Oceanic circulation: Heat loss strengthens the gyre circulation

South of Greenland and Iceland the oceanic circulation goes mainly anti-clockwise, similar to the winds around a low-pressure system in the atmosphere.

This oceanic circulation is called the subpolar gyre. An observational study shows that when the subpolar gyre strengthens, warmer and more saline water masses will be carried northward toward the Norwegian coastline (Hátún et al. 2005, Science).

From warm to cold water masses

The continuation of the Gulf Stream -- the North Atlantic Current -- carries warm and saline water masses northward. To reach the Norwegian Coastline, the current must cross an ocean ridge that stretches from the southernmost part of Greenland to Scotland (the Greenland -- Scotland Ridge). The warm Atlantic water masses changes properties in the ocean between Greenland and Norway (the Nordic Seas) before they return southward. Cold Arctic winds and freshwater input gives the water masses an Arctic character. The Bergen Climate Model simulates the exchange of Atlantic and Arctic water masses over the ocean ridge surprisingly realistic.

The returning cold current continues southward in the deep ocean along the American continental slope. This current does not only consist of Arctic water masses spilling over the ocean ridge, but also of cold water masses produced in the Labrador Sea between, Greenland and Canada. The production of these water masses increases when the Icelandic low pressure is stronger than normal and cold Arctic winds blow over the Labrador Sea.

Gyre circulation strengthens when the return current of cold water masses increases

The oceanic circulation south of Greenland and Iceland (the subpolar gyre) strengthens both when the Arctic transport over the ocean ridge increases and with stronger production of cold water masses in the Labrador Sea.

By combining the variations in these two factors with the variations in the wind over the ocean between Canada and England, the scientists from Bjerknes can explain about half of the decadal variations in the subpolar gyre.