Upper layer currents in the western tropical North Atlantic (1989-1991)

Shipboard acoustic Doppler current profiler (ADCP) measurements and hydrographic observations of temperature, salinity, and dissolved oxygen are used to examine the upper water column flow field in the North Brazil Current (NBC) retroflection region of the western tropical Atlantic Ocean. Observations are presented from six cruises, one conducted in August 1989 and the other five conducted during the Western Tropical Atlantic Experiment between January 1990 and September 1991. The upper water column is divided into two layers, an upper thermocline layer located between the surface and the 24.5 sigma(theta) isopycnal surface and a lower subthermocline layer located between the 24.5 and 26.75 isopycnals. In the upper layer the NBC retroflects north of the equator to form the eastward flowing North Equatorial Countercurrent (NECC). During the six cruises the retroflection appeared complete. However, data coverage did not extend shoreward of the 200-m isobath, so the possibility of a continuous flow over the shelf still remains. There were also indications of several NBC rings that had apparently separated from the NBC retroflection and drifted to the northwest toward the eastern Caribbean Sea. North of the NBC retroflection and the NECC, the North Equatorial Current (NEC) flows west as the southern limb of the subtropical gyre. Part of the NEC is observed to retroflect cyclonically to join the eastward NECC flow. In the lower layer, beneath the NBC, the North Brazil Undercurrent retroflects to feed the eastward North Equatorial Undercurrent (NEUC). To the north a deeper component of the NEC recurves to also contribute to the NEUC.