Variability Of Sea Ice Extent Research Articles

Mechanisms for the variation of sea ice extent in the northern hemisphere

Using daily sea ice data derived from satellite‐borne sensors and atmospheric data, we examined processes controlling the variation of sea ice extent in the Northern Hemisphere. The daily ice motion field was computed from imagery of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) for seven winters (December to April) from 1991/1992 to 1997/1998, by employing the maximum cross‐correlation method. In order to examine mechanisms of the temporal variation of the ice extent, we analyzed 50 specified lines across the daily ice edge. Although a high correlation between the ice motion and the geostrophic wind speed was observed in all the ice edge areas, the degree for correlation between the speed of the ice edge displacement and the wind speed varied with region. The degree for response of the ice edge speed to the wind speed largely depended upon that of the ice edge speed to the ice motion. The following mechanisms controlling the variation of ice extent for regions in the Northern Hemisphere were anticipated. In the Barents Sea, Bering Sea, and the Sea of Okhotsk the ice extent advances by wind‐driven ice advection and the daily scale variation of the ice extent were also controlled by the variation in wind speed. In contrast, the ice extent in the Labrador Sea and the Greenland Sea seemed to be considerably affected by oceanographic factors such as the location of the thermal front and was not related to the variation in wind speed. The regional difference of the variation mechanism was also reflected in the interannual variation in maximum ice extent.

Ocean circulation off east Antarctica affects ecosystem structure and sea-ice extent.

Sea ice and oceanic boundaries have a dominant effect in structuring Antarctic marine ecosystems. Satellite imagery and historical data have identified the southern boundary of the Antarctic Circumpolar Current as a site of enhanced biological productivity. Meso-scale surveys off the Antarctic peninsula have related the abundances of Antarctic krill (Euphausia superba) and salps (Salpa thompsoni) to inter-annual variations in sea-ice extent. Here we have examined the ecosystem structure and oceanography spanning 3,500 km of the east Antarctic coastline, linking the scales of local surveys and global observations. Between 80 degrees and 150 degrees E there is a threefold variation in the extent of annual sea-ice cover, enabling us to examine the regional effects of sea ice and ocean circulation on biological productivity. Phytoplankton, primary productivity, Antarctic krill, whales and seabirds were concentrated where winter sea-ice extent is maximal, whereas salps were located where the sea-ice extent is minimal. We found enhanced biological activity south of the southern boundary of the Antarctic Circumpolar Current rather than in association with it. We propose that along this coastline ocean circulation determines both the sea-ice conditions and the level of biological productivity at all trophic levels.

Concordance of interannual fluctuations in acoustically estimated densities of Antarctic krill around South Georgia and Elephant Island: biological evidence of same-year teleconnections across the Scotia Sea

Acoustic estimates of the densities of Antarctic krill, Euphausia superba, in areas around South Georgia (SG) and Elephant Island (EI) were compared for seven austral summers between 1981 and 1997. Estimated densities of krill at SG were most often lower than at EI, although this may simply have been a function of differences in the survey and data-analysis techniques used at each site. More interestingly, the magnitudes of density and between-year gradients of density at each site were mirrored by those at the other location; for example 1991 and 1994 were years of very low krill density at both SG and EI. There was no apparent lag in changes in density between sites, and ranked between-year gradients in density at both locations were closely correlated. These pronounced similarities suggest that densities of krill at both locations are linked directly, and may be impacted by the same gross physical and biological factors (e.g. recruitment, dispersal and environmental variability) acting over the same temporal and spatial scales. The observed concordance also implies that the pelagic ecosystems at these widely separated sites (≃1500 km distant at opposite sides of the Scotia Sea) are not operating in isolation. Fluctuations in krill density were investigated with reference to cyclical variations in sea ice extent, and in air and sea-surface temperature. The resulting model suggests that the 1999/2000 austral summer will be one of low krill density.

