Satellite Passive Microwave Observations of Sea Ice

Abstract

Sea ice is the frozen part of the world's oceans. On average, it covers up to ~15% of the world's oceans for at least some of the year, with strong and opposing seasonal variability in the Arctic and Antarctic regions. Passive microwave observations from satellite-borne sensors have provided information on sea ice cover since the early 1970s and have produced a consistent and near-complete climate record of sea ice concentration and extent since October 1979. Passive microwave sensors are optimal for sea ice because the microwave emission they detect is sensitive to the phase state of water (liquid vs. frozen). Also, the emission is independent of sunlight, which allows continuous coverage through the long polar winter nights. Atmospheric emission (clouds, water vapor, aerosols) is also generally low over sea ice, enabling the surface emission to be clearly detected by the orbiting sensors. Finally, the sensor swath width and sun-synchronous polar orbits yield near-complete daily coverage of all sea ice covered regions. The long-term record from passive microwave data indicates a significant downward trend in Arctic sea ice cover, while the Antarctic time series is more complex and is dominated by strong interannual variability. Other geophysical parameters can also be derived, including temperature, motion, snow depth, and melt/freeze. New passive microwave sensors are also now providing thickness estimates of newly-formed sea ice. The record from passive microwave sensors show a dramatically changing Arctic with large decreases in extent and age, faster ice motion, earlier melt and later freeze-up. The Antarctic is more variable over the satellite record, with small positive trends in extent.