Wind‐forced variations in the Brazil‐Malvinas confluence region as simulated in a coarse resolution numerical model of the South Atlantic

Abstract

The observed seasonal and interannual fluctuations in the Brazil‐Malvinas confluence region are investigated using a wind‐driven isopycnic coordinate model of the South Atlantic Ocean south of 10°S. The model is configured on a rotated Mercator grid with 2° horizontal resolution and five constant‐density layers in the vertical. In order to model the passage of the Antarctic Circumpolar Current (ACC) across the basin, the grid is augmented by a channel extension to the west of Drake Passage and east of 50°E, having the width of Drake Passage. A series of benchmark experiments with annual mean climatological forcing shows that (1) when bottom topography is included, one observes a reduction in Drake Passage transport in agreement with previous studies, as well as a northward shift in the Brazil Current separation latitude, (2) an increase in Drake Passage transport to realistic values does not cause any further northward shift in the separation point, and (3) the model is relatively insensitive to the choice of lateral boundary conditions. A second set of experiments, in which the forcing is by seasonal climatological wind data and the Drake Passage transport is relaxed to a constant annual mean value, indicates the presence of a semiannual signal in the annual transport cycles for the Malvinas Current and for the ACC through Drake Passage. That signal is significantly damped in the Brazil Current region, and the amplitude of each cycle is reduced in comparison to observations. When the value to which the Drake Passage transport is relaxed is allowed to vary in time, the semiannual wind‐forced oscillation in the Malvinas region remains evident, with additional superimposed variations related to the variations in the transport forcing. The final experiments are forced by a 10‐year data set of realistic wind stress values, providing 10‐year time series of model output for analysis of the interrelationships of the principal Southwestern Atlantic currents. A high correlation at the semiannual period is found to exist among the cycles of Drake Passage transport, Malvinas Current transport, and seasonal movements of the Brazil‐Malvinas confluence latitude, while the Brazil Current transport cycle exhibits a significant energy peak only at the annual period. We conclude that the locally wind‐forced semiannual signal south of the confluence is significantly damped before reaching the Brazil Current region by several factors: friction, the opposing flow of the current itself, and the inability of the Malvinas to penetrate the subtropical circulation that is confined to the upper model layers.