Polar mesosphere and lower thermosphere dynamics: 2. Response to sudden stratospheric warmings

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd008127, 2007 Polar mesosphere and lower thermosphere dynamics: 2. Response to sudden stratospheric warmings Andrew J. Dowdy, 1 Robert A. Vincent, 1 Masaki Tsutsumi, 2 Kiyoshi Igarashi, 3 Yasuhiro Murayama, 3 Werner Singer, 4 Damian J. Murphy, 5 and D. M. Riggin 6 Received 9 October 2006; revised 15 March 2007; accepted 5 June 2007; published 11 September [1] The dynamical response of the polar mesosphere and lower thermosphere (MLT) to sudden stratospheric warmings is investigated using MF radars at Davis (69 S, 78 E), Syowa (69 S, 40 E) and Rothera (68 S, 68 W) in the Antarctic and Poker Flat (65 N, 147 W) and Andenes (69 N, 16 E) in the Arctic. Mean winds, gravity waves and planetary waves are investigated during sudden stratospheric warmings, and comparisons are made with climatological means. The available MF radar data set includes six major sudden stratospheric warmings in the Northern Hemisphere and the unprecedented 2002 Southern Hemisphere major stratospheric warming. Three of the six northern events are relatively weak and could almost be classed as minor warmings, while the larger three have similar characteristics to the event in the Southern Hemisphere. Zonal wind reversals associated with the major warmings in both hemispheres are generally weaker and earlier by several days in the mesosphere than in the stratosphere. There are, however, significant differences between locations in their response to stratospheric warmings. The zonal winds are remarkably weaker than average during both winter and spring around the time of the southern major warming of 2002, but these effects are not observed for the Northern Hemisphere events. Gravity wave activity is found to vary significantly between individual stratospheric warming events and also between individual locations. Citation: Dowdy, A. J., R. A. Vincent, M. Tsutsumi, K. Igarashi, Y. Murayama, W. Singer, D. J. Murphy, and D. M. Riggin (2007), Polar mesosphere and lower thermosphere dynamics: 2. Response to sudden stratospheric warmings, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] Major stratospheric sudden warmings are large-scale transient events in the winter middle atmosphere. During major warmings polar stratospheric temperatures increase rapidly, leading to both a reversal in the zonal mean latitudinal temperature gradient and to a change in the direction of the zonal mean zonal wind, which reverts to a summer-like circulation (see Andrews et al. [1987] for a review of earlier observations and theory). [3] The classification scheme of the Stratospheric Research Group of the FU Berlin [Labitzke and Naujokat, 2000] is often used to assess the strength of warmings. A major warming must have a zonal mean temperature increase poleward from 60 latitude at 10 hpa or below, with an associated circulation reversal poleward of 60 1 Department of Physics, University of Adelaide, Adelaide, South Australia, Australia. 2 National Institute of Polar Research, Tokyo, Japan. 3 National Institute of Information and Communications Technology, Koganei, Japan. 4 Leibniz-Institute of Atmospheric Physics, Kühlungsborn, Germany. 5 Australian Antarctic Division, Kingston, Tasmania, Australia. 6 Colorado Research Associates, a division of North West Research Associates, Boulder, Colorado, USA. Copyright 2007 by the American Geophysical Union /07/2006JD latitude (i.e., net mean westward (easterly) winds poleward of 60 ). The circulation reversal means that a requirement for a major warming is a breakdown or a splitting of the winter polar stratospheric vortex. If the zonal mean zonal winds (at the 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 area of the wintertime hemisphere (as measured by radiosonde or satellite), then the warming is classed as minor. [4] Major warmings occur about once every 2 years in the Northern Hemisphere, but no major stratospheric warming had ever been observed in the Southern Hemisphere middle atmosphere until September In this unprecedented event the Antarctic ozone hole split into two parts [Varotos, 2002; Baldwin et al., 2003]. [5] The hemispheric difference in the prevalence of major stratospheric warmings relates to hemispheric differences in planetary wave activity. Stronger orographic and thermal forcing in the Northern Hemisphere leads to larger planetary wave amplitudes and stronger wave mean flow interactions [Andrews et al., 1987]. Planetary waves propagating up from the troposphere can be focused into the stratosphere causing rapid mean flow changes due to rectified nonlinear effects resulting in warming events [Matsuno, 1971; Andrews et al., 1987]. [6] There was an anomalously large amount of planetary wave forcing in the Southern Hemisphere during the of14

2 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 winter. This activity weakened and warmed the polar vortex. When an extremely large planetary wave event occurred in September 2002, the stratosphere was already preconditioned to a certain degree, resulting in the major warming event. The unusual state of the atmosphere in winter 2002 and the conditions that led to the warming are discussed in detail in a sequence of papers that appeared in a March 2005 special issue of the Journal of Atmospheric Sciences. [7] The occurrence of minor stratospheric warmings also displays hemispheric differences. Minor warmings are observed in the Southern Hemisphere [Shiotani et al., 1993], although they tend to be confined to the middle and upper stratosphere. In the Northern Hemisphere, it is more typical for minor warmings to involve the entire stratosphere [Andrews, 1989]. [8] After some stratospheric warmings the polar stratospheric temperatures decrease and the zonal winds accelerate eastward, returning the stratosphere back to a state typical of winter. Sometimes, however, a stratospheric warming leads directly into the changeover to the summer circulation, with warmer polar stratospheric temperatures