Ocean response to Greenland melting in a hierarchy of model configurations: Relevance of eddies and an interactive atmosphere

Both Greenland Ice Sheet mass loss and Atlantic Meridional Overturning Circulation weakening are considered tipping elements of the climate system under global warming. Ocean and climate models of varying complexity are widely applied to understand and project the future evolution of the two processes and their connection. The results are prone to model uncertainty however. Especially the role of regional mesoscale processes in the subpolar North Atlantic is still being investigated. We ran a systematic set of eight dedicated 60 to 100-year long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated, with regular and significantly enlarged Greenland runoff to reconcile findings of the regional ocean and global climate modeling communities.

The most prominent result is a major impact by an interactive atmosphere for limiting the AMOC weakening through enabling a compensating temperature feedback. Coupled experiments yield an AMOC decline of <2Sv to a freshwater perturbation of 0.05Sv whereas the AMOC weakens by >4Sv in the ocean-only runs. In addition to this large-scale effect, we find that the Labrador Sea and the Northwest Corner (off Flemish Cap) are critical regions for the role of mesoscale eddies in redistributing Greenland meltwater and affecting the timing of its impact. We show that an ocean grid at 1/10˚–1/12˚, which is currently used in global high-resolution climate simulations, can already significantly improve the path of the meltwater along the North American coast and into the wider North Atlantic. But the same resolution still falls short in providing sufficient dynamical exchange between the boundary current and the interior Labrador Sea and especially lacks capability in restratifying the Labrador Sea after deep convection. Our experiments demonstrate where an eddy parameterization works quite successfully and where only high resolution (>1/12˚) yields a realistic ocean response. This underlines the necessity to advance scale-aware eddy parameterizations for next-generation climate models.