Applications of time series of microwave backscatter over the Antarctic region

The 500 km wide swath of the ERS Wind scatterometer and its orbit geometry allows the measurement of the microwave backscatter of all the Antarctic region between latitudes 55°S and 79°S every three days. The values of the backscatter coefficient depend on the incidence angle and look direction of the antenna beams. By removing the contributions to the backscatter of these two effects we derive normalised values of the backscatter coefficient, and from these synthesise a sequence of composite radar images of the surface. The almost continuous operation of the AMI [Active Microwave Instrument] in wind-mode provides a regular series of images commencing in August 1991. The image sequence shows the interannual variability of melting of the surface snow cover. The increase in moisture content of the snow with warmer temperatures reduces the backscatter coefficient by as much as 25 dB below its usually very stable long term value. This signal provides a sensitive indicator of the intensity of surface melt, which is related to daily mean or maximum temperatures, as well as an estimate of the spatial extent of melting and its temporal variability. The sequence of images clearly shows the drift of large icebergs and a major calving event from the West Ice Shelf in May 1994, which generated two very large icebergs. The first has drifted westward around Antarctica for 5000 km. The drift rate indicales a variation in speed of the westward continental slopc current between 5 and 20 cm sec -1 . The Drift tracks of other large icebergs show the variability in the ocean current around the continent and north trending currents that link to the Antarctic Circumpolar Current further nonh. The backscatter from the icebergs is influenced by their surface conditions, including melting, and their size. A decrease with time in backscatter from smaller icebergs provides a measure of the reduction of their surface area by erosion.