Showing papers on "Antarctic sea ice published in 1996"

Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn

Abstract: A record of atmospheric CO2 mixing ratios from 1006 A.D. to 1978 A.D. has been produced by analysing the air enclosed in three ice cores from Law Dome, Antarctica. The enclosed air has unparalleled age resolution and extends into recent decades, because of the high rate of snow accumulation at the ice core sites. The CO2 data overlap with the record from direct atmospheric measurements for up to 20 years. The effects of diffusion in the firn on the CO2 mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close-off depth. The uncertainty of the ice core CO2 mixing ratios is 1.2 ppm (1 σ). Preindustrial CO2 mixing ratios were in the range 275–284 ppm, with the lower levels during 1550–1800 A.D., probably as a result of colder global climate. Natural CO2 variations of this magnitude make it inappropriate to refer to a single preindustrial CO2 level. Major CO2 growth occurred over the industrial period except during 1935–1945 A.D. when CO2 mixing ratios stabilized or decreased slightly, probably as a result of natural variations of the carbon cycle on a decadal timescale.

Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula

Abstract: IN 1978 Mercer1 discussed the probable effects of climate warming on the Antarctic Ice Sheet, predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the Antarctic Peninsula. Analyses of 50-year meteorological records have since revealed atmospheric warming on the Antarctic Peninsula2,3, and a number of ice shelves have retreated4–8. Here we present time-series of observations of the areal extent of nine ice shelves on the Antarctic Peninsula, showing that five northerly ones have retreated dramatically in the past fifty years, while those further south show no clear trend. Comparison with airtemperature data shows that the pattern and magnitude of ice-shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability, the isotherm associated with this limit having been driven south by the atmospheric warming. Ice shelves therefore appear to be sensitive indicators of climate change.

Rapid Collapse of Northern Larsen Ice Shelf, Antarctica

Abstract: In January 1995, 4200 square kilometers of the northern Larsen Ice Shelf, Antarctic Peninsula, broke away. Radar images from the ERS-1 satellite, complemented by field observations, showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days. This breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming. The observations imply that after an ice shelf retreats beyond a critical limit, it may collapse rapidly as a result of perturbated mass balance.

Antarctic Ice Sheet melting in the southeast Pacific

Abstract: The first oceanographic measurements across a deep channel beneath the calving front of Pine Island Glacier reveal a sub-ice circulation driven by basal melting of 10–12 m yr−1. A salt box model described here gives a melt rate similar to that of ice balance and numerical models, 5–50 times higher than averages for the George VI and Ross Ice Shelves. Melting is fueled by relatively warm Circumpolar Deep Water that floods the deep floor of the Amundsen and Bellingshausen Sea continental shelves, reaching the deep draft of this floating glacier. A revised melt rate for ice shelves in the Southeast Pacific sector raises circumpolar ice shelf melting to 756 Gt yr−1. Given prior estimates of surface accumulation and iceberg calving, this suggests that the Antarctic Ice Sheet is currently losing mass to the ocean.

Recent decreases in Arctic summer ice cover and linkages to atmospheric circulation anomalies

Abstract: Sea ice data from November 1978 through September 1995 for the Arctic Ocean and peripheral seas indicate that summer ice coverage has been below normal in recent years, with extreme minima in 1990, 1993, and 1995. The net trend in summer ice cover over the 17-year period is −0.6% per year, with the extent of the perennial ice pack reduced by 9% in 1990–1995 compared with 1979–1989. The reductions are greatest in the Siberian sector of the Arctic Ocean. Linkages are proposed between these ice anomalies and a sharp increase since 1989 in the frequency of low pressure systems over the central Arctic.

A History of Sea Ice in the Canadian Arctic Archipelago Based on Postglacial Remains of the Bowhead Whale ( Balaena Mysticetus )

