Polar mesosphere and lower thermosphere dynamics: 1. Mean wind and gravity wave climatologies

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd008126, 2007 Polar mesosphere and lower thermosphere dynamics: 1. Mean wind and gravity wave climatologies Andrew J. Dowdy, 1 Robert A. Vincent, 1 Masaki Tsutsumi, 2 Kiyoshi Igarashi, 3 Yasuhiro Murayama, 3 Werner Singer, 4 and Damian J. Murphy 5 Received 9 October 2006; revised 14 April 2007; accepted 5 June 2007; published 11 September [1] Mean wind and gravity wave climatologies are presented for the polar mesosphere and lower thermosphere (MLT). The data were derived using MF radars at Davis (69 S, 78 E) and Syowa (69 S, 40 E) in the Antarctic and Poker Flat (65 N, 147 W) and Andenes (69 N, 16 E) in the Arctic. The dynamics of the Antarctic MLT are found to be significantly different from the Arctic MLT. Summer maxima in both the westward and equatorward winds occur closer to the solstice in the Antarctic than in the Arctic. The greater symmetry around the solstice suggests radiative effects may play a greater role in controlling the state of the Antarctic MLT than in the Arctic, where dynamical effects appear to be more important. Gravity wave observations also suggest that wave drag may be greater in the Arctic than in the Antarctic. The equatorward flow near the mesopause persists later in summer in the Arctic than in the Antarctic, as do observations of polar mesospheric clouds and polar mesospheric summer echoes. All three phenomena begin at about the same time in each hemisphere, but end later in the Arctic than in the Antarctic. It is proposed that the magnitude of the meridional winds can be used as a proxy for gravity wave driving and the consequent adiabatic cooling in the MLT. Seasonal variations in gravity wave activity are predominately combinations of annual and semiannual components. Significant hemispheric differences are observed for both the timing and magnitude of these seasonal variations. Citation: Dowdy, A. J., R. A. Vincent, M. Tsutsumi, K. Igarashi, Y. Murayama, W. Singer, and D. J. Murphy (2007), Polar mesosphere and lower thermosphere dynamics: 1. Mean wind and gravity wave climatologies, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] The middle and upper regions of the atmosphere are recognized as important and sensitive indicators of the health of our atmosphere as a whole [e.g., Roble, 1993]. In particular, the importance of the polar mesosphere and lower thermosphere (MLT) is now better appreciated, and there is debate as to whether changes in the occurrence of phenomena such as polar mesospheric clouds (PMCs), polar mesospheric summer echoes (PMSEs) and noctilucent clouds (NLCs) are possible indicators of anthropogenically induced climate change [Thomas, 1996; Shettle et al., 2002; Klostermeyer, 2002; von Zahn, 2003]. In spite of this, the polar middle atmosphere remains one of the least understood regions of the Earth s atmosphere. 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 Government Antarctic Division, Kingston, Tasmania, Australia. Copyright 2007 by the American Geophysical Union /07/2006JD [3] The dynamics and thermal structure of the Arctic MLT region have been studied extensively using a number of different in situ and ground-based techniques [Balsley et al., 1984; Lübken and von Zahn, 1991; Lübken, 1999; Mitchell et al., 2002]. It is only recently that the structure and dynamics of the Antarctic MLT region has been investigated with a wide range of instrumentation. Lübken et al. [1999] and Lübken et al. [2004] reported rocket measurements of summer temperatures and winds made at Rothera (68 S, 68 W), while Gardner et al. [2001] studied temperatures over the South Pole using lidar techniques. Vincent [1994], Baumgaertner et al. [2005] and Hibbins et al. [2005] provided Antarctic MLT wind climatologies for individual stations. [4] Temperatures at the Arctic mesopause are about 60 K colder in summer than in winter [Lübken and von Zahn, 1991] and are low enough to lead to the formation of PMSE, PMC and NLC. Early interhemispheric comparisons between these phenomena suggested that there were significant differences between hemispheres [e.g., Balsley et al., 1995; Huaman and Balsley, 1999]. Although satellite measurements suggest that the Southern Hemisphere summer mesopause is a few degrees warmer in summer than in the north [Huaman and Balsley, 1999; Hervig and Siskind, 2006], rocket observations show that temperatures are just 1of16

