Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd007711, 2007 Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) observations Jiyao Xu, 1 H.-L. Liu, 1,2 W. Yuan, 1 A. K. Smith, 3 R. G. Roble, 2 C. J. Mertens, 4 J. M. Russell III, 5 and M. G. Mlynczak 4 Received 29 June 2006; revised 14 November 2006; accepted 17 December 2006; published 2 May [1] Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/ Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) temperature observations are used to study the global structure and variability of the mesopause altitude and temperature. There are two distinctly different mesopause altitude levels: the higher level at km and the lower level below 86 km. The mesopause of the middle- and high-latitude regions is at the lower altitude in the summer hemisphere for about 120 days around summer solstice and is at the higher altitude during other seasons. At the equator the mesopause is at the higher altitude for all seasons. In addition to the seasonal variation in middle and high latitudes, the mesopause altitude and temperature undergo modulation by diurnal and semidiurnal tides at all latitudes. The mesopause is about 1 km higher at most latitudes and 6 9 K warmer at middle to high latitudes around December solstice than it is around June solstice. These can also be interpreted as hemispheric asymmetry between mesopause altitude and temperature at solstice. Possible causes of the asymmetry as related to solar forcing and gravity wave forcing are discussed. Citation: Xu, J., H.-L. Liu, W. Yuan, A. K. Smith, R. G. Roble, C. J. Mertens, J. M. Russell III, and M. G. Mlynczak (2007), Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) observations, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] The mesopause, defined as the temperature minimum that separates the middle atmosphere from the thermosphere, is the coldest region in the terrestrial atmosphere. The mean mesopause location and temperature are established by radiative and dynamical processes [e.g., Leovy, 1964; Holton, 1983] and also display large variability due to gravity waves, planetary waves and tides. Both lower atmospheric and thermospheric processes can contribute to the variability. The temperature profile is the most important parameter for characterizing this transition from the mesosphere to the thermosphere; two ground-based methods that are currently used for determining it are in situ rocket measurement [e.g., Lübken and von Zahn, 1991; Fritts et al., 2004; Lübken and Müllemann, 2003; Lübken, 1999] and sodium lidar measurements. The lidars have very high vertical and temporal resolution in the altitude region of km [e.g., von 1 State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing, China. 2 High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA. 3 Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA. 4 NASA Langley Research Center, Hampton, Virginia, USA. 5 Department of Physics, Hampton University, Hampton, Virginia, USA. Copyright 2007 by the American Geophysical Union /07/2006JD007711$09.00 Zahn et al., 1996; Yu and She, 1995; She et al., 1993] but exist at only a few sites. [3] A 10-year climatology from lidar measurements indicates that there are two distinct mesopause altitudes, one at about 100 km during winter and the other at 86 km during summer [von Zahn et al., 1996; She et al., 1993; Yu and She, 1995; Leblanc et al., 1998; She and von Zahn, 1998]. These observations, however, were from separate local sites based on a limited number of observation days, and most were taken during nighttime. [4] The NASA Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) satellite was launched in December 2001, carrying four instruments for investigating the region between 60 and 180 km. The Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) instrument obtains global profiles of temperature and other parameters [Russell et al., 1999; Mertens et al., 2001]. In this paper we analyze the global altitude and temperature of the mesopause and their latitudinal, seasonal and diurnal variations as measured by SABER. Simulations with the thermosphere-ionosphere-mesosphere electrodynamics global climate model (TIME-GCM) help in the interpretation of the results and the implications for mesospheric dynamics. 2. Data Processing [5] The data used in this paper for calculating the global distribution of the mesopause altitude are temperature pro- 1of11

