Presenter: Torge Martin (GEOMAR Helmholtz Centre for Ocean Research Kiel)

Description:
Enhanced freshwater release from the Greenland Ice Sheet into the subpolar North Atlantic is anticipated to enhance upper ocean stratification, impede deep convection and weaken the AMOC. We show that both air-sea coupling and ocean mesoscale processes significantly affect the AMOC response. We conducted a hierarchy of model experiments using forced ocean-only and coupled climate simulations with the ocean model NEMO3.6-LIM2 embedded in our Flexible Ocean and Climate Infrastructure (FOCI). Making use of 2-way ocean nesting, we refined grid resolution from 1/2˚ to 1/10˚ enabling an eddy rich simulation of the North Atlantic. The four model configurations were applied to a single-perturbation sensitivity experiment, enhanced Greenland runoff at 0.05 Sv over 60-100 years, which we compare to standard reference runs. Across all experiments winter mixed layer depth (MLD) is reduced in the subpolar North Atlantic coinciding with the overall freshening but also with a large-scale cooling. The cooling in the forced experiments exceeds that in the coupled ones. The spatial patterns of MLD and SST reduction are tightly linked and depend on ocean dynamics. Peak cooling occurs in highly dynamic areas such as the North Atlantic Current (NAC) and boundary flows, which are more realistic in the nested configurations. Surface heat loss by the ocean in winter is reduced broadly in the two forced experiments whereas the coupled ones also show increases. The pattern depends again on ocean dynamics and heat loss reduction occurs farther east in the non-eddying experiments with a too zonal NAC.  An integral of all this is the AMOC response: its decline—ranging between 1.0 and 5.1 Sv after a couple of decades—is stronger by 1.2 Sv in the two nested experiments but consistently weaker in the coupled simulations than the forced ones by 3 Sv. Our results suggest that ocean-only models lacking atmospheric feedbacks tend to overestimate the impact of future Greenland Ice Sheet–melting.