and westward zonal winds persisting after the warming event. In this case, the warming is classed as a final warming. Final warmings are common when a stratospheric warming occurs toward the end of winter. Final warmings also show important hemispheric differences with final warmings in the Southern Hemisphere often being stronger and more rapid than in the Northern Hemisphere [Yamazaki, 1987; Mechoso, 1990; Rao et al., 2003]. [9] Although warmings and their effects in the stratosphere have been well studied using satellite observations and in situ techniques, such as balloons and rockets, there have been fewer studies of the response in the mesosphere lower thermosphere (MLT). With the wider deployment of ground-based instruments to high-latitude sites in both hemispheres this deficiency is now being overcome. There is now better understanding of the stratwarm effects in the polar middle atmosphere [e.g., Whiteway and Carswell, 1994; Walterscheid et al., 2000; Hoffman et al., 2002; Bhattacharya et al., 2004]. Multistation studies, such as those by Chshyolkova et al. [2006], provide a better perspective of similarities and differences in the dynamics of the high-latitude middle atmosphere of both hemispheres. [10] The significant hemispheric difference in the occurrence of stratospheric warmings indicates a high level of independence between the Northern and Southern Hemispheres, although evidence for cross-equatorial penetration by planetary waves has been reported [Chshyolkova et al., 2006]. The unprecedented Southern Hemisphere major warming is therefore particularly valuable as it provides a unique perspective from which to compare major stratospheric warmings between hemispheres and the response of the MLT. Stratospheric observations during the southern major warming have been compared with major warmings in the Northern Hemisphere [Krüger et al., 2005]. However, hemispheric comparisons at mesospheric heights during major stratospheric warmings remain relatively unreported. [11] In an accompanying paper [Dowdy et al., 2007, hereinafter referred to as Paper 1] we use long-term MLT wind measurements made with MF radars located at high latitudes in each hemisphere to compare seasonal variations in mean winds and gravity wave energy densities. The observations reported in Paper 1 were made at Poker Flat (65 N, 147 W) and Andenes (69 N, 16 E) in the Arctic and Davis (69 S, 78 E) and Syowa (69 S, 40 E) in the Antarctic. In this paper we use observations made at these locations together with wind measurements made with a similar MF radar located at Rothera (68 S, 68 W). We use these climatologies to investigate short-term dynamical changes associated with major stratospheric warmings in each hemisphere. The use of measurements that are made at similar latitude, but well spaced in longitude, enables both hemispheric and global perspectives. 2. Data Analysis [12] All five MF radars are similar in construction and operation. The spaced antenna method with the Full Correlation Analysis [Briggs, 1984] is used to derive horizontal wind speeds. The analysis techniques used to obtain the mean wind and gravity wave data from the MF radar observations are described in more detail in Paper 1. In the following analyses, daily average winds at each station were used to study the response to stratospheric warmings. For the gravity wave studies discussed in section 7, time series derived from 10-min MF radar wind averages were used. [13] MF radar data were available for the northern winters of 1998/1999 until 2002/2003 at both Poker Flat and Andenes. Six major stratospheric warming events occurred during these five winters [Manney et al., 2005]. This occurrence rate is about double the long-term average, but this interval follows an unprecedented period of nine consecutive northern winters during which no major stratospheric warmings were observed [Manney et al., 1999]. For the Southern Hemisphere (SH), data were available from 1994 to 2004 at Davis and from 1999 to 2003 at Syowa. About four months of MF radar data from Rothera were available from the start of July 2002 until early November 2002 for the study of the September 2002 warming event. 3. Overview of Major Warming Events [14] Figure 1 shows zonal mean zonal winds at 60 S and 60 N obtained from the UKMO assimilated data set (see Lorenc et al. [2000] for details) at a pressure level of 10 hpa. Data are shown for winters during which a major stratospheric warming occurred and for which MF radar data are available. [15] The first major stratospheric warming ever observed in the Southern Hemisphere occurred toward the end of September The circulation in the stratosphere was characterized by a series of large planetary wave events that weakened the vortex, preconditioning the atmosphere for the major warming event in late September [Baldwin et al., 2003; Newman and Nash, 2005]. The wind reversal reached a peak westward wind speed of 21 m s 1 and lasted from 26 September until 29 September, after which date the winds reverted back to the eastward direction on 30 September. These details are summarized in Table 1. This event is not simply an exceptionally early final warming because the polar vortex reforms and persists throughout October. The major warming occurs about one month earlier than any 2of14

3 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 1. Zonal mean zonal winds in m s 1 at 10 hpa derived from the UKMO assimilated data set. The first panel shows data for 60 S in winter The second through fifth panels are for 60 N and are, in descending order, for the winters of 1998/1999, 2000/2001, 2001/2002 and 2002/2003. The vertical dashed lines indicate the start and end of the major stratospheric warmings. final warming previously observed in the Southern Hemisphere [Baldwin et al., 2003]. [16] For the Northern Hemisphere (NH) there are six major warming events that coincided with the Arctic MF radar data set. These occurred in December 1998, February 1999, February 2001, December 2001, February 2002 and January 2003 [Manney et al., 2005]. The dates of the zonal mean zonal wind reversals at 10 hpa and 60 N are listed in Table 1. It can be seen from Table 1 that there are significant differences in the strength and duration of these major warming events. The first three events have the strongest zonal wind reversals and are similar in strength to the reversal associated with the SH warming. The last three northern events are weaker and shorter in duration at the 10 hpa level so that they almost fall into the minor warming category. [17] In the following discussion we describe the observations during all the events included in this study, but have chosen to illustrate the results with detailed descriptions of the SH September 2002, and the NH December 1998 and 3of14

