Water exchange between the continental shelf and the cavity beneath Nioghalvfjerdsbrae (79 North Glacier)

The mass loss at Nioghalvfjerdsbrae is primarily due to rapid submarine melting. Ocean data obtained from beneath the Nioghalvfjerdsbrae ice tongue show that melting is driven by the presence of warm (1 degrees C) Atlantic Intermediate Water (AIW). A sill prevents AIW from entering the cavity from Dijmphna Sund, requiring that it flow into the cavity via bathymetric channels to the south at a pinned ice front. Comparison of water properties from the cavity, Dijmphna Sund, and the continental shelf support this conclusion. Overturning circulation rates inferred from observed melt rates and cavity stratification suggest an exchange flow between the cavity and the continental shelf of 38mSv, sufficient to flush cavity waters in under 1year. These results place upper bounds on the timescales of external variability that can be transmitted to the glacier via the ice tongue cavity.