Seasonal to inter-annual variability of the Norwegian Atlantic Current: Connection between the northern North Atlantic and the Norwegian Sea

Causes for measured variability in the Norwegian Atlantic Slope Current (NwASC) during the period from 1995-2000 are investigated by considering complementary data providing information on the upstream condition and atmospheric forcing. These data are fields of sea surface height (ssh) variability from the TOPEX altimeter, reanalysed mean sea level pressure (mslp), climatological hydrography and repeated hydrographic measurements both in the northern North Atlantic and in the Norwegian Sea. The analysis indicate two different mechanisms associated with different time scales that contribute to the variabilities of the NwASC.

Firstly, for time scales of 2-3 months and 6-12 months, the analyses show significant coherences between the NwASC and the along-stream sea surface slope, and the ssh and the mslp. This can be interpreted as a relation between the wind forcing and NwASC through modulation of the sea surface height. In other words, the variability in the westerly winds modulates the sea surface slope from the northern North Atlantic into the Norwegian Sea, and thus provides a barotropic forcing term for the NwASC. The phase relation from the coherence analysis is such that increased westerly winds occur in phase with an increase of the sea surface slope from the North Atlantic into the Norwegian Sea, and furthermore, this coincides with an increase in the NwASC. The order of this wind effect on the sea level slope is 10^-7 , and is similar to the basin-scale downward sea surface slope from sub-tropical to high latitudes in the North Atlantic Ocean. This differential effect on the ssh in the eastern part of the North Atlantic and in the Norwegian Sea can be connected to the variability in the Sverdrup transport. Stronger atmospheric cyclones (westerlies) increase the magnitude of the northward Sverdrup transports in both the northern North Atlantic and in the Norwegian Sea. Combined with the orientation of the isobaths, this lead to increased ssh in the eastern part of the North Atlantic and decreased ssh in the Norwegian Sea.

Secondly, on inter-annual time scales, changes in the hydrographic conditions in the northern North Atlantic coincide with changes in the NwASC. The change in hydrography is also found in the TOPEX ssh data. At inter-annual scales, the ssh in the Iceland Basin (reflecting hydrographic changes) and the NwASC show similar variability. A conceptual mechanism relating the observed anomalies to the NwASC is proposed. Under the assumption of a limited southward extent a cold/warm anomaly in the Iceland Basin would be associated with an anomalously strong/weak eastward geostrophic current to the to south of the anomaly toward the continental slope. Here, the upper (Atlantic) layer becomes thicker, and thus transfers into a northward slope current through conservation of relative vorticity in the upper layer.