A Diagnosis of Rainfall over South America during the 1997/98 El Niño Event. Part II: Roles of Water Vapor Transport and Stationary Waves

Abstract

A diagnosis of rainfall over South America (SA) during the 1997/98 El Niño year is made examining the roles of water vapor transport and stationary waves. It is found that the low-level jet (LLJ) on the eastern side of the central Andes is stronger during the El Niño event and transports more moisture. This seems to be the source for higher rainfall over southeast SA noted during the El Niño years. A calculation of three-dimensional stationary wave activity (Fs) for 1997 and 1998 showed that in the summer of 1996/97 stationary waves propagate poleward and eastward from midlatitudes into the higher latitudes to the west of south SA and then propagate equatorward to the east of SA. During the autumn of 1997, the vertical component of Fs is consistent with a blocking high over the southeast Pacific. To the east of this high cold air advection from Antarctica occurs, and to the west warm air advection occurs. This is consistent with negative and positive centers seen in the vertical component of Fs to the east and west of south SA. The rainfall anomalies during a particular season seem to be due to multiple causes and this complicates a direct connection between them. Although over the southeast of SA, the higher rainfall during 1997 and 1998 can be attributed to some specific causes such as higher water vapor transport by LLJ in summer of 1997/98, in other parts such an association was not possible.