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).
Figure 1: Schematic of the oceanic circulation south of Greenland and Iceland (the subpolar gyre). The warm North Atlantic Current (big red arrow) bifurcates into two currents when it approaches the ocean ridge, stretching from southernmost of Greenland to Scotland (marked as the GSR). The cold Arctic southward current, spilling over the ocean ridge (black curve), and the cold current from the Labrador Sea (green curve) constitute a deep oceanic current that follows the American continental slope (also called the Deep Western Boundary Current).
A simulation over 600 years from the Bergen Climate Model shows that this circulation varies on decadal time scales. What is the reason for these variations?
Scientist from the Bjerknes Centre show in a recently published article (Langehaug et al. 2012, Journal of Climate) that a combination of three different factors contributes significantly to the decadal variations; cold water masses from the Nordic Seas and Labrador Sea, and the wind over the subpolar gyre.
This model study therefore gives an increased understanding of which factors that can cause changes in the strength of the circulation. This is important to understand, since this circulation is essential in the distribution of heat and salt in our ocean region.
From warm to cold water masses
The continuation of the Gulf Stream – the North Atlantic Current – carries warm and saline water masses northward (Figure 1). 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 (Figure 1). 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 (Figure 2).
Figure 2: Variations in the oceanic circulation south of Greenland and Iceland (the subpolar gyre; black curve). About one third of these variations can be explained by combining the variations in the production of cold water masses in the Labrador Sea and the Arctic current spilling over the ocean ridge (green curve). By including the variations in the wind over the ocean between Canada and England, 44% of the variations in the subpolar gyre can be explained (red curve).
References
Hátún, H., A. B. Sandø, H. Drange, B. Hansen, and H. Valdimarsson (2005), Influence of the Atlantic subpolar gyre on the thermohaline circulation, Science, doi:10.1126/science.1114777
Langehaug, H. R., I. Medhaug, T. Eldevik, and O. H. Otterå (2012), Arctic/Atlantic exchanges via the Subpolar Gyre, Journal of Climate, doi:10.1175/JCLI-D-11-00085.1