CHAPTER 3 State of polar boreal winter stratosphere (1993-2009) 3.1 Introduction The middle and upper regions of the atmosphere are now recognized as important and sensitive indicators of the polar middle atmosphere with changes in occurrence of phenomena such as polar mesospheric summer echoes (PMSEs), polar mesospheric clouds (PMCs) and noctilucent clouds (NLCs) being possible indicators of global climate change [Thomas, 1996; Shettle et al., 2002; Klostermeyer, 2002]. In spite of this, the polar middle atmosphere remains one of the least understood regions of the Earth s atmosphere. The importance of dynamics at high latitude is instrumental in determining the extent and timing of polar processes and ozone loss, since these processes are strongly dependent on temperature and vortex containment. In addition, dynamics plays a large role in determining the composition of the middle atmosphere via transport processes. The normal winter stratosphere circulation is qualitatively summarized below. After the fall equinox, the heating at high latitudes due to ozone absorption of UV radiation decreases as the polar night expands equatorward. Temperatures over the winter pole decline and strong westerlies develop, driven by the meridional temperature gradient between the equator and the pole and the Coriolis force. The core of strong westerly winds which thus forms in the upper stratosphere is called the polar vortex. The stratospheric polar vortex is the main dynamical feature of the winter middle atmosphere (20-100 km) at middle and high latitudes. During 66
winter, on some occasions, strong perturbations in the temperature field occur which cause sudden rise in temperature in the stratosphere. As a result, a temporary summer condition is established in the stratosphere. Such a sudden rise in temperature in the stratosphere is identified as sudden stratospheric warming. The temperature rise appears primarily to be due to an increase in eddy transport of heat associated with a vertical flow of energy from the troposphere by planetary scale waves [Mahlman, 1969]. It is known to occur over middle and high latitudes. It was first discovered by Scherhag [1952] from sudden increase in the radiosonde 10 hpa temperature on 30 January 1952 over Berlin. It is seen as a latitudinal mean temperature increase poleward of 60 o N and associated circulation reversal at 10 hpa level or below. Major warmings do not occur every winter. They tend to occur every three or four years, but there have been several years in the past without a major warming. SSWs are most common in mid to late winter, but they have been observed as early as December in Northern Hemisphere. From the mid- 1950s to 1991, major warming occurred almost every other year, but between 1992 and 1998 there was no major warming. In this chapter, the winter stratospheric state is described using conventional zonal mean parameters such as temperature and zonal wind for the 16 Arctic winters since 1993. The characteristics of SSW events are studied in the context of different types of warming and their vertical structure. Long term studies provide a sound climatology and information about interannual variability in the winter middle atmosphere. The most common long term periodicities are associated with the 11 year solar cycle and the quasi biennial oscillation (QBO). It has been suggested that the stratosphere, in particular, the polar stratosphere, is a sensitive indicator for long term global 67
changes in the atmosphere. Thus, efforts are made to identify the relation between high latitude stratosphere on one hand and QBO and 11 year solar cycle on the other hand. 3.2 Classification of stratospheric warming Following the classification system of the stratospheric research group of the Free University (FU) of Berlin [Labitzke and Naujokat, 2000], a major stratospheric warming must have a zonal mean temperature increasing poleward from 60 o latitude and time-mean easterly wind poleward of 60 o latitude. The stratospheric circulation reversal means that the requirement for major stratospheric warming is a breakdown or splitting of the winter polar stratospheric vortex. Major warming occurs mostly in January or February. This type of warming is often referred to as wave-1 warming or wave-2 warming. If the zonal-mean zonal winds at 10 hpa level do not reverse, but the temperature increases by at least 25 K in a period of a week or less at any stratospheric level in any region of the winter hemisphere then the warming is called minor warming. It occurs every winter in both hemispheres. After the warming event, the polar stratospheric temperature decreases and the zonal winds accelerate eastwards returning the stratosphere back to a state typical of winter. However, a warming sometime can lead directly to a summer type circulation, with warmer polar stratospheric temperatures and westward zonal winds persisting even after the sudden warming event. In this case the warming is classified as final warming. Final warming events are common when SSW occurs towards the end of winter. 68
Another class of SSW is termed Canadian Warming. Canadian warming is characterized by strong wave-1 amplification in the middle stratosphere resulting in the Arctic vortex shifting off the pole towards Europe (i.e. away from Canada). Canadian warming rarely leads to a major stratospheric warming with zonal wind reversals having been observed only six times since regular observations began about 50 years ago [Manney et al., 2001]. 3.3 High latitude stratosphere during winter The middle atmosphere dynamics is characterized by a strong variability during winter which can lead to strong planetary wave activity and disturbances in the stratospheric circulation. This section examines various SSW events when MF radar data were available from the low latitude station Tirunelveli (8.7 o N, 77.8 o E). The warming events are classified in terms of their strength and duration. 69