2 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 as cold near the Antarctic midsummer mesopause as they are over the Arctic [Lübken et al., 1999]. Nevertheless, there are interhemispheric differences in the strength, seasonal variation and height of occurrence of PMSE and PMC [Morris et al., 2004; Chu et al., 2004]. Olivero and Thomas [1986], Thomas and Olivero [1989] and DeLand et al. [2006] report hemispheric differences in the occurrence of PMCs, with the Northern Hemisphere clouds generally being brighter and occurring over a larger range (by about 5 latitude) than in the Southern Hemisphere. [5] It is now well established that gravity waves play a crucial role in determining the state of the middle atmosphere [e.g., Fritts and Alexander, 2003] and especially in determining the temperature structure of the polar mesopause region. Modeling studies such as those by Garcia [1989], McIntyre [1989] and Fritts and Luo [1995] investigated how gravity wave forcing drives the atmosphere away from radiative equilibrium by inducing a meridional circulation from the summer to winter pole at the mesopause. While a number of factors may be important it is likely that differences in gravity wave forcing contribute to hemispheric differences. Gruzdev and Brasseur [2005] recently highlighted how long-term changes in gravity wave activity can contribute significantly to the observed longterm changes in the thermal structure and chemical composition of the mesosphere. [6] This paper presents mean wind and gravity wave climatologies for the MLT region obtained using Medium Frequency (MF) radars located at Davis (69 S, 78 E) and Syowa (69 S, 40 E) in the Antarctic, and Poker Flat (65 N, 147 W) and Andenes (69 N, 16 E) in the Arctic. These stations are of very similar design, operate at similar frequencies and use the same software to analyze the raw data. Being located as they are at very similar latitudes around the periphery of the Antarctic and Arctic makes these four radars well suited for comparative studies of winds and waves. The Davis radar started operation in 1994, while the other systems started operation in the late 1990s. Sufficiently long-term data have been acquired so that the effects of interannual variability in winds and wave activity are reduced allowing meaningful climatologies to be produced. In each hemisphere the stations are quite well separated longitudinally, so the results give some insight into spatial as well as temporal and hemispheric variability. [7] The comparisons also provide insights into the various dynamical aspects of the polar MLT region, including mean winds and gravity wave activity and any interhemispheric differences. They provide benchmarks against which short-term variations induced by, for example, stratospheric warmings, can be assessed. Gravity waves are currently poorly represented in global climate models (GCMs) [McLandress, 1998]. One aim of this paper is to provide a better understanding of gravity wave activity that could benefit modeling studies and the predictions that they make. 2. Data Analysis [8] All four MF radars are similar in construction, e.g., Murayama et al. [2000]. They use the spaced antenna technique, with raw data analyzed with the conventional full correlation analysis (FCA) method to determine horizontal wind speeds [Briggs, 1984]. Transmission uses pulse widths of 30 ms, corresponding to range resolutions of about 4 km, but echoes are oversampled at 2 km height intervals in the km height range. Data were acquired with a time resolution of 2 min when suitable scattering irregularities were present. The winds at each height were postprocessed to improve the data quality using a moving data window of 1 hour duration, so that individual values that lay more than 2 standard deviations from the mean value were excluded. Outlier removal may have caused small amounts of genuine wind data to be rejected, but contamination by external RF interference or by poor signal-to-noise echoes is considerably reduced. A degree of caution should be used in the interpretation of some observations made at apparent vertical ranges shorter than about 70 km. During summer, echoes can be received horizontally from drifting icebergs and sea ice. This situation is particularly prevalent at Davis. [9] Mean zonal and meridional wind climatologies were compiled from daily average values. At least 20 hours of data must be available on a particular day to reduce the tidal effects. The acceptable data were then averaged over all available years, and are shown only when 2 or more years were available for a particular height and day. Running means of 15-day duration in time and 4-km in height were applied to reduce the influence of interannual variations. Data were available during the years at Davis, at Syowa, and at Poker Flat and Andenes. [10] Wind variances are used to study gravity wave activity [e.g., Vincent and Fritts, 1987]. Time series of zonal and meridional winds averaged over 10-min intervals were first constructed. To reduce the effects of large amplitude 24-hour and 12-hour tides, the time series of one week duration for each wind component at each height were harmonically analyzed and the resultant tidal harmonic components subtracted from the time series. Power spectra were then computed for each time series and Wiener filtered by computing the noise floors and subtracting them from the spectra at each height [Press et al., 1992]. Finally, the spectra were integrated to find the variances for each wind component. [11] The variance calculations were restricted to motions with periods of 8 hours or less since motions due to transient polar normal modes or intradiurnal modes with periods in the range 8 12 hours are a significant feature of the wind field at high latitudes [Forbes et al., 1999; Kovalam and Vincent, 2003]. Two period ranges ( min and min, respectively) were used in order to provide some information on spectral content. Although the two period ranges are different in bandwidth they are reasonably similar in total power because of the approximate f 5 3 nature of the gravity wave spectrum, where f is frequency [VanZandt, 1982; Kovalam and Vincent, 2003]. Variances for the zonal u 02 and meridional v 02 components as well as the covariance u 0 v 0 were calculated to allow investigation of the polarization of the gravity wave field. [12] Results from the four stations examined in this study have 5 12 years of continuous MF radar winds. Although there may be year-to-year variability, the climatological analysis presented here provides a reasonable representation of the mean situation prevalent at each station. An indica- 2of16

