Geophysical signatures from precise altimetric height measurements in the Arctic Ocean

The use of altimeter data in the polar regions has previously been limited by the presence of permanent and seasonal ice cover. Changes in the radar echo shape received by the altimeter over sea ice, as compared with the open ocean, cause problems in the on-board estimates of surface height, making the data unusable. The majority of noise on the signal can be reduced by retracking the full waveform data set (WAP). Careful quality control is applied to ensure that only those return echoes from which accurate height measurements can be obtained are retained. Consideration of possible backscattering mechanisms from ice and water, and comparisons with imagery, suggest that the specular waveforms typically found in altimeter data over sea ice originate from regions of open water or new thin ice exposed within the altimeter footprint. However, diffuse waveforms similar to those found in ice free seas have been observed in areas of consolidated ice, and may be used to measure ice freeboard. Until recently, even retracked heights contained substantial residual errors due to the interaction of the on-board tracking system with the complex return echoes over sea ice. Software simulation of the tracking system has led to the development of new ground processing algorithms, which further reduce the short wavelength (/spl sim/26 km) noise, from 30-50 cm to around 7 cm. This provides, for the first time in ice covered seas, the capability for accurate mean sea surface generation, measurement of tidal and oceanographic signals and determination of sea ice freeboard. The authors present the results of comparisons of sea surface height variability from ERS-2 radar altimetry in the Arctic with the output from a high resolution Arctic Ocean circulation model.