Showing papers on "Antarctic sea ice published in 2000"

Timing of the Last Glacial Maximum from observed sea-level minima

Abstract: During the Last Glacial Maximum, ice sheets covered large areas in northern latitudes, and global temperatures were significantly lower than today. But few direct estimates exist of the volume of the ice sheets, or the timing and rates of change during their advance and retreat. Here we analyze four distinct sediment facies in the shallow, tectonically stable Bonaparte Gulf, Australia - each of which is characteristic of a distinct range in sea level - to estimate the maximum volume of land-based ice during the last glaciation and the timing of the initial melting phase. We use faunal assemblages and preservation status of the sediments to distinguish open marine, shallow marine, marginal marine and brackish conditions, and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio-hydroisostatic modelling. Our results indicate that from at least 22,000 to 19,000 (calendar) years before present, land-based ice volume was at its maximum, exceeding today's grounded ice sheets by 52.5 x 10 exp 6 cu km. A rapid decrease in ice volume by about 10 percent within a few hundred years terminated the Last Glacial Maximum at 19,000 +/- 250 years.

The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula

Abstract: A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis.

Arctic Sea Ice Variability in the Context of Recent Atmospheric Circulation Trends

Abstract: Forty years (1958–97) of reanalysis products and corresponding sea ice concentration data are used to document Arctic sea ice variability and its association with surface air temperature (SAT) and sea level pressure (SLP) throughout the Northern Hemisphere extratropics. The dominant mode of winter (January–March) sea ice variability exhibits out-of-phase fluctuations between the western and eastern North Atlantic, together with a weaker dipole in the North Pacific. The time series of this mode has a high winter-to-winter autocorrelation (0.69) and is dominated by decadal-scale variations and a longer-term trend of diminishing ice cover east of Greenland and increasing ice cover west of Greenland. Associated with the dominant pattern of winter sea ice variability are large-scale changes in SAT and SLP that closely resemble the North Atlantic oscillation. The associated SAT and surface sensible and latent heat flux anomalies are largest over the portions of the marginal sea ice zone in which the tr...

The influence of Antarctic sea ice on glacial-interglacial CO2 variations

Abstract: Ice-core measurements indicate that atmospheric CO2 concentrations during glacial periods were consistently about 80 parts per million lower than during interglacial periods. Previous explanations for this observation have typically had difficulty accounting for either the estimated glacial O2 concentrations in the deep sea, 13C/12C ratios in Antarctic surface waters, or the depth of calcite saturation; also lacking is an explanation for the strong link between atmospheric CO2 and Antarctic air temperature. There is growing evidence that the amount of deep water upwelling at low latitudes is significantly overestimated in most ocean general circulation models and simpler box models previously used to investigate this problem. Here we use a box model with deep-water upwelling confined to south of 55 degrees S to investigate the glacial-interglacial linkages between Antarctic air temperature and atmospheric CO2 variations. We suggest that low glacial atmospheric CO2 levels might result from reduced deep-water ventilation associated with either year-round Antarctic sea-ice coverage, or wintertime coverage combined with ice-induced stratification during the summer. The model presented here reproduces 67 parts per million of the observed glacial-interglacial CO2 difference, as a result of reduced air-sea gas exchange in the Antarctic region, and is generally consistent with the additional observational constraints.

Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning

Abstract: Aircraft laser-altimeter surveys over northern Greenland in 1994 and 1999 have been coupled with previously reported data from southern Greenland to analyze the recent mass-balance of the Greenland Ice Sheet. Above 2000 meters elevation, the ice sheet is in balance on average but has some regions of local thickening or thinning. Thinning predominates at lower elevations, with rates exceeding 1 meter per year close to the coast. Interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise sea level by 0.13 millimeter per year-approximately 7% of the observed rise.

Widespread complex flow in the interior of the Antarctic Ice Sheet

Abstract: It has been suggested that as much as 90% of the discharge from the Antarctic Ice Sheet is drained through a small number of fast-moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas. Here, evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet. This finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing.

