Sudden, local temperature increase above the continental slope in the Southern Weddell Sea, Antarctica

Abstract. Around most of Antarctica, the Circumpolar Deep Water (CDW) shows a warming trend. At the same time, the thermocline is shoaling, thereby increasing the potential for CDW to enter the shallow continental shelves and ultimately increase basal melt in the ice shelf cavities that line the coast. Similar trends, on the order of 0.05 °C and 30 m per decade, have been observed in the Warm Deep Water (WDW), the slightly cooled CDW derivative found at depth in the Weddell Sea. Here we report on a sudden, local increase in the temperature maximum of the WDW above the continental slope north of the Filchner Trough (25–40° W), a region identified as a hotspot for potential changes in the flow of WDW towards the large Filchner-Ronne Ice Shelf. A combination of new and historical Conductivity-Temperature-Depth profiles and mooring records show that, starting in late 2019, the temperature of the warm water core increased by about 0.1 °C over the upper part of the slope (700–2750 m depth). The increased temperature of the WDW is accompanied by an unprecedented (in observations) freshening of about 0.1 g kg^-1 in the overlying Winter Water. Mooring records from the continental shelf further south, in the inflow pathway, do not show increased temperatures during the same period, suggesting that factors other than the WDW core temperature over the slope determine the variability in the heat content on the shelf.