Dynamics of the Brazil Current Meandering along Southeast Brazil

The Brazil Current (BC) presents intense meandering and eddy activity as it flows along the Brazilian Southeast Coast. The presence of large amplitude meanders has been reported from the region adjacent to Cape Sao Tome (22^oS) to Cape Santa Marta Grande (30^oS).

In this oceanic region, the BC eddy activity seems to occur in two different forms. The first is the formation of large amplitude either cyclonic or anticyclonic meanders off Cape Sao Tome and Cape Frio that eventually pinch off and form rings [ Mascarenhas et al., 1971; Signorini, 1977, and Garfield, 1990]. The second occurs south of Cape Frio with the formation of a mushroom-like structure composed of pairs of cyclonic and anticyclonic vortices on the BC within the so-called South Brazil Bight (25^oS) [ Campos et al., 1996]. In this location, the formed eddies seem not to separate from the current axis.

At the latitude of Cape Frio (23 S), the coastline orientation changes abruptly from a nearly north-south to a east-west direction. This inflection point at Brazilian coast is thought to be the location where the flow associated with Antarctic Intermediate Water (AAIW) bifurcates at subpycnocline levels [ Muller et al., 1998]. Hence, north of Cape Frio, the AAIW flow northward opposing the southward flowing BC formed by Tropical Water (TW) and South Atlantic Central Water (SACW) above it. South of Cape Frio, the AAIW flows in the same direction of the TW and SACW above and the NADW underneath it. Therefore, it is likely that instability processes are baroclinic north of Cape Frio where the vertical shear of the BC and AAIW is evident. Within the South Brazil Bight, where BC and AAIW flow poleward along the continental slope, it has been speculated that barotropically unstable topographic Rossby waves account for the eddy mushroom-like structure depicted in quasi-synoptic hydrographic data and satellite images.

In this work, we test the ideas above using historical hydrographic data sets and others from ongoing efforts by the Brazilian and international community to investigate the still relatively unexplored dynamics of the BC, as it flows along the Brazilian Southeast Coast. We employ the quasi-geostrophic theory to decompose the Brazil Current region into dynamical normal modes and evaluate linear stability properties via theoretical models such as Johns [1988].