Dynamics and Variability of the Circulation in the North-Atlantic Subpolar Seas

Abstract: This thesis deals with the dynamics and circulation in the northern North Atlantic and the Nordic Seas, processes of crucial importance for the mild climate of Scandinavia and Northern Europe. High-resolution ADCP scans of currents from Greenland to Scotland in the top 400 m demonstrate that the Reykjanes Ridge is a very effective separator of flow towards the Nordic and Labrador Seas, respectively. It was found that the meridional overturning circulation has weakened by ~1.7 Sv (1 Sv = 106 m3 s-1) during the 18-year period when altimetric data were available. This trend may be an effect of the Atlantic Multidecadal Oscillation, but is certainly not due to the North Atlantic Oscillation (NAO). By studying the circulation in the Faroe-Shetland Channel, which is an important choke point for the global thermohaline circulation, it was concluded that the contraction of the Norwegian-Sea gyre during low NAO periods plays an important role for disturbing the flow pattern. This specifically affects the regional ocean climate by leading to an accumulation of warm and saline Atlantic waters in the channel. During high NAO phases the circulation is strongly topographically controlled. The Norwegian Atlantic Slope Current (NwASC) is the main flow branch linking the North Atlantic to the Arctic and Barents Sea. It was found that the NwASC is largely coherent over seasonal to interannual time-scales. However, on shorter time-scales the coherency of the flow shows a sustained and pronounced weakening downstream of Lofoten. Intense eddy-shedding from the slope into the Lofoten Basin damps the coherent structure of the flow. The eddies take about two months to propagate to and to merge with the semi-permanent anticyclonic vortex above the deepest part of the Lofoten Basin. These results have implications for how flow/hydrographic anomalies are transferred through the Nordic Seas towards the Arctic. Anomalous transports of warm water into the Arctic and Barents Sea via the NwASC are found to be driven by a combination of the NAO and the other two leading modes of atmospheric variability in the North Atlantic. The results reported in the thesis may be of importance for achieving a correct representation of the heat conveyed polewards in climate models.