Observations of ice thickness and frazil ice in the St. Lawrence Island polynya from satellite imagery, upward looking sonar, and salinity/temperature moorings

[1] For the 1999 winter, this paper examines the behavior of the Bering Sea St. Lawrence Island polynya using a combination of Advanced Very High Resolution Radiometer (AVHRR), RADARSAT synthetic aperture radar ( SAR), 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 0.01degreesC 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.