Thermohaline fronts and baroclinic flow in the Argentine basin during the austral spring of 1984

Abstract

Thermohaline fronts, structure, and baroclinic flow in the central Argentine basin are investigated on the basis of a 1984 field experiment. The Brazil Current, after initial overshoot, meanders northeastward toward subtropical latitudes with speeds of the order of 0.3 m s−1. The meanders have a wavelength of about 400 km and an amplitude of 200 km. Brazil Current signatures, as expressed by dynamic height, are recognizable to depths of several kilometers. The Brazil and Antarctic Circumpolar currents do not meet in the central Argentine basin to form common eastward flow, as was expressed in classical descriptions, but instead diverge sharply near 42°W. This is seen also in the trajectories of satellite‐tracked drifters. The region between the currents is marked by cyclonic and anticyclonic eddies. Strong thermohaline fronts accompany the boundaries of these currents. The Brazil Current and subantarctic fronts are well separated in the central basin. Brazil Current density fronts are deep and extend from the surface to 3000 m, while the associated temperature and salinity fronts are intermittent over this depth interval. Temperature fronts virtually vanish at the interface between the Antarctic Intermediate Water and the North Atlantic Deep Water. Salinity fronts reverse their polarity beneath the core of the former. At depths between 3000 and 4000 m, abyssal temperature and salinity fronts are observed which are largely density compensating. At the subantarctic and cold core eddy fronts, horizontal temperature and salinity gradients in the upper mixed layer compensate each other in such a way that no density front is found. Deep subpycnocline mixed layers occur in the poleward lobes of the Brazil Current during austral spring, suggestive of previous winter convection.