Circulation on the western South Atlantic continental shelf: 1. Numerical process studies on buoyancy

Buoyancy‐driven currents are investigated in the western South Atlantic Ocean, where a major river plume, La Plata, interacts with a minor one, Patos Lagoon. A series of simulations were performed with a three‐dimensional, free‐surface numerical model, to better understand the processes which control both plumes evolution and their interaction. The simulations are focused on the importance of turbulent mixing to the evolution of the La Plata plume and its downshelf penetration toward the Patos Lagoon. Satellite observations and historical data show that the La Plata plume has a tendency to expand along the river southern margin, before it turns north in the direction of coastal trapped waves propagation. Our results, on the other hand, show that in the absence of the tides and the wind, this plume tends to leave the estuary attached to its northern margin (downstream), unless the model parameters are tuned to produce unrealistically high turbulent mixing. We then address the role of each parameter that controls mixing, and show that by tuning the model parameters we can get plumes similar to those observed in nature. However, the plumes produced in the experiments with excessively high mixing parameters showed unbounded southward (upstream) expansion. By introducing the tides we show that the plumes produced in the presence of tidal stirring, and normally set mixing parameters, are more realistic, suggesting that tidal stirring is more effective than artificially tuned turbulent mixing on causing the plume lateral expansion.