The recirculation of the intermediate western boundary current at the Tubarão Bight – Brazil

Lagrangian floats and current meter measurements from two moored arrays are analyzed, in combination with altimetry data, in order to investigate the recirculation of Antarctic Intermediate Waters (AAIW), and of the Intermediate Western Boundary Current (IWBC) at the Tubarao Bight, in the vicinity of the Vitoria-Trindade Ridge (VTR), Brazil. Results from a high-resolution numerical simulation provide a complementary view of the flow at intermediate and surface levels. The data depict a topographically-induced cyclonic recirculation at intermediate levels, and five Argo floats are successively trapped inside the bight for two-and-a-half years, performing a total of 10 closed clockwise gyres during this period of time. In situ measurements at the western side of the bight show an intense alongshore flow at intermediate levels, with averaged velocities at 800 m of ~30 cm/s, and peak velocities exceeding 50 cm/s, magnitudes comparable to the Brazil Current (BC) flow at surface levels. The recirculation extends from at least 1000 m deep up to 370 m, reaching sometimes depths as shallow as 150 m, but is mostly uncoupled from the surface flow during the one-and-a-half year long current meter record. Three different flow patterns are observed, and simulated, at surface levels inside the bight during the time the recirculation is well established at intermediate levels: a shallow cyclonic circulation, somewhat akin to the Vitoria Eddy; a recurrent anticyclonic flow that encompasses the entire bight; and a southwestward-oriented circulation, associated with the BC being reorganized in a coherent flow after negotiating its way through the VTR channels. A significant portion (about 50% according to the model) of the inflow of intermediate waters recirculates, enhancing the flow of the IWBC within the bight, and increasing the age of AAIW that will eventually cross the VTR on its way to lower latitudes. Although the data are not conclusive about a preferential pathway of the intermediate flow across the VTR, the simulation indicates that up to 70% will exit through the main channel located between the Besnard Bank and the Vitoria Seamount, while 30% will follow eastward to the south of the VTR, in a region of intense mesoscale activity, and then cross the ridge farther east.