Abstract: The bowhead whale ( Balaena mysticetus ) is a planktivore of the baleen group of whales adapted to live in the loose edges of the north polar sea ice. Its annual migrations roughly track the advance and retreat of the floe edge. The distribution and radiocarbon ages of bowhead subfossils in the Canadian Arctic Archipelago show that the range of the whale has expanded and contracted abruptly several times over the last 10.5 thousand years (ka). Each expansion or contraction was followed by nearly stable conditions that persisted for millennia. These changes in the geographic range of the bowhead are defined by >400 radiocarbon dates. The paleo-ranges are the basis for reconstructing summer sea-ice minima. Using this criterion, postglacial time is divided into four intervals: (1) 10.5-8.5 ka B.P. - A large bowhead population extended in the summer all the way to retreating glacier margins and ultimately from the Beaufort Sea to Baffin Bay; meltwater-driven outflows probably cleared the inter-island channels of sea ice; this interval terminated when the present interglacial circulation pattern was established; (2) 8.5-5 ka B.P. - Bowheads were excluded from most of the archipelago because the channels failed to clear of sea ice; summer sea-ice conditions for most of this time were more severe than during historical times; (3) 5-3 ka B.P. - Bowheads reoccupied the central channels of the Arctic Islands, and their range extended beyond historical limits; and (4) 3-0 ka B.P. - Sea ice excluded whales from the central channels, as it does today. This paleoenvironmental record based on bowhead whale distributions is more complex than that revealed in the delta 18O, conductivity or the percent-melt records of the Devon and Agassiz ice cores. A reconciliation of the two data sets may indicate the following general summer climatic conditions: 10-8 ka B.P. - warm summers with maximum postglacial warmth; 8-5 ka B.P. - cool, dry summers; 5-3 ka B.P. - cool, wet summers; 3-0 ka B.P. - cold, dry summers. Key words: bowhead whale, sea ice, ocean currents, climate change

Chronology of late Wisconsin ice retreat from the western Ross Sea, Antarctica

Abstract: Lithologic data from marine sedimentary cores and accelerator mass spectrometer (AMS) radiocarbon dates indicate that grounded ice did not advance to the western Ross Sea continental shelf edge during the last glacial maximum (LGM). A chronology of the timing of ice retreat was provided by 26 AMS dates, obtained from 12 cores. Dates ranging from 20 to 29 ka suggest that the outer continental shelf (beyond ca. lat 74°S) was not covered by grounded glacial ice prior to and during the early stages of the LGM. 14C dates just above transitions from subglacial diamictons to marine muds indicate that the area around the Drygalski ice tongue was deglaciated by at least 11.5 ka. The Ross Ice Shelf reached its present-day position near Ross Island by about 7 ka. A hiatus of about 15 ka is apparent in radiocarbon dates in cores from the outer continental shelf. The hiatus is interpreted to represent an absence of sedimentation caused by the presence of an ice shelf.

Variability and climate sensitivity of landfast Arctic sea ice

Abstract: A one-dimensional thermodynamic sea ice model is used to perform multiyear simulations of land-fast sea ice and its snow cover. The model is applied to two locations in the Canadian high Arctic: Resolute and Alert, Northwest Territories. The sites are chosen because meteorological observations and ice and snow depth measurements spanning the period 1955–1990 are available. Bulk formulae are used to convert standard meteorological observations into surface fluxes, snowfall is specified from observed annual accumulation, and a crude oceanic mixed layer is included to allow heat storage during ice-free summer months. The model is successful at reproducing seasonal and interannual variability in maximum ice thickness, snow depth, freeze-up, and breakup dates. Sensitivity studies demonstrate the relative importance of variables such as air temperature and snowfall rate on interannual variability of maximum ice thickness and the duration of the ice-free season. Multiple equilibria, corresponding to thin, seasonal ice and thick, perennial ice, are shown to exist in the model. Climate sensitivity experiments illustrate the potential effects of changes in air temperature and snowfall on ice thickness and open water duration in the Canadian Archipelago.

Late Cenozoic history of the Scandinavian and Barents Sea ice sheets

Abstract: The oldest ice rafted material (IRD) on the Voring Plateau is dated to about 11 Ma, and is regarded as evidence that glaciers extended to sea level somewhere around the Nordic Sea at this time We estimate that the major glaciations of Scandinavia and the Barents Sea-Svalbard area started at 25–28 Ma, when the amount of IRD on the Voring Plateau increased strongly, the deep sea δ18O curves indicate that onset of Northern Hemisphere glaciations, and the climate in The Netherlands was so cold that a major glaciation of Scandinavia is inferred Most of the time until 09 Ma, the ice sheets were of intermediate size; they probably reached the coastal zone of western Norway for long periods The center of glaciation is inferred to have been further north than during the later glaciations, and we speculate that this was the period of maximum glacial erosion of the Barents Sea The largest glaciations, and also the warmest interglacials occurred during the last 900 kyrs, when the 100 kyr astronomic cycle became important For the last glaciation, the Weichselian, the glacial fluctuations are known in greater detail both for Scandinavia and the Svalbard-Barents Sea region In Scandinavia the glacial fluctuations apparently followed the 23 kyr precession cycle, whereas in the Barents Sea they followed the 41 kyr tilt cycle In both areas more than one advance reached beyond the coast We use the Weichselian record to “calibrate” the interpretation of more indirect evidences of glacial fluctuations, and apply the latter to the Scandinavian and Barents Sea glacial history since 25 Ma

