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Abstract. Analyses of 32 yr of Arctic sea ice extents and areas derived from satellite passive microwave radiometers are presented for the Northern Hemisphere as a whole and for nine Arctic regions. There is an overall negative yearly trend of −51.5 ± 4.1 × 10 3 km 2 yr −1 (−4.1 ± 0.3 % decade −1 ) in sea ice extent for the hemisphere. The yearly sea ice extent trends for the individual Arctic regions are all negative except for the Bering Sea: −3.9 ± 1.1 × 10 3 km 2 yr −1 (−8.7 ± 2.5 % decade −1 ) for the Seas of Okhotsk and Japan, +0.3 ± 0.8 × 10 3 km 2 yr −1 (+1.2 ± 2.7 % decade −1 ) for the Bering Sea, −4.4 ± 0.7 × 10 3 km 2 yr −1 (−5.1 ± 0.9 % decade −1 ) for Hudson Bay, −7.6 ± 1.6×10 3 km 2 yr −1 (−8.5 ± 1.8 % decade −1 ) for Baffin Bay/Labrador Sea, −0.5 ± 0.3 × 10 3 km 2 yr −1 (−5.9 ± 3.5 % decade −1 ) for the Gulf of St. Lawrence, −6.5 ± 1.1 × 10 3 km 2 yr −1 (−8.6 ± 1.5 % decade −1 ) for the Greenland Sea, −13.5 ± 2.3 × 10 3 km 2 yr −1 (−9.2 ± 1.6 % decade −1 ) for the Kara and Barents Seas, −14.6 ± 2.3 × 10 3 km 2 yr −1 (−2.1 ± 0.3 % decade −1 ) for the Arctic Ocean, and −0.9 ± 0.4 × 10 3 km 2 yr −1 (−1.3 ± 0.5 % decade −1 ) for the Canadian Archipelago. Similarly, the yearly trends for sea ice areas are all negative except for the Bering Sea. On a seasonal basis for both sea ice extents and areas, the largest negative trend is observed for summer with the next largest negative trend being for autumn. Both the sea ice extent and area trends vary widely by month depending on region and season. For the Northern Hemisphere as a whole, all 12 months show negative sea ice extent trends with a minimum magnitude in May and a maximum magnitude in September, whereas the corresponding sea ice area trends are smaller in magnitude and reach minimum and maximum values in March and September.

A method of determining sea ice parameters using dual‐polarized multispectral radiance data obtained with the NIMBUS 7 Scanning Multichannel Microwave Radiometer (SMMR) is presented. Sea ice concentration is determined both at a 60‐km resolution from the polarization at the 1.7‐cm wavelength and at a 30‐km resolution using the polarization at the 0.81‐cm wavelength. Multiyear sea ice fraction is obtained from the spectral gradient ratio, which is the difference of the 0.81 cm and the 1.7 cm vertically polarized radiances divided by their sum. In addition, an ice temperature is calculated from the 4.6‐cm vertical channel radiances. The use of radiance ratios greatly reduces uncertainties in the derived parameters resulting from temporal and horizontal spatial variations of ice temperature. Observed SMMR radiances from selected areas in the Arctic region for the period February 3–7, 1979, are used in computing algorithm coefficients. Polar maps of sea ice concentration, multiyear fraction, and ice temperature are illustrated for this period. The variation of the mean and standard deviation of ice concentration and multiyear ice fraction for a region of perennial ice cover over the first 11 months of SMMR operation is also presented. The standard deviation about the mean for the computed concentration varies from 2 to 5% over the 11‐month period, while that of the multiyear fraction is about 8% for all but the summer months. Discrimination between first‐year and multiyear sea ice during the summer period is indeterminate largely because the general surface melt conditions mask the distinguishing properties of the two ice types. Based on the time variation of ice concentration and multiyear fraction for the central Arctic region and on an analysis of histograms of these parameters, the precision of the calculated ice concentration is estimated to be in the range of 5–9% and that of the multiyear fraction in the range of 13–25%. Comparisons are made between the calculated sea ice parameters and information obtained from previous studies using aircraft, submarine, and surface observations. From these comparisons it is concluded that the absolute accuracy of the SMMR parameters remains uncertain. The precision of the sea ice concentration is sufficient to provide useful data on the large‐scale polar ice cover, but further study is required to increase the confidence in the multiyear ice fraction which at present contains significant uncertainties.

