Vulnerability of Denman Glacier to Ocean Heat Flux Revealed by Profiling Float Observations

Denman Glacier, which drains a marine‐based sector of the East Antarctic Ice Sheet with an ice volume equivalent to 1.5 m of global sea level rise, has accelerated and undergone grounding line retreat in recent decades. A deep trough and retrograde bed slope inward of the grounding line leave this glacier prone to marine ice sheet instability. The ocean heat flux to the ice shelf cavity is a critical factor determining the susceptibility of the glacier to unstable retreat. Profiling float observations show modified Circumpolar Deep Water as warm as −0.16°C reaches a deep trough extending beneath the Denman Ice Tongue. The ocean heat transport (0.77 ± 0.35 TW) is sufficient to drive high rates of basal melt (70.8 ± 31.5 Gt y−1), consistent with rates inferred from glaciological observations. These results suggest the Denman Glacier is potentially at risk of unstable retreat triggered by transport of warm water to the ice shelf cavity. Plain Language Summary The Denman Glacier is a vast river of ice that drains the East Antarctic Ice Sheet. The Denman holds a volume of ice equivalent to 1.5 m of global sea level rise, so changes in the glacier could have a large impact on future sea level rise. The vulnerability of the Denman Glacier to melting by warm ocean waters has been difficult to assess because very few oceanographic observations have been collected in the region. We use new profiling float measurements to show warm water reaches a deep trough that extends inland beneath the glacier, exposing the base of the ice to ocean‐driven melting. We estimate that the amount of warm water entering the cavity is sufficient to melt 70.8 billion tons of ice each year. These observations suggest that the Denman Glacier is potentially at risk from unstable retreat driven by warm water flowing into the cavity and melting the ice from below. Key Points Float profiles show warm (up to −0.16°C) modified Circumpolar Deep Water at depth in a trough extending below the Denman Ice Tongue A bottom‐intensified current on the eastern side of the deep trough transports 138 ± 65 mSv of water warmer than −1°C into the cavity Ocean heat transport into the cavity is sufficient to drive high basal melt (70.8 ± 31.5 Gt y−1), consistent with glaciological observations