Evolution of individual equatorial atmospheric Kelvin waves in the stratosphere from FORMOSAT-7/COSMIC-2 temperatures

Abstract

Temperatures obtained from Formosa Satellite-7/Constellation Observing System for Meteorology, Ionosphere and Climate-2 (FORMOSAT-7/COSMIC-2) in the stratosphere are analysed to investigate equatorial atmospheric Kelvin waves (KW) during the period from October 2019 to March 2021. Least square fitting followed by a two-dimensional fast Fourier transform are employed to extract the characteristics of these waves. Comparison with ERA5 mean zonal winds clearly indicates wave-mean flow interactions. KW activity was stronger from November 2019 to July 2020 and observed in the entire stratosphere due to the prevailing westward wind. After July 2020, although the ambient winds were favourable, wave activity was weaker and is attributed to the La Nina state of El Nino Southern Oscillation (ENSO) in the equatorial Pacific. The emphasis of the current study is on individual KW events and their evolution as they propagate eastward and upward. Waves of many periods are excited simultaneously in the lower atmosphere and as they propagate upward with different speeds, disperse along the path. During November–December 2019, KW periods ranged from 8 to 21 days. Vertical wavelength of the slow waves (> 15 days) is found to be ~ 3 km just above the tropopause that increased to 10–12 km at ~ 40 km. This increase in vertical wavelength is due to Doppler effect of the mean wind on the wave. For the fast waves of periods 8, 6.5 and 4.5 days, the wavelengths are larger and 18, 24 and 30 km, respectively. COSMIC-2 data, with its high vertical resolution, provided this opportunity to investigate the fine structure of the KW delivering many insights into the atmospheric dynamics involved.