Surface wave decay and directional spectra in the marginal sea ice zone measured by an autonomous underwater vehicle

The March 2003 deployment of Autosub in the Antarctic was the first field study under the Autosub Under Ice program of the UK Natural Environment Research Council. Several missions were run under sea ice in the western Bellingshausen Sea at depths ranging from 90 to 200 m. Data from the upward-looking ADCP on the autonomous underwater vehicle (AUV) indicate a strongly oscillating horizontal velocity of the ice underside due to ocean swell. Swell period, height, direction, and directional spread are computed every 800 m from the ice edge to 10 km inward. To our knowledge, these are the first scalar and directional wave data collected by an AUV under sea ice. We observe exponential, period-dependent attenuation of waves propagating through sea ice. Mean period increases with distance from the ice edge. Directional spectra show gradual changes in swell properties during propagation through the ice pack. The wave field appears to refract during propagation. The spread does not seem to relate to distance from the ice edge. More under-ice runs and modeling are needed to confirm these observations. If suitably deployed, an ordinary ADCP may be used with this technique to study waves in open or ice-covered water, both scalar and directional properties.