70
71
72
Figures 3.1(a) to (c) depict the daily zonal mean temperature difference between the North Pole and 60 o N and zonal mean zonal wind at 10 hpa from 1 November to 31 March for sixteen winters since 1993. During 1992/1993 winter shown in the bottommost panel of Figure 3.1 (a), there was a strong eastward wind at 60 o N (plus symbol) and cold condition (negative temperature) at 10 hpa (open circle) observed up to the end of January 1993. An abrupt increase in temperature of ~10 K was observed during end of February 1993 (day number 120). The zonal wind did not reverse then but it weakened considerably. No reversal was found during the entire observational period of 1992/1993. The response of temperature and zonal mean zonal wind for 1993/1994 winter is shown in the second panel from bottom of Figure 3.1(a). Cold condition that prevailed until end of December was followed by an increase in temperature from ~-20 K to ~12 K at the beginning of January 1994. The raise in temperature was accompanied by a weak eastward wind. The zonal wind did not show any reversal during this period. This winter serves as an example for a minor warming. During 1994/1995 winter (third panel from bottom of Figure 3.1(a)), there was a strong warm pulse magnitude of ~20 K noticed during the end of January and it sustained for a few days only. Strong eastward winds were observed till end of December that weakened considerably by end of January. This winter also did not show any significant reversal in the zonal wind in the stratosphere and also satisfies the criteria of minor warming. The 1995/1996 winter (fourth panel from bottom of Figure 3.1(a)) was similar to the previous winter but a series of warm temperature were observed between middle of February (day number 100) and March. No reversal in the zonal wind was observed but the eastward flow 73
weakened when a warm condition was observed in the stratosphere. The 1996/1997 winter (second panel from top of Figure 3.1(a)) showed a normal winter pattern of the polar stratosphere. The 1997/1998 winter (topmost on the left column of Figure 3.1(a)) showed a moderately raise in temperature of ~8K during the month of December (day number 55) accompanied by a weak eastward wind. No reversal was observed in the zonal wind at stratospheric heights. This was a minor warming event. Manney et al. [1999] reported that no major warming occurred for nine consecutive winters from 1989-1990 till 1997-1998. Results for 1998/1999 winter are shown in the bottommost panel of Figure 3.1(b). Two predominant warm temperatures were observed during this winter, the first one was during the middle of December (day number 46) and other one was during the end of February (day number 120). An eastward flow of ~40 m/s reversed to westward (-20 m/s) direction when the temperature attained its peak during middle of December 1998. This was considered as a major warming event that had occurred after a period of eight years, and only the second major warming that had ever been observed in early December [Manney et al., 1999]. This warming may also be regarded as a Canadian warming since it occurred in December. Canadian warming sometimes leads to major warming. Another rise in temperature began from -15 K to 20 K was observed during the end of February which is known to be final warming. During 1999/2000 winter (second panel from bottom in Figure 3.1 (b)), a cold condition was observed during the course of this winter. A strong wintertime eastward flow was found during this winter by Manney and Sabutis [2000] and Rex et al. [2004]. During 2000/2001 winter (third panel from bottom of Figure 3.1(b)), a sudden increase in temperature from -20 K to 15 K 74
was noticed during the middle of December (day number 50) that persisted for only three days. The wintertime eastward flow weakened. Another warming event of about 12 K was observed during the end of January (day number 88). Around this time the eastward wind decelerated and turned westward (-20 m/s). The reversal lasted more than a week. This winter was categorized as midwinter major warming. The 2001/2002 winter shown in the fourth panel from bottom on right column of Figure 3.1(b), had two dominant warm pulses. A steep increase in temperature of ~ 20 K was detected in the end of December (day number ~57) and the second pulse during the beginning of February. The December event was only the third major warming ever to be observed this early in a winter, along with December 1987 and December 1998. The eastward wind decelerated and reversed to weak westward flow (-5 m/s). Moreover, the reversal persisted only for two days in both of the cases. The 2002/2003 northern winter behaviour shown in the fifth panel from bottom on the right column of Figure 3.1(b) was characterized by stratospheric temperatures that were initially very low, although a major warming occurred during mid January dominated by a wave-2 planetary wave event [Kleinohl et al., 2004]. Minor warming was observed during January (day number 70) and February (day number 120) followed by a major warming. A zonal mean zonal wind reversal of -5 m/s at 10 hpa was associated with the warm condition whereas weak eastward wind was observed during minor warming. The 2003/2004 winter is shown in the topmost panel of Figure 3.1(b). There were two predominant positive temperatures of ~22 K observed during the end of December (day number 60) and during beginning of January (day number 70). In addition, warm condition continued more than two weeks of period during this winter. The eastward flow decelerated slowly during December and turned westward (-25 m/s) in 75
