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Controls on Eurasian coastal sea ice formation, melt onset and decay from ERS scatterometry: regional contrasts and effects of river influx

Regional and local patterns in the formation, melt onset and disappearance of coastal first-year sea ice (FYI) are observed in the Eurasian Arctic using the C-band (5.3 GHz) European Remote Sensing Satellite (ERS) radar scatterometers. Near-daily time-series (1991–1999) of the radar backscatter coefficient normalized to an incident angle of 40° (σ 0 40 ) and the backscatter- incident angle relationship (B) are examined for test sites near the Severnaya Dvina, Mezen, Pechora, Ob’, Yenisei, Khatanga, Lena, Indigirka and Kolyma rivers. Melt onset of the sea ice surface is associated with abrupt changes in σ 0 40 , with values converging towards ∼–17dB. As such, whether backscatter increases or decreases at melt onset is largely determined by contrast with pre-melt backscatter levels. The presence or absence of FYI is designated from low or high values of B, respectively, whereas the addition of an anisotropy criterion further improves discrimination of FYI from open water. A strong regional pattern is seen in the daily temporal variability of both melt onset and ice cover, with maximum variability in the Barents and Kara seas and decreasing variability eastward. Similarly, significant contrasts in the seasonal duration of ice cover are found between western and eastern sites. Seasonal ice cover persists ∼144 days for sites in the Barents Sea, but ∼293 days for sites farther east in the East Siberian Sea. We speculate that our observed west-east contrasts are due to North Atlantic modulation of salinity, air temperature and cyclone density. Rivers also exert a local effect on ice cover, causing earlier formation (∼4 days) and earlier disappearance (∼17 days) near river mouths. The hydrological influence of rivers is potentially strong in the Kara, Laptev and East Siberian seas, but weak or absent in the Barents Sea.