3 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 1. Zonal wind climatologies for Davis (first panel), Syowa (second panel), Poker Flat (third panel) and Andenes (fourth panel). The data were smoothed in time with a 15- day running mean. The 0 m s 1 contour is represented by the solid line. tion of interannual variability is provided by showing the standard error of the mean where appropriate. 3. Mean Winds 3.1. Summer Zonal Wind Climatologies [13] Climatologies of zonal winds at the four stations are displayed in Figure 1. The zonal winds are westward in the summer mesosphere (heights below 90 km) at all four locations, with eastward winds in the lower thermosphere above 90 km. In the Southern Hemisphere (SH) the transition from winter eastward winds to summer westward winds occurs in mid-october. The transition is almost simultaneous at all heights, although at Davis it appears to start a little earlier near 70 km than at other heights. In contrast, at both Northern Hemisphere (NH) stations the transition appears to descend in height at a rate of about km day 1 in March from near 100 km. [14] At all four locations the summer westward winds maximize at a height about 5 10 km beneath the mesopause (85 km). The date, height and magnitude of this peak are shown in Table 1 for each of the four locations. The westward winds at Syowa appear to have a just significant double peak during summer (see Figure 2), with the first peak occurring before the summer solstice at about the same time as the westward flow maximizes at Davis, while the second peak occurs shortly after the summer solstice. For Syowa the average date, height and magnitude of the two maxima are used for the information presented in Table 1. The peak magnitudes of the westward wind at Davis, Syowa and Poker Flat are similar in value to each other, but all are about 50% larger than at Andenes. [15] A striking result from Table 1 is that the peak westward winds occur about 30 days earlier on average (i.e., closer to the solstice) at the SH locations than at the NH stations. The zonal winds are more symmetric about the solstice in the Antarctic than in the Arctic, as detailed in Figure 2. However, this asymmetry is not as strong in the zero-crossing times, given in Table 2, which occur about a week earlier and end about three days later on average at the Antarctic locations than at the Arctic locations. This is much smaller than the 30-day hemispheric difference observed for the timing of the peak westward winds, which implies greater acceleration of the westward zonal winds in the early SH summer. From Tables 1 and 2 the accelerations are estimated to be on average 0.70 m s 1 day 1 at the Antarctic locations compared with 0.44 m s 1 day 1 in the Arctic MLT. Conversely, the deceleration from the peak to wintertime eastward flow transition is larger in the NH (0.58 m s 1 day 1 ) than it is in the SH (0.45 m s 1 day 1 ). [16] Another interesting feature of Figure 1 is that the zonal winds at Syowa and Poker Flat become westward in the lower thermosphere for a brief period in late summer (early March at Syowa, and late September at Poker Flat). This feature is not seen at either Davis or Andenes, and is an example of similarities and differences between locations on a more local scale Winter Zonal Wind Climatologies [17] On average, the zonal winds decrease with increasing height, as shown in Figure 3 so that, in contrast to the situation during summer, the wintertime zonal winds maximize at heights below the lowest level of observation. The shear is significantly stronger in the SH MLT than in the NH, so that the difference in wind speed is about m s 1 at heights near 75 km, and about 5 8 m s 1 at heights near 95 km. The relatively weak zonal winds in the Arctic winter are associated with hemispheric differences in planetary wave activity and sudden stratospheric warmings, which in turn is due to stronger orographic and thermal forcing than in the SH [Andrews et al., 1987; Becker and Schmitz, 2003]. [18] So far only one major warming event has been observed in the SH, in late winter In the period May to November the zonal winds in the MLT were weaker than the longer-term average (see Dowdy et al. [2004] for details). The climatology for Syowa will be affected most since there are only 5 years of data available for this site. However, the impact of the anomalous winter does not Table 1. Timing, Height, and Magnitude of the Peak Westward Zonal Winds During Summer at Davis, Syowa, Poker Flat, and Andenes as Determined From the Climatologies Shown in Figure 1 Location Timing (Date) Timing (Days After Solstice) Height, km Magnitude, ms 1 Davis 4 Dec Syowa 18 Dec Poker Flat 16 Jul Andenes 10 Jul of16

