The tropical tropopause



The physical meaning of several different tropical tropopause definitions is examined using atmospheric data from a variety of sources, and model output. The conventional lapse-rate definition of the tropopause appears to have little physical relevance in the tropics, although it is easy to use operationally. A four year ‘climatology’ of the tropical tropopause from the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses is presented. The zonal mean picture shows the annual cycle in properties that has been related to the extratropical stratospheric wave pump. However, there are large and important zonal asymmetries. These include a relatively low pressure and temperature at the tropopause near the west Pacific heating region during December—January—February, and a striking region with low pressure on the tropopause over India during June—July—August. Results from a baroclinic model with imposed diabatic heating are used to support the hypothesis that both these features can be attributed to the direct response of the atmosphere to a large-scale region of tropospheric diabatic heating. It is proposed that the stratospheric pump provides the general picture for the upper troposphere/lower stratosphere region, but that tropospheric convection is crucial in determining the important zonal asymmetries in this region.