Origin of the first global meltwater pulse following the Last Glacial Maximum

Abstract: Well-dated sea level records show that the glacioeustatic rise following the last glacial maximum was characterized by two or possibly three brief intervals of rapid sea level rise separating periods with much lower rates. These very high rates of sea level rise indicate periods of exceptionally rapid deglaciation of remaining ice sheets. The Laurentide Ice Sheet is commonly targeted as the source of the first, and largest, of the meltwater pulses (mwp-IA between ∼14,200 (12,200 14C years B.P.) and 13,700 years ago (11,700 14C years B.P.)). In all oceanic records of deglaciation of the former northern hemisphere ice sheets that we review, only those from the Gulf of Mexico and the Bermuda Rise show evidence of low δ18O values at the time of mwp-IA, identifying the southern Laurentide Ice Sheet as a potential source for mwp-IA. We question this source for mwp-IA, however, because (1) ice sheet models suggest that this sector of the ice sheet contributed only a fraction (<10%) of the sea level needed for mwp-IA, (2) melting this sector of the ice sheet at the necessary rate to explain mwp-IA is physically implausible, and (3) ocean models predict a much stronger thermohaline response to the inferred freshwater pulse out of the Mississippi River into the North Atlantic than is recorded. This leaves the Antarctic Ice Sheet as the only other ice sheet capable of delivering enough sea level to explain mwp-IA, but there are currently no well-dated high-resolution records to document this hypothesis. These conclusions suggest that reconstructions of the Laurentide Ice Sheet in the ICE-4G model, which are constrained to match the sea level record, may be too low for time periods younger than 15,000 years ago. Furthermore, δ18O records from the Gulf of Mexico show variable fluxes of meltwater from the southern margin of the Laurentide Ice Sheet which can be traced to the opening and closing of eastward draining glacial-lake outlets associated with surging ice sheet behavior. These variable fluxes through eastern outlets were apparently sufficient to affect formation of North Atlantic Deep Water, thus underscoring the sensitivity of this process to changes in freshwater forcing.

Heat flux through sea ice in the western Weddell Sea: Convective and conductive transfer processes

Abstract: The heat flux through the snow and sea ice cover and at the ice/ocean interface were calculated at five sites in the western Weddell Sea during autumn and early winter 1992. The ocean heat flux averaged 7 ± 2 W/m2 from late February to early June, and average ice/air heat flux in the second-year floes depended on the depth of the snow cover and ranged from 9 to 17 (±0.8) W/m2. In late February, three of the five sites had an ice surface which was depressed below sea level, resulting, at two of the sites, in a partially flooded snow cover and a slush layer at the snow/ice interface. As this slush layer froze to form snow ice, the dense brine which was rejected flowed out through brine drainage channels and was replaced by lower-salinity, nutrient-rich seawater from the ocean upper layer. We estimate that about half of the second-year ice in the region was covered with this slush layer early in the winter. As the slush layer froze, over a 2- to 3-week period, the convection within the ice transported salt from the ice to the upper ocean and increased total heat flux through the overlying ice and snow cover. On an area-wide basis, approximately 10 cm of snow ice growth occurred within second-year pack ice, primarily during a 2- to 3-week period in February and March. This ice growth, near the surface of the ice, provides a salt flux to the upper ocean equivalent to 5 cm of ice growth, despite the thick (about 1 m) ice cover, in addition to the ice growth in the small (area less than 5%), open water regions.