2 Figure 1. The global distribution of mesopause (a) altitude (units are km) and (b) temperature (units are K), for a 60-day period centered around March based on 4-year-averaged Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) temperature. files from the SABER instrument onboard the TIMED satellite. The height of the TIMED orbit is about 625 km, the inclination is 74.1, and the period is about 1.6 h. Because TIMED is a slowly precessing satellite, it takes about 60 days to complete a full 24-hour coverage of local time. [6] SABER measures temperature from the lower stratosphere to the lower thermosphere by limb observation of CO 2 infrared emission. The inversion takes into account departures from local thermodynamic equilibrium (LTE) and variations in CO 2 mixing ratio (measured by SABER during daytime). Mertens et al. [2004] present comparisons of preliminary SABER observations to in situ falling sphere measurements at high latitudes. The comparisons indicate that the mesopause temperatures are similar between the two but that the SABER mesopause altitude is lower than that determined by the falling sphere method by about 2 4 km. This difference persists in more recent versions of the SABER retrieval (Version 1.06). The exact cause of this discrepancy is not clear, though recent study suggests that non-lte retrieval with additional CO 2 v-v exchange processes can bring the SABER summer mesopause altitudes closer to ground base observations [Kutepov et al., 2006]. The present study uses Version 1.06 Level 2A data. Four years of SABER data from February of 2002 to February of 2006 are used in this paper. [7] The SABER temperature inversion provides profiles on pressure levels. The effective field of view of the SABER instrument gives a vertical resolution of about 2 km[mertens et al., 2001]. Altitude information for each temperature measurement is a standard data product that was obtained by integrating the temperature hydrostatically from a reference position in the stratosphere, using NCEP analyses. In our analysis, the mesopause location for each profile is identified by determining the minimum temperature. Mesopause temperature and altitude data are binned in 10 degrees of latitude (from 90 S 90 N) and 1 hour of local time, and the binning is performed every 5 degrees. There are two hours of missing data around noon in almost every latitude [Zhu et al., 2005]; at high latitudes, the length of the missing midday measurements becomes longer: up to 3 4 hours varying with season. The SABER data not 2of11 marked as missing were used in this study. In other words, we did no extra screening to remove profiles or individual temperature measurements that were outliers. [8] In order to analyze the mesopause altitude for the full 24-hours local time, we apply 60-day windows on the SABER data. Averages over shorter periods include only a subset of local times, which can affect the mean altitude and temperature of the mesopause. Analysis labeled by a particular month includes 30 days data in that month plus 15 days before and after the month. The data used extend from February 2002 through February 2006, with averaging periods centered on each month. [9] For averaging over all local times, the mesopause temperatures and altitudes were first sorted into the 24 local time bins and averaged. Then values for all local time bins that were not empty were averaged. Note that a few bins around local noon were never filled. This introduces a bias into the diurnal means, especially in the tropics where the diurnal temperature perturbations are largest. 3. SABER Mesopause Altitude and Temperature [10] In this study we are going to focus only on the climatological mean temperature structure and its variation with solar local time. We thus average the temperature along the latitude circle for each local time hour and each latitude bin over the 60-day period. This averaging will remove or reduce the nonmigrating tidal components, but the migrating components will remain. The confidence level of the averaged temperature can be estimated. At June solstice at the mesopause for the local time bin 0 1 hour, for example, the analysis error estimates for a confidence level of 95% are 2.4 K, 2.4 K, and 5.8 K for the latitudes of 0,45 N, and 75 N, respectively. The mesopause altitude and temperature are then calculated from these averaged profiles. It should be noted here that, in this analysis, we take the minimum temperature of the vertical profile as the mesopause. When more than one local minimum is present in a profile, as can happen in the presence of tides or other waves, the smallest among these local minima determines the mesopause. Figures 1 and 2 give the results for the March and

