Long‐term tendencies in the mesosphere/lower thermosphere mean winds and tides as observed by medium‐frequency radar at Tirunelveli (8.7°N, 77.8°E)

Abstract

The present study examines changes in monthly mean winds and tides observed by MF radar at Tirunelveli (8.7°N, 77.8°E) for the years 1993–2007 with respect to long‐term variabilities, namely, the stratospheric quasi‐biennial oscillation (QBO), the El Niño–Southern Oscillation (ENSO), the solar cycle (SC), and the long‐term trends, using regression analysis. Both zonal and meridional winds show negative QBO response in the altitude region 84–94 km and negative ENSO response (∼1 m s−1 per Southern Oscillation Index) above 90 km. However, only the latter show a notable positive SC response. The response of meridional diurnal tide is positive to ENSO (above 90 km) and stratospheric QBO and is negative to SC. The tides in both wind components show positive trends. The meridional winds show a significant negative trend (0.3 m s−1 yr−1). Further, monthly variations of the responses of mean winds and tides to QBO, ENSO, and SC are presented and discussed.