Synopsis A discussion of the tidal~ wind-driven~ and density-driven aspects of the circulation in Bay of Ilha Grande and Bay of Sepetiba (BrazilJ ~ based on historical data and hydrographic and current mete'!> data coUected ·in Septerriber of 19??~ is present ed in this paper. The data analysis has shown a remarkable contrast between the circulations of these two adjacent Bays. The circulation in Bay of Sepetiba is strongly tidal~ whereas Bay of Ilha Grande has a weak tidal circulation

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... sed by a quasi-steady flow induced by density gradients. The stratification is maintained because Bay of Ilha Grande~ beeing deeper~ allows colder and saltier shelf water to flow eastward towards the fresher and warmer water flowing out of Bay of Sepetiba~ highly influenced by river runoff. The velocity and density fields have shown seiche osciUations that agree very weU with theoreticaUy derivedmodes inherent to the local geometry and density stratification.