Abstract

The zonal mean mass circulation of the stratosphere (the Brewer-Dobson circulation) is characterized by upwelling in the tropics and downwelling at high latitudes. The slow ascent across the tropical tropopause largely controls the composition of air entering the stratosphere. Despite its relevance, tropical upwelling is poorly constrained by observations and its magnitude, variability and specific forcing remain uncertain.

The variability in upwelling can especially influence tracers with steep vertical gradients across the tropical tropopause, such as ozone and carbon monoxide. Satellite observations of these tracers are used to examine the relation with indirect estimates of tropical upwelling based on reanalysis data. Complementary analyses using the Whole Atmosphere Community Climate Model allow explicitly evaluating the effect of eddy transport on tracer variability, in addition to upwelling. The large annual cycle observed in the tracers and in temperature in the tropical lower stratosphere is mainly linked to upwelling, while eddy transport is important around the tropopause.

In addition to the annual cycle, the upwelling estimates and the tracers show substantial coherent fluctuations on sub-seasonal timescales, and the specific dynamical forcing of such variability is investigated. The statistical patterns obtained in the reanalysis manifest the response of the zonal mean circulation extending away from the transient forcing regions, as expected from theory. Sub-seasonal variability in upwelling is mostly linked to wave drag in the extratropical winter stratosphere and the subtropical upper troposphere, with stronger effects on the deep and the shallow branches of the Brewer-Dobson circulation, respectively.