Large-scale circulation and flushing characteristics of the North Sea under various climate forcings

With the aid of a numerical model it is shown that, in addition to the residual effects of tides and winds, the meridional density distribution in the North-East Atlantic Ocean has a significant effect on the large-scale, residual circulation in the North Sea. This effect is due to the fact that the currents along the continental slope, which are mainly forced by density gradients and tides, intrude into the deeper parts of the shelf sea and, as a consequence, oppose or enhance the wind-driven circulation. From dispersion-model experiments it is demonstrated that the interplay of the tide, ocean density, and winds results in efficient flushing of the basin if their effects coincide. However, the residual circulation may break up into closed cells on the shelf if forcings counteract. In that case, efficient renewal of material is inhibited. The characteristics of the dispersion of material in the shelf sea are quantified by computing the spatial distributions of residence times. In addition, basin-wide flushing characteristics are typified by calculating turn-over and flushing time scales. The ratio of these time scales is used as a measure for the relative retention strength of material within the basin by closed circulation cells.