4 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Table 1. Starting and Ending Dates of the 10 hpa Zonal Mean Zonal Wind Reversals at 60 S and 60 N Associated With Major Stratospheric Warmings (From the UKMO Assimilated Data Set) a Region Start Date End Date Duration, days Peak Magnitude, ms 1 South 26 Sep Sep North 15 Dec Dec North 25 Feb Mar North 10 Feb Feb North 29 Dec Jan North 17 Feb Feb North 17 Jan Jan a The durations and peak magnitudes of the reversals are also shown. 2001/2002 warmings. This allows us to compare the response throughout the middle atmosphere during major warmings that had different strengths and durations. 4. Observations During Major Stratospheric Warmings 4.1. The 2002 SH Major Warming [18] Daily average zonal winds constructed by averaging MF radar data from Davis, Syowa and Rothera are shown in Figure 2 (top) ( km), centered around the time of the major warming event. Averaging the data from all three stations provides a quasi zonal mean view of the response of the mesosphere to the warming. Figure 2 (bottom) shows the daily (12 UT) UKMO zonal mean zonal winds for the same period. [19] Eastward zonal winds normally dominate the winter mesosphere at these locations (as shown in Paper 1). During the winter of 2002 many brief periods of westward zonal winds occurred in the mesosphere (see Figure 2 and Dowdy et al. [2004]). The strongest and longest of these zonal wind reversals occurs around the time of the major stratospheric warming at all three MF radar stations. At 85 km the reversal starts on day 262 (19 September) and ends on about day 271 (28 September). [20] The zonal winds in the mesosphere revert back to being eastward in direction following the major warming. This period of eastward winds is shorter in duration in the mesosphere than in the stratosphere since the final changeover to the summer westward circulation occurs about a month earlier in the MLT than in the stratosphere. As discussed later, the meridional winds also display features relating to the warming event Northern Hemisphere Major Warmings The 1998/1999 Northern Winter [21] The first of the two major stratospheric warmings during the 1998/1999 northern winter was unusual for a number of reasons. It was the first major warming to have occurred in about 9 years and it was only the second major warming ever observed as early as December [Manney et al., 1999]. The other major warming for this winter occurred during February 1999, which is a more common month for the occurrence of these events. [22] Figure 3 is similar to Figure 2, but for Poker Flat and Andenes during the December 1998 event. It is evident that the zonal wind reversal associated with the warming reaches from the lower thermosphere (and possibly higher) down to heights near 20 km. In the stratosphere the reversal to a westward circulation lasts from about day 349 to day 354 (15 20 December). However, there is some evidence for a more variable response in the mesosphere where the winds Figure 2. Daily average zonal winds for Southern Hemisphere winter The winds from the ground up to a height of 0.3 hpa (56 km) are from the UKMO assimilated data set, with MF radar data shown from heights of km (see text for more details). The solid line indicates the 0 m s 1 contour. 4of14

5 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 3. As for Figure 2 but for the December 1998 NH observations. The major warming is centered on day 351 (17 December) at the 10 hpa level. The winds from the ground up to a height of 0.3 hpa (56 km) are from the UKMO assimilated data set, and MF radar data averaged for Poker Flat and Andenes are shown from km. revert to a weak eastward flow for about 2 days before becoming quite strongly westward again between about days 351 and 354. [23] The February 1999 event had a somewhat different response again, with the zonal wind reversal appearing to reach all the way down to the lower troposphere. The polar vortex reformed following both warming events. The zonal wind reversal of the final warming descended in height over a period of about a month at both locations, starting first in the mesosphere around the end of March and reaching the stratosphere toward the start of May. [24] Planetary wave signatures are evident in the meridional winds (not shown) as alternating regions of northward/ southward winds around the times of both major warmings of the 1998/1999 northern winter, with the NS flow at Poker Flat and Andenes generally being opposite in direction to each other. This is consistent with zonal wave-1 planetary wave activity since the two locations are separated by almost 180 in longitude The 2000/2001 Northern Winter [25] A major stratospheric warming did not occur during the 1999/2000 northern winter, with the next event occurring in February Zonal wind reversals were observed in the mesosphere at both Poker Flat and Andenes for this major warming. The reversal at Poker Flat appears to be very short in duration, although this finding is complicated by the fact that there was a short interval of no radar data during this event. The zonal winds return to an eastward direction after the warming and the final warming does not begin until April in the mesosphere. It descends with time, reaching the lower stratosphere about a month later in May. The mesospheric meridional winds are, on