Presenter: Robert Jnglin Wills (University of Washington)

Description:
Water-mass transformation in the North Atlantic plays an important role in the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Here we analyze subpolar North Atlantic water-mass transformation and its influence on AMOC in a high-resolution version of CESM1 and in several low-resolution coupled climate models. While the high-resolution simulations reproduce the climatological water-mass transformation found in a reanalysis-forced high-resolution ocean simulation more accurately than low-resolution simulations, the mechanisms of low-frequency AMOC variability are qualitatively similar to those in low-resolution models. Anomalous northwesterly winds from eastern North America over the North Atlantic act to increase upper ocean density in the Labrador Sea region, enhancing deep convection, which later increases AMOC and the associated poleward ocean heat transport (OHT). The strengthened AMOC carries warm, salty water into the subpolar gyre, reducing deep convection and weakening AMOC and OHT. This mechanism, where changes in AMOC and OHT are driven primarily by changes in Labrador Sea deep convection, holds not only in models where the climatological (i.e., time-mean) deep convection is concentrated in the Labrador Sea, but also in models where the climatological deep convection is concentrated in the Greenland–Iceland–Norwegian (GIN) Seas or the Irminger and Iceland Basins. These results suggest that despite recent observational evidence suggesting that the Labrador Sea plays a minor role in driving the climatological AMOC, the Labrador Sea may still play an important role in driving low-frequency variability in AMOC and OHT.