Where does the freshwater go?: Pathways for Greenland and Arctic waters from the east Greenland shelf to the North Atlantic

Abstract

In the Atlantic, the ocean circulation transports warm and light waters northwards at the surface, and cold and dense waters southwards at depth. This large-scale circulation is called Atlantic Meridional Overturning Circulation (AMOC) and is a key element of the climate system. Increasing fluxes of freshwater from Greenland and the Arctic to the Subpolar North Atlantic (SPNA) could contribute to the weakening of the AMOC in a changing climate by inhibiting deep convection and the formation of deep waters in the Labrador Sea, Irminger Sea and Nordic Seas. However, these fresh waters are first advected over the Greenland continental shelf by narrow western boundary currents. While west of Greenland, both eddies and winds are known to play a role in steering shelf waters into the interior of the Labrador Sea, possible pathways for freshwater into the Irminger and Nordic Seas east of Greenland have been less studied. As recent observations demonstrated the importance of overturning east of Greenland for the AMOC, it is critical to better understand how fresh shelf waters reach these regions. To do so, I used a new observational dataset, composed of 120 surface drifters deployed in five different regions of the east Greenland shelf between 2019 and 2022. Trajectories from these surface drifters show very limited advection of fresh shelf waters into the Irminger Sea and the Nordic Seas. Some areas of the east Greenland shelf are more favourable to export, and eddies and winds can drive local exchanges between the shelf and neighbouring seas. For instance, I show that at the southern tip of Greenland, strong westerly wind events can drive fresh surface waters off-shelf. This new drifter dataset also allowed to investigate the surface circulation over the shelf in areas where observations were so far sparse. The western boundary current that flows along the east Greenland coast is composed of two distinct branches, and I show that the complex shape of the seabed causes these branches to interact. This leads to further constrain of fresh waters towards the coast along the south-east Greenland shelf, and to a spreading of fresh coastal waters towards the edge of the continental shelf at the southern tip of Greenland. The small scale of the east Greenland shelf is a major challenge for the ability of climate models to correctly represent the circulation of fresh shelf waters and where they exit the shelf. Using the HadGEM3 climate models at different resolution, I show that while the one-degree resolution model is unable to represent the circulation around Greenland, increasing the resolution to 0.25 and 0.12 degrees is not sufficient to properly represent cross-shelf exchanges, even though it allows for a better representation of the circulation. This misrepresentation can lead to under or overestimation of freshwater export into the interior seas of the SPNA. Further observational and modelling studies are necessary to understand exchanges between the north-east Greenland shelf and the Nordic Seas, the role of sea ice, and to quantify the input of freshwater to deep convection regions.