The vertical structure of time-mean estuarine circulation in a shallow, rotating, semi-enclosed coastal bay: A Liverpool Bay case study with application for monitoring

Abstract

A reduced physics Ekman boundary layer solution is developed to infer the vertical structure of time-mean circulation in a shallow tidal environment when the horizontal density and surface slope gradients are misaligned. This generalisation of the classic Heaps (1972) model shows that the time-mean depth weighted flow, or the residual circulation, is usefully constrained by knowledge of the surface velocity, instead of freshwater flux, and the horizontal density gradient.The generalised model is applied to Liverpool Bay. In regions where the Ekman depth scale is less than half the mean fluid depth the residual circulation is well modelled by a water column of uniform density, constant eddy viscosity and linear bottom drag. Lateral variability in long-term mooring observations of depth varying residual flow are attributed to the misalignment of sea surface slope and haline controlled density gradients.A method to infer 3D time-average residual currents in regions of misaligned freshwater density and sea surface slope gradients is presented. The method blends CTD survey data with HF radar surface currents and simulation estimates of viscosity and friction. It is validated against ADCP data in Liverpool Bay. It is speculated that this method could be applied more generally, to correct model biases, as part of a coastal monitoring system.