The concept of wave-turbopause layer and its signature in the global mesosphere-lower thermospheric gravity wave activity

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2012ja018172, 2012 The concept of wave-turbopause layer and its signature in the global mesosphere-lower thermospheric gravity wave activity Sherine Rachel John 1 and Karanam Kishore Kumar 1 Received 1 August 2012; accepted 24 August 2012; published 9 October [1] The concept of the wave-turbopause layer is introduced and discussed in this study, which defines the wave-turbopause as a layer having a lower and upper boundary, where maximum dissipation of atmospheric waves happens. The analysis is carried out using three years of SABER temperature measurements ( ) and seasonal and latitudinal variability of the wave-turbopause layer is established. On an average, it is found that there is a maximum of 5 20 km difference in altitude between the lower and upper boundaries of the wave-turbopause. Seasonally, the winter hemispheric high latitudes have the highest wave-turbopause which decreases toward the summer hemisphere with a secondary maximum at the summer hemisphere midlatitude. Moreover, the monthly variability of the wave-turbopause and mesopause is studied for 50 N, 0 and 50 S latitudes. On global scale, the latitude-height sections of zonal mean gravity wave potential energy of the MLT region is compared with the wave-turbopause characteristics. The wave-turbopause pattern seems to be superposed on the potential energy transition pattern which exposes an intermediate layer sandwiched between low and high magnitudes of potential energy. Thus the present study proposes the concept of the wave-turbopause layer and substantiates the same by discussing its latitudinal structure, seasonal and monthly variability, correlation with the cold point mesopause and gravity wave potential energy in the MLT region. Citation: John, S. R., and K. Kishore Kumar (2012), The concept of wave-turbopause layer and its signature in the global mesosphere-lower thermospheric gravity wave activity, J. Geophys. Res., 117,, doi: /2012ja Introduction [2] Vertical propagation of waves in the atmosphere play a significant role in coupling various regions of the atmosphere right from the troposphere to the mesosphere lower thermosphere (MLT) by transfer of energy and momentum [Lindzen, 1981; Garcia and Solomon, 1985]. As waves propagate, they increase in amplitude and undergo breaking and dissipation at various heights. This is very important in the vertical coupling process as well as global circulation of the atmosphere. Therefore it becomes important to identify the height of maximum dissipation and its latitudinal and seasonal structure. The height at which this dissipative regime ends, further beyond which free propagation is enhanced is conventionally called the turbopause. Locating the turbopause is a necessary but difficult task. There have been some studies in the past that tried to identify the turbopause in terms of diverse factors. The altitude where molecular dissipation becomes stronger than 1 Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, India. Corresponding author: K. Kishore Kumar, Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram , India. (kishore_nmrf@yahoo.com) American Geophysical Union. All Rights Reserved /12/2012JA turbulent dissipation was identified as the turbopause [e.g., Hall et al., 1998]. Another approach was by looking at the mixing ratios of constituents and the altitude at which this ratio started changing [e.g., Danilov et al., 1979;Offermann et al., 1981]. This is called the mixing turbopause or homopause. These two need not be necessarily the same and can vary between 80 to 120 km [Chabrillat et al., 2002 and references therein]. This can be due to difference in the observational technique employed, along with seasonal, latitudinal and longterm changes in the MLT region that reflects as changes in the turbopause height as discussed by Offermann et al. [2007]. [3] There has been recent interest in turbopause studies, with the introduction of an atypical and novel concept called the wave-turbopause using satellite measurements by Offermann [Offermann et al., 2006, 2007]. They define wave-turbopause as the mesospheric altitude level where the temperature fluctuation field indicates a substantial increase in wave amplitudes in the vertical direction. In their first study, Offermann et al. [2006] used measurements from CRISTA 1 and CRISTA 2 to explore the wave-turbopause with SABER observations providing auxiliary data. They found that the altitudes of the wave-turbopause range between 85 and 105 km (mostly around km). Their extended study using 4 years of SABER temperature measurements showed that the wave-turbopause is mostly found near to a zero-wind 1of8

