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Abstract

The Atlantic Meridional Overturning Circulation (AMOC) transports relatively warm and saline waters northward in the upper ocean. At the high latitudes, strong winter cooling increases the density of these waters to the point where they are mixed down in a process called deep convection. Eventually, these waters join the deep southward return flow of the AMOC. With a series of moorings, deployed in the central Irminger Sea since 2002, we investigated deep convection and its interannual variability. Over this period, there were winters with very weak convection (around 400m deep) as well as several with very strong convection (around 1600m deep). There is an ongoing discussion on whether this variability is caused by changes in winter cooling, or whether occasional strong water column stratification caused by anomalous fresh water at the surface has an impact. These fresh years could be representative of a future under climate change, with increased input of freshwater from the Arctic and Greenland. The 19-year time series suggests that the strength of the winter cooling is most important in setting the depth of deep convection. This is further investigated using the 1D PWP mixed layer model, with simulations with different forcings and stratifications conditions. We find that, in this simple model, the effect of winter cooling is dominant over that of stratification. This has implications for the future, as changing surface forcing may need to be taken into account as well as changing stratification.