the beginning of January. This reversed wind persisted for more than a month in the polar stratosphere. Manney et al. [2005] summarized the prominent characteristics of the 2003/2004 events. Their results showed that this event was characterized by an extraordinarily long vortex disruption in the lower and middle stratosphere with strong and rapid recovery of the vortex in the upper stratosphere. The winter conditions during 2004/2005 depicted (bottommost panel in Figure. 3.1 (c)) showed a cold and strong polar vortex without any disturbance in the polar stratosphere. It was another example for normal wintertime polar stratosphere. These winter characteristics were described by Manson et al. [2008] and Manney et al. [2006]. During 2005/2006 winter (shown in the second panel from the bottom of Figure 3.1(c)), there were series of warming events, which can be inferred three sequential warming events were observed during January 2006. These warming events can be classified as major or minor depending on whether there is any wind reversal at latitude 60 o N. Though there are three successive warming events, only the third event is the major warming event, as it is accompanied by the reversal of zonal winds from eastward to westward. One of the strongest and most prolonged warming events on record began during January 2006. The winter of 2006/2007 is shown in the third panel from the bottom of Figure 3.1(c), in which the polar stratosphere was very cold during November-January. After the first week of February (day number 95), a rise in temperature of ~10 K accompanied by a weakening of eastward wind was observed. Another increasing temperature signal of more than ~10 K was noticed during the end of February 2007 (day number ~117) which persisted for several days whereas the weak eastward flow turned westward by the end of February. 76
In 2007/2008 winter, there were several disturbances that caused alternating occurrences of warming and cooling at 10 hpa heights during January and February and these features are shown in the fourth panel from the bottom in Figure 3.1(c). While the intense eastward wind was slowly decelerating from January 2008 onwards, it turned westward during end of February 2008. During 2008/2009 winter, a peak in the stratosphere temperature appeared during mid- January 2009 and is shown in the topmost panel in Figure 3.1(c). The zonal mean zonal wind shows a reversal from eastward to westward flow accompanied by enhanced temperature during mid-january 2009. The 2004 and 2006 SSW events were strong and long lasting [Manney, et al., 2005]. The diagnostic results depicted in Figure 3.1(c) show that the intensity of the SSW in 2009 surpassed that during 2004 and 2006 and had a more profound and lasting effect on the lower stratosphere [Manney et al., 2010]. 3.4 Vertical structure of temperature and zonal wind driven by SSW events One of the well known characteristics of the SSW event is the downward progression of a warm layer from about 45 km to the lower stratosphere. The SSW disturbance appears to move downward because of the secondary circulation driven by the wave interaction with the critical level that extends below the level of wave breaking, decelerating the mean flow below the wave breaking zone. The extension of the secondary circulation below the level of wave breaking is sometimes called downward control. It is important to study the vertical structure of temperature and zonal wind during major SSW events. Here, four major warming events are discussed in this context. 77
3.4.1 Major warming during 1998/1999 The bottom panel of Figure 3.2 shows the time-pressure cross section of the United Kingdom Meteorological Observation (UKMO) zonal mean zonal wind, while the upper panel shows the zonal mean temperature difference between North Pole and 60 o N. The altitude variation in the range 20 to 55 km (31-0.3hPa) is investigated. Figure 3.2 shows that during normal winter conditions, the stratosphere is dominated by a negative temperature gradient and strong eastward zonal mean zonal wind. During disturbed condition, a positive temperature difference (warm) was observed between day numbers 43 and 55 and between day numbers 115 and 125. The first of them occurred around day number 48 and propagated down to lower stratosphere (31hPa). This disturbance was accompanied by a reversal in the eastward wind (bottom panel of Figure 3.2). The second one was centered on day number 120 and it propagated down to lower stratosphere. The wintertime eastward flow turned westward around day number 120. After day number 125, the warm condition reverted to normal winter condition whereas the normal wintertime eastward flow was observed little later on day number 135. The first one is an example for a Canadian warming event and the second one for final warming. Figure 3.2 also reveals that the downward penetration of disturbance in the stratosphere occurred at all levels in the same time and this effect was visible in both the component. 78
Figure.3.2.Pressure-time cross section of the UKMO zonal mean temperature difference between pole and 60 o N (upper plot) and zonal mean zonal wind at 60 o N (bottom plot) from 01 November 1998 to 31 March 1999. 3.5.2 Major warming during 2003/2004 Figure 3.3 shows an enhanced temperature gradient can be noticed between day number 45 and 70. During 2003/2004, three warm pulses were observed. The first of them occurred around day number 45 and reached only ~40 km (3.5 hpa) height. This disturbance was accompanied by a rapid deceleration of the zonal mean zonal wind (bottom panel of Figure 3.3). The second pulse was centered on day number 55 and was the strongest among the three 79