2 line or in regimes of low zonal wind speed [Offermann et al., 2007]. Further, their studies reveal that the seasonal variations are strongly latitude dependent, i.e., the wave-turbopause follows annual oscillation with a pronounced summer minimum at high latitudes, semi-annual at middle and low latitudes, with a sharp transition at about 60 latitude. Encouraging qualitative agreement is also found between observations and HAMMONIA model. This study was followed up by Hall et al. [2008]. They used radar data at two locations at 52 N and 70 N and have employed a different technique to locate the turbopause. Apart from the variation in the annual amplitude, the results for 52 N are remarkably similar to those presented by Offermann et al. [2007] including the semiannual signatures, similar height structure, and the appearance of the winter maximum in November. Similarly, for 70 N, there is exceptional agreement in the annual variation of the turbopause. [4] Though the wave-turbopause is proposed with convincing arguments in all these studies, it is often referred to as tentative. This implies that the altitudinal or structural features of the wave-turbopause may vary spatially or (and) temporally. This also indicates that a single point waveturbopause maybe subjective to the choice of height. This calls for the need to further investigate this aspect, taking into account the factors responsible for the anticipated variability. These reasons stirred up further interest in our investigations on this intriguing concept of wave-turbopause. Thus the present study would serve as a trail to the three major and seminal studies on wave-turbopause so far with an emphasis on what we find as a wave-turbopause layer. We adopt the method followed by Offermann et al. [2006] to identify the wave-turbopause using standard deviation of zonal mean temperature profiles which are a measure of temperature fluctuations and hence the wave activity. The central objective of the present study is to discuss the concept of waveturbopause layer and their latitudinal structure. An attempt is also made to discuss the wave-turbopause layer in terms of gravity wave potential energy in the MLT region for the first time. The new contributions of the present study as compared to earlier studies on wave-turbopause are (1) the identification of lower and upper boundaries of wave-tropopause layer, (2) the latitudinal and seasonal variation of waveturbopause layer, and (3) the signature of wave-turbopause layer in gravity wave potential energy. Section 2 describes the data used and methodology followed in detail. Section 3 gives the results and discussions and Section 4 provides the summary and conclusions. 2. Data and Methodology [5] The data used for the present study are from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the satellite Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED). The SABER instrument is a limb viewing multichannel (1.27 mm to17mm) infrared radiometer designed to measure heat emissions by the atmosphere over a broad altitude and spectral range. The kinetic temperature data used in the present study come from 15 mm CO 2 infrared limb radiance from approximately km altitudes measured by SABER and retrieved using a non-lte inversion method [Mertens et al., 2001; Remsberg et al., 2003]. Above km altitude the accuracy of SABER temperatures retrieved using Non-LTE methods is found to be more accurate than LTE and therefore this method is followed for higher altitudes [Mertens et al., 2004]. The errors, in addition to those associated with instrumental noise, are also estimated by Mertens et al. [2001] and the errors are found to between 1.4% at 80 km and 22.5% at 110 km. Over the course of one orbit of the satellite, SABER observes between about 52 Sand83 N if in a northwardviewing yaw, switching after 60 days to a corresponding southward-viewing yaw. The SABER therefore provides data with near-global coverage spanning 24 h in local time over a period of 60 days. So we require more than 60 days of data to get global coverage temporally and latitudinally or else we will have lack of data at the high latitudes in any one hemisphere. [6] It is generally assumed that temperature profiles of the atmosphere are comprised of a background temperature on which there is an imposed fluctuating component. These temperature fluctuations (zonal mean removed) are thus considered to be reflections of the wave activity at that particular height and the standard deviation of temperature profiles will give us a measure of the extent of wave activity [Offermann et al., 2006]. But an important aspect that one has to keep in mind is that as SABER is a limb viewing satellite with a horizontal resolution >200 km, smaller scale gravity wave fluctuations may not be captured by the satellite. Therefore the standard deviation profiles would indicate activity of planetary waves and gravity waves in the longer horizontal wavelength region. 