How fast is landfast sea ice? A study of the attachment and detachment of nearshore ice at Barrow, Alaska

During the two winters between 2003 and 2005, a land-based marine radar observed the nearshore ice motion during the development and decay of landfast ice near Barrow, Alaska. The radar imagery captured individual events at high temporal resolution, revealing deformation processes and allowing calculation of ice velocity and acceleration. Atmospheric forcing during these events appeared to be irrelevant since no corollary was found in local meteorological observations. Detailed examination of the radar imagery showed that backscatter from sea ice targets oscillated in signal strength (flickered) prior to detachment, as previously observed by (Shapiro, L.H. (1987), Mechanical Properties of Sea Ice Deformation in the near Shore Zone, in OCSEAP Final Reports, V.72, pp. 357–584, U.S. Dept. of Commerce, NOAA). Determination of ice acceleration after detachment allowed estimation of water drag beneath the ice.The distribution of grounded ridges at the end of each annual cycle was determined from field measurements of ice elevation, ice thickness and water depth. Applying a simple theoretical treatment of coupling between a first-year ridge keel and the seabed, we calculate that the anchoring strength provided by ridges was 2–3 orders of magnitude greater than typical wind or water stresses. Therefore, we conclude that additional decoupling processes, such as sea level surges or thermal erosion of keels, must occur in addition to offshore current stress in order to cause the landfast ice to detach. Although the nature of these processes is not clear, they are likely to be the cause of the flickering observed in radar imagery, which could therefore be useful in short-term prediction of detachment events.