Abstract. In sharp contrast to the decreasing sea ice coverage of the Arctic, in the Antarctic the sea ice cover has,

[1] The principal characteristics of the variability of Antarctic sea ice cover as previously described from satellite passive microwave observations are also evident in a systematically calibrated and analyzed data set for 20.2 years (1979 -1998). The total Antarctic sea ice extent (concentration >15%) increased by 11,180 ± 4190 km 2 yr À1 (0.98 ± 0.37% (decade) À1 ). The increase in the area of sea ice within the extent boundary is similar (10,860 ± 3720 km 2 yr À1 and 1.26 ± 0.43% (decade) À1 ). Regionally, the trends in extent are positive in the Weddel Sea (1.4 ± 0.9% (decade) À1 ), Pacific Ocean (2.0 ± 1.4% (decade) À1 ), and Ross (6.7 ± 1.1% (decade) À1 ) sectors, slightly negative in the Indian Ocean (À1.0 ± 1.0% (decade) À1 ), and strongly negative in the Bellingshausen-Amundsen Seas sector (À9.7 ± 1.5% (decade) À1 ). For the entire ice pack, ice increases occur in all seasons, with the largest increase during fall. On a regional basis the trends differ season to season. During summer and fall the trends are positive or near zero in all sectors except the BellingshausenAmundsen Seas sector. During winter and spring the trends are negative or near zero in all sectors except the Ross Sea, which has positive trends in all seasons. Components of interannual variability with periods of about 3-5 years are regionally large but tend to counterbalance each other in the total ice pack. The interannual variability of the annual mean sea ice extent is only 1.6% overall, compared to 6 -9% in each of five regional sectors. Analysis of the relation between regional sea ice extents and spatially averaged surface temperatures over the ice pack gives an overall sensitivity between winter ice cover and temperature of À0.7% change in sea ice extent per degree Kelvin. For summer some regional ice extents vary positively with temperature, and others vary negatively. The observed increase in Antarctic sea ice cover is counter to the observed decreases in the Arctic. It is also qualitatively consistent with the counterintuitive prediction of a global atmospheric-ocean model of increasing sea ice around Antarctica with climate warming due to the stabilizing effects of increased snowfall on the Southern Ocean.

Abstract. We have generated consistent sea ice extent and area data records spanning 18.2 years from passive-microwave radiances obtained with the Nimbus 7 scanning multichannel microwave radiometer and with the Defense Meteorological Satellite Program F8, Fll, and F13 special sensor microwave/imagers. The goal in the creation of these data was to produce a long-term, consistent set of sea ice extents and areas that provides the means for reliably determining sea ice variability over the 18.2-year period and also serves as a baseline for future measurements.We describe the method used to match the sea ice extents and areas from these four multichannel sensors and summarize the problems encountered when working with radiances from sensors having different frequencies, different footprint sizes, different visit times, and different calibrations. A major obstacle to adjusting for these differences is the lack of a complete year of overlapping data from sequential sensors. Nonetheless, our procedure reduced ice extent differences during periods of sensor overlap to less than 0.05% and ice area differences to 0.6% or less. IntroductionThe study of long-term geophysical changes on multidecadal timescales requires the generation of consistent sets of measurements taken over many years. For satellite data this will generally involve similar sensors aboard different satellites. One of the longest satellite data records currently available is derived from four multichannel passive-microwave sensors operating aboard various polar orbiting satellites. These sensors provide global radiance measurements with which to map, monitor, and study the Arctic and Antarctic polar sea ice covers. The data span more than 18 years, starting with the launch of the scanning multichannel microwave radiometer (SMMR) on NASA's Nimbus 7 satellite in 1978 and continuing with the Defense Meteorological Satellite Program (DMSP) special sensor microwave/imager (SSMI) series beginning in 1987. It is anticipated that the DMSP SSMI series will continue into the 21st century. The SSMI series will be augmented within a few years by new, improved passive-microwave sensors to be flown on Japanese and U.S. space platforms.Over the last few years, researchers have made use of these data sets either separately or combined. Because of difficulties in combining the two data sets, Johannessen et al.

Abstract. We investigated circulation and water mass modification processes in the Chukchi Sea using (1) temperature, salinity, and velocity data collected between September 1991 and September 1992 from moorings in Bering Strait and the northeast shelf, and (2) meteorological data and ice concentrations derived from special sensor microwave imager (SSM/I) imagery. In October 1991 and from February to August 1992 the mean monthly circulation was steady and northward. From November to January, strong northeast winds diverted the low-salinity Bering Inflow onto the western shelf, and weakened and reversed flow over the northeast shelf. The winds also opened extensive polynyas, wherein cold hypersaline (salinity > -34) waters formed. The fall/winter circulation enhanced dense water formation by diverting low-salinity waters away from, and prolonging the residence time of water within, the polynyas. Alongshore convergence of the coastal flow swept some of the cold hypersaline water offshore (across isobaths), but the coastal current carried most of it into the Arctic Ocean through Barrow Canyon. The dense outflow mixed little during its descent through the upper canyon, where bottom friction and rotation, but not entrainment, were important in the plume momentum balance. If the outflow is not diluted by mixing in the lower canyon (where theory suggests this will occur), then the densest shelf waters can ventilate layers deeper than the halocline. Salt rejection estimates using SSM/I data and surface heat budget calculations agree, within a factor of 2, with independent estimates from the mooring. Hence remote sensing techniques can monitor winter salt rejection rates in the Chukchi Sea.

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