THE TROPICAL TROPOPAUSE

The physical meaning of several different tropical tropopause definiti ons 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 tropic al tropopause from the European Centre for Medium-Range Weather Foreca sts (ECMWF) analyses is presented. The zonal mean picture shows the an nual cycle in properties that has been related to the extratropical st ratospheric wave pump. However, there are large and important zonal as ymmetries. These include a relatively low pressure and temperature at the tropopause near the west Pacific heating region during December-Ja nuary-February, and a striking region with low pressure on the tropopa use over India during June-July-August. Results from a baroclinic mode l with imposed diabatic heating are used to support the hypothesis tha t both these features can be attributed to the direct response of the atmosphere to a large-scale region of tropospheric diabatic heating. I t is proposed that the stratospheric pump provides the general picture for the upper troposphere/lower stratosphere region, but that troposp heric convection is crucial in determining the important zonal asymmet ries in this region.