The St. Lawrence polynya is a persistent wind-driven polynya that forms along the southern coast of St. Lawrence Island in the Bering Sea.
The above image is a false-color composite image formed using data from two separate instruments on two different satellites. The value, or light-dark content of the image, is controlled by data from the Synthetic Aperture Radar (SAR) instrument aboard the Canadian Space Agency's RADARSAT satellite, which measures radar backscatter from the surface to form a high resolution image of surface features. The hue, or color content of the image, is proportional to ice surface temperature (IST), calculated from infrared radiance measured by the Advanced Very High Resolution Radiometer (AVHRR) aboard the National Oceanic and Atmospheric Administration's NOAA-12 satellite. The two component images were acquired within approximately 35 minutes of eachother on January 9, 1999. The horizontal scale of the image is approximately 160 km.
On the day of this image the wind is blowing from the northeast at about 15 m/s (about 30 mph), advecting ice away from the coast and opening the relatively warm (-1.8 deg. C) water to the colder (-14 deg. C) air, where it quickly refreezes into a slurry of small ice crystals known as frazil. The long, feathery streaks visible in the polynya are called Langmuir cells, formed by wind-herding of the newly formed frazil ice. The red color in this area of the image, indicating a relatively warm surface temperature, stands in contrast to the colder surrounding surface temperatures indicated by yellows and greens. The structure of the surrounding ice is also revealed by the SAR, which shows the characteristic round or oval shapes of floes, solid "islands" of older ice that have been advected southward by the wind.
For more about the St. Lawrence polynya, see
Drucker, R., S. Martin and R. Moritz,
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
Copyright 2003 American Geophysical Union. Further
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© 2003 Robert Drucker, University of Washington. This material is based upon work supported by the National Science Foundation under Grant No. 9811097.