Recent and projected shifts from surface density compensation to warming, freshening and stronger stratification in the subpolar North Atlantic

Abstract

The hydrography and stratification of the subpolar North Atlantic is highly variable, with convection activating and deactivating across parts of the Labrador and Irminger seas. Likely consequential for the Atlantic Meridional Overturning Circulation (AMOC), this variability is examined in an eddy-resolving ocean model hindcast spanning 1958-2021 and in 1950-2050 simulations with four climate models, spanning differences in ocean resolution (eddy resolving or permitting), code and implementation. Stratification of the Labrador and Irminger seas is quantified with the Potential Energy Anomaly (PEA) in the upper 1000 m of the water column. Monthly PEA anomalies are evaluated alongside corresponding anomalies of sea surface temperature, salinity, and density. For 30-year windows, moving correlations between PEA and surface properties are obtained over the 100-year simulations to characterize the evolving relationships. As climate change progresses, stratification in three of the four models is increasingly associated with variable surface salinity, in both regions. Lagrangian analyses of surface flow pathways in the decades preceding 1990 and 2040, for one of the models in which surface salinity grows in influence, indicate that the subpolar presence of fresh Arctic waters and saline Atlantic waters increase and decrease respectively by 2040. Furthermore, in three of the four models, surface density compensation associated with correlation of surface temperature and salinity anomalies is progressively replaced by combined surface warming and freshening, lowering surface density, and strengthening stratification. The extent of these model-dependent changes and processes are of consequence for the projected fate of the AMOC by the mid 21st century.