Buoyancy-stirring interactions in a subtropical embayment: a synthesis of measurements and model simulations in Maputo Bay, Mozambique

Abstract

Maputo Bay, on the coast of Mozambique, is a tidally energetic, subtropical embayment in a region subjected to strong seasonal rainfall. Data from moored current meters, tide gauges and monthly bay-wide surveys were used to characterise the evolution of the density structure on seasonal, fortnightly and semi-diurnal time-scales and its relation to tidal forcing. The bay is subjected to large seasonal variations in freshwater input (10–10³ m³ s^–1) and pronounced fortnightly variations in tidal amplitude with a spring:neap tide ratio varying between 3.6 and 9.0 with a corresponding variation in tidal stirring power input (10^–3–1 W m^–3). During the dry season, the water column was continuously fully mixed with weak horizontal density gradients. In contrast, during the wet season, freshwater buoyancy induced marked horizontal salinity gradients and stratification, which was pronounced around the time of neap tides. This stratification was largely eroded at spring tides but semi-diurnal, periodic stratification was still evident. A potential energy anomaly model was used to demonstrate that this periodic component of stratification was largely a result of tidal straining acting locally, with an additional contribution of stratified water advected from outside the bay during the last stages of the flood tide. Simulations using the Delft3D-Flow hydrodynamic model gave a realistic account of the tidal regime in the bay, but achieved only qualitative success in the hindcasting of the changes in water column structure and horizontal exchange. The model indicated a wet season salinity deficit of 2–3 times that of the observed values and wet season temperatures were ~2–3 °C higher than those observed. The model also underestimated the intensity of stratification in the wet season. These shortcomings all suggest an overestimate of vertical mixing by the model’s turbulence closure scheme.

Keywords: buoyancy; density gradient; Maputo Bay; seasonal variation; subtropical zone; tidal mixing

African Journal of Marine Science 2010, 32(1): 95–107