Does a Monsoon Climate Exist over South America?

Abstract

The climatology and the basic state of the summertime circulation and rainfall over South America are studied using assimilation products from the data assimilation system of Goddard Earth Observing System-1 (GEOS-1) and satellite-derived rainfall. Results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical South American highland centered at the Altiplano Plateau. Sensitivity of the results to the assimilation scheme is tested by comparing that with the National Centers for Environmental Prediction (NCEP) reanalysis and with satellite rainfall estimates. Results show general agreement between the model produced rainfall anomaly and the satellite estimates, as well as consistency between the basic circulation features in the GEOS-1 and the NCEP reanalyses. A case study of 1989–90 South American summer monsoon (SASM) reveals the following characteristics. 1) In late spring, the onset of SASM is signaled by an abrupt merging of the upper-tropospheric double westerly jets, one in the subtropics and the other in the subpolar region, into a single jet in the midlatitudes. This is followed by the establishment of a vortex to the southeast of Altiplano and occurrence of heavy precipitation over subtropical eastern Brazil. 2) During the mature phase of SASM, the heavy rainfall zone moves over the Altiplano Plateau and the southernmost Brazilian highland. The fully established SASM features are the following: (a) an enhancement of equatorial North Atlantic trade wind, which emanates from the Sahara high and crosses the equator over the South American continent; (b) a buildup of strong northwesterlies along the eastern side of the tropical Andes;and (c) development of the South Atlantic convergence zone in the southernmost position with strong convective activity. Meanwhile, the upper-tropospheric return flow emerges from an anticyclone formed over the Altiplano Plateau, crosses the equator, and sinks over northwestern Africa. 3) The withdrawal of SASM in late summer is signaled by the resplitting of the midlatitude westerly jet. At the same time, the low-level northwest monsoon flow diminishes, reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland. Results also show that the above-mentioned characteristics of SASM are clearly linked to the tropospheric temperature changes over the central South American highland. Sensible versus latent heating over the highland are bound to play an important role in the evolution of SASM. To provide further support of presence of a monsoon climate over South America, SASM is compared and contrasted to the “classic” east Asian summer monsoon (EASM). Many similar features, including evolution characteristics between the two systems, have been identified. Contrasting aspects of the SASM from the EASM are also discussed. It is pointed out that a number of monsoonal characteristics of the climate of South America, such as the seasonal reversal of the low-level wind, become apparent only when the strong annual mean wind is removed. Based on the characteristic features and their evolution, the authors conclude that a monsoon climate does exist over South America.