The Arctic Sea Ice - Melting During Summer or not Freezing in Winter?
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- Geophysical Institute 
The Arctic sea ice cover has retreated rapidly during the last three decades, concurrent with recent global temperature increase both in the atmosphere and in the ocean. The sea ice cover has experienced a retreat in extent and a reduction in thickness, hence the sea ice volume is declining. As a consequence, the multi-year sea ice is decreasing, and first-year ice is now the dominating ice type in the Arctic Ocean. This study uses the Norwegian Earth System Model (NorESM), forced at the surface with atmospheric reanalysis data from the Coordinated Ocean-ice Reference Experiments phase II (CORE-II), during the period from 1948 to 2007. The model simulation is compared to available observations, both from satellites and in-situ observations, for model evaluation. Since there are large regional and seasonal differences of the sea ice cover in the Arctic, this study provides an evaluation of the regional and seasonal variations in 12 different Arctic regions, where March represents the winter (maximum sea ice cover in the Northern Hemisphere) and September represents the summer season (minimum sea ice cover in the Northern Hemisphere). There are particularly two 20-year periods in the model simulation with noticeable trends in sea ice extent, Period I (1948-1967) and Period III (1988- 2007), which are analyzed in more detail. The Barents Sea is in particular focus in this study, and the decreasing sea ice in this region is found to be associated with observed changes in temperature during these periods, both in the atmosphere and the ocean. The model simulation shows significant negative trends over the last 20 years, and there has been a loss of sea ice in all Arctic regions in the winter season as well as the summer season. However, the sea ice declines regionally and seasonally at different rates. Seven of 12 regions in the interior Arctic Ocean have more or less a full sea ice cover in all winters, and these regions are thus only contributing to the summer sea ice extent trends in the Northern Hemisphere. It is suggested that the loss of sea ice in these regions is dominated by increased melting towards summer. Two of 12 regions located in the north of the Pacific are completely ice-free during summer, and the diminishing sea ice cover in these regions are hence suggested to be due to reduced freezing of sea ice towards winter. For the remaining three regions, and for the Northern Hemisphere in total, variations are seen in all months of the year. The trends in Baffin Bay are greatest in winter, which indicates that the region is dominated by reduced freezing. For the Greenland Sea, the trends in March and September are on the same order of magnitude, and in the Barents Sea the trends in sea ice concentration and sea ice extent contradict each other. However, because these two latter regions approach ice-free conditions in the summer season, decreased winter freezing is seen in recent observations in the Barents Sea. As a result, decreased winter freezing will therefore likely be the dominating cause of sea ice loss in these regions in the model simulation for the years following 2007. For the Northern Hemisphere in total, the interannual variability and the long-term trend (both in sea ice concentration and in sea ice extent) are clearly greatest in the summer season, suggesting that the total loss of sea ice in the Northern Hemisphere is due to increased melting towards the summer.