Coastal landfast sea ice decay and breakup in northern Alaska: Key processes and seasonal prediction

Seasonal break-up of landfast sea ice consists of movement and irreversible ice detachment in response to winds or oceanic forces in the late stages of ice decay. The break-up process of landfast sea ice in the Chukchi Sea at Barrow, Alaska was analyzed for the years 2000 through 2010, based on local observations of snow and ice conditions, weather records, image sequences obtained from cameras, coastal X-band marine radar, and satellite imagery. We investigated the relation of break-up to winds, tides, and nearshore current measurements from a moored Acoustic Doppler Current Profiler (ADCP). Two break-up modes are distinguished at Barrow based on the degree of ice decay. Mechanical break-up due to wind and oceanic forces follows ablation and weakening of the ice. Thermal break-up is the result of ice disintegration under melt ponds, requiring little forces to induce dispersion. Grounded pressure ridges are pivotal in determining the break-up mode. The timing of thermal break-up of the near-shore ice cover was found to correlate with the measured downwelling solar radiation in June and July. This linkage allows for the development of an operational forecast of landfast ice break-up. Results from forecasts during two years demonstrate that thermal break-up can be predicted to within a couple of days two weeks in advance. The cumulative shortwave energy absorbed by the ice cover provides for a measure of the state of ice decay and potential for disintegration. Discriminating between the two modes of break-up bears the potential to greatly increase forecasting skill.