4 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 2. Time series of the zonal wind climatologies shown in Figure 1 at heights corresponding to the peak summer westward winds as listed in Table 1. The dotted lines indicate the standard error in the mean. Figure 3. Height profiles of the zonal wind climatologies at the winter solstice at Davis (first panel), Syowa (second panel), Poker Flat (third panel) and Andenes (fourth panel). The dotted lines indicate the standard error of the mean. appear to be large since at 80 km, for example, the impact of including winter 2002 is to reduce the strength of the eastward winds by only 2 3 m s 1 compared with the average winter values derived from the other four years of data Summer Meridional Wind Climatologies [19] The meridional winds (Figure 4) are generally weaker in strength and more variable in direction than the zonal winds. At each site the meridional wind climatologies show an equatorward flow at heights near the summer mesopause (85 km). The values of the height, timing and magnitude of the equatorward jet peak at each location are listed in Table 3. [20] The equatorward jet peak occurs at a similar height at all four locations (86 88 km), but about a week earlier and therefore closer to the solstice in the Antarctic than in the Arctic. The peak magnitude is larger at Poker Flat than at Table 2. Start and End Dates of the Summer Westward Zonal Winds at Davis, Syowa, Poker Flat, and Andenes, as Determined From the Data Shown in Figure 1 for the Heights Shown in Table 1 Location Start Date Start Date (Days From Solstice) End Date End Date (Days From Solstice) Davis 17 Oct Mar 91 Syowa 15 Oct Mar 95 Poker Flat 13 Apr Sep 91 Andenes 14 Apr Sep 90 Figure 4. As for Figure 1 but for the meridional winds. 4of16

5 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Table 3. Timing, Height, and Magnitude of the Peak Equatorward Meridional Winds During Summer at Davis, Syowa, Poker Flat, and Andenes, as Determined From the Climatologies Shown in Figure 4 Location Timing (Date) Timing (Days After Solstice) Height, km Magnitude, ms 1 Davis 25 Dec Syowa 27 Dec Poker Flat 1 Jul Andenes 5 Jul Table 4. Start and End Dates of the Equatorward Meridional Winds During Summer at Davis, Syowa, Poker Flat, and Andenes, for the Heights Shown in Table 3 Location Start Date Start Date (Days From Solstice) End Date End Date (Days From Solstice) Davis 17 Nov Feb 54 Syowa 31 Oct Feb 56 Poker Flat 9 May Aug 68 Andenes 19 May Aug 68 Andenes, similar to the situation for the relative strength of the westward winds during summer. The start and end dates of the equatorward meridional winds during summer are given in Table 4. The equatorward jet persists for about 13 days longer on average in the NH than in the SH. This is significantly larger than the hemispheric timing difference for both the start and the peak of the equatorward jet. [21] Given the role that gravity wave drag plays in closing the zonal jet and driving the meridional circulation, it is interesting that the peak equatorward meridional winds during summer occur near 88 km. This is about 10 km higher than the peak westward zonal winds during summer and at the height where the zonal wind shear is strongest (see Figure 1), as also reported by Manson et al. [1991] and Vincent [1994] Winter Meridional Wind Climatologies [22] An interesting feature at Poker Flat and Andenes is that the meridional winds during winter (October to March) are often opposite in sign to each other at any given height. For example, at heights near 70 km the meridional winds during midwinter are generally southward at Poker Flat and northward at Andenes, with the opposite situation being the case at heights near 90 km. This is consistent with quasistationary zonal wave-1 planetary wave activity since the two Arctic locations are almost 180 different in longitude to each other. [23] The mean meridional winds at the two Antarctic locations are generally not as strong as at the Arctic locations. Significant quasi-stationary planetary wave signatures are not seen in the meridional wind climatologies of the Antarctic locations, although this may be because the two locations are only separated in longitude by Gravity Waves 4.1. Gravity Wave Climatologies [24] Climatologies of u 02 + v 02 in the period range min at Davis, Syowa, Poker Flat and Andenes are shown in Figure 5 at heights from km. Daily variances are smoothed with 15-day and 4-km running means in time and height and averaged over all available years of data. Results are shown when at least 2 years are available for a particular height and day. [25] There is considerable variation in gravity wave activity between the four locations. Andenes generally has weaker variances than the other three stations. Davis is somewhat different from the other locations as the variances are relatively large at heights below 80 km during summer, with a minimum occurring around 80 km. This may be a consequence of data contamination from horizontal radar transmissions and spurious motions due to iceberg movement or sea scatter at larger ranges. However, analysis techniques were applied to identify and remove these latter influences from the data prior to calculating the variances. We believe that the relatively larger values below 80 km at Davis is a real feature. [26] Figure 5 shows that the variances near 80 km are generally similar in magnitude at all locations, but are significantly larger in the SH than in the NH at altitudes above 90 km. This feature is brought out more clearly in Figure 6 which shows height profiles of the average variances during summer and winter. The dotted lines in Figure 6 are an estimate of the expected height profiles if wave energy is conserved. They consist of the variance Figure 5. Climatologies of u 02 + v 02 at Davis (first panel), Syowa (second panel), Poker Flat (third panel) and Andenes (fourth panel) in the period range min. They consist of variances computed from values acquired over all years and have been smoothed using windows that are 15-day in time and 4-km in height. Units are m 2 s 2. 5of16

