Radar observations of long‐term variability of mesosphere and lower thermosphere winds over Tirunelveli (8.7°N, 77.8°E)

Abstract

The horizontal wind data acquired by MF radar at Tirunelveli (8.7°N, 77.8°E) for the period January 1993 to July 2006 are used to study long‐term variability of equatorial mesosphere and lower thermosphere (MLT) winds. The monthly mean zonal wind exhibits dominant semiannual variability with westward winds during equinox and eastward winds during solstice. The first westward phase, which occurs during spring equinox, undergoes interannual variability with larger westward winds during the years 1993, 1995 and 1997. This interannual variability has been interpreted as quasi‐biennial oscillation (QBO) in the MLT winds. However, this QBO is absent during the year 1999, as the next large westward wind phase occurs during the year 2000. Thus the period of QBO is extended from nearly 2 to 3 years (a). During these years, the period of stratospheric QBO winds is also extended to 3 a. Besides SAO and QBO, as the zonal winds undergo intraseasonal oscillations with periodicities resembling those of Madden‐Julian Oscillation (MJO) in the troposphere, the relation between the two is examined on the basis of the existing hypothesis. The present study reveals long‐term variation in the amplitude of the annual oscillation noticed in monthly mean meridional winds. A decreasing trend is observed in the annual mean northward winds during the years 1993–2006 in contrast to decreasing trend in the annual mean southward winds reported from midlatitudes. These observations are discussed in the context of our current understanding of long‐term variability of MLT winds.