Investigation of seasonal prediction of the South American regional climate using the nested model system

[1] The National Centers for Environmental Prediction (NCEP) regional Eta model was nested in the NCEP Atmosphere General Circulation Model (AGCM) to provide an assessment of the dynamic downscaling method in seasonal climate simulations over South America (SA). This study examined 15 seasons, summer and winter, of significant El Nino-Southern Oscillation years. Results showed that the nested model improved the simulation of seasonal mean lower and upper level circulations in both warm and cold seasons. The summer mean meridional wind along 20 degrees S in the nested model simulation exhibited two jet cores, one located just east of the Andes associated with the South American low level jet (SALLJ) and a second, shallower jet located over eastern Brazil. Conversely, winter cross sections showed only the SALLJ, with higher wind speeds. These features are consistent with observation. AGCM results did not show any noticeable jet structure in either season. The nested model also produced better simulations of seasonal mean rainfall and 2-m temperature. Summer (winter) precipitation root-mean-square error averaged over the entire continent was reduced from 2.09 (2.97) mm d(-1) in the AGCM to 1.37 (2.33) mm d(-1) in the nested model. The improvement persisted during the entire simulation period over selected areas of the continent. Overall, the Eta model reduced the rainfall positive bias over northern SA and negative bias over the subtropics in the AGCM simulation because of better representation of moisture transport by the SALLJ. However, the simulation of the Intertropical Convergence Zone by the nested model was weak because of weak moisture flux convergence. Its possible causes are discussed in the paper.