The Impact of Different Forcing Agents on the Residual Circulation in a Tropical Estuary (Baia de Todos os Santos, Brazil)

This study investigated the forcing of the residual circulation in a tropical estuary, Baia de Todos os Santos, Brazil (BTS). A numerical model (Regional Ocean Modeling System), forced with climatological means, was used to investigate the seasonal roles of the tide, wind, net heat, surface water fluxes, and river discharge on the residual circulation. The tide is the main driver of the circulation and its residual flow is structured at the bay mouth with a net-landward flow in the channel center and a net-seaward flow on the shoulders. The addition of wind drag affected the surface circulation, forcing W-bound (N-bound) currents in the spring-summer (fall-winter) months and generating a positive sea-level slope toward the continent. The addition of heat and surface water fluxes to the simulation established an incipient gravitational circulation, mainly because of the large (approximately 0.2 degrees C/km) temperature gradients in the summer. The river discharge, added last, was second to the tide in driving the residual circulation in the BTS and established the gravitational circulation through most of the bay, including the bay mouth. The residual velocities attained magnitudes smaller than 0.13 ms(-1) in the main channel and were seasonally controlled, with stronger currents during the summer when the river discharge peaked. The onset of the baroclinic forcing (heat and surface water fluxes plus river discharge) reduced the e-folding flushing time by a factor of five relative to the experiment with barotropic forcing (wind and tides) only.