3 Figure 2. (a, b) Similar to Figure 1 but for September. September equinoxes for the 4-year-averaged results. The mesopause altitude is between 95 and 100 km around equinoxes, but dips below 90 km for a few hours at low latitudes during the morning and at middle latitudes during night. The range of mesopause temperature is from 160 to 185 K; it is cooler in the equatorial region and warmer toward both poles. [11] The temporal and spatial variations of the mesopause altitude and temperature suggest that these variations are probably associated with atmospheric tides, particularly the diurnal tide. Both mesopause altitude and temperature have a 24-hour periodicity at low latitudes and midlatitudes. The rapid latitudinal change of the mesopause altitude and temperature at around 20 is consistent with the phase changes of the diurnal (1, 1) mode. The strongest modulation of both the altitude and temperature occurs at the equator (peak-to-peak 15 km and 30 K, respectively, for March) and also at latitudes between 30 40, again consistent with the diurnal (1, 1) mode. The modulation is larger in March than in September, agreeing with the variations of the diurnal tide amplitude [McLandress, 2002]. The semidiurnal signature is also more evident in September (Figure 2) than in March (Figure 1). The tidal modulation of mesopause height and temperature is also consistent with the model study by Berger and von Zahn [1999]. These features are similar for every year from 2002 to [12] Figure 3 demonstrates the rapid shift of mesopause altitude due to tidal perturbation. The vertical distance between the two low-temperature values (at 30 N in March) is 20 km, similar to the vertical wavelength of the observed migrating diurnal tide [McLandress et al., 1996]. At LT 0.5 hour, the minimum temperature occurs at 84.7 km, that is, at the lower elevation of the two temperature minima. In the following hour (LT 1.5 hour), the higher elevation minima is now colder, and therefore the mesopause altitude jumps to 101 km. [13] For the solstice period at June (Figure 4), there are two distinct ranges of mesopause altitude, with the higher range between km and the lower between 82 and 85 km. The boundary between the two altitude ranges is located between N during the day and N at night. The nighttime mesopause positions are in good qualitative agreement with the bimodal mesopause altitudes derived from nighttime potassium lidar measurements in June of 1996 [von Zahn et al., 1996], except that the summer mesopause altitude is lower in SABER than in the lidar observations by about 2 4 km. This discrepancy is similar to that between the SABER results and rocket measurements as mentioned in section 2. Note from Figure 4 that the lowest mesopause altitude (83 km) is not at the highest latitude of the observation, but at around N, while the lowest temperature is at the highest latitude sampled ( K, depending on local time, at 80 N). Figure 3. The temperature profiles at four local times at the latitude of 30 N in March of the 4-year average. Note that LT 23.5, 0.5, 1.5, and 2.5 hours refer to hourly means between LT 23 24, 0 1, 1 2, and 2 3 hours, respectively. 3of11

4 Figure 4. (a, b) Similar to Figure 1 but for June. At middle and low latitudes, both mesopause altitude and temperature display modulations with a 12-hour period and a clear northward progression with strongest modulation near 40 S (peak-to-peak 5 km and 25 K, respectively). These characteristics suggest that the modulation is closely tied to the migrating semidiurnal tide, which peaks around solstice. [14] The general features are similar in December (Figure 5), although the minimum summer mesopause in the polar region ( K) is warmer than that in June ( K), and the lowest mesopause altitude is about 1 km higher. A thorough comparison of winter mesopause altitude and temperature between December and June is not possible because there are no observations at most local times poleward of 50 in the winter hemisphere. With the data available, it appears that the winter mesopause temperature in December is generally warmer than that in June. [15] Figure 6 shows the latitudinal and annual variation of mesopause altitude and temperature, where the mesopause is determined from temperature profiles averaged over the 24 hours of local time for each month. The modulation by tides is thus in principle removed although a tide that varies significantly during the 60-day period can leave a residual signal. From May to August poleward of 40 N, the diurnally averaged mesopause altitude is between 83 and 84.5 km. The equatorward extension of the region of low mesopause altitude is especially prominent in May July (to about 35 N). In August, the lowest mesopause altitude of 82 km is found between 45 N and 55 N. During the boreal summer, the lowest diurnally averaged mesopause temperature is 126 K (at 80 N in July). This is in general agreement with a set of falling sphere temperature profiles measurement at Longyearbyen (78 N) by Lübken and Müllemann [2003], although the diurnal average mesopause altitude from SABER is again slightly lower than those from the falling sphere measurements. In the austral winter, the mesopause altitude increases to 98 km and the temperature minimum reaches as high as 188 K in the polar region. In November, December, January and February the region of low mesopause altitude extends poleward from 40 S, with the minimum value of about 82 km at 55 S. The lowest mesopause temperature of about K is found at 80 Sin January. [16] Hemispheric asymmetry is evident in Figure 6: the summer mesopause altitude at high latitudes is higher in the SH than in the NH by about 1 km, and the summer Figure 5. (a, b) Similar to Figure 1 but for December. 4of11