observed between day numbers 30 and 70. The effects were visible down to 31 hpa. The zonal mean circulation decelerated rapidly towards west around day number 50. Figure 3.3 Same as Figure 3.2 except for the time interval 01 November 2003 to 31 March 2004. The reversal of the zonal mean zonal wind, however, did not extend below 3 hpa (~ 40 km) height then. The third warm pulse prevailed between day numbers 60 and 70 and it was accompanied by another reversal of mean zonal flow that extended down to 31 hpa. The zonal mean zonal wind became westward at 10 hpa over 60 o N around day number 65. After around day number 70, the stratospheric temperature gradually decreased and the westward circulation reverted to eastward. Another reversal of the mean circulation occurred around day number 90 and the disruption of the polar vortex is noticed ~24 km (~31 hpa) during this winter. Quiroz 80
[1971] suggested that the disturbance started in the upper stratosphere (> 40 km) and moved down to lower heights. From Figure 3.3, it is observed that there was a time delay between the variations in the zonal mean zonal wind (bottom panel of Figure 3.3) from higher to lower heights. Figure 3.4 Cross-correlation between UKMO zonal mean zonal wind at 0.15 hpa and pressure level between 31 to 0.15 hpa for the period 01 December 1998 to 30 January 1999. In order to investigate the delay in detail, a cross correlation analysis was performed between the zonal mean zonal wind at 0.1 hpa pressure level (~64 km) and all other pressure levels for the disturbed period (between day number 31 and 92). Figure.3.4 shows the contours for the cross correlation function in the altitude range of 31-52 km with a maximum time lag of 30 days. From Figure 3.4, it may be inferred that there was no delay up to 40 km and a delay of ~3 days was present at an altitude of 30 km. The secondary maxima situated at time lags +20 days indicate the presence of planetary wave disturbances noticeable in the UKMO zonal mean zonal wind (bottom plot of Figure 3.3). 81
3.4.3 Major warming during 2005/2006 Figure 3.5 depicts the same parameters as in Figure 3.3 except for the period 01 November 2005 to 31 March 2006. As can be seen in Figure.3.5, three sequential warm pulses were observed between day numbers 60 and 95. This event was identical to the SSW event that occurred during 2003/2004 winter. The first warm pulse occurred around day number 62 and it reached only 3.4 hpa (~40 km) level. The eastward wind decelerated rapidly but did not turn westward. The second warm pulse occurred around day number 75 and it extended down to 31 hpa (24 km). This was accompanied by a reversal of eastward wind at 3 hpa. The third and final warm pulse that occurred on day number 86 which was the strongest of the three penetrated down to lower stratospheric heights (~31 hpa). The westward wind regime persisted up to day number 110. After day number 120, the existing temperature and zonal mean zonal wind conditions were replaced by normal winter conditions. The results presented in Figure 3.5 show that as the wind regime descended downwards, their magnitudes reduced similar to 2003/2004 winter. This event was also characterized by downward penetration of disturbances in the stratosphere with some time delay with respect to the upper levels. 82
Figure 3.5 depicts same as Figure.3.3 except for 01 November 005 to 31 March 2006 In order to obtain further information on this behaviour, a cross correlation analysis was applied to the zonal mean zonal wind for the disturbed period of 50 days during this winter. It is noticed in Figure 3.6 this was similar to the 2003/2004 event except that the time delay at 32 km was ~6 days. This provides credence to the hypothesis that SSW disturbance propagates from higher altitudes to lower levels. 83
Figure 3.6 depicts same as Figure.3.4 but from 01 January 2006 to 20 February 2006. 3.4.4 Major warming during 2007/2008 Figure 3.7 is similar to Figure 3.4 except for the period 1 November 2007 to 31 March 2008. The polar stratosphere was relatively quiet until late January when it warmed up quickly and several episodes of SSW occurred then. The zonal mean temperature increased up to 30 K within a couple of days before end of January 2008 at 10 hpa. Four warm pulses were observed during this winter. The first warm pulse occurred on day number 85 and it extended to lower to stratosphere pressure level of~14 hpa (top panel in Figure 3.7). The eastward wind regime reversed to westward but reached only down to 6 hpa (35 km). The third and fourth warming events were observed during the end of February (day number 115) and they reached the lower stratosphere. The fourth was the strongest of all and was accompanied by a reversal of zonal wind at the lowest pressure level (31 hpa). 84
Figure 3.7 same as Figure.3.5 but for 01 November 2007 to 31 March 2008 The westward wind regime persisted in the lower stratosphere after the withdrawal of major warming event. All the warm pulses and the wind regime associated with them showed the downward progression in the respective parameters. From Figure 3.8, it can be concluded that there was no significant delay as in previous cases. 85
Figure 3.8 same as Figure.3.4 but for 14 January 2008 to 03 March 2008. 3.4.5 Major warming during 2008/2009 Figure 3.9 depicts the results for the time interval from 1 November 2008 to 31 March 2009. Two warm pulses were observed around day number 50 and day number 85. The first warm pulse occurred at higher heights (50 km). The final warm pulse occurred on day number 85 and it was strongest of the two and penetrated down to the lower stratosphere (~31 hpa). There was a transition to zonal mean westward flow when the temperature attained its peak. The westward circulation continued till the end of February 2009 when the temperature reverted back to normal winter condition. This was similar to the 2005/ 2006 event. 86
