Recent sea ice increase and temperature decrease in the Bering Sea area, Alaska
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We analyzed the sea ice conditions in the Bering Sea for the time period 1979–2012, for which good data based on microwave satellite imagery, being able to look through clouds and darkness, are available. The Bering Sea, west of Alaska, is ice-free in summer, but each winter, an extensive sea ice cover is established, reaching its maximum normally in March. We found a slight increase in ice area over the time period, which is in stark contrast to the significant retreat observed in the Beaufort Sea north of Alaska and the Arctic Ocean as a whole. Possible explanation might be found in the Pacific Decadal Oscillation (PDO), which went from dominantly positive values to more negative values in the last decade. The PDO is related to the sea surface temperature (SST) in the North Pacific, negative values indicated cooler temperatures and cooler SST weakening the semipermanent Aleutian Low. When comparing the circulation pattern obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyzed data set for years of heavy ice against light ice years, an additional vectorial northerly wind component could be deduced from the pressure data. Hence, less relatively warm air is advected into the Bering Sea, which becomes of special importance in winter, when solar radiation is at its minimum. Surface observations confirmed these findings. Atmospheric pressure increased in Cold Bay, located close to the center of the semi-permanent Aleutian Low, the N–S pressure gradient (Nome–Cold Bay) in the Bering Sea decreased, wind speeds of the coastal stations became weakened, and the temperature of coastal stations decreased.
KeywordsArctic Ocean Pacific Decadal Oscillation Pacific Decadal Oscillation Index Multivariate ENSO Index
The sea ice conditions are very different between the Bering and Beaufort Seas, the latter one being located north of the northern coast of Alaska (Serreze and Barry 2006; Bridgman and Oliver 2006). The Bering Sea, open to the Pacific Ocean, is ice-free in summer (Zhang and Hibler 1991; Zhang et al. 2010). Danielson et al. (2011) wrote recently a substantial paper on the oceanography of the Bering Sea with 180 citations from which additional literature can be obtained. The ice-free season lasts through most of autumn and extensive sea ice, here taken at 10 % of the maximum ice extent in March, is normally not established until late November, and is again reduced to below this value around the beginning of June. In contrast to this, the Beaufort Sea has year-round ice coverage. For the winter season, the ice concentration is around 97 %, and at that time of the year only minimal changes occur from year to year. The observed decrease occurred in the melting season, which normally results in the minimal sea ice extent in late August, early September. The length of the melting season as well as the amount of open water has increased, which was shown in a previous study for an area of 200 km wide and 500 km along the northern coast of Alaska (Wendler et al. 2010). This area is of special interest as Prudhoe Bay, the center of oil activities in Alaska, is located along the coast and offshore activities are impacted strongly by the presence of sea ice. We found a strong decrease in the sea ice concentration. Over a time period of 36 years, the mean annual amount of open water increased from 12 to 32 %, as could be deduced from the best linear fit through the time series. This is in agreement with the general strong retreat of ice in the Arctic Ocean and only lately (September 2012), the lowest ice observation with 3.6 million km2 was observed, surpassing the previous record low of 2007. Analyses of sea ice data can be obtained from the National Snow and Ice Data Center in Boulder (Fetterer 2006; Benner 1996), which produce daily maps covering the whole Arctic Ocean (Cavalieri et al. 2006).
When we look at the extremes, very large variations can be observed. All three annual courses gave a maximum in ice area during the month of March, with an average value of 523,000 km2. Annual areas for March, however, varied between 307,000 and 790,000 km2. Percentage-wise, the break-up in June is even more extreme. The mean value is around 14,000 km2 and consists of values ranging between no sea ice at all and a maximum monthly coverage of up to 48,000 km2.
While the temperature/sea–ice relationship is weaker for the southern part of the Bering Sea, it improved substantially in the later season (January–April); however, the relationship is not as tight as for Nome. St. Paul Island demonstrates well this relationship (not shown).
In Fig. 2, we presented the mean annual sea ice extent, which showed a slight increase. When correlating these values with the PDO, a correlation coefficient of −0.41 was calculated (not shown). We looked also at the multivariate ENSO Index, which is dependent on the E–W pressure gradient in the tropics and influences the SST in the North Pacific. However, the correlation coefficient between ENSO the sea ice extent in the Bering Sea was weak (r = −0.08).
The sea ice cover of the Bering Sea has increased slightly over the last decades, which is in strong contrast to the Arctic Ocean, where a substantial decrease in sea ice has been observed, accompanied by higher temperatures. We offered as explanation the shift in the PDO index, which went to more negative values, bringing colder temperatures to Alaska (Wendler et al. 2012). This decrease in temperature was especially pronounced in Western Alaska (Overland et al. 2012), which lead to a weakening of the semipermanent Aleutian Low. Hence, less warm air from the South is advected into the Bering Sea region. Both, NCEP/NCAR reanalyzed data as well surface observations from existing stations agree with these assumptions.
We are thankful for financial support given by Dr. Mark Myers, Vice Chancellor of Research, and by Dr. Robert McCoy, Director, Geophysical Institute, for State of Alaska funds designated for ACRC. Further, John Walsh, William Chapman, Peter Bieniek, and Kevin Galloway helped us in various ways improving the manuscript.
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