6 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 6. Height profiles of (u 02 + v 02 ) for the period range min averaged for summer and winter seasons. Summer (winter) values are the means during December and January (June and July) in the Southern Hemisphere and June and July (December and January) in the Northern Hemisphere. For comparison, the dotted lines show how the mean square amplitudes would grow if energy is conserved, assuming a density scale height of 5 km. strength at a height of 70 km multiplied by an exponential growth term: u 02 ðþþv z ðþ¼ z u 02 ehr z 70 þ v02 70 where H r = 5 km, is the approximate density scale height, based on July values of temperature at 69 N [Lübken et al., 1999]. 6of16

7 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 7. Time series of (u 02 + v 02 ) for the period range min plotted as a function of days relative to the summer solstice in each hemisphere. Data are shown averaged for the height ranges and km. The data are smoothed with a 7-day running mean, with the data averaged over all available years. The dotted lines represent least-squares harmonic fits to the data consisting of the sum of the mean, annual, semiannual and terannual components Seasonal Variations in Gravity Wave Activity [27] Figures 7 and 8 provide information on the temporal variations in gravity wave activity for the period ranges min and min, respectively. At all four locations strong seasonal variations are apparent that appear to be combinations of annual (maxima in winter and minima in summer) and semiannual (with maxima around the solstices and minima around the equinoxes) components. 7of16

8 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 8. As for Figure 7 but for variances in the period range min. Details of the timing and magnitude of the winter maxima and the following minima (in spring or summer) are provided in Table 5. For each period range, data are averaged over heights from 76 to 84 km and from 86 to 94 km. [28] For the shorter-period range the seasonal variation at all stations changes from mainly a semiannual component at low heights to a predominantly annual cycle at upper levels. For the longer-period range the wave activity shows a stronger annual cycle at all heights, except at Andenes. The peak in wave activity in winter occurs about days later at the SH stations compared to the situation in the NH. This is true for both period ranges. The amplitude of the seasonal cycle is also larger in the SH (see Table 5) and hence the rate of change of variance with time is correspondingly larger. [29] Annual and semiannual variations in gravity wave activity are consistent with the results reported by, for 8of16

9 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Table 5. Timing and Magnitude of the Winter Maxima and Spring/Summer Minima Using a Least Squares Harmonic Fit of the Mean, Annual, Semiannual, and Terannual Components to the Data Shown in Figure 7 and 8 a Variance Period, min Height, km Maxima (Days From Summer Solstice) Minima (Days From Summer Solstice) D Magnitude, m 2 s 2 Rate of Change, m 2 s 2 day 1 SH NH SH NH SH NH SH NH a The change in magnitude and the rate of change between the maxima and minima are also shown. Data are averaged for Davis and Syowa (SH) and for Poker Flat and Andenes (NH). example, Vincent [1994]. Using MF radar data acquired at Mawson (68 S, 63 E) Antarctica he found a semiannual variation in gravity wave activity, with maxima in summer and winter Gravity Wave Polarization [30] The seasonal and height variations in wave activity described above are determined by both the state of the atmosphere through which the waves propagate and by the source spectrum. In order to provide insight into possible hemispheric differences in the wave spectrum, the wave polarization was investigated using the Stokes parameter methodology [Vincent and Fritts, 1987; Eckermann and Vincent, 1989]. The parameter d, defined by ð d D2 þ P 2 Þ 1=2 I provides an estimate of the degree of polarization of the wave field, where I =(u 02 + v 02 ), D =(u 02 v 02 ) and P = 2u 0 v 0. The degree of polarization lies in the range 0 d 1, where d = 1 means perfectly polarized and d = 0 means an unpolarized wave field. The polarization angle, defined as the orientation (north of east), q, of the major axis of the elliptical gravity wave motions, is given by tan 2q ¼ P D Note that there is a 180 ambiguity in q derived from (2) The Min Period Wave Polarization [31] Figure 9 shows d and q during summer and winter. Summer (winter) values are defined here as the mean value during December and January (June and July) in the Southern Hemisphere, and June and July (December and January) in the Northern Hemisphere. [32] Small values of d imply that the wave field is not strongly polarized but some care is required in interpreting these values. The relatively broad frequency bands that are used to compute d means that the degree of polarization is likely to be underestimated [e.g., Vincent and Fritts, 1987]. The polarization angles still show well defined features even for values of d < 0.1 (i.e., less than 10% polarization), which suggests significant organization of the wave fields. The polarization angles change systematically in direction with increasing height from an east/west (E/W) orientation ð1þ ð2þ at the lowest heights to a more north/south (N/S) orientation at higher altitudes. This is true at all locations for both winter and summer, with the exception of Poker Flat during winter where an approximate N/S orientation persists throughout the entire MLT. [33] The N/S polarization starts at a lower height during winter than in summer at all four sites. A difference between the sites, however, occurs in the km height range in summer. The polarization angle is aligned in a SE/NW direction at the Southern Hemisphere SH locations, but in a NE/SW direction at the NH locations. The height variation of d shows that the waves are generally more strongly polarized in summer than in winter at heights below 90 km, with some exceptions such as at heights from about km at Davis (which may relate to data contamination from scatter from ice bergs or sea waves) The Min Period Wave Polarization [34] Figure 10 is similar to Figure 9, but for the longerperiod wave motions. Polarization angles behave in a similar manner to that described for the short-period range, changing from an approximate E/W orientation at the lower heights to a more N/S orientation at higher altitudes. Exceptions to this arrangement occur at Andenes where the wave orientation is generally slightly NE/SW at all heights in summer and winter, and at Poker Flat during winter at the lower altitudes where the waves are oriented more in a N/S direction. [35] At the Southern Hemisphere locations, the angle of polarization changes from a somewhat SE/NW alignment at lower heights toward a more NE/SW orientation at higher altitudes. The directional change occurs at about 80 km in summer and about 70 km or below in winter, in contrast to the shorter-period range where this change occurs at about 90 km in summer and about 80 km in winter. These heights are similar to the heights where the variances begin to increase rapidly for each period range during summer or winter, suggesting that the change in polarization angle with height may be due to anisotropic wave dissipation. [36] The long-period waves are generally more strongly polarized in summer than during winter at all locations, i.e., large values of d. They are generally more polarized in the SH than in the NH during both winter and summer. 5. Discussion [37] MF radar wind measurements made over several years at two sites in each of the Arctic and Antarctic are 9of16

