Antarctic Sea Ice Thickness Project
Alfred-Wegener-Institut für Polar- und Meeresforschung
Sea ice plays an important role in the climate system since it modifies the surface radiation balance due to its high albedo, and it influences the exchange of momentum, heat and matter between atmosphere and ocean because of its insulating behaviour. During cooling periods the freezing of sea ice initiates brine expulsion and subsequent convection with deepening of the surface mixed layer and the formation of deep and bottom water. During the melting period relatively fresh water stratifies the oceanic surface layers. Melting generally occurs in places far away from the formation areas because of the sea ice motion. The net freezing rate in a certain area averaged over one seasonal cycle is therefore zero, but rather positive or negative depending on the divergence or convergence of the sea ice flow.
The most important sea ice variables relevant for climate are drift, thickness and concentration. There are several data sets for sea ice concentration and drift. Unfortunately, observations of sea ice thickness are rather limited, mainly because these data have to be taken in situ. Presently various of methods to determine sea ice thickness from satellites are in development and require appropriate validation by in situ observations from upward looking sonars. The most promising technique to obtain time series of sea ice thickness at certain crucial locations is provided by upward looking sonar systems deployed on oceanographic moorings. In order to support and co-ordinate international efforts the World Climate Research Programme (WCRP) has started under the Arctic Climate System Study/Climate and Cryosphere (ACSYS/CLIC) the Antarctic Ice Thickness Project (ANSITP) in 1990.
The programme started with six moorings in 1990. Up till now 34 ULS records have been obtained successfully from 59 ULS deployments, and 6 moorings equipped with ULS are still active.
Participated institutions are the Alfred-Wegener-Institut für Polar- und Meeresforschung (AWI, Bremerhaven, Germany) with ULS moorings in the Weddell Sea, the Antarctic Cooperative Research Centre (ACRC, Hobart, Australia) with ULS moorings off East Antarctica, the Norsk Polarinstitutt (NPI, Tromsø, Norway) with ULS moorings off the Filchner-Ronne Shelf Ice, and the Woods Hole Oceanographic Institution (WHOI, Woods Hole, MA, USA) with ULS moorings east off the Antarctic Peninsular.