An Intercomparison of DMSP F11- and F13-Derived Sea Ice Products

Passive microwave satellite data provide an extended time series for monitoring of polar processes, such as variability in sea ice extent. To ensure consistent data sets for time series analysis of sea ice cover, differences between ice concentration estimates from similar sensors on successive spacecraft must be understood. In this study, the effects of changing from the SSM/I F11 to the F13 satellite are examined for a 5-month overlap period. In terms of hemispheric averages of mean ice concentration, the biases introduced by the switch from F11 to F13 are slight and are not statistically significant in most areas, although relatively large and significant differences are seen in some regions. Furthermore, differences in sea ice extent and total ice covered area between the two platforms were found to be statistically significant. Previous efforts to reduce such differences in geophysical parameters between earlier SSM/Is have focused on establishing relationships between the brightness temperatures. However, we find the relations between the F11 and F13 brightness temperatures to be highly sensitive to the region chosen for the analysis. Consequently, the choice of sample has a substantial effect on the sensor-to-sensor adjustment and on the resulting sea ice concentrations. Furthermore, analysis of ice concentrations between F8 and F11 and between F11 and F13 shows that current attempts at a relative calibration of the two instruments do not offer a significant improvement in corresponding ice fractions.

Climate change in the western Antarctic Peninsula since 1945: observations and possible causes

Temperature records from slations on the west roast of the Antarctic Peninsula show a very high level of interannual variability and, over the last 50 years, larger warming trends than are seen elsewhere in Antarctica. in this paper we investigate the role of atmospheric circulation variability and sea-ice extent variations in driving these changes. Owing to a lack of independent data, the reliability of Antarctic atmospheric analyses produced in the 1950s and 1960s cannot be readily established, but examination of the available data suggests that there has been an increase in the northerly component of the circulation over the Peninsula since the late 1950s. Few observations of sea-ice extent are available prior to 1973, but the limited data available indicate that the ice edge to the west of the Peninsula lay to the north of recently observed extremes during the very cold conditions prevailing in the late 1950s. The ultimate cause of the atmospheric-circulation changes remains to be determined and may lie outside the Antarctic region.

ANNUAL SEA LEVEL VARIABILITY INDUCED BY CHANGES IN SEA ICE EXTENT AND ACCUMULATION ON ICE SHEETS: AN ASSESSMENT BASED ON REMOTELY SENSED DATA

Changes of mean annual net accumulation at the surface on the grounded ice sheets of East Antarctica, West Antarctica and Greenland in response to variations in sea ice extent are estimated using grid-point values 100 km apart. The data bases are assembled principally by bilinear interpolation of remotely sensed brightness temperature (Nimbus-5 ESMR, Nimbus-7 SMMR), surface temperature (Nimbus-7 THIR), and surface elevation (ERS-1 radar altimeter). These data, complemented by field data where remotely sensed data are not available, are used in multivariate analyses in which mean annual accumulation (derived from firn emissivity) is the dependent variable; the independent variables are latitude, surface elevation, mean annual surface temperature, and mean annual distance to open ocean (as a source of energy and moisture). The last is the shortest distance measured between a grid point and the mean annual position of the 10% sea ice concentration boundary, and is used as an index of changes in sea ice extent as well as of mean concentration. Stepwise correlation analyses indicate that variations in sea ice extent of ± 50 km would lead to changes in accumulation inversely of ± 4% on East Antarctica, ± 10% on West Antarctica, and ±4% on Greenland. These results are compared with those obtained in a previous study using visually interpolated values from contoured compilations of field data; they substantiate the findings for the Antarctic ice sheets (±4% on East Antarctica, ±9% in West Antarctica), and suggest a reduction by one half of the probable change of accumulation on Greenland (from ±8%). The results also suggest a reduction of the combined contribution to sea level variability to ±0.19 mm a-1 (from ±0.22 mm a-1).

