Oceanographic and environmental characteristics of a coupled coastal bay system: Baía de Ilha Grande-Baía de Sepetiba, Rio de Janeiro, Brazil

Abstract

Baía de Ilha Grande and Baía de Sepetiba are two connected coastal bays in southeastern Brazil just west of Rio de Janeiro. They are favored for tourism and recreation, provide most of the potable water for the city of Rio de Janeiro, are the site of the only two nuclear reactors in Brazil, a deep-water oil terminal, a Federal container port, and hundreds of industries. The bay system experiences mixed mainly semidiurnal tides and currents. Hundreds of sediment samples and cores plus side scan-sonar surveys were used to map the bottom sediment distribution. Long-term measurements from two moored instruments yielded two sets of time series of water level, currents, temperature, and salinity, which were used to analyze the salient ocean processes in the two bays. The temperature time series and extensive CTD profiling indicated the occurrence of cold-water intrusion events into Baía de Ilha Grande, with temperatures ranging from 16 °C to 20 °C, in spring to late summer (October–April). The intrusions are ascribed to coastal upwelling of South Atlantic Central Water as a result of synoptic northeasterly winds and the westward translation of cold core eddies and meanders from the region of Cabo Frio. The calculated mean sea level varies annually by 50 cm with a peak in June and a trough in November due to atmospheric pressure variations (5%), steric changes (28%), but mostly due to wind and Ekman effects. Spectral analysis was applied to analyze the water level and current time series. The results indicated both tidal asymmetry and the existence of a pronounced quarter-diurnal seiche oscillation in the coastal bay-system parallel to the coast.