Monthly Sea Ice Research Articles

Long-Range Statistical Forecasting of Ice Severity in the Beaufort–Chukchi Sea

Abstract Interannual variations in Beaufort Sea summer ice conditions influence a wide range of socioeconomic activities, including merchant shipping in the Beaufort Sea and subsistence lifestyles on the Alaskan North Slope. Each year, the National Ice Center quantifies Beaufort Sea summer ice conditions based on the Barnett severity index (BSI), which is based on distances from Point Barrow, Alaska, to the sea ice edge, as well as characteristics of the shipping season from Prudhoe Bay to the Bering Sea. Long-range forecasts (monthly to seasonal) of the BSI would be valuable for the above-mentioned users, provided that the forecasts are communicated effectively and used properly. Utilizing mean monthly sea ice and atmospheric data from 1979 through 2000, multiple linear regression models are developed here to forecast the BSI at monthly intervals from October of the previous year through to July of the prediction year. The final models retain between three and five variables, with decreased multiyear sea...

Sea ice, atmosphere and upper ocean variability in the Weddell Sea, Antarctica

A frequency domain singular value decomposition is performed on 20 years (1979–1998) of monthly sea ice concentration, sea ice drift and sea level pressure data in the Weddell Sea, Antarctica. Interannual oscillations with periods of around 3–4 years are found to dominate the variability in this region. Anomalous atmospheric patterns periodically reach the Weddell Sea from the west and perturb the sea ice circulation and distribution in the Weddell Gyre through changes in the intensity and direction of the climatological winds. Sea ice accumulates in the southeastern Weddell Sea every 3–4 years owing to two atmospherically driven processes: (1) weak ice export to the north due to a weak northward branch of the gyre (driven by weak southerly winds) and (2) large ice import from the east due to a strong East Wind Drift (driven by strong easterly winds along the coast). The opposite situation gives rise to a depletion of sea ice in the same region half a cycle later. Sea ice anomalies are then advected north‐northeastward before turning eastward in the gyre circulation. The eastward propagation of ice anomalies along the ice margin accounts for the passage of the Antarctic Circumpolar Wave through the Atlantic sector of the Southern Ocean. A low frequency signal is also detected in the Weddell Sea variations, albeit rather speculatively in this 20‐year‐long record. Sea ice variability on this timescale appears to be associated with a change in the shape and characteristics of the Weddell Gyre circulation around 1990. This mode of variability implicates feedbacks between the gyre and Weddell Deep Water temperature variations, whose impact is observed near the Maud Rise topographic feature.

Variability of Arctic Sea Ice: The View from Space, An 18-year Record

A recently compiled 18-year record (1979 to 1996) of sea ice concentrations derived from four passive-microwave satellite instruments has allowed the quantification of a variety of measures of Arctic sea ice variability. Earlier maps generated using data through August 1987 have been updated to 18-year summaries of the annual range of sea ice distributions, the interannual variability of average monthly sea ice distributions, the frequency of sea ice coverage over the 18 years, the length of the sea ice season, and trends in the length of the sea ice season. Linear least squares trends over the 18-year record show the sea ice season to have lengthened over some sizeable regions, especially in the Bering Sea, Baffin Bay, Davis Strait, the Labrador Sea, and the Gulf of St. Lawrence, but to have shortened over a much larger area, including the Sea of Okhotsk, the Greenland Sea, the Barents Sea, and all the seas along the north coast of Russia. The area with trends showing sea ice seasons shortening by over 0.5 days/year is 7 500 000 km², over 2.5 times the area experiencing a lengthening of the sea ice season by over 0.5 days/year. Neither the shortening nor the lengthening, however, is uniform or monotonic over the 18-year record. Instead, the ice cover exhibits widespread interannual variability, not just in the length of the sea ice season but for each month-a fact well illustrated by the monthly average September ice coverage, which was at its lowest extent in 1995 but at its second highest one year later, in the final year of the record. The maps of ice frequency and ice variability can help identify how anomalous individual years are. In some cases, they can help forestall unnecessary concern over seemingly unusual conditions which, upon examination of the maps, are found to fall well within the observed variability.

Antarctic sea-ice relationships with indices of the atmospheric circulation of the Southern Hemisphere

A link between the Antarctic sea-ice extent and low-frequency atmospheric variations, particularly ENSO, has been suggested by recent modeling and empirical studies. This question is examined here using a high-resolution (by week, by region) data base of Antarctic sea-ice extent for the 1973–1982 period. Although of relatively short duration by Northern Hemisphere standards, such a data base offers an opportunity rare in Southern Hemisphere climate studies. The seaice variations are examined in the context of longer-term indices of the large-scale atmospheric circulation. These are a Southern Oscillation Index (SOI) and an index of sea-level pressure (SLP) wavenumber one in the Southern Hemisphere extratropics. The indices are updated through 1982, and their associations with regional-scale pressure indices in the Australia-New Zealand sector are also examined. The 1973–1982 period is anomalous when compared with the period 1951–1972. Correlation analysis of the monthly sea ice and circulation index values reveals that much of the apparent link between the ice and the SOI suggested in previous studies arises from autocorrelations present in both data sets and the strong annual cycle of sea-ice extent. Removing these effects from the data and re-running the correlations reveals that most of the resulting “significant” associations between the ice and one or other of the circulation indices can probably be explained on the basis of chance. In order to reconcile these findings with previous studies that show some strong ice-circulation interactions on regional scales, only those months in which significant correlations occur between both largescale circulation indices and the sea ice are examined further. These occur preferentially in the Ross and Weddell sectors, which constitute the regions contributing most to the variability of Antarctic sea ice. The analysis suggests that the sea-ice-extent changes lag the SOI by several months but may precede changes in extratropical SLP wavenumber one. Confirmation of these tentative regional ice extent-circulation teleconnections necessarily awaits the forward extension of the high-resolution sea-ice data base beyond the 10 years available here.