Sea ice is an important component in the highly interactive global climate system, reflecting and influencing conditions in both the atmosphere and oceans. Although typically overlying approximately 7 percent of the world's oceans, the sea ice cover experiences considerable seasonal variability in both hemispheres: in the eight regions described in this atlas, encompassing most of the Northern Hemisphere sea ice area, the total extent of sea ice varies from a minimum of about 7.8 x 10^6 square kilometers in September to a maximum of about 14.8 x 10^6 square kilometers in March, and in the Southern Hemisphere the extent varies from about 4 x 10^6 square kilometers in February to about 20 x 10^6 square kilometers in September. The ice significantly reduces the amount of solar radiation absorbed at the Earth's surface, greatly restricts exchanges of heat, mass, and momentum between ocean and atmosphere, and affects the density structure of the upper ocean through the salt and heat fluxes associated with the freezing and melting processes. The changes in density structure at times lead to deep-water and even bottom-water formation, and the net equatorward advection of sea ice provides a transport of cold, low-salinity water out of the polar regions. In this document, the sea ice cover of the Northern Hemisphere over the 4-year period 1973 through 1976 is described from the passive microwave data of the Electrically Scanning Microwave Radiometer (ESMR) on the Nimbus 5 satellite. The book is a companion volume to Antarctic Sea Ice, 1973-1976: Satellite Passive-Microwave Observations (Zwally et al., 1983a), and the two volumes together provide a global picture of sea ice conditions in the mid- 1970s.
Data collected by the Nimbus 5 ESMR from its launch in December 1972 through most of the next 4 years provide the earliest all-weather, all-season imagery of global sea ice. For 39 months of the 4-year period, good quality Northern Hemisphere data were transmitted, and these data are the basis of the sea ice maps, plots, and analysis in this volume. The data have been interpolated for spatial and temporal gaps, averaged on a monthly basis into monthly averaged microwave brightness temperatures, and displayed in color-coded polar maps. The large contrast in microwave emissivities between sea ice and open water enables a conversion of the brightness temperatures to sea ice concentrations (percentages of the ocean area covered by sea ice) providing that all sea ice in the field of view has approximately the same emissivity. In many of the seas and bays peripheral to the Arctic Ocean, the ice is predominantly first-year sea ice, with an emissivity near 0.92, so that a straightforward conversion from brightness temperatures to sea ice concentrations is possible. In the Arctic Ocean itself and some of the immediately adjacent waters, multiyear sea ice with an emissivity near 0.84 exists in addition to first-year sea ice, complicating the interpretations. In these areas, sea ice concentrations can be calculated from the brightness temperatures as a function of multiyear ice fraction. The same function is valid for first-year sea ice regions as well, with the multiyear ice fraction set at zero. Color-coded maps of the resulting sea ice concentrations are presented in monthly averaged formats, with associated nomograms relating the color scale, multiyear ice fraction, and sea ice concentration. Various other color-coded images, including ice concentration yearly averages, 4-year averages for individual months, and differences from month to month, are also presented, as are plots of a variety of variables such as total areal extent of sea ice, area of ice in various ice concentration categories, and area of actual ice coverage. The plots are presented for each of eight regions, covering most of the Northern Hemisphere sea ice area, and for the sum of the eight regions. The eight regions are the Arctic Ocean, the Sea of Okhotsk, the Bering Sea, Hudson Bay, Baffin Bay/Davis Strait, the Greenland Sea, the Kara and Barents Seas, and the Canadian Archipelago.
The ESMR data reveal many of the details of the distribution and dynamics of the Northern Hemisphere sea ice cover, including considerable interannual variability and interregional contrasts. At the time of maximum ice extent in March, the ice cover is nearly complete Greenland, in the Arctic Ocean, Hudson Bay, the Kara Sea, and the Canadian Archipelago, and is extensive for large portions of the other peripheral seas and bays. The springtime retreat of the ice edge tends to begin first in Davis Strait and the northern North Atlantic and last in the Bering Sea and Hudson Bay. At the time of minimum ice extent in September, the ice pack is mostly confined to the central Arctic Ocean and portions of the Greenland Sea, the Kara Sea, and the Canadian Archipelago. Essentially no ice remains in the Bering Sea, Hudson Bay, the Sea of Okhotsk, or Baffin Bay/Davis Strait. Noticeable autumn ice-edge advance begins first in the Greenland Sea, between August and September, then becomes apparent throughout the remainder of the ice-covered region between September and November.
Certain consistent latitudinal asymmetries in the extent of the ice are readily explained by major ocean currents, whereas many regional interannual contrasts in the ice are explainable by interannual differences in the atmospheric pressure and wind fields. Currents with major impacts include the warm, north-flowing Norwegian, West and West Kamchatka Currents, which prevent or delay ice formation in the Barents Sea, immediately southwest of Greenland, and along the west coast of Kamchatka Peninsula, respectively, and the cold, south-flowing East Greenland and Labrador Currents, which transport ice far to the south along the east coasts of Greenland and Canada. Among the phenomena which appear closely connected to atmospheric conditions are the interannual variabilities in the timing of maximum sea ice extent in the Bering and Greenland Seas.
The ESMR data reveal an approximately symmetrical growth/decay cycle of the ice in the Northern Hemisphere and no systematic trend in the overall area of ice coverage over the 4 years 1973 through 1976. This contrasts with the situation revealed for the same 4 years for the Southern Hemisphere, where the seasonal ice decay proceeded far more rapidly than the seasonal ice growth and a marked decreasing trend was apparent in the overall ice area.