Evidence Of Environmental Change Since The Last Glacial Maximum Inferred From Chemical Analysis Of An Ice Core From Law Dome, Antarctica

Evidence Of Environmental Change Since The Last Glacial Maximum Inferred From Chemical Analysis Of An Ice Core From Law Dome, Antarctica

Evidence Of Environmental Change Since The Last Glacial Maximum Inferred From Chemical Analysis Of An Ice Core From Law Dome, Antarctica

Evidence Of Environmental Change Since The Last Glacial Maximum Inferred From Chemical Analysis Of An Ice Core From Law Dome, Antarctica

An index of sea ice variations in the Nordic seas

AbstractTwentieth century sea ice data from the Nordic Seas (the Atlantic Sector of the Arctic) are examined using a simple index of variations in sea ice extent. The index measures overall sea ice extent variations in the Nordic Seas between 1901 and 1977. Variations in the marginal ice zone during the melt season are shown to reflect regional air temperature changes. Reduction of the index data using principal component analysis highlights sub‐regional heterogeneity in melt season sea ice extent variations. An important spatial pattern of variation is one in which sea ice extent in the Barents Sea varies out of phase with that in the central Greenland Sea. This alternating pattern has a quasi‐periodicity of around three years. Time series analyses of other sea ice records indicate significant quasi‐periodicities in the region of between 3 and 4 years and 2.5 years. Three years may be a key time scale of variation in the Atlantic Sector of the Arctic seasonal sea ice zone.

Seasonal Variations of Sea Ice Extent in the Davis Strait-Labrador Sea Area and Relationships with Synoptic-Scale Atmospheric Circulation

Using published data sources for south-eastern Baffin Island, Ungava Bay and the northern Labrador Sea area, a study of the general patterns of sea ice growth and decay has been made for the years 1964 to 1974. From a comparison of individual years an "early" and a "late" pattern of both ice advance and ice retreat are recognized. Mean daily sea level pressure patterns for June - July and for October - mid-November are examined and a relationship is established between the type of ice advance or retreat pattern and the synoptic circulation over the area. In the years of early ice retreat there is an increased frequency of southerly airflow over the region. Strong winds and the advection of warm air leads to the more rapid removal of the ice compared to years of late ice retreat. Similarly for the years of early ice advance there is an increased frequency of northerly and westerly flow, bringing lower temperatures and an influx of second-year and multi-year ice into the area.

Antarctic Sea-Ice Variations from Satellite Sensing in Relation to Climate

AbstractAn analysis of records of annual mean temperatures around Antarctica shows large-scale anomalies of thousands of kilometers extent with typical variations of a deg from one year to another From 1967 on, composite satellite photographs are available which show considerable variation in the sea-ice extent in different years up to about 5º of latitude. These largest differences seem to persist over entire seasons. In general there seems to be considerable association between the region around the Antarctic with the coldest temperatures and the regions of greatest sea-ice extent. An analysis of long-term records at a single location near the edge of the Antarctic sea ice indicates a strong correlation between variations in the annual mean temperature and the duration of the sea ice, such that a change of 1 deg in the annual mean temperature corresponds to about 70 d variation in the duration of the sea ice. A relation is obtained between variations of annual mean temperature and the mean extent of the sea ice, viz. a 1 deg change corresponds to approximately 2.5º latitude variation in the maximum sea-ice extent. The magnitude of the variations in the sea-ice extent observed from the satellite data in comparison with the large-scale temperature anomalies is compatible with the above relations, although some rotational shifts appear to take place.

Antarctic Sea-Ice Variations from Satellite Sensing in Relation to Climate

AbstractAn analysis of records of annual mean temperatures around Antarctica shows large-scale anomalies of thousands of kilometers extent with typical variations of a deg from one year to another From 1967 on, composite satellite photographs are available which show considerable variation in the sea-ice extent in different years up to about 5º of latitude. These largest differences seem to persist over entire seasons. In general there seems to be considerable association between the region around the Antarctic with the coldest temperatures and the regions of greatest sea-ice extent. An analysis of long-term records at a single location near the edge of the Antarctic sea ice indicates a strong correlation between variations in the annual mean temperature and the duration of the sea ice, such that a change of 1 deg in the annual mean temperature corresponds to about 70 d variation in the duration of the sea ice. A relation is obtained between variations of annual mean temperature and the mean extent of the sea ice, viz. a 1 deg change corresponds to approximately 2.5º latitude variation in the maximum sea-ice extent. The magnitude of the variations in the sea-ice extent observed from the satellite data in comparison with the large-scale temperature anomalies is compatible with the above relations, although some rotational shifts appear to take place.