Recent changes in the microwave scattering properties of the Antarctic ice sheet

Time series, satellite microwave data are used to monitor and quantify changes in the scattering properties of the Antarctic ice sheet. Daily ERS scatterometer (EScat) and Special Sensor Microwave/Imager (SSM/I) image data, acquired since 1992, are analyzed to understand the seasonal and interannual changes over the ice sheet. For regions of the ice sheet where azimuthal modulation is negligible, seasonal cycles are observed in both the EScat (amplitude /spl sim/0.5 dB) and SSM/I data (amplitude /spl sim/10 K). These cycles are attributed to seasonal changes in surface temperature. Interannual variability in the time series signatures appears to be associated with accumulation. There is also evidence to suggest that shifts in the wind direction can alter the backscatter through azimuthal modulation. Over the period 1992-97, large trends are observed in the EScat(<-0.3 dB yr/sup -1/) and SSM/I (>1 K yr/sup -1/) signatures over several regions in Antarctica. These changes typically occur over ice shelves and at the margins of the ice sheet where previous melt events have occurred, and where accumulation is relatively high (>300 mm yr/sup -1/). It is likely the large changes result from the successive burial of an efficient scattering layer formed by refreezing after a melt event prior to 1992. There is also evidence to suggest that similarly large changes can be observed in the interior of the ice sheet due to the burial of depth hear layers. In order to monitor long term change in the properties of the Antarctic ice sheet, it is necessary to remove the seasonal cycle from time series microwave data. Such anomaly data can then be used to understand the link between EScat and SSM/I with accumulation and wind shifts.