Observations of middle atmospheric seasonal variations and study of atmospheric oscillations at equatorial regions

Abstract

The equatorial atmospheric regions are unique and one of the most important parts of the Earth's atmosphere. These regions receive the maximum energy from the Sun, and are highly turbulent and influenced by the processes of the Northern Hemisphere as well as by the Southern Hemisphere. Atmospheric temperature is one the most sensitive parameters and having imprints of dynamical, radiative and chemical processes. Thermal structure, seasonal variability and atmospheric oscillations occurring in the equatorial middle atmosphere have been studied here using temperature profiles retrieved from the Sounding of the Atmosphere using the Broadband Emission Radiometry (SABER) instrument on-board Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite during January 2002 to December 2015 (14 years). In the present work, three geographical locations over the Equatorial latitudes and ±120° longitude have been chosen, considering 5° × 15° latitude-longitude grid between the altitudes ranging from 20 to 100 km. A detailed time series and climatology analyses have been performed over these regions and significant longitudinal differences have been observed. Characteristics of Semi Annual oscillation (SAO), Annual Oscillation (AO) and Quasi Biennial Oscillation (QBO) have been investigated using Lomb Scargle Periodogram and wavelet transform analysis techniques. It has been noted the longitudinal differences in the spectral amplitudes of equatorial oscillations relative to the zonal mean spectrum. The longitudinal asymmetry might be due to the vertically propagating planetary waves. From the analysis, it is also observed that SAO is showing the highest amplitude at 45 km and 75–85 km, and QBO is showing significant amplitude in the lower stratosphere. At higher altitudes, amplitudes of different oscillations (viz. SAO, AO, and QBO) are very weak, and temperatures variations are not revealing any significant pattern. Moreover, from the climatology and time series of the temperature measurements, high temperature (∼260 K) has been noticed at 55–65 km height range. In addition to that, seasonal variations over equatorial regions have been studied and the strength of seasonal differences are found to be about 9 K (summer-winter) at higher altitudes.