3. Results and Discussions [7] For this study, SABER temperature measurements for three consecutive years ( ) are used and the observations are gridded into latitude and longitude bins of Temperature profiles in each grid are clustered according to the Northern Hemispheric (NH) Seasons, namely Winter (DJF), Summer (JJA), Vernal Equinox (MAM) and Autumnal Equinox (SON). For a given viewing yaw of SABER, each of the grids will have a minimum of 2 or 3 temperature profiles per day. So taking a season comprising of three months, there will be approximately temperature profiles in each grid, which are statistically significant for the present study. We have followed the novel method described by Offermann et al. [2006, 2007] for the identification of the wave-turbopause. Two height regions are identified, one in the stratosphere-lower mesosphere (40 75 km) and the other in the MLT region ( km), and separate linear fits are carried out to the standard deviations from zonal mean temperatures in these height regions. These heights are chosen because the standard deviations vary linearly in these regions. The wave-turbopause is then identified as the height of intersection of these two extrapolated straight lines. Offermann et al. [2006] extensively discussed and validated the concept of the wave-turbopause and also discussed the general characteristics of standard deviation profiles across the globe right from the stratosphere to the MLT region. [8] In the present study, we concentrate on the new concept of the wave-turbopause layer. Figure 1 shows the height profile of standard deviation from the zonal mean temperature corresponding to 80 S latitude during the month of July, As discussed earlier, one can notice linearly increasing standard deviations, one in the stratosphere and the other above 95 km region in the MLT. However, in the MLT region, we 2of8

3 Figure 1. Height profile of standard deviation (blue) from zonal mean temperature corresponding to 80 Slatitudeduring austral winter (July, 2004). The green and red lines correspond to straight-line fits for MLT region and black line represents straight-line fit for stratosphere and lower mesosphere. find that the standard deviation profiles have two different slopes instead of a single slope. This led to further investigation and proposal of wave-turbopause layer, which is the central objective of the present study. In Figure 1, a linear fit is made for the height km in the lower region and two separate linear fits are made in the upper region, one in km and another in km height region. These height regions are arrived at by examining the standard deviation profiles in the MLT region, which show two different linear trends. The extrapolation of these two linear fits in the MLT region are intersecting the linear fit of the lower region at two different heights, one at 92 km and another at 103 km. These two heights are regarded as the boundaries of the waveturbopause layer in this case. The difference between the lower and upper boundary heights shown in Figure 1 is 11 km, which is more than the error in detecting the wave-turbopause. The error in detecting the wave-turbopause height was given by Offermann et al. [2006] as 3.3 km. Similar to Offermann et al. [2006], we also carried out the error analysis by varying the height limits for linear fits in the MLT region and found that the variations are within the 3 km range. This analysis thus verified the subjectivity in choosing the height region in the MLT for linear fit and found that the errors are not affecting the results. Thus from Figure 1, it is evident that the waveturbopause is not confined to a single altitude but is a layer with upper and lower boundaries. The concept of the waveturbopause layer is further tested using a large number of standard deviation profiles across the globe (individual figures not shown) and it is found that this is a feature that exists throughout with differences in the altitudes of the lower and upper boundaries along with seasonal and latitudinal variability. Over the globe, on an average, there is a thickness of 5 20 km for the wave-turbopause layer which is again beyond the error of detecting the wave-turbopause height. This thickness tapers toward the summer poles and increases toward the winter poles, which will be further discussed in the following paragraph. Thus we have a lower and an upper boundary for the wave-turbopause and the region between these boundaries should be considered as the turbopause layer as wave-turbopause cannot be a single point logically. i.e., waves cannot suddenly