Abstract
The southern Weddell Sea features a vast perennially ice-covered continental shelf with polynyas, strong sea ice formation, first- and multi-year ice. Sea ice and the general ocean circulation maintain predominantly near-freezing waters on the shelf, which help to maintain the comparatively moderate basal melt rates of the Filchner-Ronne Ice Shelf (FRIS), Antarctica’s largest ice shelf by volume. In contrast to FRIS, other West Antarctic ice shelves show strong basal melt rates, caused by warm intruding ocean waters. In the southern Weddell Sea, however, warm water inflows occur episodically and spatially limited, when modified warm deep water enters the continental shelf through incisions in the shelf break and flows southward towards the FRIS front. Overall, the majority of the shelf is dominated by dense and cold water masses such as High Salinity Shelf Water (HSSW) and Ice Shelf Water (ISW), which are precursors of Antarctic Bottom Water and thus relevant for the global ocean circulation. In 2018, a comprehensive CTD survey found unprecedented (in the available observations) volumes of ISW in Filchner Trough. The ISW was exported from underneath the Filchner Ice Shelf (FIS) following a shift to enhanced cavity circulation due to strong sea ice formation in front of the Ronne Ice Shelf. These Filchner Trough conditions are summarized as the “Ronne-mode”, which is in contrast to the “Berkner-mode”, characterized by a greater influence of locally-formed waters. In this presentation, we introduce new multi-year time series from an international mooring network from various Southeast Weddell Sea locations (sub-FIS, Filchner Trough and Sill), to highlight the temporal and spatial extent of the recent Ronne-mode event, which lasted from 2018-2020, before shifting back into a Berkner-mode. The dominance of either circulation mode is controlled by large-scale atmospheric forcing and has implications on ice shelf basal melt and dense water export into the Weddell Sea. 
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