10 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 9. Degree and angle of wave polarization in the period range min at Davis, Syowa, Poker Flat and Andenes during winter (asterisks) and summer (triangles). used to construct climatologies of mean winds and gravity wave activity in the polar MLT region ( km). While there are a number of similarities in the dynamical state of the two hemispheres there are also a number of significant differences. [38] 1. The seasonal changes in zonal winds near the SH mesopause are more symmetrical about the summer solstice than at NH locations (e.g., Figure 2). [39] 2. Zonal winds in the wintertime lower MLT are stronger in the SH and the vertical shears are correspondingly larger than they are in the NH. 10 of 16

11 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 10. As for Figure 9 but for the period range min. [40] 3. Meridional winds are usually much smaller than the zonal winds and more variable. A persistent jet-like feature exists near the summer mesopause. This feature persists for a longer period in the NH. [41] 4. Height profiles of the gravity wave energy density (Figure 6) show that the waves have similar amplitudes at 80 km in both hemispheres, but wave energy grows more rapidly with height in the SH. [42] 5. Short-period ( min) wave activity has a strong semiannual component at lower heights, but becomes predominantly annual in character at heights of 90 km and above Mean Winds [43] The stronger zonal winds in the winter SH MLT are consistent with a less disturbed middle atmosphere com- 11 of 16