Figure 3.9 similar to Figure 3.5 except for 01 November 2008 to 31 March 2009 The results from the cross correlation function of this winter 2008/2009 was plotted in Figure 3.10 shows a behaviour similar to 2005/2006 event except that the delay at 30 km was ~5 days. Secondary maxima noticed around time lags of 12 to 15 days are attributed to the presence of the planetary wave disturbances visible in the UKMO zonal mean zonal wind (bottom panel of Figure 3.9). The differences and similarities between the dynamics of 2006 and 2009 SSW events are now discussed. The breakdown of vortex was wave -1 type during 2006 and wave- 2 type during 2009. The zonal mean zonal wind at 60 o N decelerated rapidly in 2009 compared to 2006. In 87
2006, the normal stratospheric winter conditions occurred very late after warming compared to 2009. These differences in dynamics were consistent with differing characteristics of vortex split and vortex displacement described by Charlton and Polvani, 2007. During 2006, the disturbance exhibited downward progression and there was time delay in the lower stratosphere with respect to the upper stratosphere level. Figure 3.10 same as Figure.3.4 but for 01 January 2009 to 19 February 2009. 3.5. Sudden stratospheric warming (SSW) and its relation with QBO phase The occurrence of SSW is significantly affected by other forcing: agents such as the quasi-biennial oscillation (QBO), the 11-year solar cycle and sea surface temperature (SST) anomalies. These forcings influence the variability of extratropical stratosphere. The dominant source of variability in the tropical stratosphere is the QBO, which involves the quasi-periodic reversal of zonal winds from westerly to easterly and back again. Theory shows that due to interaction between planetary waves and zonal mean winds, the zonal winds exhibit alternating 88
patterns of westerly and easterly winds that repeat every 26 months on average. Several observational reports show that the equatorial QBO has significant effects on high latitude stratospheric circulation (Holton and Tan 1980, 1982 Baldwin and Dunkerton 1991n Baldwin et al 2001 and few). When the QBO is in its westerly phase in the tropical lower stratosphere, the planetary waves are refracted away from polar region towards the tropics. The stratospheric polar vortex should therefore be cold and strong because of the absence of wave forcing. On the other hand, when the QBO is in easterly phase, the waves are more confined to middle and polar latitudes. The polar vortex will be warmer and weaker and therefore is more susceptible to be broken down by SSWs. The phase of the QBO effects the both the position of the zero wind line and the structure of the zonal mean wind field. The Northern Hemisphere (NH) winter temperature and circulation are likely to be influenced both by the QBO and the 11 year solar cycle. The mechanisms of the interaction of the QBO with the polar stratosphere have been analyzed statistically, and yet many of the reported observational results have not been explained. For example, Holton and Tan [1980] have shown observational evidence that the QBO modulates the extratropical circulation at 50 hpa, and they speculate that the latitudinal shift of the critical line (u=0) might be an important link between the QBO and high latitude circulation. The hypothesis of Holton and Tan [1980] was motivated by the analysis of 16 years of National Meteorological Center (NMC) data (1962-1977) in which they presented statistical evidence for the association between QBO and the extratropical flow. By compositing the data according to the phase of the QBO at 50 hpa, they showed westerly phase of the QBO (QBO-W) to be associated with anomalously low 50 hpa geopotential height at high latitudes corresponding to a strong polar vortex in both early (Nov- 89
Dec) and late (Jan-Mar) Northern Hemisphere winter. Anomalies of the opposite sense were associated with the easterly QBO phase (QBO-E). This correlation between QBO and stratospheric polar vortex was referred to as the HT effect. Labitzke [1977] investigated the interannual variability of the Northern Hemisphere winter using zonal harmonic wave analysis of daily 30 hpa temperatures. She identified the characteristics of disturbed winter months and undisturbed months. The undisturbed patterns were frequently observed during the QBO westerly phase. In contrast major stratospheric warmings were shown to occur during QBO easterly phase. Labitzke and Van Loon [1988] have shown that the major warming occurred during solar maximum with QBO westerly phase and during solar minimum with QBO easterly phase. As the zero wind line moves more poleward during easterly QBO, the easterly waveguide for stationary waves is narrowed and located more poleward. This tends to focus the planetary waves more poleward, making the polar vortex more disturbed and hence the region warmer. A physical mechanism for a solar cycle influence on polar variability was suggested by Kodera [1991, 1995], who analyzed 12 years of Northern Hemisphere extratropical NMC data and concluded that the four different combinations of QBO and solar cycle phase (Westerly/maximum, Westerly/minimum, Easterly/maximum and Easterly/minimum) appeared to induce different patterns of anomalous stratospheric mean zonal wind in early winter, which then became similar pattern in late winter. He suggested that the early winter influence of those forcings at low latitudes in the upper stratosphere created a polar signal by means of downward and poleward propagation of the mean zonal wind anomaly due to wave-mean flow interaction. Baldwin and Dunkerton [1991] diagnosed QBO-extra-tropical correlations using 13 years of NMC data extending up to 1hPa. They showed high correlation in the layer above 10hPa 90
