Transformation of light to dense waters by atmospheric cooling is key to the Atlantic Meridional Overturning in the Subpolar Gyre. Convection in the center of the Irminger Gyre determines the transformation of the densest waters east of Greenland. We present a 19-year (2002-2020) weekly time series of hydrography and convection in the central Irminger Sea based on (bi-)daily mooring profiles supplemented with Argo profiles. A 70-year annual time series of shipboard hydrography shows that this mooring period is representative of longer term variability. The depth of convection varies strongly from winter to winter (288-1500 dbar), with a mean March climatogical mixed layer depth of 470 dbar and a mean maximum density reached of 27.70 ± 0.05 kg m-3. The densification of the water column by local convection directly impacts the sea surface height in the center of the Irminger Gyre and thus large-scale circulation patterns. Both the observations and a Price-Weller-Pinkel (PWP) mixed layer model analysis show that the main cause of interannual variability in mixed layer depth is the strength of the winter atmospheric surface forcing. Its role is three times as important as that of the strength of the maximum stratification in the preceeding summer. Strong stratification as a result of a fresh surface anomaly similar to the one observed in 2010 can weaken convection by approximately 170 m on average, but changes in surface forcing will need to be taken into account as well when considering the evolution of Irminger Sea convection under climate change.

Miriam Frauke Sterl

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The Irminger Sea is one of the few regions in the ocean where deep (> 1000 m) convection occurs. Convection is followed by restratification during summer, when the stratification of the water column is reestablished and the convectively formed water is exported to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). We investigate the interannual variability and physical drivers of restratification in the upper 600 m of the central Irminger Sea using reanalysis data for the years 1993–2019. We find that there are distinctly different restratification processes in the upper 100 m of the water column (the upper layer) and the water below it (the lower layer). In the upper layer, the stratification is dominated by a strong seasonal cycle that matches the cycle of the surface heat flux. In 2010 and 2019, there were peaks in upper layer stratification, which could be related to strong atmospheric heat and freshwater fluxes. By contrast, in the lower layer the seasonal cycle is weaker and there is strong interannual variability. Restratification can continue for up to 5 months after the surface heat flux has become negative, indicating a role for lateral advection. The strength of the restratification is strongly correlated with the eddy kinetic energy in the eastern Irminger Sea. This suggests the lateral advection is driven by warm, saline eddies from the Irminger Current. In the future, surface warming and freshening of the Irminger Sea due to anthropogenic climate change are expected to increase stratification.