North Atlantic Deep Water Formation
Vage, K., Pickart, R.S., Thierry, V., Reverdin, G., Lee, C.M., Petrie, B., Agnew, T.A., Wong, A. and Ribergaard, M.H. 2008. Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007-2008. Nature Geoscience 2: 67-72.
The Canadian, Danish, French and US scientists observed winter mixing to depths of 1800 m in the Labrador Sea, 1000 m in the Irminger Sea, and 1600 m south of Greenland, which values are to be compared to base-period (the winters of 2001-2006) mixing depths of less than 1000 m. They also determined, via analyses of heat flux components, that "the main cause of the enhanced heat flux was unusually cold air temperatures during [the 2007-2008] winter." More specifically, they tell us "the air temperature recorded at the Prins Christian Sund meteorological station near Cape Farewell was 2.8°C colder in the winter of 2007-2008 than the corresponding mean of the base period." And they say the cooling was "not a local phenomenon," noting that "the global temperature dropped 0.45°C between the winters of 2006-2007 and 2007-2008," and that across northern North America "the mean winter temperature was more than 3°C colder." In addition, they report that "storm tracks, the flux of freshwater to the Labrador Sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air, making the surface water sufficiently cold and dense to initiate deep convection," which phenomenon was aided by "very strong westerly winds off the Labrador ice edge" that "boosted the advection of cold air towards the region of deep convection," thereby providing a sort of perfect storm situation, where everything came together at the same time to create an oceanic overturning, the likes of which had not been seen since the late 1980s to early 1990s.
In the words of the nine scientists who comprised the research team, "the return of deep convection to the Labrador and Irminger seas in the winter of 2007-2008 was a surprise." One reason for this reaction, as they describe it, was that "contrary to expectations [italics added] the transition to a convective state took place abruptly, without going through a phase of preconditioning."
Editorializing for a moment, these observations give pause to wonder if there might not be still other surprises in store for humanity, especially for climate modelers and people who put so much trust in what that fellowship predicts that they are willing to accept massive changes in the way the world is energized and its business is conducted.