Retroflections of the North Brazil Current (NBC) are examined using observational data collected in the western tropical Atlantic between 35W-44W and 10S-7N. ADCP, CTDO2 and XBT 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 isopycnals. With respect to earlier work, the data show that (1) the South Equatorial Undercurrent (SEUC) is primarily fed by waters from a recirculation gyre on its northern boundary with some minor transport retroflecting from the North Brazil Undercurrent (NBUC); (2) the Equatorial Undercurrent (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 North Equatorial Countercurrent (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 44W 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 44W 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 44W and 41W separates into the NEUC and EUC by 35W, with the former composed of northern hemisphere waters and the latter southern hemisphere waters; (3) demonstration that the precursor to the EUC (NEUC) at 44W and 41W accelerates (decelerates) by 35W; and (4) depiction of the potential vorticity (PV) field showing that the PV front at 44W has separated into a front of higher PV values coincident with NEUC and a low PV core coincident with the EUC at 35W.

On the second part of this thesis a reduced-gravity model was applied to the upper tropical Atlantic basin with a simplified domain. Three main experiments were done, in which the wind-driven and thermohaline circulation (MOC) interaction could be studied: (1) MOC-only experiment ({\bf MOC}); (2) Wind-only experiment ({\bf VENTO}); and (3) Wind+MOC experiment ({\bf VMOC}). In {\bf MOC} a 6 Sv MOC flux was applied along the western boundary. Eddies were generated by barotropic instability and detached northward within 100 km from the equator. The eddy generation presented a hysteresis-like behavior with a critical Reynolds number of about Re_c=60. In {\bf VENTO}, Subtropical and Tropical Cells (STCs and TCs) were generated, and they feed the equatorial region almost symmetrically between the northern and southern hemisphere. Their interaction with the MOC ({\bf VMOC}) cause a stronger contribution from the southern hemisphere and, in the model, the SEUC and the NEUC are stronger and may caused by a meandering jet along the equator, modulated by Tropical Instability Waves. The temperature pattern in the mixed layer for the {\bf VMOC} experiment shows lower temperatures over the whole tropical basin, but the heat transport increases northward compared to {\bf VENTO}.
Simulations of extreme warm and cold events were made, changing the climatological wind forcing by the composites of warm and cold years. For the cold (warm) year, due to the increase (decrease) of the equatorial upwelling, there is a reinforcement (weakening) of the TCs and STCs along the basin.