extinguish at one height as rightly pointed by Offermann et al. [2006]. Further, the lower boundary can be regarded as the height at which the maximum dissipation begins and the upper boundary as the height where the maximum dissipation ceases beyond which free propagation begins. The importance of wave-turbopause being a layer rather than a single-point has some consequences in MLT dynamics. This concept can be used to explain the extent of the turbulence layer, which is a consequence of wave dissipation and the height region where maximum wave drag can be noticed. The wave-turbopause layer thickness can be used to constrain the wave parameterization schemes also. [9] As a next step, we looked into the variability of the wave-turbopause layer thickness with season and latitude. Figures 2a and 2b show the latitudinal variation of the upper and lower boundary of the wave-turbopause height for the boreal winter and summer seasons, respectively. We note that the maximum thickness of the wave-turbopause layer is seen at the winter pole and minimum at the summer pole. As the difference between the boundaries taper, we see that they almost coincide at the summer pole for the NH as shown in Figure 2b. It can also be noticed from this figure that at 70 N, the lower boundary is higher than that of the upper boundary of wave- turbopause layer. This may be due to the narrow Figure 2. Variation of the two-level turbopause heights with latitude for (a) boreal winter and (b) boreal summer. 3of8

4 wave-turbopause layer over that latitude, which could not be resolved by the present method. Further, beyond 70 N of the summer pole, the wave-turbopause layer thickness decreases drastically and it is within the error limits of the present method, which is 3 km. However, in general, both the upper and lower boundaries of the wave-turbopause follow a similar signature having maximum height at the winter pole with secondary maxima in the summer midlatitude and minimum at the summer pole with secondary minima at the equator. Both lower and upper boundaries of wave-turbopause show distinct heights during summer and winter season at high latitudes. It is also interesting to note that the summer variability of the upper and lower boundaries in the Southern Hemisphere (SH) shown in Figure 2a is much smaller than the variability in the NH shown in Figure 2b. This can be attributed to differences in landmass distribution over Southern and Northern hemispheric midlatitude. The SH midlatitudes are dominated by oceanic regions where as the NH midlatitudes are dominated by landmass, which will result in more wave activity as a result of continental convective activity in the NH. It is also known that the differences in landmass distribution and orographical features between the NH and SH are responsible for observed differences in the planetary wave activity, which have direct implications in wave-turbopause height. For example, the waves having higher phase speeds can reach higher heights without dissipation compared to the lower phase speed waves. This assertion is supported by previous studies, which state that SH planetary wave activity is weaker compared to that of NH wave activity [Garcia et al., 1992; Siskind et al., 2003]. Thus the enhanced wave activity over the NH midlatitudes results in variability of the wave-turbopause. This may be a possible reason for the observed differences in the variability of wave-turbopause over NH and SH midlatitudes during summer. [10] Earlier studies by many researchers reported that the cold point mesopause also shows distinct heights during winter and summer. Von Zhan and Neuber [1987] found at 69 N that the mesopause was consistently near 100 km during winter. In summer, at the same observational site, the mesopause height was found to be around 88 km by von Zhan and Meyer [1989]. Lübken and von Zhan [1991] reported a bimodal character of the polar mesopause with the winter mesopause being higher by more than 10 km than the summer mesopause. This similar feature is seen in our observations of the wave-turbopause where the turbopause is higher by more than 15 km in winter than in summer. Further studies in midlatitudes (41 N) by She et al. [1993] and Yu and She [1995] showed prevalence of two distinct mesopause heights with the lower height (86 km) persisting all through the summer with reduced occurrence in winter. i.e., other than the cold point mesopause, there exists a secondary minimum in the temperature structure at a lower height giving rise to a two-level mesopause structure similar to wave-turbopause discussed in the present study. At the tropics, Venkat Ratnam et al. [2010] showed that the equatorial cold point mesopause is at 100 km with a secondary minimum in temperature seen between 75 and 80 km. Similarly, we