The residual mean circulation in the tropical tropopause layer driven by tropical waves

Recent variations in stratospheric water vapor with unknown causes have been implicated in global decadal-scale surface temperature variations. Most notably, a sudden decrease in stratospheric water vapor in 2001 coincided with a cooling in tropical tropopause temperatures observed in radiosondes, and the change persisted and only slowly recovered through the first decade of the 21st century. These decadal-scale changes to both temperature and water vapor suggest that there was an increase in tropical upwelling across the tropical tropopause layer (TTL). The tropical upwelling circulation is driven by waves and deep convection and stratospheric water is further controlled by tropopause temperatures. Hence it is natural to suspect that there is an increase in wave driving, either through enhanced wave forcing or through changes in the pattern of wave propagation that would lead to enhanced wave-flux divergence. Although extratropical wave pumping clearly influences the zonal mean tropical upwelling in the lower stratosphere, several recent studies have found that global-scale equatorial waves, forced by convection, play an important role in modulating tropical tropopause temperatures and upwelling in the tropopause layer that control stratospheric water vapor.

We use latent heating estimates derived from rainfall observations to construct model experiments that isolate equatorial waves forced by tropical convection from mid-latitude synoptic-scale waves. These experiments are used to demonstrate that quasi-stationary equatorial Rossby waves forced by latent heating drive most of the observed residual mean upwelling across the tropopause transition layer within 15o of the equator. The seasonal variation of the equatorial waves and the mean meridional upwelling they cause is examined for two full years from 2006 to 2007. We find that changes in equatorial Rossby wave propagation through seasonally varying mean winds is the primary mechanism for producing a seasonal variation in the residual mean upwelling in the tropical tropopause layer. Seasonal variations in latent heating have only a minor effect on seasonal variations in tropical tropopause upwelling, but that interannual variations in the heating can have a significant effect.