Mapping of Active Frazil and Sea Ice Production in the Northern Hemisphere, With Comparison to the Southern Hemisphere

Frazil ice in the coastal polynyas has been increasingly recognized as important for efficient sea ice production, associated dense water formation, transport of particulate matter and consequent biological production. However, it has not been well understood where and to what degree active-frazil areas occur in the Northern Hemisphere polynyas. We presented the first mapping of active frazil in the Northern Hemisphere, using sea ice data during September-May for 2002/2003-2010/2011, created from an Advanced Microwave Scanning Radiometer for the Earth Observing System thin ice algorithm that can detect active frazil areas. The North Water, Northeast Water, Anadyr, and St. Lawrence Island polynyas were found to have a relatively high occurrence rate of active frazil compared with other polynyas. We improved the estimation of sea ice production by discriminating between active frazil and thin solid ice and found that previous algorithms tended to overestimate sea ice production, particularly for polynyas dominated by thin solid ice such as the Okhotsk Northwestern polynya. We also made the first global comparison of polynya characteristics for all the major coastal polynyas. The annual ice production rate is 3-4 m yr(-1) in the Northern Hemisphere polynyas and 6-9 m yr(-1) in the Antarctic coastal polynyas. This large difference is mainly explained by the less frequent occurrence of active frazil and thin ice areas in the Northern Hemisphere polynyas owing to a lower occurrence of strong wind events. Plain Language Summary Coastal polynyas, which are high-latitude coastal regions of thin ice or open water surrounded by thick sea ice, are important components of the global climate system. High production of sea ice in coastal polynyas generates dense water, which then influences physical and biological processes in the surrounding regions. Thin ice areas in a polynya are roughly classified into two ice types: active frazil, a mixture of grease/frazil ice and open water formed under turbulent conditions, and thin solid ice, a relatively uniform thin ice region formed under calm conditions. In this study, we focused on measuring the active frazil, which indicates areas of efficient ice production. Frazil ice also incorporates and transports sediments or micro-nutrients, possibly leading high biological productivity. We presented the first mapping of active-frazil areas in the Northern Hemisphere, and improved estimates of total ice production taking account of the ice type. We also discussed the characteristics of major coastal polynyas in the world. The rates of ice production and active frazil occurrence are generally lower in the Northern Hemisphere than in the Antarctic coastal polynyas because of lower wind speeds in the Northern Hemisphere high-latitude coastal regions.