Acoustically-tracked subsurface floats and network of mooring in the Filchner Trough provide observations of pathways and interannual variability of the warm inflow onto the continental shelf in the southern Weddell Sea

Abstract

Bottom water formation in the Weddell Sea and mass loss from the Filchner-Ronne Ice Shelf are tightly linked by the supply of Warm Deep Water to the continental shelf. Heavy sea ice cover and icebergs restrict ship access and upper ocean measurements by moorings, compelling us to try new sampling methods. We present results from two unprecendented and complementary datasets of the Filchner-Ronne continental shelf,  spanning 2017-2021: (i) the first dedicated under-sea-ice float experiment tracking circulation on the continental shelf; and (ii) an extensive network of moorings installed at key sites along the Warm Deep Water inflow pathway. The floats capture the southward inflow of Warm Deep Water over the eastern continental shelf and a cold Ice Shelf Water regime with a recirculation of ISW in the Filchner Trough throughout the four years of observations. In the present circulation regime, the warm inflowing water is blocked from reaching the ice shelf cavity due to the presence of the thick Ice Shelf Water layer inside the Filchner Trough. The floats' trajectories and hydrography reveal the dynamically active front, flow reversal, and eddy generation between these two water masses along the eastern flank of the Filchner Trough. The moorings complement the floats information by revealing internannual variability of the warm inflow. The inflow is regulated by the heaving of isopycnals over the continental slope, and the southward propagation toward Filchner Ice Shelf is twice faster during the warm 2017-2018 years. Furthermore, the warm years coincide with low summer sea ice concentration, which enhances surface stratification through increased freshwater input and modifies sea ice-ocean stresses that both act to lift the warm water layer and increase the temperatures on the continental shelf. The recent record low sea ice conditions around the Antarctic emphasize the importance of our findings and raise concerns regarding a potentially increasing presence of Warm Deep Water on the southern Weddell Sea shelf.