Seasonal variabilities of low-latitude mesospheric winds

R. Rajaram,S. Gurubaran

doi:10.1007/s00585-998-0197-4

Abstract

Abstract. Observations of mesospheric winds over a period of four years with the partial reflection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70–98 km. The meridional winds exhibit overall transequatorial flow associated with differential heating in the Northern and Southern Hemispheres. At lower altitudes (70–80 km) the mean zonal winds reveal easterly flow during summer and westerly flow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80–98 km) and at all altitudes during equinox periods, the mean zonal flow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at specific harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation.Key words. Meteorological and atmospheric dynamics · General circulation · Middle atmosphere dynamics · waves and tides.

Highlights

The equatorial and low-latitude mesosphere hosts a variety of dynamical processes some of which are con®ned to this region where the Coriolis frequencyCorrespondence to: S
Recent eorts by many workers have fully utilized the potential of ground-based radar measurements in examining the low frequency motions in the equatorial and low-latitude mesosphere and evaluating their contributions to the observed dynamical behaviour of the mesosphere and lower thermosphere region (Vincent and Lesicar, 1991; Harris and Vincent, 1993; Fritts and Isler, 1994; Palo and Avery, 1995, 1996)
There are many unresolved problems that need to be quantitatively understood, to name a few, eects of longperiod waves on the background meanow, gravity wave ± tide ± planetary wave interactions and their eects, and contribution of tides and gravity waves to the momentum budget of the equatorial mesosphere. This calls for a number of radar measurements that are separated in longitude in the equatorial region

Summary

Introduction

The equatorial and low-latitude mesosphere hosts a variety of dynamical processes some of which are con®ned to this region where the Coriolis frequencyCorrespondence to: S. There are many unresolved problems that need to be quantitatively understood, to name a few, eects of longperiod waves on the background meanow, gravity wave ± tide ± planetary wave interactions and their eects, and contribution of tides and gravity waves to the momentum budget of the equatorial mesosphere. This calls for a number of radar measurements that are separated in longitude in the equatorial region.

Objectives

Results

Conclusion