Evidence for Glacial Control of Rapid Sea Level Changes in the Early Cretaceous

Abstract: Lower Cretaceous bulk carbonate from deep sea sediments records sudden inputs of strontium resulting from the exposure of continental shelves. Strontium data from an interval spanning 7 million years in the Berriasian-Valanginian imply that global sea level fluctuated about 50 meters over time scales of 200,000 to 500,000 years, which is in agreement with the Exxon sea level curve. Oxygen isotope measurements indicate that the growth of continental ice sheets caused these rapid sea level changes. If glaciation caused all the rapid sea level changes in the Cretaceous that are indicated by the Exxon curve, then an Antarctic ice sheet may have existed despite overall climatic warmth.

Changes in the configuration of ice stream flow from the West Antarctic Ice Sheet

Abstract: Surface-based ice-penetrating radar profiles on the northeast flank of Siple Dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream. The scar defines the southwestern boundary of an ice stream flowing from ice stream C to ice stream D, close to where it enters the Ross Ice Shelf. Our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across, underlain by a zone of disturbed internal stratigraphy revealed by the radar. Burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1.3 ka. The configuration of the ice streams draining the West Antarctic Ice Sheet into the Ross Ice Shelf evidently changes with time, and attempts to predict the evolution of the ice sheet must incorporate this observation.

The thickness distribution of sea ice and snow cover during late winter in the Bellingshausen and Amundsen Seas, Antarctica

Abstract: Data collected from a voyage of RV Nathaniel B. Palmer to the Bellingshausen and Amundsen Seas during August–September 1993 are used to investigate the thickness distribution of sea ice and snow cover and the processes that influence the development of the first-year pack ice. The data are a combination of in situ and ship-based measurements and show that the process of floe thickening is highly dependent on ice deformation; in particular, rafting and ridging play important roles at different stages of floe development. Rafting is the major mechanism in the early stages of development, and core structure data show the mean thickness of individual layers of crystals to be only 0.12 m. Most ice 0.6 m having some surface deformation. Blocks within ridge sails are typically in the range 0.3–0.6 m thick, and ship-based observations estimate approximately 25% of the pack exhibits surface ridging. When corrected for biases in the observational methods, the data show that the dominant ice and snow thickness categories are >0.7 m and 0.2–0.5 m, respectively, and account for 40% and 36% of the surface area of the pack ice. Approximately 8% of the pack is open water. An estimate of the effects of ridging on the distribution of ice mass within the pack suggests that between 50 and 75% of the total mass is contained within the 25% of the pack that exhibits surface ridging.

Limits on the areal extent of the Barents Sea ice sheet in Late Weichselian time

Abstract: Observations of Lateglacial and Holocene sea levels from the Barents Sea region provide constraints on the grounded ice sheet during Late Weischselian time. Ice sheets that were restricted primarily to the Svalbard islands and the immediate shallow sea floor are inadequate to explain the observed age-height relations across the region. Instead, the ice sheet extended out to the edge of the shelf and attained a maximum thickness of the order of 3000 m over the central region of the Barents Sea. Ice volumes to the east of Novaya Zemlya have been small compared to the ice over the Barents Sea. The raised shoreline information from western Spitsbergen implies that retreat of ice over this area was initially slow until about 13,000 yr B.P. following which the remaining deglaciation appears to have been rapid.

Seismic-reflection evidence for a deep subglacial trough beneath Jakobshavns Isbræ, West Greenland

Abstract: Seismic-reflection methods were used to determine the ice thickness and basal topography of Jakobshavns Isbrae, a large, fast-moving ice stream/outlet glacier in West Greenland. A method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source; the method yields the depth, horizontal position and slope of the basal reflector. A deep U-shaped subglacial trough was found beneath the entire length of the well-defined ice stream. The trough is incised up to 1500 m into bedrock, and its base lies 1200–1500 m below sea level for at least 70 km inland. Center-line ice thickness along most of the channel is about 2500 m, or about 2.5 times that of the surrounding ice sheet. This prominent bedrock trough was not apparent in existing radio-echo-sounding data. Reflection coefficients indicate that much of the basal interface is probably underlain by compacted, non-deforming sediment. The large ice thickness, coupled with relatively steep surface slopes, leads to high basal shear stresses (200–300 k Pa) along the ice stream. The large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of Jakobshavns Isbrae.

