On the origins of the North Brazil Current

Abstract

The analysis of the hydrographic data obtained during the “Nordeste III” (NEIII) survey by the R/V Almirante Saldanha (Brazilian Navy) between May 4 and July 12, 1986, is reported in this work. Distributions of geostrophic speeds and transports (relative to 1000 m) reveal the North Brazil Current (NBC) and other major features of the tropical Atlantic circulation west of 30°30′W between 4°S and 10°30′S. The well‐developed NBC appears in all analyzed crossisobath sections bordering the Brazilian continental slope between 10°S and 5°S, transporting an average of 21 Sv. The velocity core of the NBC is found to be centered at about 100–200 m, instead of at the surface, as previously thought. The central branch of the South Equatorial Current (CSEC) enters the study region at about 5°S–6°S and separates into two portions. The northern portion of the CSEC transports about 15 Sv northwestward, and north of 5°S, it coalesces with the NBC to form a 300‐km‐wide current that transports more than 36 Sv equatorward. The southern branch of the CSEC transports 14 Sv southwestward until about 9°30′S, where it makes a cyclonic turn to the north and merges with northward flow from the southernmost branch of South Equatorial Current (SSEC). The high‐salinity signature of waters from the SSEC appears in the NBC, indicating that it is formed to the south of the study area. By contrast, historical ship drift and Lagrangian drifter observations suggest that the NBC and the Brazil Current are formed at the bifurcation of the CSEC. This apparent contradiction is explained by the surface Ekman drift which obscures the geostrophic flow patterns.