Variability of South American Convective Cloud Systems and Tropospheric Circulation during January–March 1998 and 1999

Abstract

A comparison of the submonthly variability of atmospheric circulation and organization of convection in South America during January-February-March of 1998 (JFM98) and January-February-March of 1999 (JFM99) is presented. According to the National Centers for Environmental Prediction reanalysis, the South American low-level jet (SALLJ) was about twice as strong during JFM of the 1998 El Nin o episode than during JFM of the 1999 La Nin a episode. The difference in SALLJ strength between these two years translated into stronger transport of moist tropical air into the subtropics during JFM98 than during JFM99. An objective tracking technique was used to identify large, long-lived convective cloud systems in infrared imagery. The stronger SALLJ was accompanied by larger and more numerous long-lived convective cloud systems and nearly twice as much rainfall in subtropical South America (parts of southern Brazil, Uruguay, and Argentina) during JFM98 than during JFM99.