SOUTH AMERICA DURING THE LGM: A REGIONAL MODELING STUDY

Despite considerable reconstruction efforts, the LGM climate of the Amazon basin is not well understood. There is some general agreement that the region was some 3-6ºC cooler, but conflicting evidence and interpretation about the precipitation. Rainfall reconstructions from adjacent regions such as Lake Titicaca are helpful, but the relationship to the Amazon is not clear. An improved understanding of the physical mechanisms that relate precipitation in the Amazon and the high Andes would help resolve this issue. Such a problem is well-suited for a modeling investigation. Global models of climate (general circulation models, or GCMs) have been used extensively to simulate the climate of the LGM. However, they are of limited use for South America because of their relatively coarse resolution and related difficulties in resolving the sharp Andean topography. In this study, a higher-resolution regional climate model (RCM) is used to contrast the rainfall regimes over South America for the present day and LGM. Extensive testing and evaluation are used to optimize the RCM for simulations of the South America climate through choice of physical parameterizations and boundary conditions (surface and lateral). The RCM provides an improved simulation of South American precipitation regimes compared with a GCM, and resolves some of the smaller-scale features of the circulation that are important in connections between the Amazon basin and high Andes. Multiple seasonal climate-mode simulations are conducted to understand the physical processes of moisture transport and wind convergence in and between the Amazon and high Andes regions, and to evaluate the roles of the various LGM forcing factors (e.g., orbital parameters, CO2, sea surface temperatures, vegetation changes) individually.