average, opposite in direction at Poker Flat and Andenes around the time of the February 1999 warming event, consistent with the findings for the 1998/1999 northern winter. [26] This northern winter of 2000/2001 is also of interest because a strong Canadian warming occurred in November Canadian warmings occur when a strong Aleutian high moves poleward. During a Canadian warming the meridional temperature gradient can reverse and sometimes briefly change the zonal wind direction over the polar cap, but nevertheless they do not lead to a breakdown of the cyclonic polar vortex. They rarely lead to major warmings [Labitzke and Naujokat, 2000]. However, during the November 2000 event a zonal mean zonal wind reversal did occur for about 9 days in the lower and middle stratosphere from about 65 N to the pole, reaching a peak on 24 November [Manney et al., 2001]. It can be seen from Figure 1 that the zonal mean zonal winds at 60 N reduce, but do not completely reverse, at this time meaning that this event is not classed as a major warming. [27] A stratospheric zonal wind reversal was observed at Andenes at the time of the Canadian warming, but only a very short and weak reversal occurred at Poker Flat over a narrow height range around the end of November. The zonal winds in the mesosphere do not reverse in direction at either Poker Flat or Andenes at this time, but do reverse in early December at the time of a minor stratospheric warming The 2001/2002 Northern Winter [28] Two major warmings took place in the 2001/2002 northern winter, one in December and the other in February. The December 2001 event was only the third major warm- 5of14

6 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 4. As for Figure 3 but for the period December 2001 to February The major warmings are centered on day 365 (31 December) and day 48 (17 February). ing ever to be observed this early in a winter, along with the December 1987 and December 1998 events [Naujokat et al., 2002]. [29] Figure 4 is similar to Figure 3, but for the 2001/2002 northern winter. The zonal wind reversals associated with these two major warmings do not extend down as far into the lower stratosphere as the 1998 event, but are clearly quite large in the middle and upper stratosphere. There is excellent continuity in the wind contours between the stratosphere and the mesosphere. As was noted from Figure 1, the zonal mean zonal wind reversals for the December 2001 and February 2002 warmings are smaller in magnitude and shorter in duration than for the major warmings of the 1998/1999 and 2000/2001 winters at 60 N and 10 hpa. This also appears to be the case in the stratosphere and the mesosphere at the latitude of the Arctic MF radars The 2002/2003 Northern Winter [30] Stratospheric temperatures in the 2002/2003 northern winter were initially very low, although a major warming dominated by a wave-2 planetary wave event occurred in mid-january [Kleinböhl et al., 2005], followed by minor warmings in February and March. Unfortunately, data were not available from Poker Flat during the major warming, but in the mesosphere at Andenes there is a zonal wind reversal that occurs slightly earlier than the zonal wind reversal in the stratosphere. This reversal may or may not be related to the major stratospheric warming. It is relatively weak in magnitude and short in duration, as was also the case in the stratosphere for this event (see Table 1). 5. Zonal Wind Response to Major Warmings [31] Despite the relative difference in the strength of the events discussed here there is a relatively similar response in zonal winds in the MLT to major stratospheric warmings. Table 2 summarizes the main features at a height of 80 km, representative of the MLT, and at 10 hpa, representative of the middle stratosphere. It shows that the zonal wind reverses on average 4 days earlier in the MLT than in the middle stratosphere and so the zero wind line descends with time. In general, however, the reversal lasts longer in the stratosphere and is larger in magnitude than in the MLT. [32] Dowdy et al. [2004] reported that the Antarctic MLT zonal winds in winter 2002 were consistently weaker than the ten-year average values. Furthermore, the transition to the 2002/2003 summer circulation was anomalously weak. The winter to summer zonal wind transition normally proceeds with remarkably little interannual variability at both Davis and Syowa. However, during 2002 the zonal winds are remarkably different from those observed during all other years, being weaker (more eastward) by about m s 1 during November. It is not until the start of December that the zonal westward flow becomes comparable in strength to other years. [33] Figure 5 shows time series of the daily average zonal winds measured at 80 km at both Poker Flat and Andenes for all years used in this study. The results for winters 1998/ 1999 and 2000/2001 are highlighted. Following the large major warming events of February 1999 and February 2001 the zonal winds during the winter-to-summer transition are not noticeably weaker than in other years, in contrast to the situation in the SH event. If anything, they show the opposite effect with a stronger acceleration than normal to the summer circulation. [34] About one month before the summer solstice at the Arctic locations, the average zonal wind strength is about 20 m s 1. This is similar to the average zonal wind strength at the Antarctic locations about one month before 6of14

