Seasonal variability in the southwestern Atlantic

Abstract

The circulation of the southwestern Atlantic Ocean is dominated by the Subtropical Gyre and the confluence of the Brazil and Malvinas currents. Observations indicate that the latitude of this confluence changes seasonally, lying farther north during the austral winter than during the summer. This phenomenon has important consequences for the local climate and marine population, as the latitude of the confluence also marks the boundary between the warm waters of the subtropical gyre and the cold waters of the Antarctic Circumpolar Current. We present evidence that these seasonal migrations may be related to changes in the transport of both the Brazil and Malvinas currents. A numerical model forced by climatological wind stress indicates that the transport of the Brazil Current decreases during winter months and increases during summer months. Geosat altimeter data corroborate the model results and also indicate that the transport of the Malvinas Current undergoes a seasonal cycle with phase opposite to that of the Brazil Current. Our hypothesis is that during the austral summer, a southward displacement of the latitude of the confluence is coincident with an acceleration of the flow in the subtropical gyre and a weakening of the transport of the Malvinas Current. This situation reverses during the winter when the Malvinas Current grows stronger, the Brazil Current transport decreases, and the latitude of the confluence of these two currents moves northward.