5 Figure 6. The diurnally averaged global distribution from the 4-year-averaged results of (a) the mesopause altitude and (b) the mesopause temperature. Each monthly data point represents 60 days of observation centered on the specific month. Contour intervals are 1 km (Figure 6a) and 5 K (Figure 6b). mesopause temperature in the polar region is higher in the SH than in the NH by about 5 10 K. To better illustrate this asymmetry, we overplot the latitudinal distribution of the mesopause altitude (Figure 7a) and mesopause temperature (Figure 7b) derived from the 60-day average around December solstice (at day 350) and June solstice (at day 170). It is clearly seen from Figure 7 that the mesopause altitude during austral summer (solid line) is 1 km higher than that during the boreal summer (dashed line), and the summer mesopause temperature in the polar region is warmer in the SH than in the NH. Figure 8 shows that the austral summer is warmer not only at the mesopause but also at all altitudes between 20 and 90 km at the polar region. The hemispheric difference in summer mesopause temperature agrees with that found by Huaman and Balsley [1999] from analysis of HRDI temperature data. Hemispheric differences of about 1 km are also found in the polar mesospheric cloud (PMC) altitudes from lidar measurement [Chu et al., 2003, 2006]; the PMC altitudes are strongly affected by temperature but also are quite sensitive to water vapor, nucleation processes, and vertical wind. [17] It is evident from Figure 7 that around December solstice in both hemispheres, the mesopause altitude is higher at most latitudes, and that the mesopause is warmer at middle and high latitudes. The results for high latitudes in the winter hemisphere should be treated with caution because of very poor local time coverage there (Figures 4 and 5). At middle-low latitudes, the mesopause altitude is between 96 and 100 km for the whole year, and the distribution of mesopause temperature is relatively uniform. [18] The transition time between the two mesopause altitudes is also studied using SABER data. Figure 9 shows the daily mesopause altitude and temperature at 5 latitudes for 4 years. From Figure 9a we can see that at the middleand high-latitude regions there are about days centered around the summer solstice when the mesopause altitude is at the lower level, which is around 85 km in both hemispheres. The summer mesopause altitude in the SH Figure 7. Latitudinal distribution of (a) mesopause altitude and (b) mesopause temperature, from the diurnally averaged temperature profiles for December (solid line) and June (dashed line). The June curves have been flipped so that left to right on the latitude axis indicates south to north for the December curves and north to south for the June curves. 5of11

6 Figure 8. Profiles of diurnally averaged temperature from SABER at 80 for both hemispheres (solid line for Southern Hemisphere and dashed line for Northern Hemisphere) and two summer months (left profile shows December and June; right profile shows January and July). shows more variability than in the NH. In other seasons the mesopause altitude is at the high level in middle and high latitudes. At the equator the mesopause altitude is at the high level in all seasons. Owing to the missing data at the latitudes of 80 N and 80 S, we cannot determine the time of the transition between the two mesopause levels so we will investigate this at 50 N and 50 S. We take the variation of the temperature at 50 N in 2005 as an example (Figure 10). Figure 10 shows that from day 125 to day 130, the mesopause altitude jumps from the higher mesopause level to the lower level. During the period from day 250 to day 255, the mesopause altitude jumps back from the high level to the low level. Therefore the transitions between the two levels of mesopause are fast (several days) and they occur 40 days after the spring equinox and 15 days before the fall equinox. The temperature transitions, on the other hand, are more gradual (Figure 9b). [19] We recognize that the summer mesopause altitudes derived from SABER temperature profiles may be systematically low by 2 4 km, especially in the high latitude, as mentioned earlier. This bias, however, should affect the determination of summer mesopause in both hemispheres similarly. [20] Larger error may occur at higher latitudes (>52 ) in the analysis because the polar regions are only alternatively observed half of a year owing to the yawing of the TIMED satellite. The yaw cycle is 60 days, and only one polar region is observed in each yaw cycle. 