Ice forming nuclei in the high Arctic

Abstract: Concentrations of ice forming nuclei detected by the membrane filter technique were measured at temperatures of − 12.5, − 15 and − 17.5 °C during a voyage on the icebreaker Oden to the North Pole between 1 August and 6 October 1991. Geometric mean concentrations ranged from 13 m −3 at − 15 °C during the first 17 days of the expedition to 2.9 m −3 on the last 17 days of the voyage in good agreement with surface measurements made earlier north of latitude 70°N. On average, concentrations increased by a factor of 4.5 for a 5 °C fall in temperature, less than is common in continental regions. Although air trajectory analysis showed that land sources occasionally influenced concentrations strongly, the time since the air had been over the open ocean was clearly the most important determining factor other than the seasonal decline. This implies an oceanic origin of the nuclei, the relationship being consistent with a halflife of 48.5 h ( e -folding time of 70 h), about 60% longer than that of condensation nuclei. An apparent decrease in ice nucleus concentrations with temperature was mainly due to the seasonal change in concentrations but partly to the air trajectories associated with low temperatures. Elapsed time since the air was above the planetary boundary layer or over land also influenced concentrations, suggesting that the upper troposphere was deficient in ice nuclei while land was a weak source. Changes in the mixed depth of the atmosphere appeared to affect ice nucleus concentrations in the same way as condensation nucleus concentrations although poor time resolution limited this to two good examples. North of latitude 80°N, a few ice crystals were present near the surface for a considerable proportion of the total time. Single stellar crystals constituted about 80% of the total, implying growth near − 15 °C in predominantly supercooled clouds. Their concentrations were usually but not always consistent with surface ice nucleus concentrations. DOI: 10.1034/j.1600-0889.1996.t01-1-00007.x

Seasonal and interannual variations of the oceanic heat flux under a landfast Antarctic sea ice cover

Abstract: A multilayer thermodynamic model is used to simulate sea ice growth for 12 years between 1958 and 1986 in the vicinity of the Australian station Mawson on the coast of East Antarctica. The atmospheric forcing data for the model are derived from radiosonde profiles and from surface measurements. Global radiation data are available for 4 years, and we use these measurements for comparison with the results of a Zillman-type model for global radiation. Combining the thermodynamic model with sea ice thickness measurements for 12 years, we solve the energy balance equation for the oceanic heat flux. The oceanic heat flux is not constant but changes with time within the year and from year to year. The oceanic heat flux averages 7.9 W/m2, and the yearly means vary between 5 and 12 W/m2. Seasonal values of the oceanic heat flux range from 0 to 18 W/m2. From the yearly averaged values a decadal trend is evident: During the first years that were analyzed the yearly average lies well above 10 W/m2; then in the mid-1970s a decrease to 9 W/m2 occurs, while for all later years the values are ∼6–8 W/m2. In general, the oceanic heat flux increases from the start of the fast ice formation season in early April until it breaks out in December or January. To compare the calculated oceanic heat fluxes for different years, we divide the total ice season into three characteristic time regimes of the sea ice growth and calculate the averaged oceanic heat fluxes for each regime. For the first regime (through August) the mean flux is 2.7 W/m2, for the middle regime (September) it is 8.4 W/m2, and for the final regime (October–January) it is 17 W/m2. We discuss the results of our model calculations in conjunction with current meter observations, which give evidence of seasonally varying intrusions of relatively warm Circumpolar Deep Water into Prydz Bay. Comparison of passive microwave data of sea ice extent and concentration (from the scanning multichannel microwave radiometer sensor) with the model results reveals a correlation between the magnitude of the oceanic heat flux and local features such as polynyas.

Low-Frequency Variability in the Arctic Atmosphere, Sea Ice, and Upper-Ocean Climate System

Abstract: The low-frequency natural variability of the arctic climate system is modeled using a single-column, energy balance model of the atmosphere. sea ice, and upper-ocean system. Variability in the system is induced by forcing with realistic, random perturbations in the atmospheric energy transport and cloudiness. The model predicts that the volume of perennial sea ice varies predominantly on decadal timescales, while other arctic climate variables vary mostly on intraannual and interannual timescales. The variance of the simulated sea ice volume is most sensitive to perturbations of the atmospheric forcing in late spring, at the onset of melt. The variance of sea ice volume increases with the mean sea ice thickness and with the number of layers resolved in the sea ice model. This suggests that much of the simulated variance develops when the surface temperature decouples from the sea ice interior during the late spring, when melting snow abruptly exposes the sea ice surface and decreases the surface ...

