OBSERVATION OF AUTUMN FREEZE-UP IN THE BEAUFORT AND CHUKCHI SEAS USING THE ERS-1 SYNTHETIC-APERTURE RADAR

We present observations of the transition in sea ice backscattering si gnatures from late summer into early winter in the Beaufort and Chukch i Seas during 1991 and 1992, using data from the ERS 1 synthetic apert ure radar (SAR). We employ both analyzed surface temperature fields an d direct observations of near-surface temperature from drifting buoys. Consistent with previous surface-based observations, backscattering f rom sea ice surviving at the end of summer increases strongly and rapi dly when temperatures fall below freezing for the final time in the au tumn, apparently because of increased volume scattering from bubbles i n the upper part of the ice. Areally averaged backscattering sometimes overshoots typical winter multiyear ice values by 1-2 dB, before sett ling back over a period of approximately a week. The backscattering-te mperature link forms a sufficient basis for an algorithm to estimate r etrospectively, from time series of SAR images, the date of freeze-up (defined here to be the date on which all liquid water in the bubbly u pper layer of surviving sea ice freezes and remains frozen for the dur ation of the ensuing autumn and winter). We present a prototype algori thm and use it to estimate freeze-up dates in nine Lagrangian cells in the Beaufort Sea during the autumn of 1992. We observe a 12-day sprea d in freeze-up dates between latitudes of approximately 73 degrees N a nd 82 degrees N, with dates in the northernmost cells of August 29-30 and those in the southernmost cells of September 7. Two cells at latit udes of 75 degrees N-77 degrees N appear to freeze-up earlier than cel ls further north but present uncertainties limit the significance of t his observation.