Measurements under Fimbulisen Using an Autonomous Underwater Vehicle

Antarctic ice shelves are the floating extensions of the Antarctic Ice Sheet, covering around 40% of the continental shelf. The cavities beneath the ice shelves are important to the climate system through their contribution to the production of the globally important Antarctic Bottom Water. However, they remain some of the most difficult areas of the world ocean to access. Many of the processes that take place under larger ice shelves can be observed more conveniently beneath smaller ice shelves such as Fimbulisen, an ice shelf in the eastern Weddell Sea. In February 2005 an Autonomous Underwater Vehicle (AUV), Autosub-2, was the first AUV successfully to perform measurements beneath an ice shelf when it was sent under Fimbulisen. The data from this mission include temperature, salinity, and current measurements, as well as an acoustic image of the underside of the ice. The data from Autosub show that (relatively) warm water can access the base of the ice shelf; this inflow is probably controlled by winds near the ice front. Measurements from the edge of the ice shelf provide some clues to the route taken by the warmer water, and help present the ice shelf in its oceanographic context. Although large parts of the base of the ice shelf are relatively smooth, there are also large patches of rough ice evident in the sonar data. These unexpected rough areas appear to correspond to smooth surface features known as flow traces. The roughness of the ice shelf base could have significant implications for the way we model the interaction between ice shelves and the ocean, the results of which are used to parameterize the effect of ice shelves in global climate models. Our measurements show that we may need to reassess many of our long-held beliefs about ice shelf/ocean interaction in general, and that the circulation under the eastern Weddell ice shelves is very sensitive to changes in local climatic conditions.