Under ice measurements by seals carrying a miniaturized conductivity–temperature–depth (CTD) instrument fill an important gap in existing observations. Here we present data from an instrumented Weddell seal that spent 8 consecutive months (February–September) for aging in close proximity to Filchner Ice Shelf, thus providing detailed information about the evolution of mixed layer hydrography during the austral autumn and winter. The resultant time series of hydrography shows strong seasonal water mass modification, dominated by an upper–ocean (0–300 m) salinity increase of 0.31, corresponding to 3.1 m sea ice growth, and the development of a 500 m thick winter mixed layer. Observations furthermore highlight a gradual salinity increase in a slow (3–5 cms−1 ) southward flow on the continental shelf, to wards the site, and suggest that the inferred ice production is better considered as a regional average rather than being purely local. No clear seasonality is observed in the properties of the underlying Ice Shelf Water.
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