Atmospheric and oceanic forcing of Weddell Sea ice motion

Abstract: The data from sea ice buoys, which were deployed during the Winter Weddell Sea Project 1986, the Winter Weddell Gyre Studies 1989 and 1992, the Ice Station Weddell in 1992, the Antarctic Zone Flux Experiment in 1994, and several ship cruises in Austral summers, are uniformly reanalyzed by the same objective methods. Geostrophic winds are derived after matching of the buoy pressure data with the surface pressure fields of the European Centre for Medium Range Weather Forecasts. The ratio between ice drift and geostrophic wind speeds is reduced when winds and currents oppose each other, when the atmospheric surface layer is stably stratified, and when the ice is under pressure near coasts. Over the continental shelves, the spatial inhomogeneity of tidal and inertial motion effectively controls the variability of divergence for periods below 36 hours. Far from coasts, speed ratios, which presumably reflect internal stress variations in the ice cover, are independent of drift divergence on the spatial scale of 100 km. To study basin-scale ice dynamics, all ice drift data are related to the geostrophic winds based on the complex linear model [Thorndike and Colony, 1982] for daily averaged data. The composite patterns of mean ice motion, geostrophic winds, and geostrophic surface currents document cyclonic basin-wide circulations. Geostrophic ocean currents are generally small in the Weddell Sea. Significant features are the coastal current near the southeastern coasts and the bands of larger velocities of ≈6 cm s−1 following the northward and eastward orientation of the continental shelf breaks in the western and northwestern Weddell Sea. In the southwestern Weddell Sea the mean ice drift speed is reduced to less than 0.5% of the geostrophic wind speed and increases rather continuously to 1.5% in the northern, central, and eastern Weddell Sea. The linear model accounts for less than 50% of the total variance of drift speeds in the southwestern Weddell Sea and up to 80% in the northern and eastern Weddell Sea.

Boundary-layer modification in wintertime cold-air outbreaks from the Arctic sea ice

Abstract: During the field experiment ARKTIS 1993 ten cases of boundary-layer modification in wintertime cold-air outbreaks from the Arctic sea ice in the Spitsbergen region were observed by aircraft over a distance ranging from about 50 km over the ice to about 300 km over the water. The modification depends decisively on the initial conditions over the ice, the boundary conditions at the bottom and top of the boundary layer and on the conditions of the large-scale flow. The modification of the bulk boundary-layer characteristics in relation to these conditions is presented.

High-latitude climate change in a global coupled ocean-atmosphere-sea ice model with increased atmospheric CO2

Abstract: A global atmospheric general circulation model (GCM) coupled to a global 1-degree, 20-level ocean GCM with dynamic and thermodynamic sea ice is integrated with CO2 increasing at 1% per year compounded for 75 years (CO2 doubles at about year 70). Flux correction is not used in the experiment. The increase of globally averaged surface air temperature at the time of CO2 doubling is 3.8°C. The warm subsurface Atlantic layer at intermediate depths in the Arctic is maintained mainly by the sinking and intrusion of water from the West Spitsbergen Current in the model and the observations. With increased CO2 in the model, the warmer surface waters are intruded into the upper portion of the Atlantic layer producing an anomalous warming in the model at depths between 200 and 400 m. This resembles an anomalous warm layer near those depths recently observed in the Arctic. As the climate warms and sea ice retreats, low clouds increase over the newly exposed water. Yet the consequent increase of cloud albedo over these regions is more than compensated for by the decrease of surface albedo due to the melting of sea ice. This produces a net decrease of planetary albedo in the Arctic that contributes to a strong ice-albedo feedback and the comparatively high sensitivity of the model.

Response of Baltic Sea ice to seasonal, interannual forcing and climate change

Abstract: The objectives of the present paper are to formulate and explore a coupled sea ice-ocean model and to examine the sensitivity of ice in the Baltic Sea to climate change. The model treats the Baltic Sea as 13 sub-basins with vertical resolution, horizontally coupled by estuarine circulation and vertically coupled to a sea ice model which includes both dynamic and thermodynamic processes. The reducing effect on the barotropic exchange due to sea ice in the entrance area is also added. The model was first verified with data from 3 test periods representing one mild, one normal and one severe ice winter. The maximum seasonal ice extent was then examined on the basis of simulated and observed data for the period 1980–1993. After that, some climate scenarios (both warm and cold) were examined. The seasonal, regional and interannual variations of sea ice were well described by the model, and the thermal response in the Baltic Sea can be realistically simulated applying forcing data from rather few stations. The Baltic Sea system is highly sensitive to climate change, particularly during the winter season. Warming may drastically decrease the number of winters classified as severe, forcing the climate towards more oceanic conditions. On the other hand, cooling will increase the number of severe winters, forcing the climate towards more sub-arctic conditions. DOI: 10.1034/j.1600-0870.1996.t01-4-00004.x