4. Asymmetry Between the Northern and Southern Hemispheres [21] As shown in section 3, the SABER measurements indicate that the mesopause is warmer around December solstice than June solstice at middle and high latitudes (Figure 7) at and below the altitude of the summer mesopause altitude (Figure 8). Possible mechanisms for this hemispheric asymmetry are discussed in this section. [22] The thermal structure of the mesosphere/mesopause is affected by radiative processes, by heating through exothermic chemical reactions, and by dynamical processes. Adiabatic warming and cooling due to vertical motion is responsible for the strong departure from the radiative equilibrium [e.g., Brasseur and Offermann, 1986; Mlynczak and Solomon, 1993; Berger and von Zahn, 1999; Holton, 1983]. Variation in any of the processes could lead to the hemispheric asymmetry of the mesopause temperature and altitude. Chu et al. [2003] showed that at most altitudes the summer mesosphere is warmer at the South Pole (SP) than that at the North Pole (NP) in simulations made with the TIME-GCM numerical model. It was determined from the simulations that the difference is mainly caused by the greater solar flux in January than in July due to the Earth s orbital eccentricity. Therefore the warmer mesosphere during austral summer is a consistent feature in multiple-year TIME-GCM simulations. [23] Figure 11 shows the latitudinal and seasonal variation of the mesopause altitude and temperature derived from monthly mean temperature profiles of TIME-GCM simulation for the year The variation is in general agreement with those of the SABER measurements (Figure 6). The mesopause temperature from the simulation is generally warmer at most latitudes in December/January than in June/ July. Similar to the SABER observations (Figure 8), the temperature below the mesopause is also higher during austral summer in the simulation. Detailed comparisons, however, show disagreement between the observation and simulation. For example, in the simulation the temperature is generally lower at low latitudes and the polar mesopause is colder in summer and warmer in winter than those in the 6of11

7 Figure 9. Daily (a) mesopause altitude and (b) mesopause temperature, at five latitudes during 2002 to February of of11

8 Figure 10. The variation of temperature at 50 N around (left) March equinox and (right) September equinox. The dashed lines show the mesopause altitudes. observations. The meridional temperature gradients are generally larger at high latitudes in the simulation that in the observations. It is also difficult to resolve the 1 2 km hemispheric difference of mesopause altitude with certainty in the model because the vertical resolution of the model near the mesopause is about 2 km. [24] An asymmetry in the gravity wave forcing of the circulation is another possible cause of the hemispheric asymmetry of the mesopause altitude and temperature, especially in the summer hemisphere. As is well established in previous studies, the gravity wave breaking in the mesosphere drives a circulation from the summer to the winter hemisphere; this circulation generates adiabatic cooling and warming, respectively, in the summer and winter mesosphere [e.g., Andrews et al., 1987]. The winter mesopause location is not controlled by gravity wave forcing in any significant way, but rather tied to the minimum temperature near 100 km owing to weak solar heating. This is because (1) the adiabatic warming associated with the gravity wave driven circulation is spread over a deeper vertical layer because of the broader spectrum of waves [Garcia and Solomon, 1985] and is not sufficient at any single level to change the sign of the thermal gradients, and (2) most models indicate the bulk of the gravity wave momentum driving and associated adiabatic warming occurs below 100 km, which will accentuate rather than alter the temperature minimum above. [25] In the summer hemisphere, however, the mesopause is determined by the lower temperature from two competing cooling mechanisms: the net radiative balance and adiabatic cooling [Chu et al., 2005]. From both observations and model simulations, it is clear that the summer mesopause altitude at high latitudes is always at lower altitudes (below 90 km) and thus that the adiabatic cooling is reliably strong. Model studies [e.g., Garcia and Solomon, 1985] also indicate that the summer mesopause altitude coincides with the strongest upward vertical wind of the residual circulation (Figure 1 in that paper) and occurs near the altitude of the strongest forcing by gravity wave breaking (Figure 6 in that paper). Therefore the summer mesopause altitude and temperature are dependent closely on the strength of the gravity wave sources in the troposphere and on the wave filtering by the wind below the mesopause. Studies by Vincent [1994] and Dowdy et al. [2001] suggested that the Figure 11. (a, b) Similar to Figure 6 but from thermosphere-ionosphere-mesosphere electrodynamics global climate model (TIME-GCM) simulation of the year of11

