Ocean Observing and Climate
LSLTOC Theme 2 - The North Atlantic Sub-Polar Gyre
Schematic of the subpolar gyre. Orange/yellow arrow denotes northward flow of warm Gulf stream waters; blue/yellow arrow southward flow of fresh, cold waters, and light blue line the southward flow of deep, dense waters. Dark blue shading is water deeper than 3000 m, light blue deeper than 1500 m, and white is shallower than 200 m.
Warm subtropical waters enter the northeastern Atlantic from the eastward extension of the Gulf Stream and circulate northward and westward in a broad anticlockwise gyre - the Subpolar Gyre. The vast quantities of heat given up to the atmosphere by the waters of the North Atlantic Subpolar Gyre moderate Britain's climate, giving us winter temperatures some 10 C warmer than the average for our latitude. This heat is supplied by northward flow of warm waters, counterbalanced by the southward flow of cooler waters: the thermohaline circulation (THC). This exchange of waters is driven both by winds and by the natural tendency of cold waters to sink and spread under warmer, lighter waters.
The very cold, dense waters formed in the polar seas north of Iceland are penned back by the ridge that lies between Greenland, Iceland and Scotland. They can only escape by flowing over the deepest parts of this ridge - a gap in the Denmark Strait between Greenland and Iceland, and a narrow channel skirting east and south of the Faroe Islands: the Faroe Bank Channel (FBC). The northern seas gain freshwater both by ice melt, and an excess of precipitation over evaporation. This freshening makes the water lighter and so works against the cooling in making water less likely to sink. Models suggest that anthropogenic global warming will both warm the polar atmosphere and increase precipitation and ice-melt at high latitudes, doubly weakening the THC.
Crucial questions include:
-
What are the pathways along which warm, light waters flow through the Subpolar
Gyre and pass into the polar seas north of the Greenland - Iceland - Scotland
Ridge? How much variability is there in the strength and properties of these flows?
- What limits the cold overflows through the gaps in the ridge? How much mixing
of overlying waters occurs as the overflow waters spill down the ridges?
- How much freshening is there in the Subpolar Gyre and Arctic? Does freshening happen in the same places as cooling? Does the freshened water mix with the cooled water, thus making it less dense, or does it escape by different pathways?
Links to LSLTOC Theme 2 Research
Rotating Channel Flow Faroe Bank Channel Ellett Line SPG in 1/12 OCCAM Irminger Sea Convection Greenland Coastal Current
