The Arctic Ocean–Nordic Seas thermohaline system

Abstract

The Arctic Mediterranean Sea is located north of the Greenland–Scotland Ridge and allows warm water from lower latitudes to penetrate beyond the Polar Circle. The northward flowing water is cooled in the Norwegian Sea and its density increases. In the Arctic Ocean the high river runoff and the net precipitation lead to a density decrease in the surface layers and heat loss at the sea surface results in the formation and maintenance of a permanent sea-ice cover. Brine ejected by freezing creates dense waters on the Arctic Ocean shelves, which sink as convecting boundary plumes into the deeper layers. In the Eurasian Basin the water column primarily reflects the interaction between the two inflows from the Norwegian Sea: through Fram Strait and over the Barents and Kara Sea and their different transformation histories. In the Canadian Basin the water transformations are dominated by the boundary convection, which makes the Canadian Basin water column different from that of the Eurasian Basin already at levels shallower than the now known sill depth of the Lomonosov Ridge. In the Greenland Sea deep-reaching, open-ocean convection occurs, partly rehomo-genising the water column. The waters entering the Arctic Mediterranean are thus transformed partly into a low salinity, cold upper layer, partly into cold, dense deep waters which all re-cross the Greenland–Scotland Ridge. The dense waters sink into the deep North Atlantic to supply the North Atlantic Deep Water. A reduction of the deep convection in the Greenland Sea has recently been inferred and the Greenland Sea deep water renewal presently occurs by advection of deep waters from the Arctic Ocean. Observed changes in the temperature and salinity of the Greenland Sea Deep Water are used to estimate the vertical diffusion coefficient in the deep layers and the renewal time of the deep salinity maximum layer, which originates from deep water outflow from the Eurasian Basin through Fram Strait. A weaker convection in the Greenland Sea is found to influence primarily the deep water circulation internal to the Arctic Mediterranean. The supply of dense overflow water from the upper layers in the Greenland Sea and from the other sources is not expected to be reduced.