9 Figure 12. Zonal mean temperature at December solstice from three otherwise identical TIME-GCM simulations, with the gravity wave momentum fluxes at the lower boundary in the (left) weak gravity wave case and the (right) strong gravity wave cases 50% and 200% of that in the (center) base case. The dashed lines indicate the mesopause heights at corresponding latitudes. Contour interval is 10 K. gravity wave activity is weaker in the austral summer; the weaker gravity wave driving and warmer temperature in the mesosphere are consistent with their measurements of a smaller mean vertical wind. [26] Large uncertainties in gravity wave sources, gravity wave launch level, the parameterization of gravity waves, and limited vertical resolution make it difficult to accurately determine the summer mesopause and its hemispheric asymmetry in global models. For example, the disagreement between the observed summer mesopause structure (Figure 6) and the simulation (Figure 11) may be due to the uncertainty in the latitudinal distribution of gravity wave. This is because gravity wave drag plays a determining role in the departure of the summer mesopause from radiative equilibrium; the radiative and chemical processes are of secondary importance there. In contrast, the winter mesopause structure is mainly determined by the balance between radiative cooling, chemical heating and diffusive transport of heat from the thermosphere. The uncertainty may also explain why the summer mesopause altitudes vary among a range of global models with different gravity wave parameterization schemes [e.g., Garcia and Solomon, 1985; Berger and von Zahn, 1999; Siskind et al., 2003]. The winter mesopause, at the same time, has been consistently at 100 km in all these models. [27] The sensitivity of mesopause temperature to the gravity wave filtering was demonstrated by Siskind et al. [2003], who showed that the greater eastward wind in the Southern Hemisphere troposphere during summer leads to stronger breaking of westward gravity wave components in the troposphere and lower stratosphere and greater filtering of the eastward wave components. As a result of the greater filtering, there are fewer eastward wave components that can propagate into the mesosphere. The eastward forcing is thus weaker in the summer mesosphere and the summer mesopause in the SH is warmer. Because the tropospheric wind mainly affects the gravity wave components with relatively lower phase speed, the hemispheric difference is more pronounced with a narrower gravity wave phase-speed spectrum. [28] The sensitivity of the mesopause to gravity wave source strength is tested here through three TIME-GCM simulations. In all three simulations, the gravity wave spectra of phase speed specified at the lower boundary of the model (10 hpa) are anisotropic with the spectral peak at 20 ms 1 (eastward). As discussed by Liu and Roble [2002], the gravity wave spectra was specified to be anisotropic in order that the jet reversal in the summer hemisphere occurred at a lower altitude than that in winter, as observed. Observational support for such an anisotropy in the equatorial region was provided recently by Kovalam et al. [2006]. The magnitudes of the gravity wave momentum fluxes are different for the three cases: that in the weak gravity wave case is half of the base case and that in the strong gravity wave case is twice as large. The simulations are otherwise identical. It is shown in Figure 12 that within this range of gravity wave fluxes, a stronger flux leads to a lower breaking altitude for the waves while a weaker flux leads to a higher breaking altitude. The position of the mesopause altitudes will change accordingly: lower for stronger gravity wave fluxes. The mesopause altitudes at high latitudes (poleward of 40 ) in the summer hemisphere are highest in the weak gravity wave case (87 90 km) and lowest in the strong gravity wave case (at km). The mesopause altitudes are between km in the base case. With the scale height near the summer mesopause around 4 km, the change of the mesopause altitudes in these three numerical experiments suggest that the mesopause altitude decreases by three quarters to one scale height when the momentum flux of the wave source doubles (increased by 100%). [29] Accompanying the lowering of the summer mesopause altitudes, the summer mesopause temperature becomes warmer. This can be associated with the increasing air density at lower altitudes. With the doubling of gravity wave momentum flux, the wave breaking altitude (and the summer 9of11

