Retroflections of the North Brazil Current during February 2002

Retroflections of the North Brazil Current (NBC) are examined using observational data collected in the western tropical Atlantic between 35°W–44°W and 11°S–7°N. Acoustic Doppler Current profiler, conductivity temperature depth oxygen and expendable Bathythermograph data were obtained during a cruise conducted in February 2002. The water column was separated into two layers, the upper layer from the sea-surface to the 24.5 isopycnal and lower layer from the 24.5 to the 26.8 isopycnal. The upper layer includes the NBC, the South Equatorial Current, the North Equatorial Countercurrent (NECC) and a portion of the Equatorial Undercurrent (EUC). The lower layer includes the North Brazil Undercurrent (NBUC), the South Equatorial Undercurrent (SEUC), the North Equatorial Undercurrent (NEUC) and another portion of the EUC. With respect to earlier work, the data show that (1) the SEUC is primarily fed by waters from a recirculation gyre on its northern boundary with some minor transport retroflecting from the NBUC; (2) the EUC is fed primarily by southern hemisphere waters that retroflect from the NBC, but there is some northern hemisphere water in the near surface flow that connects the EUC and NEUC during this cruise; (3) the NECC during this cruise has no connection to the NBC and is fed by northern hemisphere waters originating in the North Equatorial Current; and (4) the majority of the upper layer NBC that crosses 44°W does not appear to retroflect from the boundary farther north to return equatorward. New results include: (1) quantification, for this period, of the amount of transport in the single current core at 44°W that is comprised of waters from the southern (9.2 Sv) and northern hemispheres (5.2 Sv); (2) demonstration that the single core of eastward flow at 44°W and 41°W separates into the NEUC and EUC by 35°W, with the former composed of northern hemisphere waters and the latter southern hemisphere waters; (3) demonstration that the precursor to the EUC (NEUC) at 44°W and 41°W accelerates (decelerates) by 35°W; and (4) depiction of the potential vorticity (PV) field showing that the PV front at 44°W has separated into a front of higher PV values coincident with NEUC and a low PV core at 35°W coincident with the EUC.