WIND-FORCED VARIATIONS IN THE BRAZIL-MALVINAS CONFLUENCE REGION AS SIMULATED IN A COARSE RESOLUTION NUMERICAL-MODEL OF THE SOUTH-ATLANTIC

The observed seasonal and interannual fluctuations in the Brazil-Malvi nas confluence region are investigated using a wind-driven isopycnic c oordinate model of the South Atlantic Ocean south of 10-degrees-S. The model is configured on a rotated Mercator grid with 2-degrees horizon tal resolution and five constant-density layers in the vertical. In or der to model the passage of the Antarctic Circumpolar Current (ACC) ac ross the basin, the grid is augmented by a channel extension to the we st of Drake Passage and east of 50-degrees-E, having the width of Drak e Passage. A series of benchmark experiments with annual mean climatol ogical forcing shows that (1) when bottom topography is included, one observes a reduction in Drake Passage transport in agreement with prev ious studies, as well as a northward shift in the Brazil Current separ ation latitude, (2) an increase in Drake Passage transport to realisti c values does not cause any further northward shift in the separation point, and (3) the model is relatively insensitive to the choice of la teral boundary conditions. A second set of experiments, in which the f orcing is by seasonal climatological wind data and the Drake Passage t ransport is relaxed to a constant annual mean value, indicates the pre sence of a semiannual signal in the annual transport cycles for the Ma lvinas Current and for the ACC through Drake Passage. That signal is s ignificantly damped in the Brazil Current region, and the amplitude of each cycle is reduced in comparison to observations. When the value t o which the Drake Passage transport is relaxed is allowed to vary in t ime, the semiannual wind-forced oscillation in the Malvinas region rem ains evident, with additional superimposed variations related to the v ariations in the transport forcing. The final experiments are forced b y a 10-year data set of realistic wind stress values, providing 10-yea r time series of model output for analysis of the interrelationships o f the principal Southwestern Atlantic currents. A high correlation at the semiannual period is found to exist among the cycles of Drake Pass age transport, Malvinas Current transport, and seasonal movements of t he Brazil-Malvinas confluence latitude, while the Brazil Current trans port cycle exhibits a significant energy peak only at the annual perio d. We conclude that the locally wind-forced semiannual signal south of the confluence is significantly damped before reaching the Brazil Cur rent region by several factors: friction, the opposing flow of the cur rent itself, and the inability of the Malvinas to penetrate the subtro pical circulation that is confined to the upper model layers.