extending from Northern Hemisphere polar latitudes to 30 o S and altitudes covering the whole depth of the extratropical stratosphere. Figure 3.11(a) Time- series of JF NP temperature anomalies (K) at 24 km (dotted line) and zonal wind anomalies at 24 km (solid line), (b) Scatter plot of the data, (c-d) similar to (a and b) except for 1993 to 2008. In order to explore the possible links between observed polar temperature and QBO winds during winter, averaged January and February North pole temperature anomalies at 24 km for the time interval of 1965-1990 and 1993-2008 and equatorial wind anomalies at 24 km over the same time interval are plotted in Figure 3.11. The anomalies were derived by subtracting the mean value over the period from each point. It is seen in the top panel of Figure 3.11 that the polar temperature anomaly was positive (warmer conditions) during the westward phase of QBO winds and negative (colder conditions) in the eastward phase of the QBO winds. 91
The JF North Pole temperature anomalies and equatorial zonal wind anomaly at 24 km are anti-correlated during the periods 1969-70, 1980-81, 1984-85, 2003-04, 2005-06 and 2007-08 and there was no relationship noticed during the winter of 1982-83. The bottom panel of Figure 3.11 shows the scatter plot for these two parameters. The correlation coefficient was -0.25 during 1993-2008 and -0.4 during 1965-1990. The Holton-Tan relationship of negative correlation between the two signals is found to hold only for certain winters and not for other winters. In Figure 3.11, the periods of Holton-Tan (i.e., negative) correlation tend to coincide with periods under the solar minimum conditions and the periods in which Holton-Tan relation is reversed tend to coincide with periods during solar maximum conditions. In the lower stratosphere the solar signals are mainly dynamical in origin. A direct influence of dynamics on the temperature fields is reinforced by anomalous ozone transport during solar maximum. Lu et al. [2009] suggested that the anomalies in this region may involve SSWs and anomalous wave forcing in the polar region. During solar maximum/eastward QBO years there are anomalously more SSWs occurring due to stronger poleward propagating planetary waves, hence greater equatorial upwelling. The anomalously stronger equatorial upwelling during solar maximum counteracts the anomalous eastward QBO descent. It results in weaker solar signals in the tropical lower stratosphere. During solar minimum/westward QBO phase years there are also more SSWs occurring in the lower stratosphere, mainly due to stronger poleward wave forcing under westward QBO phase [Holton and Tan 1980, 1982, McIntyre 1982, Lu et al., 2008]. However, the increased equatorial upwelling due to SSWs reinforces the anomalous QBO upwelling and hence the overall solar influence is strengthened in the equatorial lower stratosphere. Recently, Labitzke et al. [2009] observed opposite correlation in two 92
different phases of the QBO between geopotential height on February 1942-2009 at 30hPa and 10.7 cm solar flux, while the winters in the east phase of the QBO are negatively correlated and the winters in the west phase are positively correlated. It still does not explain why we observe more SSWs during solar maximum/eastward QBO phase and solar minimum/westward QBO phase years in the mid and lower stratosphere and why planetary waves respond to the 11 year solar cycle differently in the early and late winters. Thus further research is needed to investigate the mechanism of the solar-qbo influence on the Northern Hemisphere polar winter stratosphere. 3.6 SSW impact on ENSO A major source of tropospheric variability is the El Nino Southern Oscillation phenomenon (ENSO). ENSO is one of the primary circulation patterns at equatorial latitudes and affects climate worldwide. Since strong El-Nino events are connected with more clouds over the tropical Pacific, this also leads to cooling of the tropopause region and lowermost stratosphere. This in turn during winter reduces the temperature gradient between lower latitudes and the polar vortex, weakens the later and enhances the probability of major stratospheric warming [Van Loon and Labitzke, 1987]. On average the polar stratospheric vortex is weaker during El- Nino years than it is during La-Nino years. Shiogama and Mukougawa [2005] suggested that ENSO has a significant effect on the life cycle of SSWs. The stratospheric wavenumber 1 wave is responsible for the onset of SSWs during the warm winters of ENSO. On other hand wavenumber 2 component also becomes important during cold winters. Kodera and Yamada [2004] observed that a major warming of the Southern Hemisphere in September 2002 produced a north-south sea-saw of tropical convection activity. Kodera [2006] also noted similar features 93