have the two-level turbopause structure which can be explained in terms of wave propagation from the lower atmosphere to the MLT region. As waves propagate vertically up, their amplitudes increase exponentially owing to fall in density. However, exponential increase in amplitude is marred by the dissipative processes such as critical level filtering, dynamical and convective instabilities etc. Owing to these dissipative forces, the wave amplitudes follow more or less a linear trend in the km height region. After this height region, the dissipation seems to be random for a few kilometers after which once again the linearity is followed by the wave activity. The height region where the wave induced perturbations are random is regarded as the wave-turbopause layer, which has a lower and upper boundary. Thus rather than a specific height, we can think of the wave-turbopause as a layer with lower and upper boundary as explained earlier. From Figure 2, it is evident that the turbopause layer is spread over km over the winter pole and it is rather shallow over the summer pole, indicating that significant wave dissipation takes place over the wider height region in the winter pole as compared to the summer pole. By now, it is wellestablished that the planetary/gravity wave activity in the winter hemisphere is relatively more than that of the summer hemisphere. So it is envisaged that the waves having different phase speeds will dissipate at different altitudes thus keeping the wave-turbopause layer higher and moving the cold point mesopause higher through adiabatic cooling due to vertical motion, which is thought to be responsible for the strong departure from radiative equilibrium [Mlynczak and Solomon, 1993; Xu et al., 2007]. As the wave dissipation takes place over a wider region over the winter pole, the convective and dynamic instabilities spread over a wider region, aiding to keep the mesopause higher [John and Kumar,2011;John and Kumar, 2012]. The wave-turbopause layer can be thought as analogous to the tropopause structure at the tropics called the Tropical Tropopause Layer (TTL). Many of the geophysical parameters measured over the tropics show that the transition from troposphere to stratosphere occurs over a transition layer rather than at a sharp boundary. Very recently, Fueglistaler et al. [2009] reviewed the concept of TTL and synthesized various existing definitions and provided a new definition of TTL with 14 km as the lower boundary and 18.5 km as the upper boundary. Thus we can think of the wave-turbopause layer analogous to the TTL physically, though the processes taking place at these two regions are totally different. [11] The latitudinal variations of the upper wave-turbopause with respect to the mesopause during the four seasons are discussed further. The lower wave-turbopause is not discussed here as it was found to be always lower than the altitude of the mesopause. Figures 3a 3d show the latitudinal variability of the upper boundary of the wave-turbopause along with mesopause variability for the three consecutive years, We make a few specific observations with regard to the variability of the mesopause and waveturbopause: [12] (i) The latitudinal pattern followed by the waveturbopause is similar in all the three years with very small change in the wave-turbopause height from year to year. We see that years 2005 and 2006 give very similar turbopause heights whereas 2004 exhibits slightly different pattern during all seasons. [13] (ii) In all seasons except boreal summer, the waveturbopause is at minimum altitude at the tropics. This means that propagating waves at the tropical latitudes will dissipate at lower heights than at high latitudes. 4of8

5 Figure 3. Latitudinal variability of the wave turbopause along with cold point mesopause height for the years during boreal (a) winter, (b) vernal equinox, (c) summer, and (d) autumnal equinox. [14] (iii) The winter pole has the maximum wave-turbopause altitude. This altitude decreases with latitude to the other pole but with a secondary maximum in the summer hemisphere midlatitudes. The maximum at the winter pole maybe related to higher wave activity in the winter hemisphere. [15] (iv) During equinoxes, the wave-turbopause has minimum altitude at the tropics which increases toward poles. Both poles have high wave-turbopause altitudes during equinoxes. [16] (v) Comparing with the pattern of cold-point mesopause height, we see that both wave-turbopause and mesopause have similar altitudes at the midlatitudes (around )in both hemispheres during all seasons, but varies toward the poles and the equator. At the summer pole, both mesopause and wave-turbopause are at similar altitudes but at the winter poles, they differ largely. In the winter hemisphere high