over much of the Southern Hemisphere, and is suggestive of a well defined large-scale pattern indicating that cyclone strength tends to be very responsive to the PDO phase in an apparent linear manner. The typical magnitude found for the cyclone DP is above hpa for the most important anomalies over mid and high latitudes, where the climatological DP typically ranges between 4 and 0 hpa (figure not shown). When this phenomenon is analysed together with the results from the SD shown in Figure 6, there is a suggestion that, in general terms, the negative phase of the PDO is associated with more numerous cyclones, but those that are observed tend to be less intense than the climatology. Conversely, the positive phase is associated with fewer cyclones, but these tend to be more intense. However, this combination is not seen in all regions. The Tasman Sea, for instance, is host to more and stronger cyclones during the negative phase, and the opposite during the positive phase. This pattern may represent a positive reinforcement of the induced rainfall and temperature anomalies due to the anomalous cyclonic activity, i.e. caused by the gain in quantity and intensity of cyclones at the same time and over the same region. The synoptic behaviour over the Tasman Sea near the Australian coast is of importance for rainfall anomalies in southeastern Australia, an area that has been facing extreme challenges of water management policies owing to a pattern of decreasing rain and increasing demand for water (Bureau of Meteorology, personal communication 006). Therefore, the findings discussed here can be of significant relevance to help understand the factors modulating the observed behaviour in key regions of the Southern Hemisphere. In particular, we would like to address why that particular region is one of the few in the Southern Hemisphere to present such a combined behaviour of SD and DP with a positive feedback-like mechanism. Possible reasons for such an occurrence may be associated with non-linear mechanisms in the atmospheric response related to the decadal variability; however, exploring these aspects is beyond the scope of this paper. The southwestern tip of Australia has also exhibited a significant decreasing trend in precipitation since the seventies (Smith, 004; Baines, 005; Hope et al., 006 and references therein), and Figure 7 also shows very strong PDO-related anomalies over an area of influence from the Indian Ocean to the southwestern Australian coast. It is known that the decreasing trend in precipitation in that part of the hemisphere started before the last change in the main PDO phase in 976 (Baines, 005), but it is interesting that the anomalies over this area, which are believed to be connected with a southward displacement of the cyclone activity related to the storm tracks over the Southern Ocean (Fyfe, 003; Hope et al., 006), presented a strong PDO-related response. Another region where there is a behaviour different from the typical large-scale signal include the subtropical PO on the western coast of South America, which presented areas with above average DP during both phases of the PDO. As discussed in the section Localized Trends and Possible Links with Key Synoptic features, that region of the globe also presents a different behaviour in terms of the observed trends over the ERA40 period. The impacts of our findings in terms of rainfall and temperature anomalies over key areas of interest will be addressed in a future article, but some insights based on recent trends over the last decades are discussed in the sections Localized Trends and Possible Links with Key Synoptic features and Final Comments. Winter anticyclone anomalies Figures 8 and 9 show the SD and DP, similar to Figures 6 and 7, but for the anticyclone tracks. Before discussing these figures, a few words on the anticyclone SD and DP climatology are warranted. The wintertime climatology Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

10 4 A. B. PEZZA, I. SIMMONDS AND J. A. RENWICK (a) (a) (b) (b) Figure 8. Anticyclone SD (System Density) anomaly for the (a) PDO and (b) PDO+ composites during the winter season (JJA). of anticyclone SD shows a large number of systems over the subtropical regions and midlatitudes of the Southern Hemisphere, with a maximum that is approximately coincident with the centre of the green belt shown in Figure 5(b) (Jones and Simmonds, 994). The typical magnitude of the climatological SD based on the whole ERA40 data series ranges between and 3 SD units over the midlatitudes, i.e. to 3 anticyclones per 0 3 (deg. lat ) with contours above 3 SD units being found over the semi-permanent subtropical highs (figure not shown). In terms of DP, the climatological maxima are displaced much farther south when compared to the SD, with values between 0 and 5 hpa over mid-to-high latitudes (figure not shown). The greater DP farther south is a reflection of the relatively greater departure from the climatological sea-level pressure for typical migratory anticyclones crossing relatively high latitudes where the background pressure is very low. Figure 8 shows that the anticyclone SD anomalies present a stronger signal over the midlatitudes than that of cyclone SD, as discussed with respect to Figure 6. This is expected because of the typical spatial distribution of cyclones and anticyclones, as seen in Figure 5 and discussed in terms of the climatological pattern. The order of magnitude of the anticyclone SD anomalies is Figure 9. Anticyclone DP (hpa) anomaly for the (a) PDO and (b) PDO+ composites during the winter season (JJA). comparable to that of the cyclones, reaching values above SD unit over some regions, which is significant when compared to the climatology. In terms of the differences between the positive and negative phases of the PDO, Figure 8 suggests predominantly positive anomalies for the PDO and negative anomalies for the PDO+, similar to