Circulation and Cross‐Shelf Exchanges in the Northern Shelf Region of the Southwestern Atlantic: Kinematics

A high‐resolution ocean model is used to characterize the circulation and cross‐shelf exchanges in the Brazilian/Uruguayan portion of the southwestern Atlantic shelf. Cross‐shelf exchanges are strongly modulated by the bottom topography. There is ∼1.2 Sv of on‐shelf transport between 21°S and 25.2°S, and ∼1.6 Sv of off‐shelf transport between 35°S and 25.2°S. North of 25.2°S, the cross‐shelf exchanges show a two‐layer structure with an off‐shelf flow in the upper 50m and on‐shelf flow deep below. A Lagrangian diagnostic shows that ∼0.15 Sv of deep waters from the Brazil Current (z > 200 m) are injected into the shelf. Mixing with ambient waters produces a spicier (warmer and saltier) water mass, which is ejected into the open ocean in the southern region. Backward in‐time particle's trajectories analysis reveals that 95% of the southward shelf transport at 32°S originates in the open ocean at 22°S. Our model diagnostics show that there is a very limited connectivity between the shelf regions north and south of Cabo Frio. Correlation analysis shows no significant influence of El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) on the time variability of the cross‐shelf transport. Cross‐shelf transports, however, are significantly correlated with the local wind stress variability. Plain Language Summary The shelf circulation off southern Brazil and Uruguay and the exchange between the shelf and the Brazil Current are investigated using the results of an ocean model. We find that the mean cross‐shelf exchange is strongly modulated by the bottom topography, while its variability is driven by the local wind stress and by the Brazil Current internal variability. South of 25.2°S, the transport across the shelfbreak is on average in the off‐shelf direction. North of 25.2°S, the cross‐shelf exchanges show a two‐layer structure with an off‐shelf flow in the upper 50 m and on‐shelf flow deep below. Using model passive tracers and floats, our model diagnostics also show that there is a very limited connectivity between the shelf regions north and south of Cabo Frio (∼22.5°S). Key Points Characterization of the shelf circulation off Southern Brazil and Uruguay Characterization of the mass exchanges between the shelf and the Brazil Current Bottom topography, local wind stress, and Brazil Current internal variability are the primary drivers of cross‐shelf mass exchanges