Spatial variations in heat at the base of the Antarctic ice sheet from analysis of the thermal regime above subglacial lakes

Abstract: Antarctic subglacial lakes provide an important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature over lakes may be assumed to be at the pressure-melting point We have used a one-dimensional vertical heat-transfer equation to determine theoretical temperature values for the ice-sheet base above 77 subglacial lakes identified from airborne radio-echo-sounding data covering 50% of Antarctica Variations in our temperature results to below the pressure-melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from (a) internal ice deformation and (b) basal sliding, in the thermal model Our results indicate that, when the geothermal heat flux is set at 54 mW m -2 , the ice-sheet base above 70% of the known Antarctic subglacial lakes is calculated to be at the pressure-melting value These lakes are located mainly around Dome C, Ridge B and Vostok station For the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide, using the same thermal model, loss of heat due to vertical advection is calculated to be relatively high In such regions, if the ice-sheet base is at the pressure-melting point, heat lost due to vertical advection must be supplemented by heat from other sources For the three lakes beneath Terre Adelie and George V Land, for instance, the basal thermal gradient calculated to produce pressure melting at the ice-sheet base is equivalent to 15-2 times the value obtained when 54 mW m -2 of geothermal heat is used as the sole basal thermal component We suggest that, as distance from the ice divide increases, so too does the amount of heat due to internal ice deformation and basal sliding Moreover, by considering the ice-sheet basal thermal characteristics above subglacial lakes which lie on the same ice flowline, we demonstrate empirically that the heat due to these horizontal ice-motion terms varies pseudo-exponentially with distance from the ice divide The location along a flowline where a rapid increase in the basal heat gradient is calculated may correspond to the onset of large-scale basal sliding

Development of seasonal pack ice in the Beaufort Sea during the winter of 1991–1992: A view from below

Abstract: Subsea sonars moored in the Beaufort Sea acquired a spatial section of draft across 941 km of sea ice during the winter of 1991–1992. These observations document the development of seasonal sea ice from an open sea surface in October to an average accumulation exceeding 2.5 m draft by early April. Initially, level ice occupied 85% of the profile, but continued ridging reduced this fraction to 50% in late winter. Three modes of level ice were tracked: that of seasonal ice whose growth was initiated at freeze-up, and two others created during openings of the ice field in January and in March. The growth of these modes can be closely matched by calculations based on a slab model of ice formation forced using surface meteorological data. Initially, ridge keels were small and widely spaced. By midwinter, an exponential dependence of keel frequency on draft was obvious (e-folding scale, 2.16 m), and the frequency and mean draft of keels had stabilized. The maximum keel draft was a modest 17.4 m. The low incidence of deep keels at all times is a statistically significant departure from the exponential dependence valid at lesser drafts. The truncation point of the exponential relation is related to the draft of the thinnest level ice present. In late winter, the formation of deeper keels from thick first-year ice was apparently precluded by the presence of younger, thinner ice, which limited the force available for ridge building. Through calculation of the seaward transport of ice over the sonars, the total production of ice in the coastal flaw lead during 1991–1992 was determined to be about 60% of earlier indirect estimates. In general, the observations revealed winter ice conditions significantly less severe than those found on the periphery of the polar pack only a few tens of kilometers to the north, but more so than conditions in a marginal first-year ice zone at the same latitude in Davis Strait.

