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The ice shelf water (ISW) found in the Filchner Trough, located in the southern Weddell Sea, Antarctica, is climatically important; it descends into the deep Weddell Sea contributing to bottom water formation, and it blocks warm off-shelf waters from accessing the Filchner ice shelf cavity. Yet the circulation of ISW within the Filchner Trough and the processes determining its exchange across the ice shelf front are to a large degree unknown. Here mooring records from the ice shelf front are presented, the longest of which is 4 years long. They show that the coldest (T = -2.3 degrees C) ISW, which originates from the Ronne Trough in the west, exits the cavity across the western part of the ice shelf front during late austral summer and early autumn. The supercooled ISW escaping the cavity flows northward with a velocity of about 0.03 m/s. During the rest of the year, there is no outflow at the western site: the current is directed eastward, parallel to the ice shelf front, and the temperatures at the mooring site are slightly higher (T = -2.0 degrees C). The eastern records show a more persistent outflow of ISW.

Plain Language Summary Antarctica is surrounded by large, floating ice shelves covering vast ice shelf cavities that are filled with sea water. The circulation of water within the cavity brings heat toward the ice shelf base, which causes the ice shelves to melt from below. To understand the future evolution of the Antarctic ice shelves and the ice sheet upstream, we need to understand the physics governing the sub-ice shelf circulation and the processes determining the heat transport across the ice shelf front. Here we present mooring records from the front of the Filchner ice shelf in the Weddell Sea, Antarctica. The unique records show that there is a seasonal outflow of water that has been cooled down below the surface freezing point temperature through interaction with the glacial ice, across the western part of the front. The outflow across the eastern part of the front is stronger, more persistent, and slightly warmer. It is hypothesized that the seasonality in the western outflow is caused by changes in the stratification. The findings reopens the question about the potential blocking effect caused by the large step in bathymetry effectuated by the ice shelf front.