7 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Table 2. Starting and Ending Dates, Duration, and Magnitude of Zonal Wind Reversals Associated With Selected Major Warmings for Which MF Radar Data Are Available Region Start Date End Date Duration, days Peak Magnitude, ms 1 SH 80 km 19 Sep Sep SH 10 hpa 23 Sep Sep NH 80 km 13 Dec Dec NH 10 hpa 15 Dec Dec NH 80 km 21 Feb Mar NH 10 hpa 25 Feb Mar NH 80 km 27 Jan Jan NH 10 hpa 2 Feb Feb the 2002/2003 summer solstice, which in turn is about 20 m s 1 weaker than all other years of data at both Antarctic locations. In this respect, the 2002 Southern Hemisphere spring transition is more similar to Northern Hemisphere climatological means than to Southern Hemisphere climatological means. The 2002 Southern Hemisphere final warming (see Figure 2) does, however, appear to be typical of the Southern Hemisphere in that it does not show the downward phase progression typical of the Northern Hemisphere (see Paper 1). [35] The interannual variability can be further quantified by forming composites of average zonal wind strengths in winter. The period May to August inclusive was used to define winter in the Southern Hemisphere and the period November through February was used for Northern Hemisphere data. It was found that the average zonal winds during winter at the Northern Hemisphere locations were generally weaker at all heights than at the Southern Hemisphere stations. This is true for all years with the exception of 2002 where the zonal winds at the Southern Hemisphere locations are significantly weaker than usual (by about 5 10 m s 1 ), and are more similar to mean values in the Northern Hemisphere. [36] It is less easy to investigate interannual variability in the Arctic MLT in the 1998 to 2003 period under investigation here because there was only one winter (1999/ 2000) when no major warmings occurred. While the zonal winds during this winter are somewhat stronger than in other years, they are not as significantly different from other winters as the winter of 2002 was in the Southern Hemisphere. 6. Planetary Waves in the MLT [37] Figure 6 shows time series of the daily average meridional winds in the MLT around the times of major stratospheric warmings in both hemispheres. Leading up to the SH major warming in 2002 the oscillations at Davis and Syowa are similar in phase, while an out-of-phase behavior occurs at Rothera. Dowdy et al. [2004] showed that the mesospheric meridional wind fluctuations at this time were consistent with the presence of a 14-day westward propagating zonal wave-1 planetary wave. Espy et al. [2005] also reported a 14-day westward propagating zonal wave number 1 in the Antarctic MLT at this time. There is very little indication of quasi-stationary planetary wave activity in the meridional winds in the MLT around the time of the warming. This can be seen from Figure 6 where the average meridional wind speed in the MLT is relatively close to zero at all three locations. This is despite the fact that a strong quasi-stationary s = 1 wave was evident in the stratosphere and which appeared to play an important role in generating the warming [Krüger et al., 2005]. [38] The 2002 Southern Hemisphere major stratospheric warming event was caused by a series of unusually large amplitude planetary waves propagating from the troposphere up into the stratosphere. Despite the apparent lack of evidence for strong stationary wave activity in the MLT there was some evidence for linkage between planetary waves in the stratosphere and the MLT, as illustrated by Figure 7, which shows NS daily wind variations obtained from the UKMO data set at the grid point closest to Davis and MF radar winds at Davis. The data have been high-pass filtered with a 20-day cutoff, so the effective period range is 2 20 days. Figure 5. Zonal winds at 80 km for (top) Poker Flat and (bottom) Andenes. Winters during which a large major stratospheric warming occurred are shown with solid (1998/ 1999) and dotted (2000/2001) lines. The fainter lines show data for other winters. A 5-day running mean has been applied to all time series. 7of14

8 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 6. Daily average meridional winds in m s 1 at 80 ± 4 km for (a) Davis, (b) Syowa, and (c) Rothera for SH winter Daily average meridional winds for (d) Poker Flat and (e) Andenes during NH winter and (f) Poker Flat and (g) Andenes during NH winter The vertical dotted lines indicate the start and end of major stratospheric warmings (Table 1). Dates are shown relative to the respective winter solstice. [39] Figure 6 shows that meridional winds at the two Arctic locations are generally opposite in direction to each other around the times of NH major warmings. This suggests that quasi-stationary zonal wave-1 planetary waves are a feature of the NH mesosphere. The meridional winds range in strength between ±30 m s 1 at Poker Flat and at Andenes. This is a larger range than occurs at the Antarctic locations around the time of the southern major warming, suggesting stronger mesospheric planetary wave activity in the NH MLT. [40] A convenient way to investigate seasonal and interannual planetary wave variability is through wavelet analysis. To this end, Morlet wavelet power spectra [e.g., Torrence and Compo, 1998] computed using daily average meridional winds measured at 80 km and 10 hpa for Andenes and Davis are shown in Figure 8. Zonal wind values were not used in order to avoid the impulse response effects caused 8of14

9 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 7. Meridional wind variations in the 2 20 day period range during SH winter (top) Winds measured with the MF radar at Davis and (bottom) UKMO winds at the nearest grid point to Davis. by the rapid zonal wind changes that occur in stratospheric warmings. Furthermore, in order to preserve the dynamic range of the plots the powers have been normalized to the largest spectral value, which occurred in September 2002 at Davis. Because the mesospheric spectral values are much weaker than those for the stratosphere they have been multiplied by a factor of 15 so that the dynamic ranges for both regions are similar. [41] In both hemispheres a strong annual variation in power is observed throughout the middle atmosphere. This is expected in that planetary waves forced in the lower atmosphere will not propagate vertically in summer when the middle atmosphere winds are westward [Charney and Drazin, 1961]. In general, the results from the NH location tend to show maximum power around the winter solstice and there tends to be more variability in the MLT than in the middle stratosphere. Careful examination of the spectra for the SH shows, however, that there tends to be two peaks in spectral power, one shortly before and one shortly after the winter solstice. This effect can be explained by the strong eastward flow in mid winter inhibiting vertical propagation of forced waves [Plumb, 1989]. [42] The spectral results for both hemispheres show that the spectral strengths are on average about fifteen times weaker in the mesosphere than in the stratosphere (see Figure 8). So it appears that planetary wave effects are only being partially transmitted to the upper atmosphere from below. 