Observations of Ice Thickness and Frazil Ice in the St. Lawrence Island Polynya 
from Satellite Imagery, Upward Looking Sonar and Salinity/Temperature Moorings
Journal of Geophysical Research, Vol.108, C5, 3149, doi:10.1029/2001JC001213, 2003

Robert Drucker, Seelye Martin
University of Washington,
School of Oceanography, Box 357940
Seattle WA 98195-7940

Richard Moritz
Polar Science Center/Applied Physics Laboratory
University of Washington
Seattle WA 98105-6698

Abstract

For the 1999 winter, this paper examines the behavior of the Bering Sea St. Lawrence Island
polynya using a combination of AVHRR (Advanced Very High Resolution Radiometer),
RADARSAT SAR (Synthetic Aperture Radar), meteorological data, over-winter moored upward 
looking sonars (ULS) and SeaBird salinity/ temperature sensors. We define a thermal ice 
thickness from the AVHRR retrieval of ice surface temperature combined with meteorological 
observations and a heat flux model. South of the island, we compare the ULS and thermal 
thicknesses for congelation and frazil ice. When the satellites observe congelation ice 
over the ULSs, the ULS and thermal ice thicknesses generally agree. When SAR observes 
Langmuir plumes over the ULSs, which indicate frazil ice formation, the ULSs show 
scatterers at 5-20 m depths in the water column and the seawater temperatures are either 
within 0.01 of freezing or are slightly supercooled. This suggests that during frazil events,
crystals either nucleate at depth or are transported to depth by the Langmuir circulation.
The combination of the SAR imagery and ULS observations also allow measurement of the pack
ice advection velocity, the polynya width and the downwind frazil accumulation thickness,
giving widths of 10 to 30 km and thicknesses of 0.1 - 0.2 m.  Substitution of these observed
values with the heat flux into the Pease polynya model yields polynya widths that
approximately agree with the observed.