Wind driven Ekman transport vs eddies – ultimate fight or peaceful cooperation in the Labrador Sea?

The Labrador Sea’s surface circulation remains important for the large-scale thermohaline circulation due to its fast response to atmospheric forcing and strong links to the North Atlantic and the Arctic Ocean’s counterparts. Its role in redistribution of heat and momentum, as well as for the biochemical exchange with the atmosphere is crucial in several time and space scales. The region is characterised by advection of freshwater originating from the combined melt of the Arctic Ocean’s sea-ice and Greenland’s glaciers around and towards the interior of the Labrador Sea. The fate of surface freshwater is an important factor that modifies ocean stratification, deep water convection and thus, ocean climate. Despite the major role of surface freshwater in the Labrador Sea, the dominant mechanism responsible for its offshore transport remains debatable, whether it is due to wind-induced Ekman transport, particularly strong in winter, or to eddy advection.

To explore this disagreement, we use surface drifters deployed in three seasons: 50 in December 2019, 50 in March 2020 and 50 in August 2020 in the shelf/slope location off Cape Desolation and near Qaqortoq, a town in the south-west Greenland. The drifters are equipped with temperature sensors and underwater drogues allowing them to follow the cyclonic surface currents: first, the along-shelf, coastal current and along-slope, boundary current west of Greenland; then, if they are able to detach from the shelf edge, the interior circulation of the central Labrador Sea that directs them south-westward from the Davis Strait; eventually, joining the coastal and along-slope boundary currents east of Labrador before circulating into the Labrador Sea’s central basins or eventually leaving the study area.

To investigate the dominant force responsible for the surface transport we use a wind product (ERA5) in a combination with daily SST (OSTIA). Detachment from boundary current is defined as crossing of the 2500 m isobath. The number of crossings varies depending on the season and weather conditions, e.g. an abrupt change in wind direction. This, in turn, may create upwelling of deep-water masses near the shelf-break. However, trajectories of drifters superimposed on SST maps indicate that besides Ekman transport, eddies carry shelf-originating water offshore as well. Auxiliary data from below (Argo floats and other CTD profiles collected near the drifters) allow to distinguish how deep both processes can leave their signature or whether they can drive a return flow.

If any substantial changes in the North Atlantic wind field occur in the future, the fate of the surface water transport in the Labrador Sea will also change, both in respect to its volume and direction. This could potentially affect the balance between Ekman transport and eddies revealed by our analysis of surface drifters data.