Emerging impacts of enhanced Greenland melting on Labrador Sea dynamics

Abstract

Meltwater input to the subpolar North Atlantic from the Greenland ice sheet has been steadily increasing in the past decades due to global warming. To identify the impacts of this enhanced freshwater input since the late 1990s, we use output from the eddy-rich model VIKING20X (1/20˚) running two nearly identical simulations from 1997–2021 only differing in the freshwater input from Greenland: one with realistic interannually varying runoff increasing in the early 2000s and the other continued after 1997 using the local, grid-cell climatology of 1961–2000 maintaining the mean seasonal runoff cycle. Here, runoff is based on the JRA55-do reanalysis (Tsujino et al., 2018, Ocn.Mod.), which includes the Bamber et al. (2018, JGR-O) Greenland runoff and calving record, where liquid and solid discharge is combined into a single liquid flux entering the ocean through the surface and coast. Apart from this, atmospheric forcing is identical between the two runs. To our knowledge this is the first set of twin experiments with a most realistic, well validated, eddy-rich ocean model to assess the impact of the current, observed increase in Greenland ice sheet mass loss.  We find that the majority of the additional freshwater remains within the boundary current. This enhances the density gradient between the fresh and cool slope current and the warm and salty waters of the interior Labrador Sea and leads to a small (.01 m/s) but significant increase in boundary current speed in our experiment. Both, the faster slope current and the enhanced shelf–interior density gradient increase the potential for intensified eddy shedding into the interior Labrador Sea. This more dynamic regime fosters the eddy-driven import of fresh boundary current waters (Polar Water and meltwater) into the nearby deep convection regions. Lastly, our experiments indicate a role of enhanced Greenland runoff in the eastward shift of deep convection reported by Rühs et al. (2021, JGR-O) for the recent period 2015–2018. The experiment with realistically increased runoff exhibits meltwater tracer mixed only to shallower depths before transferred east into the Irminger Sea leading to a weaker stratification in the upper to mid-depth Irminger Sea than in the experiment with less, climatological runoff, which would enable or at least support deep convection southeast of Greenland.