Study of tropical tropospheric and lower stratospheric zonal wind variability in the context of dry and wet Indian summer monsoon years

Abstract

This paper describes the comparative study on some features of the tropospheric and lower stratospheric circulation over the tropical Indian region during the monsoon season in the context of deficient and excess monsoon (dry and wet) years. The monthly mean of zonal wind component are computed directly with NCEP reanalysis data for the main rainy season of June–September. The composite anomalies of zonal wind circulation for monsoon season in four meteorologically important regions of lower atmosphere viz., lower troposphere, middle troposphere, upper troposphere and lower stratosphere in respect of wet and dry monsoon years were calculated and compared. The results of our analysis show major contrasts between the characteristics of winds of tropical troposphere and lower stratosphere over Indian region during the two scenarios. In the dry years, the troposphere and lower stratosphere are characterized by weaker wind flows in comparison with the normal flow of low level jet (LLJ) and tropical easterly jet (TEJ) streams . The observed weak LLJ and TEJ support the rainfall deficit in dry years. In contrast, during the wet years, the troposphere is dominated by stronger westerly flows at lower and middle tropospheric levels in concomitant with stronger easterlies in the upper troposphere and lower stratosphere. The long term variability of zonal winds averaged between 20°N and 20°S shows decreasing trend in the absolute values in speed during 1981 to 2010. Over the period of study, both tropical westerlies and easterlies and also jet streams have weakened. The results from this study serve as bases for further work on the tropospheric circulation features over the tropical Indian region for an enhanced understanding of the regional climate.