Josefino C. Comiso

Arctic sea ice extents, areas, and trends, 1978-1996

Satellite passive-microwave data for November 1978 through December 1996 reveal marked seasonal, regional, and interannual variabilities, with an overall decreasing trend of −34,300±3700 km2/yr (−2.8%/decade) in Arctic sea ice extents over the 18.2-year period. Decreases occur in all seasons and on a yearly average basis, although they are largest in spring and smallest in autumn. Regionally, the Kara and Barents Seas have the largest decreases, at −15,200±1900 km2/yr (−10.5%/decade), followed by the Seas of Okhotsk and Japan, the Arctic Ocean, Greenland Sea, Hudson Bay, and Canadian Archipelago. The yearly average trends for the total, the Kara and Barents Seas, and the Seas of Okhotsk and Japan all have high statistical significance, with the null hypothesis of a 0 slope being rejected at a 99% confidence level. Regions showing increasing yearly average ice extents are Baffin Bay/Labrador Sea, the Gulf of St. Lawrence, and the Bering Sea, with only the increases in the Gulf of St. Lawrence being statistically significant at the 99% level. Hemispheric results for sea ice areas exhibit the same −2.8%/decade decrease as for ice extents and hence a lower absolute decrease (−29,500±3800 km2/yr), with the ice-free area within the ice pack correspondingly decreasing at −4800±1600 km2/yr. Confidence levels for the trends in ice areas and ice-free water areas exceed 99% and 95%, respectively. Nonetheless, interannual variability is high, and, for instance, the Arctic Ocean ice extents have a positive trend 1990–1996, in spite of their negative trend for the time period as a whole.

Large Decadal Decline of the Arctic Multiyear Ice Cover

AbstractThe perennial ice area was drastically reduced to 38% of its climatological average in 2007 but recovered slightly in 2008, 2009, and 2010 with the areas being 10%, 24%, and 11% higher than in 2007, respectively. However, trends in extent and area remained strongly negative at −12.2% and −13.5% decade−1, respectively. The thick component of the perennial ice, called multiyear ice, as detected by satellite data during the winters of 1979–2011 was studied, and results reveal that the multiyear ice extent and area are declining at an even more rapid rate of −15.1% and −17.2% decade−1, respectively, with a record low value in 2008 followed by higher values in 2009, 2010, and 2011. Such a high rate in the decline of the thick component of the Arctic ice cover means a reduction in the average ice thickness and an even more vulnerable perennial ice cover. The decline of the multiyear ice area from 2007 to 2008 was not as strong as that of the perennial ice area from 2006 to 2007, suggesting a strong role...

Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent

Based on a new analysis of passive microwave satellite data, we demonstrate that the annual mean extent of Antarctic sea ice has increased at a statistically significant rate of 0.97% dec−1 since the late 1970s. The largest increase has been in autumn when there has been a dipole of significant positive and negative trends in the Ross and Amundsen‐Bellingshausen Seas respectively. The autumn increase in the Ross Sea sector is primarily a result of stronger cyclonic atmospheric flow over the Amundsen Sea. Model experiments suggest that the trend towards stronger cyclonic circulation is mainly a result of stratospheric ozone depletion, which has strengthened autumn wind speeds around the continent, deepening the Amundsen Sea Low through flow separation around the high coastal orography. However, statistics derived from a climate model control run suggest that the observed sea ice increase might still be within the range of natural climate variability.

Passive Microwave Algorithms for Sea Ice Concentration: A Comparison of Two Techniques

Abstract The most comprehensive large-scale characterization of the global sea ice cover so far has been provided by satellite passive microwave data. Accurate retrieval of ice concentrations from these data is important because of the sensitivity of surface flux (e.g., heat, salt, and water) calculations to small changes in the amount of open water (leads and polynyas) within the polar ice packs. Two algorithms that have been used for deriving ice concentrations from multichannel data are compared. One is the NASA Team algorithm and the other is the Bootstrap algorithm, both of which were developed at NASAs Goddard Space Flight Center. The two algorithms use different channel combinations, reference brightness temperatures, weather filters, and techniques. Analyses are made to evaluate the sensitivity of algorithm results to variations of emissivity and temperature with space and time. To assess the difference in the performance of the two algorithms, analyses were performed with data from both hemispheres and for all seasons. The results show only small differences in the central Arctic in winter but larger disagreements in the seasonal regions and in summer. In some areas in the Antarctic, the Bootstrap technique shows ice concentrations higher than those of the Team algorithm by as much as 25%; whereas, in other areas, it shows ice concentrations lower by as much as 30%. The differences in the results are caused by temperature effects, emissivity effects, and tie point differences. The Team and the Bootstrap results were compared with available Landsat, advanced very high resolution radiometer (AVHRR) and synthetic aperture radar (SAR) data. AVHRR, Landsat, and SAR data sets all yield higher concentrations than the passive microwave algorithms. Inconsistencies among results suggest the need for further validation studies.

Variability and Trends in Antarctic Surface Temperatures from In Situ and Satellite Infrared Measurements

Abstract The surface air temperatures observed from stations in Antarctica have been shown to have predominantly positive trends that are as high as 0.5°C decade−1 along the Antarctic Peninsula. To evaluate whether the trends are caused by a local or large-scale phenomenon in the Antarctic region, surface temperatures inferred from infrared satellite data from 1979 to 1998 have been analyzed in combination with data from 21 stations that have long record lengths. The surface temperatures derived from infrared data are coherent spatially and temporally and are shown to agree well with Antarctic station data with a correlation coefficient of 0.98 and a standard deviation of about 3°C. The trend analysis on station data yielded on the average 0.012 ± 0.008°C yr−1 and −0.008 ± 0.025°C yr−1 for the 45- and 20-yr record, respectively. The latter reasonably agrees with the trend of −0.042 ± 0.067°C yr−1 inferred from the satellite 20-yr record. The 20-yr record length is shown to be about the minimum length requ...