latitudes, the wave-turbopause is situated at a higher altitude than the mesopause. At the tropics, the mesopause is higher than the wave-turbopause for almost all seasons. During equinoxes, the wave-turbopause is higher at both poles, but lower at the tropics. The transition between the two happens around latitudes during all seasons. The inference is that winter high latitudes have more prominent wave activity and the waves are capable of reaching higher heights as the wave-turbopause is higher than the summer hemisphere. [17] Following the investigation of the latitudinal variability of the wave-turbopause and the mesopause, we carried out analysis for the year 2005 to investigate the monthly variation of the two structures for three different latitudes. The monthly variability of the wave-turbopause and mesopause is studied for 50 N, 0 and 50 S latitudes, representing high latitudes and equator and is shown in Figures 4a 4c. Even though it will be interesting to investigate the annual cycle of wave-turbopause over poles, due to observational constraints of SABER, we cannot get continuous observations over poles. From Figure 4, it is found that at high latitudes, the mesopause falls in between the upper and lower boundaries of the wave-turbopause all through the year except for one or two points where they coincide or the mesopause is slightly lower. But at the equator, the mesopause lies close to the upper boundary of the wave-turbopause, mostly above it. The latitudinal differences in behavior of mesopause with respect to wave-turbopause layer are yet to be fully understood. It is noteworthy that at high latitudes both structures follow a clear annual oscillation whereas at the equator it resembles more of an ambiguous semi-annual oscillation structure. To 5of8

6 with cold point mesopause and classical turbopause (only over 50 N) are established. [18] These observations about the wave-turbopause variability are consistent with the present understanding of the variability of atmospheric waves. However, to substantiate the wave-turbopause layer concept, we have estimated the gravity wave potential energy profiles from SABER temperature measurements using the following equation, E p ¼ 1 gz ðþ 2 Tz ðþ 2 ð1þ 2 Nz ðþ T 0 ðþ z Where g(z) is the acceleration due to gravity, T 0 (z) is the mean temperature at an altitude z, T (z) is the temperature fluctuations of the instantaneous temperature profile about the mean temperature T 0 (z) and N (z) is the Brunt-Väisälä frequency, given by N 2 ðþ¼ z gz ðþ T 0 ðþ z þ gz ðþ ð2þ T 0 ðþ z z C p Figure 4. Monthly variability of the wave turbopause (WTP) layer along with the cold point mesopause (CPM) height for (a) 50 N, (b) 0, and (c) 50 S latitudes. The magenta line in the top panel represents the classical turbopause over Saskatoon and is adopted from Hall et al., [2008]. compare the pattern of the classical turbopause with waveturbopause, we have used the seminal results of Hall et al. [2008] in Figure 4a. These authors have used MF radar observations over Saskatoon (52 N)andTromsø(70 N) during the years 1999 to For the present study, we use classical turbopause height information over Saskatoon (52 N) and is sown in Figure 4a. From this figure, it can be noted that the classical turbopause more or less follows the upper boundary of the wave-turbopause during all the months except for autumnal equinox. However, it has to be remembered that the classical turbopause is derived using MF radar observations over a single location whereas as SABER observations comprises of the entire longitudinal belt. Thus the monthly variations of upper and lower boundaries of wave-turbopause along C p is the specific heat capacity of air at constant pressure and has a value of [19] To estimate the temperature fluctuations induced by gravity waves alone, 0 6 zonal wave number planetary wave components are subtracted from the instantaneous profiles of SABER measured temperature measurements. The amplitudes and phases of planetary waves are estimated using a least square fit technique. The similar method was proposed by Fetzer and Gille [1994]. The gravity wave potential energies are then estimated for each instantaneous profile of SABER and are gridded into 5 20 grids over the globe. The potential energy values for the MLT region are zonally averaged in grids of 10 0 latitudes to obtain the latitude-height sections. Figures 5a and 5b show the height-latitude section of gravity wave potential energy in the MLT region for the boreal winter and summer respectively. [20] The pink shade in the Figures 5a and 5b represents the transition from smaller magnitudes of gravity wave potential energy below to higher values above in altitude. This transition region can be regarded as the lower boundary of the wave-turbopause