in his composite analysis of twelve major warming events that occurred at high latitude Northern Hemisphere. In this section, the SSW events that occurred during the years 2003-2004 and 2005-2006 are considered with the goal of investigating changes in convective activity, if any, over the equatorial region. It is known that the effect of SSWs propagates down to the troposphere within a few weeks and can last longer than two months [Baldwin and Dunkerton, 2001]. To identify the SSW events, UKMO data are used. A description of UKMO data set is provided in the earlier chapter. Besides, interpolated outgoing long-wave radiation (OLR) data are used in the present study and were obtained from the Climate Diagnostic Center (CDC) of NOAA. This parameter (OLR) is used as an indicator of deep convection in the tropical region [Liebman and Hartmann, 1982]. Kuroda [2008] used daily reanalysis data to analyze SSWs and their impact on the tropospheric climate. In the present study, we focus on the impact of the SSW on the stratosphere and troposphere in shorter time scales. We use unfiltered daily data with an assumption that the time scales associated with the SSW event can be quite short involving relatively fast interactions between the stratosphere and troposphere. Figure 3.12 (a) and (b) show the state of the winter hemisphere at 10 hpa, 1 hpa and 0.1 hpa for 1-120 days starting from 01 November 2003. The SSW event may be identified from the positive poleward temperature difference at 10 hpa during day numbers 49-69 (19 December 2003-14 January 2004) with a maximum temperature difference of 27 K occurring on day number 58 (Figure 3.13 (a)). This was a major warming event, as winds decelerated after day number 40 from 48 m/s, turned westward and attained a maximum speed of 38 m/s on day number 53 (Figure 3.12 (b)). The westward winds again turned eastward for about a week and 94
later changed to westward flow reaching a maximum speed of 20 m/s on day number 66. Though the event was identified at 10 hpa as per WMO definition [Quiroz, 1979], the warming events occurred earlier at 1 hpa and 0.1 hpa around day number 42. It may be noted that the largest temperature difference of ~40 K was observed at 1 hpa on day number 46, whereas it was only ~27 K at 10 hpa. Manney et al. [2005] summarized the prominent characteristics of the 2003-2004 Northern Hemisphere winters. Their results showed that this warming event was characterized by an extraordinarily long vortex disruption in the lower and middle stratosphere, with strong and rapid recovery of the vortex in the upper stratosphere. As discussed earlier, the meridional circulation associated with any warming event is expected to induce upwelling in the equatorial region. As OLR has been used as a proxy for tropical convection, the time variation of zonal mean OLR averaged for equatorial northern hemisphere (ENH) (5 N-15 N), equator (EQ) (5 S-5 N) and equatorial southern hemisphere (ESH) (5 S-15 S) is compared with that of poleward temperature difference. It is observed that the zonal mean OLR (Figure 3.12 (c)) do not show any notable relationship with the SSW event and they just show an increasing tendency over ENH and decreasing tendency over equator and ESH. Figure 3.12 (d-e) shows results for the time interval of 1-120 days from 1 November 2005. During this period, the temperature difference between the latitudes 90 N and 60 N reveals that there are series of warming anomalies, which can be inferred from the positive temperature difference at 10 hpa on day numbers 64 66, 69-80, and 82-98 (Figure 3.12 (d). Large temperature differences of 16 K, 20 K and 32 K were observed on day numbers 65, 72 and 83 during the three warming cases, respectively. Though there were three successive warm 95
pulses, only the third was the major warming event, as it was accompanied by the reversal of zonal winds from eastward to westward (Figure 3.12 (e). 96
Figure.3.12 (a) UKMO zonal mean temperature difference between latitude 90 o N and 60 o N, (b) UKMO zonal mean zonal wind at 60 o N for pressure levels 10hPa, 1hPa and 0.1 hpa, (c) zonal mean NOAA OLR (d) 90 o E-150 o E mean NOAA OLR averaged for the latitudes 5 o S-15 o S, 5 o N- 15 o N and 5 o S-5 o N for the time interval of 1-120 day starting from 01 November 2003. (e-h) Same (a-d) except for 01 November 2005. 97
The zonal mean wind at 60 N started decelerating after day number 55 (25 December 2005), reversed its direction to westward on day number 82 (21 January 2006), attained maximum westward wind of 26 m/s on day number 87 (26 January 2006) and reversed again to eastward direction on day number 109 (17 February 2006). Stronger warming is noticed in this case at 10 hpa than at 1 hpa and 0.1 hpa. The 2006 SSW event was a remarkable event with extended period of nearly one month of polar vortex disruption and zonal wind reversal observed in the lower and middle stratosphere [Manney, et al., 2008]. It was a polar vortex displacement event, whereas the one of 2009 was the polar vortex split event [Manney et al., 2009]. The daily zonal mean OLR shows a decreasing trend over equator and ESH whereas it shows an increasing trend over ENH (Figure 3.12 (e)). We compared the time variation of poleward temperature difference with the longitudetime cross section of ENH, EQ and ESH OLR and found that lower OLR prevailed in the longitude band 90-150 E in relation with both the SSW events (not shown). In other longitudes, no notable relationship exists between OLR and stratospheric poleward temperature difference. In the case of 2003-04, the equatorial OLR averaged for 90-150 E shows large convective activity (<200 W/m 2 OLR) during day numbers 40-53 with the lowest OLR value of 169 W/m 2 on day number 50 (Figure 3.13 (d)). It may be recalled that though peak warming day occurred at 10 hpa on day number 58, the onset of warming started much earlier at higher heights (1 hpa) on day number 43 and the peak warming (40 K) was observed on day number 46 (Figure 3.12 a). The ESH also shows less OLR value with the lowest being 190 W/m 2 on day number 48. The ENH shows convective activity, but relatively less when compared to ESH and EQ. 98
