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1/n Thread: Science Mag has a new study reporting measurements of the Atlantic overturning circulation #AMOC from the #OSNAP project. They find that this circulation happens mostly towards waters east of Greenland rather than the Labrador Sea in the west. washingtonpost.com/climate-enviro…
2/n I do not find that surprising. At least since the early 1990s the Greenland-Norwegian Seas were considered the most important site for Atlantic deep-water formation. Water there reaches the highest density, and flows over the sills near Iceland. (Graph Rahmstorf Nature 99) 
3/n This “overflow” was measured by Bogi Hansen and others and amounted to about 6 Sv, and was also captured well in models (e.g. nature.com/articles/natur…)
4/n Deep water formed on the way down the slope, where the dense water entrained and “dragged along” more water and increased in volume, as captured in this beautiful model animation by my Kiel colleague Rolf Käse.
5/n In coarse-resolution models we took special care to capture this deep water formation in the Greenland-Norwegian Sea and the overflow, it was a key diagnostic for model quality. See convection depths from my 1995 Nature article. pik-potsdam.de/~stefan/Public…
6/n All this is very consistent with the new measurements. Now to time variations. First of all the new data covers only 21 months, and variability on different time scales can have very different mechanisms, so this allows no conclusions for longer time scales.
7/n But the mechanism we proposed in Nature for abrupt glacial climate changes (DO events), based on model simulations and supported by proxy data, hinges on Greenland-Norwegian Sea convection switching on and off. pik-potsdam.de/~stefan/Public…
8/n And for the AMOC response to global warming, Schweckendiek and Willebrand (2005) have shown that this mainly depends on what happens in the Greenland-Norwegian Sea. core.ac.uk/download/pdf/1…
9/n A classic global warming study by Wood et al in Nature 1998 found that Labrador Sea convection shuts down between 2000 and 2030, which leads to a 20% reduction in AMOC at 24 North. nature.com/articles/21170 (My News&Views: nature.com/articles/21066)
10. Could the weak Labrador Sea overturning in the new data be a sign of its gradual demise being underway? Labrador Sea convection paused for nearly two decades until a few years ago. The AMOC appears to have weakened by about 15%. realclimate.org/index.php/arch… End.
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