Simulation and Diagnosis of the Regional Summertime Precipitation Climatology of South America

Abstract

The regional precipitation climatology of South America during austral summer is studied by means of an R30 general circulation model (GCM). Results from perpetual January experiments, which differ only in their distributions of topography and sea surface temperature (SST), are presented. The precipitation field of the most realistic experiment compares well with the observed January precipitation climatology of South America. reproducing, in particular, five regions of maximum precipitation. To understand how structure in the surface conditions is mapped onto the precipitation field. the results of the three GCM experiments are compared. Continentality, through the generation of a thermal low, is responsible for much of the structure in the modeled precipitation field of South America. Topography introduces orographic precipitation maxima in the Central and Southern Andes and modifies precipitation rates elsewhere. Longitudinal structure in SSTs, which is also largely an expression of continentality, is not a dominant source of structure for the South American precipitation field. However, the positions and magnitudes of some of the precipitation maxima (especially those in the cast) are moderately affected by SSTs. Analysis of the atmospheric water vapor budget associates structure in the precipitation field with structure in the moisture flux convergence field. Connections with the large-scale circulation are explored to explain the convergence of moisture flux in each region of enhanced precipitation. Comparisons with observed low-level wind and moisture fields suggest that the mechanisms responsible for the modeled precipitation maxima are, for the most part, reflective of those in the real world.