7. Gravity Wave Variability [43] Major stratospheric warmings profoundly change the wind and temperature structure of the polar middle atmosphere and hence change the propagation conditions for gravity waves whose sources are in the lower atmosphere. Since gravity wave drag is believed to be the primary cause for driving the pole-to-pole circulation that is responsible for cooling the summer polar mesopause and warming the winter mesopause region, any change in wave driving can be expected to produce significant changes in the structure of the mesopause region. A modeling study by Holton [1983] indicated that the spectrum of waves reaching the mesosphere could be significantly affected during a stratwarm, causing the winter mesopause temperatures to relax back toward summer-like conditions. An analysis by Dunkerton and Buchart [1984] of the sudden warming event of 1979 showed that it acted to reduce, but not eliminate, quasi-stationary gravity waves acting in the mesosphere. [44] In order to investigate possible changes in gravity wave activity during the warming events discussed here a simple daily index of variations in wave activity was devised. For each height z and day d the index d(d,z) was defined as dðd; zþ ¼ xd; ð zþ xz ðþ : xz ðþ [45] Here x(d, z) =(u 02 ðd; zþ + u 02 ðd; zþ) where u 0 and v 0 are the zonal and meridional perturbation velocities observed on a given day and height and xz ðþis the average variance at height z. In order to reduce strong seasonal variations in wave activity a harmonic fit of the annual, semiannual and terannual components of the variances was calculated and removed before calculating the index. 9of14

10 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 8. Normalized wavelet spectra derived (top) from MF radar meridional winds at a height of 80 km and (bottom) from UKMO meridional winds at 30 km for (left) Andenes and (right) Davis in the years 1999 to For ease of comparison the spectra at 80 km have been scaled by a factor of 15 relative to those at 30 km. The dashed lines indicate the cone of influence [Torrence and Compo, 1998]. Essentially the process acts as a high-pass filter and has the advantage of removing the exponential increase in wave fluxes with height (see Paper 1) making it easier to compare variations over a wide range of heights. [46] This technique was applied to waves in the min period range. The quantity d(d, z) observed around the time of the 2002 Southern Hemisphere major warming is shown in Figure 9. The starting and ending dates taken from Table 1 of the major stratospheric warming are indicated as vertical dashed lines. [47] Comparing the results for Davis and Syowa, there appears to be different behavior at the two stations during the warming. At Davis the wave variance was reduced by about 50% at most heights, while at Syowa there is, if anything, an increase of about 20% in variance relative to the period before the warming. [48] However, given the level of variability in wave activity evident in Figure 9 it is not unambiguously clear whether these apparently different responses at Davis and Syowa are due to the warming. Possible differences in propagation conditions at the two sites were investigated by using middle atmosphere winds derived from the UKMO data set and the MF radars themselves. [49] Figure 10 shows regions of forbidden phase speed due to critical level filtering. A critical level occurs when the ground-based phase speed of a wave is equal to the background wind speed and the wave can no longer propagate vertically. The radar winds are shown at heights above 60 km, with UKMO winds at lower altitudes. The winds are averaged during the time of the zonal mean zonal wind reversal at 68 S and 10 hpa associated with the September 2002 major stratospheric warming (see Table 2). [50] It is apparent that regions of excluded phase speeds are quite different between Davis and Syowa, despite the fact that the stations differ by only about 38 in longitude. Waves propagating southwestward with phase speeds of 50 m s 1 are unlikely to reach the mesosphere at Syowa, whereas waves with a southward orientation are excluded in the vicinity of Davis. These differences could account for some of the observed differences between the gravity wave fields at Davis and Syowa around the time of the major warming. [51] Similar analyses to those described above were carried out for Poker Flat and Andenes around the time of NH major warming events. As was found for the SH major warming a large degree of gravity wave variability was evident, to the extent that it was not clear whether changes in wave activity could be ascribed unambiguously to the warming events themselves. 8. Discussion and Conclusions [52] The response of the polar MLT to major stratospheric warmings was investigated using MF radar observations from sites in both the Southern and Northern Hemispheres. The radars were used to characterize the wind fields at 10 of 14

