Tropical precipitation patterns in response to a local warm SST area placed at the equator of an aqua planet

Aqua planet experiments are performed in order to investigate the effects of an equatorial warm sea surface temperature (SST) area on the tropical large-scale precipitation patterns. The numerical model utilized 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 distribution is zonally uniform and symmetric about the equator. The calculated tropical precipitation distributions are characterized by the appearance of an east-west asymmetry; precipitation decreases to the west of the warm SST area, while it increases in the longitudinally wide area to the east. The east-west asymmetry appears regardless of the cumulus parameterization schemes utilized (the convective adjustment scheme and the Kuo scheme). In the western region to the warm SST area, an increase of the stability due to a temperature rise in the middle layer is observed and correspondingly downward flow is recognized. These are consistent with the decrease of precipitation. In the eastern region, although a temperature rise appears in the middle layer, the stability decreases due to the increase of water vapor in the lower layer caused by the meridional moisture convergence. This destabilization is consistent with the increase of precipitation. Experiments in which the wind velocity used in the evaluation of the surface evaporation is fixed are performed to show that WISHE (Wind Induced Surface Heat Exchange) mechanism is not a principal cause for the generation of the east-west asymmetry of the precipitation pattern.