Record deep convection in the Irminger Sea in winter 2014-15

The subpolar North Atlantic is an important region for dense water production and acts as a sink of anthropogenic carbon, whose magnitude depends on the depth of convection and the Atlantic meridional overturning circulation for distribution of the recently ventilated water masses. Here we present observations from winter 2014-15 that show the deepest winter mixed layers ever recorded in the Irminger Sea. Possible oceanic and atmospheric causes for the deep convection are examined. The mean atmospheric circulation in winter 2014-15 was optimal for the generation of westerly tip jets, which efficiently extract heat from the water column. In addition to the strong atmospheric forcing, a below-average stratification in late fall resulted in a water column that was preconditioned for convection prior to the onset of winter. The oxygen and anthropogenic carbon concentrations within the mixed layer at the tail end of the convective season were close to saturation. Accordingly the winter of 2014-15 had a substantial impact on the distribution of oxygen in the interior ocean and the amount of anthropogenic carbon sequestrated in the deep ocean.