We performed lag-correlation analysis between stratospheric polar temperature difference and OLR over equator. Figure 3.13 shows the lag-correlation coefficient between temperature difference between the latitudes 90 N and 60 N at 10 hpa and 1 hpa and negative OLR averaged for 5 S-15 S (ESH), 5 N-15 N (ENH) and 5 N-5 S (EQ) for November 2003-January 2004 and December 2005-February 2006. We obtained correlation coefficients (CCs) of -0.5, -0.5 and -0.3 between the time variation of poleward temperature difference at 1 hpa and that of negative OLR variations at ENH, EQ and ESH for the period 01 November 2003-31 January of 2004, respectively. The correlation improves slightly, with a lag of 3-4 days. Similarly for the case of 2005-06 also, we obtained CCs of -0.6, -0.4 and -0.4 between the time variation of poleward temperature difference at 1 hpa and that of OLR variations at ENH, EQ and ESH for the period 01 December 2005-01 March 2006. In both cases, the poleward temperature difference at 10 hpa does not show noticeable correlation with equatorial OLR at zero lag. However, with a lag of 5 days, the CC increases to 0.5 and 0.35 with the ESH OLR, respectively, for the cases 2003-04 and 2005-06. 99
Figure 3.13 Lag-correlation coefficient between UKMO temperature difference (between 90 o N and 60 o N) and negative NOAA OLR at 10 hpa (left panel) and 1 hpa (right panel), a proxy for tropical convection averaged for 5 o S-15 o S (ESH), 5 o N-15 o N (ENH) and 5 o S-5 o N (EQ) for November 2003-January 2004 (top panel) and December 2005-Februay 2006 (bottom panel). 3.7 Discussion Stratospheric major sudden warmings were detected using NCEP Reanalysis data for seventeen years (1992-2009) in the Northern Hemisphere winter (November to March), Major sudden warming was defined when the temperature gradient between the pole and 60 o N is positive and the zonal mean zonal wind at 60 o N is negative (representing westward flow) at 10 hpa when the temperature gradient is positive. Nine major warmings were selected during 18 years. The vertical structure of major warming features observed during these events reveals that 100
the changes in zonal mean temperature gradient and zonal mean zonal wind occur first in the upper stratosphere and penetrate down with some delay to the lower heights. The delay is particularly noticeable below 40 km. The correlations during winters 2005/2006 and 2008/2009 were very similar, which perhaps due to congruities in their winter stratospheres such as the occurrence of a major warming, continuous and numerous disturbances, and consecutive and cumulative reversals of zonal mean zonal wind from westerly to easterly. Among the nine warmings, five occurred during the westward phase of the QBO, three occurred during the eastward phase and one during the QBO phase reversal period. In the case of major warmings, the largest warm anomaly observed when the QBO phase was west but in the case of minor warming the large warm anomalies was observed when the QBO was in eastward phase. In addition, we have also examined the SSW-QBO-solar cycle relationship in the Northern Hemisphere winter by using daily NCEP reanalysis data. It is found that the QBOsolar cycle relationship holds in both temperature and zonal wind anomalies throughout the period 1965-2008 and the solar signals primarily occur in late winter (February). In polar temperature anomaly, positive solar signals are found in the lower and middle stratosphere during eastward phase of QBO whereas in the zonal wind anomaly, negative signals are found in the middle stratosphere under eastward phase of QBO. It seems that the QBO east phase locks the occurrence of the major warming and solar maximum has an unlocking effect on the role of the QBO east phase. The results for the entire data set confirm the earlier findings and demonstrate the significant effect that the solar cycle changes have on the polar stratosphere temperature and on the meridional circulation. The major SSW events that occurred during the years 2003-04 and 2005-06 were considered to investigate changes in the equatorial troposphere in response to the circulation 101
changes caused by the SSW events. Results obtained in the present study pertain to the time variation of poleward temperature difference at 10hPa, 1hPa and 0.3hPa, OLR over Equatorial Northern Hemisphere (ENH), Equatorial EQ and Equatorial Southern Hemisphere (ESH). The increase in the poleward temperature difference at 1 hpa is accompanied by a decrease in OLR over EQ and ESH averaged for the longitudes 90 o E-150 o E (Indonesian sector) indicating the development of deep convective activity The time variation of OLR values suggest that ENH also shows high, but relatively less convective activity than the EQ and ESH. Kodera and Yamada [2004] investigated the impact of Southern Hemispheric major SSW that occurred during September 2002 on the Hadley circulation. They found that the warming event induced equatorial upwelling and polar downwelling and noted that the impact on the average intensity of the convective activity was very small. They also observed that the convective activity in the equatorial region shifted towards Southern Hemisphere following the SSW. Similar results were obtained by Kodera [2006] in the composite analysis of 12 Northern Hemispheric stratospheric warming events that showed that the convection in the ENH decreases and that over ESH increases following the SSW event. Unlike the inferences made earlier by Kodera [2006], the present study (see Figure 3.12) shows that the zonal mean OLR decreases (increases) with time indicating more (less) convective activity over ESH (ENH), which merely shows the southward migration of inter-tropical convergence zone and but no notable relationship with major SSW events. The results of the present study also show that the convective activity in both hemispheres over Indonesian sector drastically decreases, which can be noted by an increase in OLR after the warming event. 102