Wind-induced variability of estuarine circulation in a tidally energetic inlet with curvature

In numerous studies, the functioning of estuarine circulation has been investigated, under idealized conditions, by means of numerical models. This has led to a deep understanding of the theory of estuarine residual flows. However, the question as to how estuarine circulation is established in real estuaries, in response to their topographical and forcing characteristics, remains. The present study uses a highly accurate three-dimensional numerical model simulation to calculate estuarine circulation in a curved, tidally energetic channel of the Wadden Sea in the southeastern North Sea. The specific momentum balance of this curved inlet shows an approximate pressure-gradient-frictional balance in the longitudinal direction and a pressure gradient-centrifugal balance in the lateral direction. A local Wedderburn number is introduced to quantify the varying contributions of wind stress and gravitational forcing on estuarine circulation. A total exchange flow (TEF) analysis is combined with an analysis of the intensity of the vertical overturning circulation to understand the dynamics of estuarine exchange in this inlet. The results show how established forcing mechanisms of residual circulation, such as horizontal buoyancy gradients and wind stress, act in a combined way. In general, the strength of estuarine circulation is always positively correlated with wind stress, with frequent reversals of residual flow for wind stress directed toward higher buoyancy. Only during calm weather conditions are longitudinal and lateral estuarine circulation highly correlated with the respective buoyancy gradients.