Coastal Upwelling and Estuarine Gravitational Circulation: A Feedback System in a Tropical Estuary in the South Atlantic

This study examines the effects of coastal upwelling on the longitudinal water density gradient within the estuary of Baía de Todos os Santos (BTS), its effect on the gravitational circulation at the estuary entrance, and the reverse effect of gravitational circulation on the coastal upwelling. This investigation was based on a 1-year dataset of observed water temperature, mean velocities, and river discharge, as well as 2 years of numerical simulation of the estuarine flow. The results show that the upwelling regulates the thermohaline field in front of the BTS, decreasing water temperature (up to 3 °C), and increasing density (up to 0.3 kg/m³), and have sufficient intensity to more than double the speed, or even establish, the gravitational circulation. It was frequently observed that the water temperature falls after an increase in the subtidal flow shear, suggesting that the estuarine gravitational circulation acts as a facilitator to the upwelling process. Numerical simulations indicate that the coastal upwelling events are also capable of reestablishing the gravitational circulation at times with weak longitudinal density gradient, a scenario that tends to become more frequent and intense in the near future due to the ongoing climate changes.