what was shown for the cyclones (Figure 6) but apparently subject to higher variability. The region of New Zealand presents above average anticyclone SD over the South Island and negative anomalies over the northern tip of the country for the PDO, with an opposite pattern during the PDO+, and Australia presents above average anticyclones to the southeast for the PDO and to the south during the PDO+. The pattern of positive anomalies impinging on Southeast Australia and New Zealand for the PDO seems to connect to a region of enhanced density of activity in the eastern Pacific (around the semi-permanent Pacific High), suggesting a preferential path for anticyclone propagation between Australia and South America during the negative phase. This is consistent with the anomalies being over the region that is known to be carrying the preferential wave trains over the Pacific Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

11 SOUTHERN HEMISPHERE CYCLONE AND ANTICYCLONE LINKS WITH THE PACIFIC VARIABILITY 43 and originating polar outbreaks in South America (Vera and Vigliarolo, 000; Garreaud, 000; Marengo et al., 00; Pezza and Ambrizzi, 005), and may indicate some degree of connection. This is also consistent with the anomalies observed on the eastern coast of South America, suggesting a continuity of the same pattern after the migratory anticyclones have crossed the Andes. On the other hand, during the positive phase of the PDO the anticyclone density over the South Pacific is significantly suppressed, in particular, close to South America, apparently confining the activity much further south. In fact, comparing Figure 8(a) and (b) for the 30 S band from a large-scale perspective between the Pacific and the Atlantic, including the oceans and South America, there is a suggestion that the positive anticyclone density over Uruguay and eastern Argentina during the PDO seems to be linked to a similar area of positive anomalies over the eastern Pacific. The opposite pattern is seen for the positive PDO phase. The anticyclone winter anomalous DP patterns tend to be more uniform over the Southern Hemisphere, as seen in Figure 9. In this case there is a consistent pattern indicating below average DP for the PDO composite and above average for the PDO+, similar to what was observed in terms of the cyclones. The typical magnitude found over mid-to-high latitudes can be significantly strong, with DP anomalies above 5 hpa in some cases, representing up to 50% of the climatological strength over those latitudes. The fact that cyclones and anticyclones present changes in the same direction is not a contradiction, as it is expected from considerations of the Rossby wave theory (enhancing or decreasing both polarities of the same wave train). The opposite behaviour in terms of density versus DP would also be explained when one considers that waves of higher amplitude are usually accompanied by greater DP and would tend to last longer, and therefore one would expect a reduction in the number of systems. However, as discussed before and emphasized in the section Localized trends and possible links with key synoptic features, it is noticeable that some key areas of this hemisphere present different behaviour. The Southern Ocean between Australia and South America exhibits consistent negative anomalies stronger than 3 hpa for the PDO and positive anomalies stronger than 5 hpa for the PDO+. This means that there is a difference of the order of 0 hpa between both extremes of the PDO, which is strong compared to the climatology. The Student s t-test shows that the differences over this area are marginally significant at the 90% level. In accordance with what was discussed about the anticyclone SD anomalies, this is a key area for the propagation of wave trains associated with polar air outbreaks over South America. A comparison between Figures 8 and 9 for this sector suggests a possible link with slightly positive DP anomalies between New Zealand and South America around 35 S during the PDO, which is also seen in terms of SD, as discussed before. The strong negative DP anomalies further south in this case may be a reflection of possible trajectories further north, leading to a coherent pattern of propagation when the PDO is negative. On the other hand, when the PDO is positive the DP anomalies suggest that the connection between Australia and South America seems to occur at higher latitudes (around 50 S), with positive anomalies also appearing on the Patagonian coast of Argentina at mid-to-high latitudes. This can be also connected with the recent trends observed over the SPH, as discussed in the section Localized Trends and Possible Links with Key Synoptic features. Another intense feature observed in the DP field is the +4 hpa blocking-like anomaly observed over Tasmania during the positive phase. The significance test shows that these changes are significantly different from zero, at least at the 95% level. During the negative phase of the PDO an opposite pattern is observed, with negative anomalies over most of the region. This pattern is located in a key area with regard to the influences of anticyclones on the Australian climate, and is suggestive of drier conditions in most of extratropical Australia for the positive phase of the PDO. Although this alone cannot explain the observed decreasing precipitation in the southwestern and southeastern tips of the country (Smith, 004; Baines, 005), it is consistent with drier conditions during the recent climate in extratropical Australia. In the case of the blocking pattern given by the positive DP anomalies over Tasmania for the PDO+, Figure 8 shows that the region is also subjected to above-normal densities for the PDO+ phase, and particularly further west to the south of Australia. This pattern reinforces the same ideas discussed in connection with cyclone anomalies over the Tasman Sea, suggesting that, although the main