Deep Water Formation and Meridional Overturning Circulation in High-Resolution Simulations of the Subpolar North Atlantic

A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last five decades based on the high-resolution (1/20°) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4°-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit strong differences in the total overturning transport, i.e., the AMOC maxima vary between 14 and 22 Sv at the southern exit of the subpolar basin, reflecting its well-known sensitivity to details in model configurations and forcing. In contrast, the experiments concur in that the overturning is mainly fed by sinking (in both depth and density space) in the northeastern Atlantic, in accordance with the first results of the OSNAP observational program. In line with earlier theoretical studies and process modelling, the sinking is largely confined to the continental boundaries. The model simulations show its particular concentration along the eastern slope of Greenland, highlighting the important contribution of the descending motion and entrainment processes associated with the outflows from the Nordic Seas. The simulations further suggest that ratio between the net sinking in the western and eastern basin has not been constant during the last decades: while the contribution of the Labrador Sea was rather small during the 1970s and 1980s, and again during the last years, it was substantially enhanced following the period of exceptionally strong convection in the 1990s.