16.04.2018: FB1-Seminar

11:00 Uhr, Hörsaal, Düsternbrooker Weg 20

Abstract:  

Two recently completed studies of velocity, temperature and salinity in the northeast Atlantic, one along a line between Scotland and Cape Farewell (line 1) and the other along a composite line between Shetland, the Faroes, Iceland and the Greenland shelf 1200 km northeast of Cape Farewell (line 2) have given us accurate estimates of volume, heat and fresh water fluxes between Europe and Greenland. The velocity data for these studies came from repeat (over many years) acoustic Doppler current profiler measurements on the container vessel Nuka Arctica and the high seas ferry Norröna, respectively; both in regular service all year round.

While the Labrador Sea has received considerable attention as an important contributor to the meridional overturning circulation (MOC), the heat fluxes across these two lines indicate that the heat flux towards the Nordic Seas is far greater than towards the Labrador Sea. It is also known that the principal source of North Atlantic Deep Water is the overflow from the Nordic Seas with the Labrador Sea contributing to the lower branch of the MOC through entrainment. This suggests that the stability of the MOC is primarily governed by conditions in the Nordic Seas (including the Arctic), not the Labrador Sea. Consistent with this is the fact that the Nordic Seas contribution to the MOC was shut down, but not that of the Labrador Sea during the last glacial maximum.

It has been suggested that the MOC may weaken in the coming decades, and may already have started. This is not supported by direct observation either in the Gulf Stream off the US east coast, or where the MOC enters the Nordic Seas. There is plenty of short-term variability, but on time scales of decades to a century no trend, whether up or down, can be discerned. Thanks to the shape of the Nordic Seas the MOC may actually be rather insensitive to increased supplies of fresh water from the Arctic and Greenland ice melt.

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