Late glacial air temperature, oceanographic and ice sheet interactions in the southern Barents Sea region

Abstract: Abstract Stratigraphic records from three areas (the Andøya area, the continental slope and the continental shelf of the southern Barents Sea) are used to evaluate the history of the late glacial Barents Sea Ice Sheet and the northern Fennoscandian Ice Sheet. Two late glacial maxima are indicated: one before 22 ka BP (LGM I) and the younger after 19 ka BP (LGM II). A major deglaciation phase was initiated during a warm spell between 16 and 15 ka BP. During a drawdown of marine based ice sheets about 14.5 ka BP large parts of the southern Barents Sea Ice Sheet decayed. This deglaciation was retarded between 13.7 and 13 ka BP. Most of the eastern and northern Barents Sea was deglaciated during the Bølling-Allerød interstadial complex. During the LGM II there was an ice stream in the Bear Island Trough with an estimated velocity at the front of about 2.5 km/year. An average accumulation rate of 0.3–0.4 m/year is estimated for the drainage area of the Bear Island Trough Ice Stream. A close interrelation exists between summer air temperatures and the waxing and waning phases of the northern Fennoscandian and southern Barents Sea ice sheets. It is suggested that most of the ice sheet decay was due to climatic warming, which caused thinning of the ice sheets, making them susceptible to decoupling from the sea bed and increased calving. The subsequent halts/readvances were due to climatic deteriorations caused by huge inputs of icebergs to the Norwegian Sea which cooled the surface water and in turn promoted sea ice preservation through the summer season and thereby increased the albedo.

An ice-shelf model test based on the Ross ice shelf

Abstract: A standard numerical experiment featuring the Ross Ice Shelf, Antarctica, is presented as a test package for the development and intercomparison of ice-shelfmodels The emphasis of this package is solution of stress-equilibrium equations for an ice-shelf velocity consistent with present observations As ademonstration, we compare five independently developed ice-shelf models based on finite-difference and finite-element methods Our results suggest thatthere is little difference between finite-element and finite-difference methods in capturing the basic, large-scale flow features of the ice shelf We additionallyshow that the fit between model and observed velocity depends strongly on the ice-shelf temperature field for which there is presently little observationalcontrol The main differences between model results are due to the equations being solved, the boundary conditions at the ice front and the discretisationmethod (finite element vs finite difference)

Scales and rates of glacial sediment removal: a 20 km long, 300 m deep trench created beneath Breiðamerkurjökull during the Little Ice Age

Abstract: A 20 km long, 2-5 km wide trench, which extends to 300 m below sea level, is believed to have been created by removal of sediment during the advance of Breiðamerkurjokull in the Little Ice Age. Between 1732 and 1890, the glacier advanced by 9 km, covered an area of 45 km2 and excavated a volume of 5 × 109 m3, equivalent to an average of 110 m over this area. Average sediment-removal rate during the 158 years was 32 × 106 m3 a−1 or 0.7 m a−1 km−2 averaged over the area covered by the advancing glacier. Calculated over the whole drainage area of the eastern branch of the glacier of 750 km2, the denudation rate would be 4 × 10−2 m a−1 km−2. Fluvial processes are estimated to have carried about 30 × 106 m3 a−1, and the sediment fluxes within the ice and by the deforming subglacial till are estimated to be 105 and 106 m3 a−1, respectively. The average sediment concentration in the glacial streams would have been about 10 kg m−3. Such concentrations have been measured in Icelandic rivers during jokulhlaups and surges. Several surging events took place during the advance of Breiðamerkurjokull, and jokulhlaups drain regularly beneath the glacier from ice-dammed marginal lakes. The present rate of transport, although considerable, seems to be about 10 × 106 m3 a−1, of which 30% is transported by the river to the sea and 70% is dumped into a proglacial lake.

Climate forcing and thermal feedback of residual lake-ice covers in the high Arctic

Abstract: A discontinuous ice coverage and lake temperature record of 36-yr duration has been compiled for Colour Lake (79”25’N, 90”45’W) on Axe1 Heiberg Island, Northwest Territories. About once every 6 yr, the ice cover remains through the entire summer, creating a residual ice cover the following winter. Residual ice covers are more frequent in the last 10 yr (1986-1995) than in the 20 yr from 1959 to 1978, indicating a reduction in climate factors controlling ice decay. These factors are identified as the number of thawing degree-days, suggesting a tendency toward cooler summers over the last decade. We report year-round meteorology observations that indicate a mean annual temperature of about - 15.2”C, with - 500 thawing degree-days in summer. Following a residual ice year, spring lake-water temperatures are significantly greater than they are following nonresidual years. The increased temperatures can be attributed to the stabilizing effect of the residual ice pan the previous fall. Without an ice pan, convection cools the entire water column before the surface freezes. When the lake has an ice pan at the start of freezing, the surface freezes over quickly, effectively trapping water heated during summer. The warmer spring water temperatures act as negative feedback, decreasing the potential for a second consecutive residual ice year.