Connections between Agulhas Leakage and the South America's Climate - a study using a Coupled Ocean-Atmosphere Model

Abstract

The climatic conditions in northern and northeastern Brazil, and much of South America, are related to ocean-atmospheric interactions in the Tropical Atlantic. These interactions in the tropics are in turn affected by signals originated in extra-tropical regions, transported by ocean currents through the subsurface. Studies with low resolution coupled models show, for example, that the position of the Atlantic Tropical ITCZ can be changed in response to anomalies which arise in remote areas and propagate inside the ocean thermocline, reappearing in the equatorial belt. With the possibility of increase of heat transport from Pacific to Indian Ocean, through the Indonesian Throughflow (ITF), and of waters carriage from Indian Ocean to the South Atlantic via a phenomenon known as the Agulhas Leakage, as suggested by several recent studies, it is natural to ask what would be the impact of these increases on the import of the relatively warmer and more saline waters of the Indian on the South Atlantic circulation and, consequently, on the ocean-atmospheric interactions and also on the regional climate. Substantial part of the Agulhas leakage occurs through large rings formed by mesoscale instabilities in the Agulhas Current retroflexion. Also, the ITF region comprises an amount of different dimensions' small islands, straits and passages of different depths. Both processes are, so, strongly nonlinear ones, and they are not properly resolved by climate models. Thus, this work proposes to use general circulation models where the oceanic component has enough horizontal resolution to solve mesoscale processes ("eddy-resolving") with intent to study the ocean response to different recent changes on the global climate system and the impact of ocean's changes on the atmosphere in the tropical Atlantic and on the regional climate. Therefore, firstly some results of a high resolution global implementation of HYCOM (Hybrid Coordinate Ocean Model) in two runs, one using climatological means and the other with interannual variability in the atmospheric forcing, will be analyzed. Next, applying the same data, numerical experiments simulating Lagragian particles will be implemented, in order to identify and quantify water masses' origins compounding the ITF and to correlate variabilities signals of ITF on the Indian basin circulation and on the Agulhas Leakage. Finally, output data of a run for the XX century of the coupled model Community Earth System Model, Version 1 (CESM1), from the National Center for Atmospheric Research (NCAR), will be used to correlate the obtained results with variabilities in the South Atlantic and the climate in South America.