4.4.4 Pack Ice Motion

Pack ice motion influences ice mass locally, through deformation and creation of open water areas; regionally, through advection of ice from one area to another; and globally through export of ice from polar seas to lower latitudes where it melts. The drift of sea ice is primarily forced by the winds and ocean currents. On time scales of days to weeks, winds are responsible for most of the variance in sea ice motion. On longer time scales, the patterns of ice motion follow surface currents and the evolving patterns of wind forcing. Here we consider whether there are trends in the pattern of ice motion.

4.4.4.1 Data Sources and Time Periods Covered

Sea ice motion data are primarily derived from the drift of ships, manned stations and buoys set on or in the pack ice. Although some individual drift trajectories date back to the late 19th century in the Arctic and the early 20th century in the Antarctic, a coordinated observing program did not begin until the International Arctic Buoy Programme (IABP) in the late 1970s. The IABP currently maintains an array of about 25 buoys at any given time and produces gridded fields of ice motion from these using objective analysis (Rigor et al., 2002 and references therein).

Sea ice motion may also be derived from satellite data by estimating the displacement of sea ice features found in two consecutive images from a variety of satellite instruments (e.g., Agnew et al., 1997; Kwok, 2000). The passive microwave sensors provide the longest period of coverage (1979 to present) but their spatial resolution limits the precision of motion estimates. The optimal interpolation of satellite and buoy data (e.g., Kwok et al., 1998) seems to be the most consistent data set to assess interannual variability of sea ice motion.

In the Antarctic, buoy deployments have only been reasonably frequent since the late 1980s. Since 1995, buoy operations have been organised within the World Climate Research Programme (WCRP) International Programme for Antarctic Buoys (IPAB), although spatial and temporal coverage remain poor. A digital atlas of antarctic sea ice has been compiled from two decades of combined passive microwave and IPAB buoy data (Schmitt et al., 2004).