A modelling study on the residual circulation in the North Sea, with the focus on water fluxes through the Strait of Dover

Abstract

In this master thesis the residual circulation in the North Sea is investigated. It has been shown that the numerical models used in this thesis (DCSMv5 and DCSMv6) give a rather low representation of the actual circulation pattern in the North Sea. The dominating forces which cause residual currents in the North Sea are the tidal forcings, meteorological forcings and density variations. Their variability over the whole domain, as well as their interaction with bottom topography and the earth’s rotation, leads to a variety of different processes. Since it is difficult to quantify to total transport over the whole North Sea, the focus in this thesis lies on the transports through the Strait of Dover. Studies on fluxes through the Strait of Dover date back to the late 1920’s, where in situ velocity profiles were measured and integrated over the crosssection to make an estimate on residual flows. In later studies different methods were used to obtain valuable information on transports. Use was made of electric potentials through a telephone cable, high frequency radars and modelling studies. Because the magnitude of residual transports is an order of magnitude smaller than short scale movements like tidal oscillations, the estimated trans- ports lack accuracy. They vary between 87000 to 235000 m3/s. Dutch ecologists suggests an annual inflow in the order of 100000 m3/s. In this thesis the state-of-the-art numerical DCSM models of Deltares have been com- puted in Delft3D-FLOW to investigate the dominant processes in the Strait of Dover. The models cover the complete northwest European continental shelf and calculate depth aver- aged barotropic flow. The representation of the general North Sea circulation is qualitatively well, except for some highly stratified and density driven areas. However, the in- and outflow of water at the North Sea boundaries is considerably low. The leakage of the radioactive nuclide Antimony-125 from La Hague in 1985 till 1994 has been simulated to give an in- dication of the spatial varying transit time of the current model. It is shown that in the southern region transit times are simulated to be two times larger than other modelling studies and measurements suggest. The daily varying residual flow through the Strait of Dover has been investigated by applying a Low-Pass frequency filter to the modelled fluxes. The contribution of tidal residuals are found to be very significant with an average annual flow of 45000 m3/s. When meteorological forcings are also taken into account the annual inflow for the year of 2007 equals 56000 m3/s. Since meteorological forcings vary throughout the years, a quasi-linear approximation is formulated where the daily residual flow is a function of the wind induced shear stress and wind direction. A correlation of 0.79 is found for this linear approximation. For rapidly varying surface elevations differences between the North Sea and English Channel the approximation fails to give proper estimates. During the research, model adjustments were made to (1) assess the sensitivity of the fluxes and (2) look for candidate solutions to improve the amount of transport through the Strait of Dover. Physical adjustments have been made by implementing density differences, residual surface level boundaries and a varying bottom roughness. The numerical adjust- ments were for the largest part based on different discretization methods. In addition, a preliminary computation has been performed in DFlow-FM; a recently developed simulation package which computes on an unstructured grid. Although there are notable differences for each adjustment, the effects are small in comparison with tide and wind induced residuals.