Upper layer circulation in the western tropical North Atlantic Ocean during August 1989


  • W. Douglas Wilson,

  • E. Johns,

  • R. L. Molinari


Shipboard acoustic Doppler current profiler velocity measurements and profiles of temperature, salinity, and dissolved oxygen measurements are used to map the flow field above the σθ = 26.8 isopycnal (approximately the upper 300 m of the water column) in the North Brazil Current (NBC) retroflection region (0° to 14°N, 60° to 40°W) during August of 1989. The water column is divided into a near-surface, upper thermocline layer (above σθ = 24.5) and a main to subthermocline layer (σθ = 24.5 to σ0 = 26.8). In the upper layer the eastward flowing North Equatorial Countercurrent (NECC) is composed of 16×106 m3/s of NBC transport that has retroflected from the coast between 6°N and 8°N and 8×106 m3/s of North Equatorial Current (NEC) transport returning eastward. An anticyclonic eddy with a 10×106 m3/s transport lies northwest of the NBC retroflection. No throughflow is observed along the boundary, and only a small portion of the observed NEC transport (1.5×106 m3/s) enters the Caribbean Sea. In the lower layer the NBC transports 8×106 m3/s into the subthermocline North Equatorial Undercurrent (NEUC). In addition, 15×106 m3/s joins the NEUC from the north. At 44°W the subsurface core of the NEUC lies south of the near-surface core of the NECC. Nearly half of the NEUC transport is made up of a mixed water type with salinity-oxygen (S-O2) characteristics intermediate to the characteristics of the original component transports. There is no evidence of continuous NBC flow into the Caribbean Sea in the lower level. The closed eddy to the north of the retroflection, however, contains water masses with South Atlantic S-O2 properties.