Long-term changes in the flushing times of the ICES-boxes

Abstract

From an integration of a numerical model, an intensification of the circulation in the North Sea during the past 40 years is found. The gross influx into the ICES-boxes, calculated from monthly mean currents, yield an increase of about 10% for the northernmost boxes 1, 2, 6 and 3a, and also for the southernmost boxes 3b and 4. The gross influx for the remaining boxes 7a, 7b, 5a and 5b has increased even more, with about 20%. The flushing-times have decreased respectively.

Further insight into the causes of the changed circulation is given by a simple linear model, which links the anomaly of the gross influx of each ICES-box to changes in the wind climate over the North Sea, and the anomaly of the net inflow of Atlantic water masses at the northern entrance of the North Sea. The model shows the strong dependence on the Atlantic inflow for the northern boxes 1, 2 and 6, while the anomaly in the gross influx for the southeastern boxes 3b, 7b, 5b, 4 and 5a is mainly driven by the changing wind climate. For these boxes, apart from box 7b, offshore gradients in the salinity and the influence of the inflow from the English Channel are important; however baroclinic effects are not included in the linear model. The increasing trend in the gross influx is overestimated by the linear model for all of these boxes, so the baroclinic forcing seems to counteract the increasing trend in the gross influxes. The question remains as to whether the anomalies in the density field are coupled to the Atlantic climate system or if they are an indirect response of the North Sea hydrographie system to forced atmospheric changes in the circulation.