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Abstract

The subpolar North Atlantic provides a strong geostrophic eastward inflow to the European shelf seas, which varies across timescales with basin-scale warming and cooling. Lagrangian particle back-tracking experiments have suggested that this is a main contributor to the northward flow of the European Slope Current. As the subpolar North Atlantic has warmed by approximately 2°C over the past 4 decades, geostrophic inflow to the European shelf edge and shelf seas has decreased in strength by up to 10 Sv (nearly 50%) due to weakened meridional density gradients across subpolar latitudes. There has been a corresponding 2-3 Sv (50-70%) drop in northward along-slope Slope Current transport. Atlantic inflow to the Slope Current has become more subtropical in provenance, slower, warmer and shallower, since 1997. The North Sea is consequently freshening with decreasing Atlantic inflow, being more influenced by Baltic outflow and riverine inputs. Using a 1D physics-biology coupled model, prescribing surface nutrient supply in proportion to Slope Current transport and shelf edge exchange, we present evidence of decreasing primary production over 1988-2010. Largest changes are associated with reduced productivity during the autumn bloom, after spring bloom exhaustion of surface nutrients that are subsequently reset during winter mixing. These findings have implications for seasonality of the North Sea ecosystem, particularly fisheries, and may explain some recently observed ecological shifts.