D5 Oceanography under Ice

Topic

Under-ice oceanography measures the physical and dynamic properties of the ocean in some of the planet's most inaccessible regions: the ocean extending beneath Antarctica's floating ice shelves and the ocean flowing under Arctic sea ice. These measurements are critical because the heat transported by this ocean is the primary driver of basal melting in floating ice shelves and acts as the most active feedback mechanism regarding the instability of marine-based ice sheets.

A conductivity, temperature, and depth (CTD) instrument is lowered through holes drilled in the ice—or directly from a ship in open-water areas—measuring the vertical temperature and salinity profile of the water column as it descends; this profile determines the ocean water's density at various depths and, consequently, how the ocean is stratified into layers.

In contrast, a fixed ocean mooring—with instruments anchored directly to the seabed—records a continuous time series of water velocity, temperature, and salinity, capturing both tidal variability and occasional warm-water intrusions reaching the mooring's location.

An autonomous underwater vehicle (AUV) can operate directly beneath a floating ice shelf for several consecutive hours without maintaining a surface connection; during this time, it maps the bathymetry of the ice shelf's underside and measures the oceanic heat fluxes reaching that base. Once the data from these instruments are combined, the expected outcome is the identification of the distinct ocean water layers interacting with the base of a floating ice shelf—specifically, a layer of Circumpolar Deep Water with temperatures above freezing that penetrates the cavities beneath the ice and melts it from below at rates ranging from one to one hundred meters per year, thereby closing the link between ocean heat and the basal melting of floating ice shelves with which this description began.

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