Maputo Bay is a tidally-energetic, subtropical embayment in a region subjected to
strong seasonal rainfall. Observational data are analysed alongside modelling outputs to explain the mechanisms controlling the physical environment inside Maputo Bay. Moored current meters, tide gauges and monthly bay-wide surveys were used to characterise the evolution of the density structure and flow field on seasonal, fortnightly and semi-diurnal time scales. The bay is subjected to large seasonal variations in freshwater input ( ~ 10 - 10³ m ³ .s –¹ ) and pronounced fortnightly variations in tidal stirring power input (~10³ - 1 W.m–³ ). During the dry season, the water column was found to be continuously, fully mixed, with weak horizontal density gradient and a residual circulation, which was mainly due to tidally-rectified currents. By contrast, during the wet season, freshwater buoyancy was observed to induce marked horizontal salinity gradient and stratification, which is pronounced around the time of neap tides. This stratification is largely eroded at spring tides but semi-diurnal periodic stratification, a sum of local production and advection from offshore, was still apparent. A simple potential energy anomaly model shows that the local
stratification terms were tidal straining and estuarine circulation. The flushing time of the Bay is investigated using the observed bay averaged salinity and a modified tidal prism model, estimating the wet season mean between 9 and 17 days depending on the retention coefficient used. A series of experiments with a high resolution 3d model of the bay show that an initial increase in tidal currents, during a period of significant freshwater input, decreased the efficiency of the density driven circulation thus increasing flushing time. Under high runoff and tides, both forcing mechanisms collaborated to decrease flushing time. Depending on the variation of these two parameters, the 3d model predicted a flushing time minimum of 35 days and a maximum of 160 days. The main control of exchange during the dry season is tidal forcing mainly through rectified currents that increase in direct proportion to tidal range. During the wet season, tidal forcing and estuarine circulation generate a neap to spring cycle of the efficiency of exchange, which decreases with the amount of tidal forcing.