Time-Dependent Upwelling in the Tropical Lower Stratosphere Estimated from the Zonal-Mean Momentum Budget

Abstract

Dynamical variability in the extratropical stratosphere occurs on a broad range of timescales, from daily to seasonal. Extratropical wave transience is correlated with variations in the mean meridional (Brewer?Dobson) circulation that links the Tropics and the extratropics. In this study, the variability of observed temperature and calculated vertical velocity in the tropical lower stratosphere is examined to isolate the imprint of forcing by extratropical waves. The influence of the waves is quantified by estimating zonal-mean tropical upwelling from the zonal-mean momentum balance on a daily basis; a large fraction of the variance of tropical upwelling occurs at periods of 10?40 days, forced by transient waves. In addition, significant coherence is found between calculated upwelling and observed temperatures in the tropical lower stratosphere on weekly to seasonal timescales. This relationship is quantitatively consistent with simple thermodynamic balance, and suggests that the large annual cycle of temperature near the tropical tropopause is mainly a result of the relatively long radiative timescales in that region. The results indicate that EP flux divergence due to extratropical waves is a major determinant of zonal-mean temperatures in the tropical lower stratosphere.