| Over the past 30 years, sea ice cover, as a significant driving factor in the global climate change, has been decreasing dramatically with the extent declining and thickness thinning. Open water reflects only 7% of the incident solar radiation, compared to 85% for snow-covered sea ice and 65% for bare sea ice. As the ice cover decays, highly reflecting ice is replaced by highly absorbing ocean, resulting in more solar heat absorption and more melting. Furthermore, an ice cover thinned by excessive bottom melt transmits more solar radiation directly to the ocean than the original thicker ice cover. In this thesis, the focus has been put on the absorption of solar radiation by sea ice during the melt and freeze up season in Arctic. Meanwhile, the annual variability in sea ice cover in East Siberian Sea and the distribution of ice thickness in northern Canadian Arctic Archipelago has been discussed to improve the knowledge of the sea ice changes in Arctic.East Siberian Sea is a highly ice covered area in Arctic Ocean. During 1997 to 2001, the sea ice cover area had extended slightly, and then the rapid decline happened since 2002. The increase in the volume of runoff from the Russian Siberia was one of the elements which influence the decay of sea ice. Another significant driving factor was the southerly wind over the East Siberian Sea since 2002, which played an important role in ice drift northward.The thickness of level ice around Borden Island in Canadian Arctic Archipelago is spatially varied in the horizontal and in the vertical dimension. The thickness of multi year pack ice was 154.4±22.4cm, which is similar to that of multi year fast ice in Prince Gustaf Adolf Sea (166.6±16.2cm). The first year pack ice and fast ice on the both side of the flaw lead have appeared thinner than multi year ice with 105.1±10.9cm and 124.5±13.2cm thickness. The thickest ice in the section was the multi year fast ice which is very close to Borden Island with 205.3±8.8cm. There was a ridge ice in the multi year pack ice with the thickness of 2.5-3m. During the melt season, there was an obvious linear trend between the absorptance and thickness of the melting sea ice: the thinner the sea ice, the higher the absorptance. Most of the solar energy wsas reflected by snow covered ice, while the less of them has been input into ocean through the sea ice. Both the observation and theory indicated that radiative flux of 12.8 Wm-2 has been absorbed by sea ice in the Centre Arctic, which could melt the sea ice of 3.3mm per day.At the fall freeze-up stage, while the solar altitude was very low, the light with shorter wavelength was weakened in the atmosphere, whereas the light with longer wavelength was weakened in the sea ice. The combined effects of atmosphere and sea ice have induced the solar radiation under the sea ice much weaker. Moreover, the absorption of sea ice at the longer wave-length allows the sea ice to gain more heat to slow down the freezing process.For the freezing first year ice, the apparent attenuation coefficient was obtained from the'linear portion'of the measured logarithmic relative variation rate. With the exception of blue and red lights, the LPL attenuation coefficient changed little with wavelength, but changed considerably with depth. The vertical decrease of the attenuation coefficient was found to be correlated with salinity: the greater the salinity, the greater the attenuation coefficient. The observed attenuation coefficient of LPL was much larger than that of vertical propagation light. The preferential propagation in the vertical dimension is likely due to the columnar structure of the sea ice crystals.
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