11 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 9. Percentage difference from the mean variance (u 02 + v 02 ) as a function of height for (top) Davis and (bottom) Syowa around the time of the Southern Hemisphere major stratospheric warming in Variances are for the period range min and have been smoothed with a (1/4, 1/2, 1/4) filter in time. The dotted lines show the start and end dates of the warming event. heights above 60 km during six major warmings in the NH between 1998 and 2003 and in the singular 2002 Antarctic event. Winds and temperatures from the UKMO assimilation model were used to define the stratospheric response. The main findings are: [53] 1. The radar data from both the Arctic and the Antarctic show that the MLT region is strongly affected by major stratospheric warmings. In general, the duration of the zonal wind reversal from eastward to westward flow was between 5 and 10 days. [54] 2. Zonal wind reversals in the MLT tend to lead the reversals in the zonal mean zonal wind at 10 hpa (30 km altitude). Typically, the winds reversed 4 days earlier at 80 km than at 30 km with a descent rate of the zero wind line of about 15 km day 1. [55] 3. There was little interannual variability in the strength of zonal winds in the MLT throughout all NH winters. This is in contrast to the 2002 winter in the SH when the MLT zonal winds were significantly weaker than the long-term winter average [Dowdy et al., 2004]. [56] 4. Planetary wave amplitudes in the NH winter MLT were larger than during winter 2002 in the SH. The meridional wind fluctuations observed at Andenes and Poker Flat were consistent with wave-1 planetary waves. [57] 5. There was no significant evidence for systematic changes in gravity wave activity in the MLT during major warmings. [58] Overall, there is good agreement between the UKMO and MF radar data as to the timing and structure of the warming events in all the cases that were studied. Features such as the planetary wave signatures in the winds and the zonal wind reversal associated with each major warming event show that the UKMO data and MF radar data are reasonably consistent with each other in terms of wind direction. In the upper stratosphere and lower mesosphere, where the two data sets meet, the UKMO winds are often larger in magnitude than the MF radar data. MF radar and UKMO wind speeds are generally in better agreement in the Arctic (Figures 3 and 4) than they are in the Antarctic (Figure 2). This is a consequence of the greater amount of satellite, radiosonde and aircraft observations made at mid to high latitudes of the NH compared with the SH which are assimilated into the UKMO model forecasts. [59] A range of situations are covered in our analysis. It includes the first and only SH major warming event and six NH warmings that encompass a range of strengths. Nevertheless, the basic manifestation of the warming events in the MLT is similar in all situations. There are, however, some distinct differences in the overall dynamical situations. Firstly, the timing of the September SH major stratospheric warming event is equivalent to a March Northern Hemisphere event (which is often called an early spring warming as opposed to a midwinter warming ). The NH events studied here occur only as late as February, a difference which should be considered when making hemi- 11 of 14

12 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 Figure 10. Regions of forbidden phase speeds due to critical level filtering for (top) Davis and (bottom) Syowa. The exclusion circles were computed from the wind profiles shown in the plots on the right. Solid (dotted) lines indicate zonal (meridional) wind components. The winds are averaged during the time of the zonal mean zonal wind reversal at 68 S and 10 hpa associated with the September 2002 major stratospheric warming (see Table 2). MF radar winds are shown at heights above 60 km, with UKMO winds at lower altitudes. spheric comparisons. Secondly, there appear to be distinct differences in the dynamical states of the MLT preceding the warmings. As discussed in Dowdy et al. [2004] the mean zonal winds in the MLT during the SH winter of 2002 were significantly weaker than the long-term average values. It was also noteworthy that after the warming the transition to the 2002/2003 summer zonal circulation was slower than normal. In contrast, the NH winters discussed here did not show significant interannual variability in the MLT zonal winds. This may be because all the winters except one (1999/2000) experienced major stratospheric warmings. It does suggest, however, a difference in the basic dynamical state of the winter MLT regions of the two hemispheres, which is likely due to the very distinct hemispheric differences in the strength of planetary waves, and perhaps gravity waves, reaching the MLT in winter. [60] The state of the Antarctic MLT during the winter of 2002 was anomalous in a number of ways. As well as the weak zonal circulation noted above, hydroxyl airglow temperatures measured at heights near 87 km at Davis station showed that the 2002 winter average was about 5 K warmer than the climatological mean [French et al., 2005]. At South Pole station Hernandez [2003] reported anomalously low mesospheric temperatures, with values about 180 K, which are approximately 35 K colder than the long-term average. However, Azeem et al. [2005] also made MLT temperature measurements using optical techniques at the South Pole and report temperatures ranging from 200 to 220 K during the period from mid-july to early September These values are significantly higher than those reported by Hernandez [2003]. [61] Northern Hemisphere observations often show a cooling in the mesosphere at the time of a stratwarm [e.g., Walterscheid et al., 2000]. The observations of French et al. [2005], however, do not show any evidence of mesospheric cooling during the SH 2002 event. They do find an unusually cold interval in mid-august, about one month prior to the stratwarm, but this appears to be a climatological feature. As far as the NH stratwarm events that are discussed here are concerned we do not have any evidence concerning any associated temperature changes in the MLT. [62] The mesospheric response to stratospheric warmings depends on a number of factors that are not yet fully understood. Changes in the momentum and energy budgets of the MLT will likely be caused by changes in both planetary and gravity wave interactions with the mean flow as they propagate up through the middle atmosphere. Our observations show that planetary waves are present with significant amplitudes in the MLT winters discussed here. They support the findings of Hoffman et al. [2002], for example, who found significant middle atmosphere wave-1 activity in the NH mid and high latitudes during the majority of warming events that they studied between 1989 and They also reported enhanced wave activity in the MLT in the period range days whenever there was enhanced wave-1 or 2 amplitudes at the 30 hpa level. 12 of 14

