Summertime Intraseasonal Variability over Tropical South America

Using the Goddard Earth Observing System Reanalysis and rainfall data from CPC (Climate Prediction Center) Merged Analysis of Precipitation (CMAP) we investigate the source of intraseasonal variation (ISV) over tropical South America during summertime. Rotated EOF analysis of 30-60 day band-passed 850-hPa velocity potential reveals that the first two leading modes are closely related to the Madden-Julian Oscillation (MJO). The third mode, which accounts for a large portion of intraseasonal variances over tropical South America, suggests a local forcing mechanism. The variation of the total ISV over tropical South America is well represented by a reconstruction using the first two propagating modes and the third stationary mode. The dynamical structure and the excitation source are also investigated. The flow pattern in association with MJO is consistent with previous studies. The local forcing of the third mode corresponds to the South American summer monsoon (SASM) oscillation, which has been demonstrated in our previous 1989-1990 SASM case study (Zhou and Lau, 1998). Our results suggest that the tropical South American region possesses a source of ISV that is distinct from that due to the MJO. The physical mechanism of this ISV is being investigated.