Dynamics of the North Brazil Current retroflection region from the Western Tropical Atlantic Experiment observations

Hydrographic and velocity observations of the North Brazil Current (NBC) retroflection ree;ion during the 1990-1991 Western Tropical Atlantic Experiment (WESTRAX) are examined with the intent of extracting dynamical information about the NBC eddy shedding. A comparison is performed between the depth structure of empirical orthogonal functions and the dynamical normal modes of the NBC retroflection region on a beta plane centered on 5 degreesN. The barotropic and first two baroclinic modes account for about 75% of the vertical structure of the NBC flow. Thus, a three-layer model of the NBC region is suitable for dynamical studies of the NBC retroflection. In terms of flow structure the upper layer represents the retroflection of the surface layers of the NBC to feed the North Equatorial Countercurrent. The middle layer represents the separating subthermocline waters of the NBC which feed the North Equatorial Undercurrent. The third layer represents a weak meandering flow that may be thought of as the Deep Western Boundary Current signature in the three-layer ocean. In terms of PV, a well-defined front separates the NBC waters from the North Equatorial Current in the upper layer. Both upper and middle layers present closed PV contours associated with the eddy in the process of pinching off from the retroflecting NBC. The NBC separation region, although equatorial, complies reasonably well with the basic quasigeostrophic (QG) assumptions. Therefore QG methods are applied to investigate the NBC meander growth. By isolating the effect of PV anomalies in each of the layers on each of the layers, baroclinic growth is verified to occur during the NBC eddy shedding.