12 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 11. (top) Frequency of occurrence of PMC observed with the Summer Mesospheric Explorer satellite (G. Thomas, private communication, 2002). Data are shown for 70 N (solid) and 70 S (dotted). (bottom) Mean equatorward meridional winds averaged for Poker Flat and Andenes (solid), and for Davis and Syowa (dotted), at the heights listed in Table 4. The vertical lines indicate the starting and ending dates of PMSE observations at Andenes averaged for (solid lines), and at Davis for the 2003/2004 summer (dotted lines). pared with the NH where stronger planetary wave activity and sudden warmings lead to reductions in the zonal flow and even short-term zonal wind reversals. In turn, this is consistent with the idea that the strong zonal flow in the Southern Hemisphere polar lower middle atmosphere acts to inhibit the vertical propagation of planetary waves, whereas in the Northern Hemisphere the action of the waves themselves prevents the eastward flow becoming too large [Plumb, 1989]. The presence of a significant quasiplanetary wave signature in the winter MLT mean winds observed at Poker Flat and Andenes has already been noted in section 3. [44] With regard to the strength of the jet-like feature in the meridional winds at the summer mesopause, Dowdy et al. [2001] suggested it could be used as a proxy for gravity wave driving in computing the strength of the mean vertical motions. This should be a valid assumption in the summer MLT since forced planetary waves are excluded by the westward winds at lower heights, and transient waves such as the 2-day wave appear to induce equatorward winds of only about 1 m s 1 [Lieberman, 1999]. [45] The peak magnitude of the equatorward jet varies somewhat between the four locations, ranging from 8 to 16 m s 1. Although there is only a small sample in longitude, this range of values is reasonably consistent with the model results of Fritts and Luo [1995] who report a mean meridional jet of approximately m s 1, corresponding to a mean vertical motion of approximately 0.05 m s 1 due to gravity wave forcing [Garcia, 1989]. [46] The longer duration of the summer equatorward mesospheric jet in the Arctic than in the Antarctic (see Table 4) probably relates to hemispheric differences in gravity wave forcing. The longer duration of the jet implies an extended period of adiabatic cooling of the NH polar MLT compared with the situation in the SH. This in turn would suggest hemispheric differences in the occurrence of temperature-dependent phenomena such as PMCs, PMSEs and NLCs. [47] Figure 11 shows PMC observation frequency obtained from the Summer Mesospheric Explorer (SME) satellite at 70 N and 70 S averaged over the years (see Olivero and Thomas [1986] for details). MF radar data detailing the summer meridional jet is also shown (averaged for each hemisphere). The starting and ending dates of VHF radar PMSE observations at Davis and Andenes are also shown (see below for details). [48] With respect to the summer solstice, PMC observations begin at about the same time in each hemisphere. This is about 20 days after the start of the meridional jet at the four MF radar locations. The lag may relate to the process of PMC formation being nonlinear, so that the equatorward flow needs to be greater than a certain value in order to trigger PMC formation. Alternatively, it could be due to the availability of water vapor [Kirkwood et al., 1998]. [49] The peak PMC observation frequency occurs about 10 (20) days after the summer solstice in the SH (NH). These values are somewhat later than those given in Table 3 for the timing of the maximum in the meridional jet, but are similar in that the peak occurs at a later time in the NH. The frequency of PMC observations decreases during February in the Antarctic and during August in the Arctic, with PMC observations remaining about 15% more frequent in the Arctic than in the Antarctic. With respect to the summer solstice, the frequency of PMC observations in the Arctic is similar to the frequency in the Antarctic about 2 weeks earlier. [50] The longer persistence of PMCs in the Northern Hemisphere is, in this way, similar to the longer persistence of the summer equatorward meridional jet in the Northern Hemisphere. The Transformed-Eulerian-Mean equation theory indicates that the meridional wind is balanced with the gravity wave force and a vertical wind appears so that the continuity equation holds for the meridional wind. Hence the meridional winds can be used as a proxy for the adiabatic cooling associated with gravity wave drag in this region [Dowdy et al., 2001]. [51] A VHF radar was recently installed at Davis, with the 2003/2004 summer being the first summer of operations [see Morris et al., 2004, 2006]. The period of PMSE observations (from 19 November 2003 until 21 February 2004) is relatively consistent with the starting and ending dates of the meridional equatorward wind flow at Davis. The equatorward jet during the 2003/2004 summer at Davis starts about 2 days earlier and ends about 8 days earlier than the period of PMSE observations, suggesting a slight lag as was the case for the PMC observations. 12 of 16

