Interannual Variability in the Regional Climate Simulation over South America

The Regional Spectral Model (RSM) is used in making austral summer seasonal simulations of 1997 (normal), 1998 (El Nino) and 1999 (La Nina) using the National Center for Environmental Prediction (NCEP) reanalysis as the time varying base field. The RSM by its formulation, predicts the perturbations about the time varying base field provided by the NCEP reanalysis. There is strong evidence that the RSM enhances the NCEP reanalysis relative to the observations. The regional domain is partioned into 6 subdomains comprising the Amazon River Basin (ARB), ITCZ over Atlantic (ITCZA), South Atlantic Convergence Zone (SACZ), Nordeste, subtropical areas of Gran Chaco area and Pampas region to investigate the variability of precipitation, surface energy and water budget in greater detail. The interannual variability of precipitation in these simulations are dictated strongly by moisture flux convergence particularly over ARB and ITCZA while surface evaporation becomes equally important over Nordeste and sub-tropical areas.

The Low Level Jet (LLJ) in the simulation is observed to be largely modulated by the subtropical high over the Atlantic. A linear relationship between the LLJ and convection over Amazon River Basin, ITCZ over Atlantic, Nordeste, Gran-Chaco area, Pampas region is nearly non-existent at synoptic and diurnal time scales. To further understand their relationship we are conducting sensitivity experiments with the RSM which has a suitable feature of not having a runaway climate in the absence of deep convection.

A Simplified Simple Biosphere (SSiB) following Xue et al. (1991) is implemented in the RSM. In addition to looking at the suface energy and water budgets , the impact of SSiB will be assessed on surface, near-surface variables and the variability in the troposphere.