Deep Convection Variability in the Labrador Sea Versus the Irminger Sea over the last Decades as Simulated with an Eddy-Rich Ocean General Circulation Model

Deep convection in the Northwest Atlantic has been suggested to be a key process impacting the strength and variability of the Atlantic Meridional Overturning Circulation (AMOC) as well as the oceans 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. Despite intense research in the field, the nature of this variability as well as the nature and strength of its link to the AMOC and the oceanic CO₂ and heat uptake are not fully understood. In this work, we employ a hindcast simulation with the eddy-rich (1/20°) ocean model configuration VIKING20X to analyze the variability of deep convection in the Northwest Atlantic over the last decades (1958-2018). A special focus is set on mixed layer depth (MLD) pattern and deep water formation characteristics in the Labrador Sea versus the Irminger Sea. We emphasize that a proper representation of the mesoscale eddy field is crucial to realistically simulate MLD pattern and deep water formation in the Northwest Atlantic. VIKING20X matches observations better than (i) coarser resolution model configurations, and (ii) its precursor VIKING20 at the same horizontal resolution due to configuration details such as lateral boundary conditions. We further show that, in agreement with observations, the VIKING20X hindcast captures strong convection events with particularly deep MLDs in the winters of the late 1980s and early 1990s as well as in recent years, but with temporally-varying spatial pattern and thermohaline properties of the newly formed deep water. Most notably, in recent years convection intensity greatly 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 this simulated change.