JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. Cll, PAGES 22,501-22,512, NOVEMBER 15, 1994 On the origins of the North Brazil Current ilson C. A. da Silveira Ocean Process Analysis Laboratory, Institute for the Study of Earth, Oceans and Space University of New Hampshire, Durham Luiz B. de Miranda Departamento deOceanografia F•sica, Instituto Oceanogr•fico, Universidade deS•oPaulo, S•o Paulo, Brazil Wendell S. Brown Ocean ProcessAnalysis Laboratory, Institute for the Study of Earth, Oceans and Space University of New Hampshire, Durham Abstract. The analysis of the hydrographic data obtained during the "Nordeste III" (NEIII) survey by the R/V Altairante Saldanha (BrazilianNavy) between May 4 and July 12, 1986, is reported in this work. Distributions of geostrophic speeds and transports (relative to 1000m) reveal the North Brazil Current (NBC) and other major features of the tropical Atlantic circulation west of 30ø30'W between 4øS and 10ø30'S.The well-developed NBC appears in all analyzed cross- isobath sections borderingthe Brazilian continentalslopebetween 10øSand 5øS, transporting .anaverage of 21 Sv. The velocity coreof the NBC is foundto 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 studyregion at about 5øS-6øS and separates into two portions. The northern portion of the CSEC transports about 15 Sv northwestward, and north of 5øS,it coalesces with the NBC to form a 300-km-wide current that transports more than 36 Sv equatorward. The southernbranch of the CSEC transports 14 Sv southwestward until about 9ø30'S, where it makes a cyclonic turn to the north and merges with northward flow from the southernmost branch of SouthEquatorialCurrent (SSEC). The high-salinity signature of watersfrom the SSEC appears in the NBC, indicatingthat 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. 1. Introduction The western tropical Atlantic is a dynamically rich region and the origin of many important features of Atlantic Ocean circulation. One of those features is the eastward flowing Equatorial Undercurrent (EUC), which is fed by the North Brazil Current(NBC) [Met- calf and Sialcup, 1967]as it flowsalongthe Brazilian coast near the equator. The North Equatorial Un- dercurrent (NEUC) is fed by the subthermocline lay- ers of the NBC, which permanently retrofiect anticy- clonically between 3øN and 5øN [Cochrane et al., 1979; Peterson and Slramma, 1990]. The North Equatorial Countercurrent (NECC) is fed by the upper layersof the NBC, which retrofiect between 5øN and 8øN from Copyright 1994 by the American Geophysical Union. Paper number 94JC01776. 0148-0227]94]94JC-01776505.00 July through September [Richardson andMcKee,1984; Richardson and Walsh, 1986;Johns et al., 1990]. The NBC itself originates south of the equator where the South Equatorial Current (SEC) approaches the South American coast (Figure 1). However,the ex- act location of the NBC origin is not well defined be- cause the SEC has a rather complexstructure in this re- gion. Molinari [1982] and $tramma [1991] described the SEC as a westward flow with three distinct branches, namely, a northern branch (the NSEC), a central branch (the CSEC), and a southern branch(the SSEC). The NSEC and the CSEC are separated by the eastward flowing South EquatorialUndercurrent (SEUC), usu- ally foundbetween 3øSand 5øS[Molinari et al., 1981; Molinari, 1982]. The CSEC is bounded to the south by the South Equatorial Countercurrent (SECC), which is locatedbetween 7øS and 9øS [Molinari, 1982; Pe- tersonand Siraroma,1990; Stramma,1991]. Molinari [1982] estimated the geostrophic transport (relative to 1000m) of the CSEC to range between 7 and 26 Sv (1 22,501