Factors Maintaining the Zonally Asymmetric Precipitation Distribution and Low-Level Flow in the Tropics of an Atmospheric General Circulation Model: Diagnostic Studies

Abstract

The roles played by the large-scale motion induced by vertical diffusion of heat from the lower boundary and condensational heating due to deep convection in maintaining the precipitation zones in the Tropics of an atmospheric general circulation model (GCM) are explored. A steady linearized version of the GCM is used to diagnose the wind forced by these processes. The wind field obtained from the linear model is combined with the time-mean moisture field from the GCM in order to determine the zonally asymmetric moisture flux convergence, which is the primary factor maintaining the zonally asymmetric precipitation distribution. The role of the other diabatic heating processes is explored as is the role of the orographic forcing in maintaining the precipitation distribution. The vertically integrated moisture flux convergence forced by vertical diffusion of heat and condensational heating are found to be in phase over the ocean and 180 degrees out of phase over the land. Over the ocean, both of these forcings contribute to moisture flux convergence in the regions of largest precipitation. The moisture flux convergence forced by the vertical diffusion of heat tends to narrow the precipitation zones in the meridional direction over the ocean. Over the land, the condensational heating leads to moisture flux convergence in the regions of large precipitation, while the vertical diffusion of heat leads to moisture flux divergence. This indicates that the motions forced by the surface temperature provide a negative feedback on the precipitation. This feedback is apparently due to the relatively cool surface temperatures present in the regions of large precipitation over land. This locally cool surface temperature leads to a low-level divergent circulation from the cool region to warmer regions. Other forcing functions are found to play a minor role in the moisture flux convergence by the time-mean flow with the exception of the orographic forcing in some regions. The lowest model sigma-level wind field over the tropical Pacific Ocean is examined. In general both the zonal and meridional wind fields are dominated by the response to convective condensational heating. Exceptions include the meridional wind in the western Pacific and the zonal wind along the equator. In these regions, the response to low-level temperature gradients is found to be nonnegligible in comparison with the response to convective condensational heating. The role of the orographic forcing is also significant along the coasts of the tropical continents and in the western Pacific.