13 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 [52] VHF radar observations made at Andenes from show that the median start and end dates for PMSE are 19 May and 28 August, respectively (see Bremer et al. [2003]). Relative to the summer solstice, the Andenes PMSE observations begin at about the same time as at Davis, but persist longer at Andenes in a similar way to both the PMC observations and the equatorward jet (Figure 11). Factors other than dynamics and temperature also play a role in PMC formation. For example, hemispheric differences in water vapor concentrations could also cause hemispheric differences in PMC occurrence [Huaman and Balsley, 1999] Gravity Wave Variability and Critical Level Filtering [53] The seasonal, height and hemispheric variations of MLT gravity wave activity presented in section 4 are due to a number of factors. They include changes in sources in the lower atmosphere, changes in the middle atmosphere wind and temperature (static stability) fields through which the waves propagate and changes in saturation effects in the MLT itself. There are a number of wave sources that are likely to be important at polar latitudes, including topographic generation, wind shears and geostrophic adjustment [Fritts and Alexander, 2003]. However, there are many aspects of sources that are not well understood, including their seasonal variability and relative importance. Propagation effects are easier to understand, at least in principle, using model wind and temperature fields to assess the effects of refraction and critical level filtering. [54] Critical level filtering occurs when the wave groundbased phase speed c matches background wind speed u(z) at a given height z and the wave is removed from the spectrum. The relevant relation is c uz ðþcos f ¼ 0 ð3þ where f is the angle between the wave propagation direction and the wind direction. As f changes, those values of c that satisfy (3) trace out the so-called exclusion circle, which defines those waves that encounter a critical level at z and are removed from the spectrum [e.g., Taylor et al., 1993]. [55] In order to explore this effect we use tropospheric and stratospheric winds derived from the United Kingdom Meteorological Office (UKMO) assimilated data set (see Lorenc et al. [2000] for details) to model critical level filtering in the SH and NH polar middle atmosphere. The phase speeds forbidden by the UKMO winds are shown as exclusion circles in Figure 12 for both summer and winter. For the Southern Hemisphere, summer is defined as being from the start of December until the end of January and winter being from the start of June until the end of July, with the opposite being the case for the Northern Hemisphere. [56] As might be expected, the exclusion circles show that the westward winds in the summer middle atmosphere preferentially remove westward propagating waves while the eastward winds in winter remove eastward propagating waves. One feature evident in Figure 12 is that for the SH sites, critical level filtering is predominantly zonal in character in both seasons. In contrast, for the NH sites during winter the excluded phase speeds are more southward at Poker Flat and northward at Andenes. This indicates the effects of quasi-stationary planetary waves. Another pronounced hemispheric difference shown in Figure 12 is that the stronger polar night jet of the Antarctic excludes a larger range of eastward phase speeds than in the Arctic. [57] The forbidden phase speeds can be combined with the results of the Stokes parameter investigations (section 4.3). This combination suggests that during summer at all four locations the waves that reach the mesosphere from lower altitudes are expected to be propagating eastward, or to have westward phase speeds in excess of 40 m s 1. During winter the waves are expected to have westward (or high eastward) phase speeds at Davis, Syowa and Andenes, and northward (or strong southward) phase speeds at Poker Flat. [58] It was seen in Figure 1 that the zonal winds during summer reach their largest magnitude at heights around 80 km at all four locations. It is interesting that above this height the zonal winds decrease in strength, reaching zero magnitude at about 90 km, since it is in this region that the variances begin increasing rapidly (see Figure 6). One possible interpretation of this is that relatively little critical level filtering is occurring above about 90 km because of the weak zonal winds in this region. [59] An additional explanation arises from the fact that gravity wave variance (energy) is not conserved during propagation, but modified by background wind. Waves that are breaking (saturating) will have amplitudes that are u 0 (z) jc uz ðþj, so that hemispheric differences in the mean wind height profile (see Figure 3) could also produce differences in the wave amplitudes. 6. Conclusions [60] Wind observations made in the Arctic and Antarctic MLT are analyzed on a climatological basis. Although the stations used in this analysis are all located at similar latitudes the dynamics of the MLT at the stations often show significant differences that are often hemispheric in nature. [61] The results presented here show that hemispheric similarities exist in the height of the summer equatorward jet. In both hemispheres the peak height is estimated to be 88 km at three stations and 86 km at the fourth (Andenes). However, the height resolution of the MF radars is such that it is difficult to say whether there is a systematic variation in the jet peak between stations or even between hemispheres. [62] Polar MLT phenomena, such as the peaks in both the zonal and meridional winds during summer, occur closer to the summer solstice (earlier) in the SH than in the NH. The greater symmetry around the solstice indicates that radiative effects may play a larger role in controlling the state of the southern polar MLT than in the northern, where dynamical effects appear to be more important [Fritts and Luo, 1995]. [63] The equatorward flow near the mesopause persists longer in summer in the NH than in the SH, as do observations of PMSE and PMC. All three phenomena begin at about the same time, relative to the summer solstice, but end later in the NH than in the SH. [64] Given the role that gravity wave drag plays in closing the solsticial zonal jets and driving the meridional circulation, the hemispheric differences in the zonal and meridio- 13 of 16

14 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 Figure 12. Exclusion circles at 2 km height intervals defining regions of forbidden gravity wave phase speeds. Winds are derived from UKMO assimilated data set, averaged for the years at heights from the ground up to 0.3 hpa (56 km). nal winds reported here suggest that wave driving (and the consequent adiabatic cooling) may be different between the two hemispheres. There is some indication that this may be the case from the gravity wave climatologies presented in this study. [65] At heights around 80 km, gravity wave activity is reasonably similar in magnitude at all four locations. At heights above about 90 km the variances are significantly stronger at the Southern Hemisphere locations than at the Northern Hemisphere sites. This is true in both summer and 14 of 16

15 DOWDY ET AL.: POLAR MLT DYNAMICS, 1 winter, for both the short- and long-period gravity wave motions. [66] The variances at the Antarctic locations increase with height at a rate faster, and more similar to what would be expected if wave energy was being conserved, than the rate of increase at the Arctic locations. A possible interpretation is that more wave saturation (and the consequent wave drag) is occurring in the Arctic than in the Antarctic. [67] The gravity wave fields are found to be far from random, with regular seasonal variations occurring in magnitude, spectral content and polarization at all four locations. The seasonal variations in the magnitude of gravity wave activity are generally a combination of an annual component (with maxima in winter and minima in summer) and a semiannual component (with maxima around the times of both solstices and minima around the times of the equinoxes). Maxima in wave activity occur in winter, and minima in spring or summer. 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