TROPICAL PRECIPITATION PATTERNS IN RESPONSE TO A LOCAL WARM SST AREA PLACED AT THE AQUATOR OF AN AQUA PLANET

Aqua planet experiments are performed in order to investigate the effe cts of an equatorial warm sea surface temperature (SST) area on the tr opical large-scale precipitation patterns. The numerical model utilize d is a spherical three-dimensional primitive system with resolution of T42L16 and with simplified hydrological processes. The warm SST area is placed at the equator of an aqua planet whose basic SST distributio n is zonally uniform and symmetric about the equator. The calculated t ropical precipitation distributions are characterized by the appearanc e of an east-west asymmetry; precipitation decreases to the west of th e warm SST area, while it increases in the longitudinally wide area to the east. The east-west asymmetry appears regardless of the cumulus p arameterization schemes utilized (the convective adjustment scheme and the Kuo scheme). In the western region to the warm SST area, an incre ase of the stability due to a temperature rise in the middle layer is observed and correspondingly downward flow is recognized. These are co nsistent with the decrease of precipitation. In the eastern region, al though a temperature rise appears in the middle layer, the stability d ecreases due to the increase of water vapor in the lower layer caused by the meridional moisture convergence. This destabilization is consis tent with the increase of precipitation. Experiments in which the wind velocity used in the evaluation of the surface evaporation is fixed a re performed to show that WISHE (Wind Induced Surface Heat Exchange) m echanism is not a principal cause for the generation of the east-west asymmetry of the precipitation pattern.