layer. This dissipation regime continues till the yellow shade in the Figures 5a and 5b, after which the wave potential energies are drastically increasing (brown shade), which represent the end of dissipation regime and start of free propagation as proposed by Offermann et al. [2006]. Thus the region of pink shade to yellow shade in the Figures 5a and 5b can be regarded as the wave-turbopause layer proposed in the present study. It is also seen that the potential energy values follow similar latitudinal pattern as that of the wave-turbopause height shown in Figure 2, though there is no one to one correspondence in the altitude. The yellow line in Figure 5 is similar in structure to the waveturbopause, though it doesn t correspond to the exact altitude. This is probably because the spectrum of waves in the atmosphere ranging from gravity waves to planetary waves propagate with different phase speeds and will dissipate at different altitudes. Among these waves, gravity waves will have relatively higher phase speeds and may propagate to higher heights as compared to other waves. This may be the reason for the observed differences in the wave-turbopause and gravity wave potential energy maps. Thus the difference 6of8

7 Figure 5. Height-latitude section of zonal mean gravity wave potential energy in the MLT region for (a) boreal winter and (b) boreal summer. in height can be attributed to the contributions from planetary waves and tides, which are included in Figure 2 but not in Figure 5. Nevertheless, Figure 5 shows the summerwinter difference in wave-turbopause layer thickness similar to Figure 2 and substantiates the concept of wave-turbopause layer proposed in the present study. 4. Concluding Remarks [21] The concept of the wave-turbopause layer is introduced and discussed using SABER temperature measurements during the years The wave-turbopause is identified as the point of intersection of the two linear fits of the lower and higher regions of standard deviation from zonal mean temperature profiles as proposed by Offermann et al. [2006]. However, we have observed that there exists a wave-turbopause layer globally. The lower and upper boundaries of the wave-turbopause are revealed by the existence of two different slopes in the upper region of the standard deviation profile. It is shown that the wave-turbopause has lower and upper boundaries and the region in between is regarded as the wave-turbopause layer. It is found that the winter hemispheric high latitudes have the highest waveturbopause in altitude which decreases toward the summer hemisphere with a secondary maximum at the summer hemisphere midlatitude. The thickness of the wave-turbopause layer is minimum at the tropics which increases toward midlatitudes and is maximum at the winter poles. At the summer pole, the two boundaries almost coincide. For further investigation, the upper wave-turbopause is considered for evaluating the seasonal variations of its latitudinal structure. The variation of the cold point mesopause heights in comparison with waveturbopause height is studied in detail and some interesting observations are made. Moreover, the monthly variability of the wave-turbopause and mesopause is studied for 50 N, 0 and 50 S latitudes. It is found that at high latitudes, the mesopause falls in between the upper and lower boundaries of the wave-turbopause for almost all months whereas at the 7of8

8 equator, the mesopause lies close to the upper boundary of the wave-turbopause and more often above it. On global scale, the latitude-height section of gravity wave potential energy in the MLT region is computed. The wave-turbopause pattern seemed to be superposed on the potential energy transition trend. From the height-latitude section of gravity wave potential energy, we have shown an intermediate layer sandwiched between low and high magnitudes of potential energy, which is regarded as the wave-turbopause layer. Thus the present study proposed the concept of wave-turbopause layer and substantiated the same by discussing its latitudinal structure, seasonal variability and the height latitude section of gravity wave potential energy in the MLT region. As the wave-turbopause layer is the region where maximum wave-dissipation happens, the thickness and the height of this regions becomes more important from the MLT dynamics stand point. [22] Acknowledgments. Sherine Rachel John is grateful to ISRO for providing Research Fellowship for her work. The authors are thankful to the TIMED/SABER team for the freely downloadable data. [23] Robert Lysak thanks the reviewers for their assistance in evaluating this paper. References Chabrillat, S., G. Kockarts, D. Fonteyn, and G. Brasseur (2002), Impact of molecular diffusion on the CO 2 distribution and the temperature in the