Tropical mesopause: is it always close to 100 km?

Abstract

Global distribution of mesopause using observations of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) aboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite is studied with special emphasis on tropical latitudes. Temperature structure of the mesosphere and lower thermosphere over Gadanki (13.5°N, 79.2°E), a tropical latitude location in India, has been studied in detail. The mesopause altitude at Gadanki is observed to be in the range of 87-105 km with a mean of ~98 km, and the temperature is in the range of 160-205 K with a mean of ~170 K. A secondary temperature minimum is observed in the height region of 75-80 km with temperature in the range of 175-195 K. Although the mesopause temperature and altitude show pronounced short-term variability, on a seasonal mean basis, the variation in these two parameters is rather small. There is no significant long-term variation in mesopause altitude. The height of the secondary minimum, however, is found to show semiannual variation with a maximum in solstices and a minimum in equinoxes. Thus, the variabilities in the height and temperature of the mesopause and the secondary minimum reported here are different from the two-level mesopause reported previously for middle and high latitudes. Another significant result that comes from the time series analysis of mesopause temperature is a decreasing trend with a rate of ~0.72±0.05 K/yr. Furthermore, the long-term mean profile of temperature does not show the secondary minimum, providing strong support that it could be due to shorter scale tides and gravity waves. The possible implications of the mesopause temperature structure on various physical processes occurring in the mesosphere and the lower thermosphere are discussed.