NASA - National Aeronautics and Space Administration

Latent heating estimates derived from rainfall observations are used to construct model experiments that isolate equatorial waves forced by tropical convection from midlatitude 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 158 of the equator. The seasonal variation of the equatorial waves and the mean meridional upwelling that they cause is examined for two full years from 2006 to 2007. Changes in equatorial Rossby wave propagation through seasonally varying mean winds are the primary mechanism for producing an annual variation in the residual-mean upwelling. In the tropical tropopause layer, averaged within 158 of the equator and between 90 and 190 hPa, the annual cycle varies between a maximum upwelling of 0.4 mm s21 during boreal winter and spring and a minimum of 0.2 mm s21 during boreal summer. This variability seems to be due to small changes in the mean wind speed in the tropics. Seasonal variations in latent heating have only a relatively minor effect on seasonal variations in tropical tropopause upwelling. In addition, Kelvin waves drive a small downward component of the total circulation over the equator that may be modulated by the quasi-biennial oscillation.