mesosphere, Geophys. Res. Lett., 29(15), 1729, doi: / 2002GL Danilov, A. D., U. A. Kalgin, and A. A. Pokhunkov (1979), Variation of the turbopause level in the polar regions, Space Res., XIX(83), Fetzer, E. J., and J. C. Gille (1994), Gravity wave variances in LIMS temperatures. Part I: Variability and comparison with background winds, J. Atmos. Sci., 51, , doi: / (1994)051<2461: GWVILT>2.0.CO;2. Fueglistaler, S., A. E. Dessler, T. J. Dunkerton, I. Folkins, Q. Fu, and P. W. Mote (2009), Tropical tropopause layer, Rev. Geophys., 47, RG1004, doi: /2008rg Garcia, R. R., and S. Solomon (1985), The effect of breaking gravity waves on the dynamics and chemical composition of the mesosphere and lower thermosphere, J. Geophys. Res., 90(D2), , doi: / JD090iD02p Garcia, R. R., F. Stordal, S. Solomon, and J. T. Kiehl (1992), A new numerical model of the middle atmosphere: 1. Dynamics and transport of tropospheric source gases, J. Geophys. Res., 97, 12,967 12,991, doi: / 92JD Hall, C. M., A. H. Manson, and C. E. Meek (1998), Seasonal variation of the turbopause: One year of turbulence investigation at 60 N by the joint University of Tromsø/University of Saskatchewan MF radar, J. Geophys. Res., 103(D22), 28,769 28,773, doi: /1998jd Hall, C. M., C. E. Meek, A. H. Manson, and S. Nozawa (2008), Turbopause determination, climatology, and climatic trends using medium frequency radars at 52 N and 70 N, J. Geophys. Res., 113, D13104, doi: / 2008JD John, S. R., and K. K. Kumar (2011), TIMED/SABER observations of global cold-point mesopause variability at diurnal and planetary wave scales, J. Geophys. Res., 116, A06314, doi: /2010ja John, S. R., and K. K. Kumar (2012), TIMED/SABER observations of global gravity wave climatology and their interannual variability from stratosphere to mesosphere lower thermosphere, Clim. Dyn., 39, , doi: /s Lindzen, R. S. (1981), Turbulence and stress owing to gravity wave and tidal breakdown, J. Geophys. Res., 86, , doi: / JC086iC10p Lübken, F.-J., and U. von Zahn (1991), Thermal structure of the mesopause region at polar latitudes, J. Geophys. Res., 96, 20,841 20,857, doi: /91jd Mertens, C. J., M. G. Mlynczak, M. López-Puertas, P. P. Wintersteiner, R. H. Picard, J. R. Winick, L. L. Gordley, and J. M. Russell III (2001), Retrieval of mesospheric and lower thermospheric kinetic temperature from measurements of CO 2 15 mm Earth limb emission under non-lte conditions, Geophys. Res. Lett., 28(7), , doi: / 2000GL Mertens, C. J., et al. (2004), SABER observations of mesospheric temperatures and comparisons with falling sphere measurements taken during the 2002 summer MaCWAVE campaign, Geophys. Res. Lett., 31, L03105, doi: /2003gl Mlynczak, M. G., and S. Solomon (1993), A detailed evaluation of the heating efficiency in the middle atmosphere, J. Geophys. Res., 98, 10,517 10,541, doi: /93jd Offermann, D., V. Friedrich, P. Ross, and U. von Zahn (1981), Neutral gas composition measurements between 80 and 120 km, Planet. Space Sci., 29(7), , doi: / (81) Offermann, D., M. Jarisch, J. Oberheide, O. Gusev, I. Wohltmann, J. M. Russell III, and M. G. Mlynczak (2006), Global wave activity from upper stratosphere to lower thermosphere: A new turbopause concept, J. Atmos. Sol. Terr. Phys., 68, , doi: /j.jastp Offermann, D., M. Jarisch, H. Schmidt, J. Oberheide, K. U. Grossmann, O. Gusev, J. M. Russell III, and M. G. Mlynczak (2007), The wave turbopause, J. Atmos. Sol. Terr. Phys., 69, , doi: / j.jastp Remsberg, E., G. Lingenfelser, V. L. Harvey, W. Grose, J. Russell III, M. Mlynczak, L. Gordley, and B. T. Marshall (2003), On the verification of the quality of SABER temperature, geopotential height, and wind fields by comparison with Met Office assimilated analyses, J. Geophys. Res., 108(D20), 4628, doi: /2003jd She, C. Y., J. R. Yu, and H. Chen (1993), Observed thermal structure of a midlatitude mesopause, Geophys. Res. Lett., 20, , doi: / 93GL Siskind, D. E., S. D. Eckermann, J. P. McCormack, M. J. Alexander, and J. T. Bacmeister (2003), Hemispheric differences in the temperature of the summertime stratosphere and mesosphere, J. Geophys. Res., 108(D2), 4051, doi: /2002jd Venkat Ratnam, M. V., A. K. Patra, and B. V. Krishna Murthy (2010), Tropical mesopause: Is it always close to 100 km? J. Geophys. Res., 115, D06106, doi: /2009jd von Zahn, U., and W. Meyer (1989), Mesopause temperatures in polar summer, J. Geophys. Res., 94, 14,647 14,651, doi: /jd094id12p von Zahn, U., and R. Neuber (1987), Thermal structure of the high latitude mesopause region in winter, Beitr. Phys. Atmos., 60, 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, D09102, doi: / 2006JD Yu, J. R., and C. Y. She (1995), Climatology of a midlatitude mesopause region observed by a lidar at Fort Collins, Colorado (40.6 N, 105 W), J. Geophys. Res., 100, , doi: /94jd of8