Global and Local Circulations Associated with South American Summer Precipitation

摘要

Precipitation regimes over South America involving variations of one to two weeks are documented using 15 years of gridded data, 20 years of outgoing longwave radiation (OLR) measurements obtained from satellites and 6 years of cloud data from the International Satellite Cloud Climatology Project (ISCCP). The gridded data were produced by an assimilation system from the European Center for Medium-range Weather Forecasts (ECMWF). Comparisons of assimilated OLR fields with those obtained from satellite observations show model skill in reproducing tropical patterns of convection. The 6-month atmospheric evolution of the austral summer is discussed, based on the ECMWF reanalysis, with a focus on circulations over South America. The South Atlantic Convergence Zone (SACZ) is a typical feature of summer convection over South America, and it appears well-described by the gridded data. Fourteen-year averages are used to define the seasonal evolution every 2 weeks, and averages are removed from individual times to define anomalies. Previous research has shown the existence of a meridional 'seesaw' of alternating wet and dry events over tropical and subtropical South America. This investigation focuses on 2-week modulations of the SACZ with the goal of ascertaining the dependence of precipitation regimes over South America on convection over the Central Pacific Ocean. This is accomplished using empirical orthogonal function analyses of filtered OLR anomalies confined to two regions: one includes the South Pacific Convergence Zone (SPCZ), extending from the dateline eastward to 20 E; the other extends from 100 W to 0 W, including South America and the SACZ. Four convective patterns are identified: those with an enhanced/suppressed SACZ with a well- defined SPCZ (referred to as 'with SPCZ'), and those with an enhanced/suppressed SACZ without a clear composite.