Statistics of “Synoptic Circulation Weather” in the North Sea as Derived from a Multiannual OGCM Simulation

Abstract

A 15-yr simulation of an ocean general circulation model, exposed to atmospheric forcing as provided by the ECMWF reanalysis 1979?93, is analyzed with respect to the statistics of the surface circulation of the North Sea on timescales of days to several weeks in winter. The first two EOFs of surface circulation are found to represent the bulk of the variability (72%). They are broadly consistent with the limited observational record. The first EOF represents regimes with one gyre flushing the entire North Sea, either with clockwise orientation (15% of time) or with counterclockwise orientation (30% of time). These regimes are excited by northeasterly and, respectively, southwesterly wind. The second EOF is representative for two opposite regimes with two bipolar patterns in the northern and southern part of the North Sea (45% of time). For a certain range of both EOFs coefficients, the North Sea circulation ceases (10% of time). The circulation of the North Sea in winter is highly variable; the regimes change frequently. Only 40% of the one-gyre regimes persist for longer than 5 days, and the bipolar pattern regimes rarely extend for more than 5 days.