Physical Oceanography

Sub Ice Shelf Circulation

A large portion of the Antarctic continent is fringed with Ice Shelves, the floating extension of an Ice Sheet. Interactions between the glacial ice and the seawater entering the ice shelf cavity leads to basal melting, freezing and the formation of Ice Shelf Water – the coldest water found in the ocean.

Ice shelf cavity processes
Ice shelf cavity processes
Nicholls et al. (2009)

Main content

The Filchner-Ronne Ice Shelf (FRIS) in the southwestern Weddell Sea is one of the largest in Antarctica and its grounding line, the point where the ice sheet goes afloat and becomes an ice shelf, is found as deep as 1800m.  Most of the water entering the FRIS cavity is at the surface freezing point – but since the freezing point of water decreases with increasing depth it can be cooled further and melt ice at great depth. When mixing with the meltwater the seawater gets fresher and lighter, and starts to rise along the ice shelf base. As it rises, the in situ freezing point increases, and the rising plume will eventually become supercooled. This leads to the formation of ice crystals that aggregate at the base of the ice shelf and form layers of marine ice that may be many hundreds of meter thick. The result is a transfer of ice shelf mass from the deeper, thicker part of the ice shelf to the shallower, thinner part and a net cooling and freshening of the seawater. The resulting watermass - the Ice Shelf Water – leaves the cavity and makes up the Filchner Overflow.

To study the circulation within the FRIS cavity, scientists from GFI and British Antarctic Survey have drilled through the more than 750 m thick ice to deploy instruments within the cavity.

Further east along the Antarctic Coast, at the Greenwich meridian, the Fimbul ice shelf is an example of another type of ice shelf. The water entering under the ice here is warmer as the ice shelf is located close to the continental slope were warm Circumpolar Deep Water is found at ~300 m depth.  Sverdrup (1953) was one of the first to study the circulation in this sector, and his conclusion that it is the wind stress pushing freshwater towards Antarctica that protects the ice shelves here from the warmer waters to the north are still found to be correct. New results from the Fimbul ice shelf additionally indicate that eddies transport warm water towards the ice close to the ocean floor, and that solar heated water during summer is pushed down below the ice and melts ice in the shallow parts of the ice shelf. 

Svein Østerhus
Lars-Henrik Smedsrud

Project: Fimbul Ice Shelf – top to bottom