Simulations of the hydrological cycle over southern south America using the CPTEC/COLA AGCM

The La Plata River basin is the second largest basin of South America after the Amazon basin, and it is located in an international area that occupies territories of Argentina, Uruguay, Brazil, Paraguay, and Bolivia, areas of great economic activity. In the present work, the water budget over the region was studied and the atmospheric and terrestrial components were analyzed to investigate the Center for Weather Forecast and Climate Studies/Center for Ocean-Land-Atmosphere Studies (CPTEC/COLA) atmospheric general circulation model (AGCM) behavior in a simulation of 10 yr. The analysis was performed considering two sectors, northern and southern, because of their different behaviors, and the main characteristics were simulated by the model. The northern sector presents a well-defined annual cycle with well-established wet conditions in the summer, when there is development of the South Atlantic convergence zone (SACZ). In the southern sector, there is a weak annual cycle and the hydrological variables do not have large seasonal variations as in the northern sector. A period of maximum precipitation is identified at the end of spring in the southern sector. Moisture flux convergence (MFC) in the model occurs over southeast Brazil during summer and over northern Argentina in the spring, consistent with observations. Analysis of the meridional and zonal moisture fluxes reveals that there is an intrusion of moisture from the tropical region, southward, and also flux from the Atlantic Ocean, that feed both sectors. The flux from the Amazon region was the main source of external moisture during the summer season, while the flux from the Atlantic Ocean was dominant during winter. Additional analysis of El Nino 1982/83 and La Nina 1988/89 episodes showed the importance of Amazon and Atlantic moisture fluxes to the La Plata basin region. During El Nino 1982/83 there was more moisture flux from the Amazon region to the southern sector than during La Nina 1988/89. This feature was related to droughts during La Nina and floods during El Nino in the La Plata region.