Deep convection variability in the Labrador versus Irminger Sea over the last decades

Deep convection in the subpolar North Atlantic has been suggested to be a key process impacting the strength and variability of the Atlantic Meridional Overturning Circulation as well as the ocean’s uptake and deep storage of heat and anthropogenic CO_2. However, the spatial pattern and strength of deep convection are subject to variability on interannual-to-decadal timescales and despite intense research in the field the nature of this variability is not fully understood. In this work, we employ a hindcast simulation with the eddy-rich (1/20°) ocean/sea-ice model configuration VIKING20X to analyze the variability of deep convection in the subpolar North Atlantic over the last decades (1980-2018). A special focus is set on mixed layer depth (MLD) pattern and deep water formation characteristics in the Labrador versus Irminger Sea. We show that, in agreement with observations, the VIKING20X hindcast captures strong convection events with particularly deep MLDs in the winters of the early 1980s, late 1980s and early 1990s, as well as in recent years. Yet, there are large differences in the spatial pattern of the deep convection events, as well as in the volume and thermohaline properties of the newly formed deep water. Most notably, in recent years deep convection intensity, and in particular its spatial extent, increased in the Irminger Sea and decreased in the Labrador Sea compared to the late 1980s and early 1990s. We finally discuss potential drivers of the simulated changes, thereby contrasting the relative importance of wintertime atmosphere-ocean buoyancy fluxes and oceanic preconditioning.