Role of atmospheric feedbacks in the ocean’s response to enhanced Greenland Ice Sheet–melting

Torge Martin, Jan Harlass, Arne Biastoch and Katja Bettina Matthes, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
The currently observed increase in Greenland Ice Sheet–melt is anticipated to lead to a reduction of deep convection in the subpolar North Atlantic and ultimately a weakening of the Atlantic Meridional Overturning Circulation (AMOC). We conducted a suite of coupled climate and forced ocean-only model experiments with the Flexible Ocean and Climate Infrastructure (FOCI) in which we enhance Greenland runoff to 0.05 Sv as a step change for 100 years to enable the calculation of climate response functions. The magnitude of the freshwater flux resembles the upper envelope of estimated ice mass loss in the late 21stcentury. In all experiments we apply a two-way nested ocean between 30 ̊N and 85 ̊N refining the ocean grid from 1/2 ̊ to 1/10 ̊. The resulting eddy-rich ocean in the region of the Gulf Stream and subpolar gyre facilitates a significant reduction of well-known surface biases.

These systematic experiments are used to discuss the role of atmospheric feedbacks in setting the North Atlantic’s sensitivity to enhanced Greenland Ice Sheet–melt. Seasonality likely plays a major role in the ocean’s sensitivity to meltwater but also atmospheric feedbacks. Meltwater runoff peaks in summer, which is maintained in our experiments. We find an annual maximum of meltwater concentration in the adjacent seas in early fall, a realistic delay. In the subpolar gyre region, the strongest reduction in surface heat fluxes (-30%) is found in late fall/early winter, a change occurring already during the first decade with enhanced runoff. Simultaneously, precipitation increases significantly, almost doubles in the gyre center on long-term mean. Both changes act together in stabilizing the local stratification and inhibiting the important preconditioning for open ocean deep convection in the Labrador Sea in winter. While the reduced heat flux is a feature of both, coupled and forced experiments, the increased precipitation is only found in the coupled simulation.