hemispheric pattern is associated with DP and SD anomalies in opposite directions, those may also combine to exert a positive feedback over key regions of significant influence for continental rainfall and temperature anomalies. Figure 9 also shows positive DP anomalies to the east of New Zealand during the negative phase and negative anomalies during the positive phase, as opposed to the blocking pattern seen over Tasmania. These anomalies are also significant on a level greater than 95%, and suggest an impact of the PDO phase on the circulation over New Zealand. The influences discussed above are very important for the circulation patterns over the Southern Hemisphere, and may impact on important synoptic features leading to rain and temperature anomalies over key continental areas. Although the anomalies discussed here cannot be solely explained by the PDO, the consistency of some of the patterns found is surprising, especially in terms of the intense DP anomalies over the Southern Ocean. Anomalies during the summer time and differences between PDO/SOI The summer season presented anomalies that are similar in many ways to those discussed for the winter, which is a reiteration of the generalization of the discussed Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc

12 44 A. B. PEZZA, I. SIMMONDS AND J. A. RENWICK patterns. One of the main differences found during the summer is the presence of more pronounced anomalies over the tropical region, which is reasonable given the more intense tropical convection seen at that time of the year. This is particularly apparent for the SD but is also seen in DP. However, when comparing the anomalies associated with the PDO and the SOI composites, it is evident that the PDO presents a stronger response over higher latitudes in both winter and summer. Figure 0 shows the difference between the PDO+ and PDO cyclone DP composite (a) and the difference between the SOI and SOI+ for the same field (b) for the summer period. Comparing Figure 0(a) with Figure 7, a similar pattern is observed in terms of the DP response to the PDO phase, indicating above average conditions for most of the hemisphere when the PDO is positive, similar to the winter pattern, and again the eastern coast of Australia appears to have negative anomalies when the PDO is positive. However, in this last case the anomalies appear shifted towards lower latitudes when compared with the winter pattern. The composite associated with the SOI presents a significantly different response, as seen in Figure 0(b). The negative anomalies are still seen on the eastern coast (a) (b) Figure 0. Cyclone DP (hpa) for the (a) (PDO+) (PDO ) and (b) (SOI ) (SOI+) composites during the summer season (DJF). of Australia, but with a maximum located much further south. On the other hand, some intense anomalies in the tropical region to the northeast of Australia are observed only for the SOI composite, and the same is apparent for the tropical Indian Ocean and to the northwest of Australia. Apart from these local differences, the anomalies over high latitudes are also very different, with a totally different pattern being observed over many areas. This result reinforces the idea of at least a certain degree of independency between the PDO and SOI, suggesting that the PDO tends to be more closely associated with the anomalies over mid-to-high latitudes, similar to what is known for the Northern Hemisphere and in accordance with the pattern discussed in Figure 4. Large-scale trends in cyclone and anticyclone behaviour Figure shows the trends in cyclone and anticyclone behaviour over the ERA40 period for the summer (a) and winter (b) period. The trends are indicated for the latitude bands where the signal was found to be stronger, and this coincided with an average over the whole Southern Hemisphere latitudes for all anticyclone indicators with the exception of SD during the summer, which had a stronger signal for the midlatitude band of 0 to 35 S. In the case of cyclones, the high latitude region of 40 to 70 S exhibited the strongest trends. All trends presented in Figure are statistically significant at the 99% level. This figure shows that the behaviour of the trends between summer and winter is very similar, but the slopes and their significance are greater during the summer. There is a very consistent pattern of increasing DP for both cyclones and anticyclones, accompanied by a more modest but still significant decreasing trend in cyclone SD, which has also been noted in anticyclones but with a very small rate of decrease as seen by the angular coefficient from the regression equation. This opposed-pattern between DP and SD is feasible given the independent meaning of both variables and in agreement with the Rossby wave theory, as discussed before. These results are also in agreement with previous works found in the literature. For example, Simmonds and Keay (000) explored the variability of cyclone behaviour using the NCEP reanalysis data for the period , and they concluded that although there is a significant trend for fewer systems, the cyclones are becoming larger and deeper over many parts of the Southern Hemisphere. These authors addressed the trends found in the NCEP MSLP data set for the studied period and concluded that, even after taking the possible data set inconsistencies into account, the trends towards fewer and more intense systems are likely to be real, and possibly connected with global warming. Pezza and Ambrizzi (003) also showed a similar pattern inferred by the total count of cyclones and anticyclones during the winter time, using a slightly different methodology and period. Renwick (005) also found an upward trend in the frequency of persistent positive height anomalies in the Southern Hemisphere circulation, especially in the occurrence of zonal wave number three. Copyright 007 Royal Meteorological Society Int. J. Climatol. 7: (007) DOI: 0.00/joc