Extratropical control of tropical upwelling in the lower stratosphere

Extratropical planetary wave events which drive the global stratospheric meridional circulation are observed to occur episodically, with several distinct events per year. These bursts of wave activity are correlated with variations in the mean meridional (Brewer-Dobson) circulation, which link the tropics and extratropics. Time series of temperature and derived vertical velocity in the tropical lower stratosphere clearly reveal the imprint of extratropical variability on weekly to monthly time scales. The influence of extratropics is quantified by estimating tropical vertical velocity via 'downward control' on a daily basis, and reasonable agreement with estimates derived from thermodynamic balance is found. Inclusion of zonal wind tendencies in the 'downward control' calculations is important, and makes a significant impact on the estimated seasonal cycle in the tropics. Furthermore, significant coherence is found between the extratropically-forced vertical velocity and observed temperatures in the tropics. This relationship is quantitatively consistent with simple thermodynamic balance: the large annual cycle of temperature near the tropical tropopause is mainly a result of the very long radiative time scales in that region.