10 mesopause altitude) decreases by about scale height from z 0 (the base mesopause height) to z d (the mesopause height with doubled gravity wave fluxes). At z d, the momentum deposition nearly doubles while, with r(z d )= r(z 0 ) exp((z d z 0 )/H), the air density is about 2.11 to 2.72 times as large as the air density at z 0. Therefore the increase of the momentum flux deposition is offset by the increase of the air density at the breaking altitude z d and the acceleration/deceleration rate at z d is 94% 74% of that at z 0. The strength of the residual circulation and the adiabatic cooling rate are directly related to the acceleration/ deceleration rate and will be smaller at the lower altitude. At 70 S, the increase of the mesopause temperature is about 5 K for each doubling of the momentum flux. 5. Conclusions [30] In this study we use TIMED/SABER temperature observations to analyze the global distribution of the mesopause altitude and temperature. The results indicate that there are two distinct ranges of mesopause altitude, a higher one between 95 and 100 km and a lower one below 86 km. These results are in general agreement with the mesopause structure observed by lidar [von Zahn et al., 1996; She et al., 1993] although the summer mesopause altitude is lower in SABER than either lidar or falling sphere data. The mesopause temperature varies from 126 K in the summer polar region to 190 K in the winter polar region. Around equinox, the diurnally averaged distribution of mesopause altitudes is quite uniform and mainly in the higher altitude range and the mesopause temperature is between K. Around solstice the mesopause position at middle and high latitudes in the summer hemisphere is at the lower altitude while it is at the higher altitude at other latitudes and in the winter hemisphere. Therefore there is strong seasonal variation of the mesopause altitude and temperature in the middle- and high-latitude regions, while at low latitudes the mesopause altitude and temperature are quite uniform throughout the year. In addition to seasonal variation, the mesopause altitude and temperature are also strongly modulated by the migrating diurnal tide at equinox and the semidiurnal tide at solstice. [31] SABER temperature measurements from 2002 to February of 2006 show that the mesopause altitude is higher at most latitudes by 1 2 km and the mesopause temperature warmer by about 7 8 K at middle and high latitudes around December solstice than June solstice (136.8 K and K at 80 ). The temperature is also warmer at most altitudes between 20 km and the mesopause during austral summer than that during boreal summer. This difference is likely caused at least in part by the greater solar flux in December/ January than in June/July due to the Earth s orbital eccentricity, as discussed by Chu et al. [2003]. The mesopause altitude and temperature, especially during summer, are also sensitive to troposphere gravity wave sources and gravity wave filtering below the mesopause, and they could contribute to the hemispheric asymmetry of the mesopause. There are still large uncertainties in quantifying the hemispheric differences of gravity waves from observations and numerical studies. [32] We identify three sources of error that could affect the analysis of the SABER data: errors in the non-lte temperature retrieval due to uncertainties in parameters; slow precession in local time of the TIMED satellite; and missing data, especially at high latitudes due to yawing of the TIMED satellite. Further comparisons with other observations are thus desirable to better quantify the mesopause structure and variability. [33] Acknowledgments. We would like to thank Xinzhao Chu and Qian Wu for helpful discussions. This research was supported by the National Science Foundation of China ( , , ) and the National Important Basic Research Project (2006CB806306). This work was also supported in part by the International Collaboration Research Team Program of the Chinese Academy of Sciences. H.L.L. s effort is in part supported by the NASA Sun Earth Connection Theory Program. Support for A.K.S. and partial support for J.Y.X. to visit NCAR was provided by the NASA Office of Space Science. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under the sponsorship of the National Science Foundation. References Andrews, D. G., J. R. Holton, and C. B. Leovy (1987), Middle Atmosphere Dynamics, 489 pp., Elsevier, New York. 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