Warming Trends in the Arctic from Clear Sky Satellite Observations

Abstract Satellite thermal infrared data on surface temperatures provide pan-Arctic coverage from 1981 to 2001 during cloud-free conditions and reveal large warming anomalies in the 1990s compared to the 1980s and regional variability in the trend. The rms error of the derived surface temperatures when compared with in situ data ranges from 1.5 to 3 K over the 20-yr period. Average temperature trends are generally positive at 0.33 ± 0.16°C decade−1 over sea ice, 0.50 ± 0.22°C decade−1 over Eurasia, and 1.06 ± 0.22°C decade−1 over North America. The trend is slightly negative and insignificant at −0.09 ± 0.25°C decade−1 in Greenland with the negatives mainly at high elevations. The trends are also predominantly positive in spring, summer, and autumn causing the lengthening of the melt season by 10–17 days per decade while they are generally negative in winter. The longer-term in situ surface temperature data shows that the 20-yr trend is 8 times larger than the 100-yr trend suggesting a rapid acceleration ...

Coastal zone color scanner pigment concentrations in the southern ocean and relationships to geophysical surface features

The spatial and seasonal distributions of phytoplankton pigment concentration over the entire southern ocean have been studied for the first time using the coastal zone color scanner historical data set (from October 1978 through June 1986). Enhanced pigment concentrations are observed between 35°S and 55°S throughout the year, with such enhanced regions being more confined to the south in the austral summer and extending further north in the winter. North and south of the polar front, phytoplankton blooms (>1 mg/m3) are not uniformly distributed around the circumpolar region. Instead, blooms appear to be located in regions of ice retreat (or high melt areas) such as the Scotia Sea and the Ross Sea, in relatively shallow areas (e.g., the Patagonian and the New Zealand shelves), in some regions of Ekman upwelling like the Tasman Sea, and near areas of high eddy kinetic energy such as the Agulhas retroflection. Among all features examined by regression analysis, bathymetry appears to be the one most consistently correlated with pigments (correlation coefficient being about −0.3 for the entire region). The cause of negative correlation with bathymetry is unknown but is consistent with the observed abundance of iron in shallow areas in the Antarctic region. It is also consistent with resuspension of phytoplankton cells by wind-induced mixing, especially in shallow waters. On the other hand, in the deep ocean (especially at latitudes 30%) in average pigment concentration over the entire region during different seasons indicates possible influence of time dependent parameters.

Circumpolar Arctic Tundra Vegetation Change Is Linked to Sea Ice Decline

Abstract Linkages between diminishing Arctic sea ice and changes in Arctic terrestrial ecosystems have not been previously demonstrated. Here, the authors use a newly available Arctic Normalized Difference Vegetation Index (NDVI) dataset (a measure of vegetation photosynthetic capacity) to document coherent temporal relationships between near-coastal sea ice, summer tundra land surface temperatures, and vegetation productivity. The authors find that, during the period of satellite observations (1982–2008), sea ice within 50 km of the coast during the period of early summer ice breakup declined an average of 25% for the Arctic as a whole, with much larger changes in the East Siberian Sea to Chukchi Sea sectors (>44% decline). The changes in sea ice conditions are most directly relevant and have the strongest effect on the villages and ecosystems immediately adjacent to the coast, but the terrestrial effects of sea ice changes also extend far inland. Low-elevation (<300 m) tundra summer land temperatures, a...

Climate Variability in the Amundsen and Bellingshausen Seas

Satellite data reveal a 20% decline in sea ice extent in the Amundsen and Bellingshausen Seas in the two decades following 1973. This change is negatively correlated with surface air temperatures on the west side of the Antarctic Peninsula, which have increased ;0.58C decade21 since the mid-1940s. The recession was strongest during summer, when monthly average minima in 1991‐92 removed much of the incipient multiyear ice over the continental shelf. This would have lowered the regional-mean ice thickness, impacting snow ice formation, brine production, and vertical heat flux. The northern ice edge contracted by ;18 of latitude in all seasons from 1973‐79 to 1987‐93, returning toward mean conditions in 1993‐95. The decline included multiyear cycles of several years in length, superimposed on high interannual variability. A review of atmospheric forcing shows winds consistent with mean and extreme ice extents, and suggests links to larger-scale circulation changes in the South Pacific. Historical ocean measurements are sparse in this sector, but mixed-layer depths and upper pycnoclines beneath the sea ice resemble those in the Weddell Sea. Weaker surface currents or changes in the upwelling of Circumpolar Deep Water on the continental shelf could have contributed to the anomaly persistence.

Sea ice concentration, ice temperature, and snow depth using AMSR-E data

A summary of the theoretical basis and initial performance of the algorithms that are used to derive sea ice concentration, ice temperature, and snow depth on sea ice from newly acquired Earth Observing System-Aqua/Advanced Microwave Scanning Radiometer-EOS (AMSR-E) radiances is presented. The algorithms have been developed and tested using historical satellite passive microwave data and are expected to provide more accurate products, since they are designed to take advantage of the wider range of frequencies and higher spatial resolution of the AMSR-E microwave instrument. Validation programs involving coordinated satellite, aircraft, and surface measurements to determine the accuracies of these sea ice products and to improve further our capability to monitor global sea ice are currently underway.