13 DOWDY ET AL.: POLAR MLT DYNAMICS, 2 [63] Changes in the stratospheric circulation produce changes in the propagation conditions for gravity waves and hence in the gravity wave fluxes reaching the mesosphere. Any zonally averaged reduction in the strength or reversal of the eastward winds in the middle atmosphere will allow an increased flux of eastward propagating gravity waves, producing an eastward forcing in the MLT. The complex changes in the residual circulation in the highlatitude MLT associated with warming events are also likely to influence the composition of important minor constituents such as atomic oxygen [Liu and Roble, 2002]. Mesospheric cooling might be expected during major warmings since gravity wave momentum deposition drives the pole-topole meridional circulation that in turn drives mesospheric temperatures away from radiative equilibrium at the solstices. Holton [1983] showed how a cooling could be induced by the change to the westward flow in the stratosphere, causing a reduction of gravity wave transmission into the mesosphere. In particular, orographically forced waves, that is waves with a ground-based phase speed c 0, would be unable to propagate through the zero wind line in the stratosphere, although these effects are likely to be longitudinally dependent because of planetary wave modulation of gravity wave fluxes [Dunkerton and Buchart, 1984; Manson et al., 2003]. [64] From the MF radar data presented here, it appears that major stratospheric warmings have a variable effect on mesospheric gravity wave activity, depending on factors such as location, gravity wave frequency and the individual major warming event. It must be remembered that radars measure in a ground-based reference frame, so that lowphase speed (c 0) waves are difficult (if not impossible) to distinguish from other low-frequency fluctuations. Nevertheless, it is probable that longitudinal variations in critical level filtering associated with planetary wave activity explain some of the observed variations in mesospheric gravity wave activity evident in Figure 9. Certainly gravity wave activity around the times of the major warmings show quasi-periodic variations which are often in phase over large height ranges. Because of the 180 ambiguity in gravity wave polarization (see equation (2) in Paper 1), planetary waves with periods of about 2 weeks could cause critical level filtering observed as the periodic variations of about 1 week in gravity wave activity. [65] The influence of major stratospheric warmings on gravity wave activity has been investigated at lower altitudes. Rayleigh lidar studies during a stratospheric warming at Eureka, Canada (80 N, 86 W) show substantially greater dissipation of gravity wave energy during the warming than during the preceding and following periods [Whiteway and Carswell, 1994]. Duck et al. [1998] report increased gravity wave activity in the vortex jet during a stratospheric warming, and suggest that the decreased wave activity found by Whiteway and Carswell [1994] was in fact due to the movement of the polar vortex over Eureka so that Eureka was beneath the vortex core. Whiteway et al. [1997] found that gravity wave activity maximized within the vortex jet and at the edge of the vortex, and was a minimum inside the vortex and intermediate outside the vortex. [66] Satellite observations perhaps have a better chance of providing a hemispheric perspective on variations in wave activity. [67] Ratnam et al. [2004] analyzed gravity wave activity in the stratosphere on a global scale derived from GPS measurements made with the CHAMP satellite. They report an enhancement of wave activity during the 2002 Southern Hemisphere major stratospheric warming. Gravity wave potential energies were up to three times higher than usual near the edge and outside the polar vortex, but not inside the vortex. Their results appear to be the only observations so far of gravity wave activity during the Southern Hemisphere major warming, although they are only at heights up to 30 km. However, Pfenninger et al. [1999] and Yoshiki and Sato [2000] reported enhanced gravity wave activity with final stratospheric warmings in the SH. [68] The findings of Ratnam et al. [2004], Whiteway et al. [1997] and Duck et al. [1998] all show that the position of the vortex relative to a particular observing site plays a large role in determining the characteristics of the observed gravity wave fields during stratospheric warmings. The position of the vortex relative to a particular radar station is a factor that also explains some of the variations observed in the mean winds and gravity waves that we report. A ray tracing analysis of gravity wave propagation may provide a way of investigating longitudinal variations in wave activity. As the network of radars and lidars located at high latitudes is expanded there will be a better understanding of gravity (and planetary) wave effects in the MLT during stratospheric warmings. [69] Acknowledgments. The permissions of the Japanese National Institute of Polar Research to use their MF radar data from Syowa, the Colorado Research Associates and the British Antarctic Survey to use their MF radar data from Rothera, the Japanese National Institute of Information and Communications Technology to use their MF radar data from Poker Flat and the Leibniz-Institute of Atmospheric Physics to use their MF radar data from Andenes are all gratefully acknowledged. This research was supported by Australian Research Council grant DP and Australian Antarctic Science Advisory Committee grants scheme project 674. References Andrews, D. G. 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