Antarctic sea ice extent variability and its global connectivity

Abstract: This study statistically evaluates the relationship between Antarctic sea ice extent and global climate variability. Temporal cross correlations between detrended Antarctic sea ice edge (SIE) anomaly and various climate indices are calculated. For the sea surface temperature (SST) in the eastern equatorial Pacific and tropical Indian Ocean, as well as the tropical Pacific precipitation, a coherent propagating pattern is clearly evident in all correlations with the spatially averaged (over 12° longitude) detrended SIE anomalies (〈SIE*〉). Correlations with ENSO indices imply that up to 34% of the variance in 〈SIE*〉 is linearly related to ENSO. The 〈SIE*〉 has even higher correlations with the tropical Pacific precipitation and SST in the tropical Indian Ocean. In addition, correlation of 〈SIE*〉 with global surface temperature produces four characteristic correlation patterns: 1) an ENSO-like pattern in the Tropics with strong correlations in the Indian Ocean and North America (r > 0.6); 2) a telecon...

Potential links between surging ice sheets, circulation changes, and the Dansgaard-Oeschger cycles in the Irminger Sea, 60-18 kyr

Abstract: Surface and deepwater paleoclimate records in Irminger Sea core SO82-5 (59oN, 31 oW) and Icelandic Sea core PS2644 (68oN, 22oW) exhibit large fluctuations in thermohaline circulation (THC) from 60 to 18 calendar kyr B.P., with a dominant periodicity of 1460 years from 46 to 22 calendar kyr B.P., matching the Dansgaard- Oeschger (D-O) cycles in the Greenland Ice Sheet Project 2 (GISP2) temperature record (Grootes and $tuiver, 1997). During interstadials, summer sea surface temperatures (SSTsu) in the Irminger Sea averaged to 8oC, and sea surface salinities (SSS) averaged to -36.5, recording a strong Irminger Current and Atlantic THC. During stadials, SSTsu dropped to 2o-4oC, in phase with SSS drops by -1-2. They reveal major meltwater injections along with the East Greenland Current, which turned off the North Atlantic deepwater convection and hence the heat advection to the north, in harmony with various ocean circulation and ice models. On the basis of the IRD composition, icebergs came from Iceland, east Greenland, and perhaps Svalbard and other northern ice sheets. However, the southward drifting icebergs were initially jammed in the Denmark Strait, reaching the Irminger Sea only with a lag of 155-195 years. We also conclude that the abrupt stadial terminations, the D-O warming events, were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater-covered northwestern North Atlantic. In the 1/1460-year frequency band, benthic 1580 brine water spikes led the temperature maxima above Greenland and in the Irminger Sea by as little as 95 years. Thus abundant brine formation, which was induced by seasonal freezing of large parts of the northwestern Atlantic, may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the THC. In summary, the internal dynamics of the east Greenland ice sheet may have formed the ultimate pacemaker of D-O cycles.

A habitat for psychrophiles in deep Antarctic ice

Abstract: Microbes, some of which may be viable, have been found in ice cores drilled at Vostok Station at depths down to approximately 3,600 m, close to the surface of the huge subglacial Lake Vostok. Two types of ice have been found. The upper 3,500 m comprises glacial ice containing traces of nutrients of aeolian origin including sulfuric acid, nitric acid, methanosulfonic acid (MSA), formic acid, sea salts, and mineral grains. Ice below approximately 3,500 m comprises refrozen water from Lake Vostok, accreted to the bottom of the glacial ice. Nutrients in the accretion ice include salts and dissolved organic carbon. There is great interest in searching for living microbes and especially for new species in deepest Antarctic ice. I propose a habitat consisting of interconnected liquid veins along three-grain boundaries in ice in which psychrophilic bacteria can move and obtain energy and carbon from ions in solution. In the accretion ice, with an age of a few 10(4) years and a temperature a few degrees below freezing, the carbon and energy sources in the veins can maintain significant numbers of cells per cubic centimeter that are metabolizing but not multiplying. In the 4 x 10(5)-year-old colder glacial ice, at least 1 cell per cm(3) in acid veins can be maintained. With fluorescence microscopy tuned to detect NADH in live organisms, motile bacteria could be detected by direct scanning of the veins in ice samples.

Ice in the ocean

Abstract: The Frozen Oceans Formation, Growth and Decay of Sea Ice The Thermodynamics of Sea Ice Ice in Motion Pressure Ridges and the Ice Thickness Distribution The Marginal Ice Zone Icebergs Sea Ice, the Environment and Climate Normal Normal Default Paragraph Font Default Paragraph Font wdooley$P:\sourcecdvf7\tnf_books\TF2982doc Lyn Meany(Dropbox2:ABSTRACT:T&FAbstract:TF2982docy?? Times New Roman Times New Roman Symbol Symbol 8'xfl+{f

Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet

Abstract: During the last interglacial period (the Eemian), global sea level was at least three metres, and probably more than five metres, higher than at present1,2. Complete melting of either the West Antarctic ice sheet or the Greenland ice sheet would today raise sea levels by 6–7 metres. But the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the West Antarctic ice sheet3, with model studies attributing only 1–2?m of sea-level rise to meltwater from Greenland4,5. This result was considered consistent with ice core evidence4, although earlier work had suggested a much reduced Greenland ice sheet during the last interglacial period6. Here we reconsider the Eemian evolution of the Greenland ice sheet by combining numerical modelling with insights obtained from recent central Greenland ice-core analyses. Our results suggest that the Greenland ice sheet was considerably smaller and steeper during the Eemian, and plausibly contributed 4–5.5?m to the sea-level highstand during that period. We conclude that the high sea level during the last interglacial period most probably included a large contribution from Greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the West Antarctic ice sheet.

New grid of Arctic bathymetry aids scientists and mapmakers

Abstract: For over two decades, Sheet 5.17 of the Fifth Edition of the General Bathymetric Chart of the Oceans (GEBCO) [Canadian Hydrographic Service, 1979] has been considered the authoritative portrayal of the sea floor north of 64 N.This sheet was constructed from publicly available bathymetric data sets, which in the late 1970s were rather sparse, consisting almost entirely of underway measurements collected from ice-breakers, drifting ice islands, and point measurements obtained along snow-mobile tracks or using air support. Data coverage tended to be fairly good at lower latitudes where ice cover was not a hindrance, but at higher latitudes, where ice was more prevalent, major features such as the Amerasian and Eurasian Basins were not well delineated. This situation posed problems not only for expedition planners but also for scientific investigators, who needed an accurate description of the sea floor to design field experiments and to link their research with processes affecting or affected by the shape of the seabed (for example, sea level change, ocean circulation, sediment transport,seafloor spreading, and Pleistocene glaciation).

Evidence for a recent change in the link between the North Atlantic Oscillation and Arctic Sea ice export

Abstract: Evidence for a recent change in the link between the North Atlantic Oscillation (NAO) and Arctic sea ice export through Fram Strait during wintertime (DJFM) is presented from the analysis of simulated Arctic sea ice and observations for the period 1958–1997. Whereas no correlation between the two time series is found from 1958 to 1977 (r=0.1), the correlation increased significantly thereafter (r=0.7, 1978–1997). The increased coherency between the NAO and Arctic sea ice export through Fram Strait during the last two decades resulted from anomalous meridional wind components near Fram Strait which were associated with the more easterly position of the NAO's centers of interannual variability compared to 1958–1977.

Deep sea-floor evidence of past ice streams off the Antarctic Peninsula

Abstract: The past existence of a giant ice stream off the northern end of the Antarctic Peninsula is suggested by a convex-upward, elongated sediment body now at a mean water depth of 1000 m. Because of its morphological characteristics, i.e., a set of parallel to subparallel ridges and grooves to 100 km long, with an overall width of 25 km, we call this depositional body a bundle structure. We hypothesize that bundle structures form by accumulation of basal deformation till under their parent ice streams. From its location, size, and overall characteristics the bundle structure described here constitutes the best preserved, largest, deepest, and relatively low latitude evidence of giant ice streams that flowed offshore Antarctica during glacial maxima. Bundle structures reveal the very dynamic behavior of ice caps in the northern Antarctic Peninsula during the last glacial maximum, with catchment areas draining rapidly under marine-based critical subglacial conditions. Bundle structures represent a new megascale streamlined glacial landform.

Recent Changes in Arctic Sea Ice: The Interplay between Ice Dynamics and Thermodynamics

Abstract: It is well established that periods of high North Atlantic oscillation (NAO) index are characterized by a weakening of the surface high pressure and surface anticyclone in the Beaufort Sea and the intensification of the cyclonic circulation in the eastern Arctic Ocean. The response of Arctic sea ice to these atmospheric changes has been studied with a thickness distribution sea-ice model coupled to an ocean model. During a period of high NAO, 1989–96, the model shows a substantial reduction of ice advection into the eastern Arctic from the Canada Basin, and an increase of ice export through Fram Strait, both of which tend to deplete thick ice in the eastern Arctic Ocean and enhance it in the western Arctic, in an uneven dipolar pattern we call the East–West Arctic Anomaly Pattern (EWAAP). From the period 1979–88 with a lower-NAO index to the period 1988–96 with a high-NAO index, the simulated ice volume in the eastern Arctic drops by about a quarter, while that in the western Arctic increases by ...

Further evidence of ice thinning in the Arctic Ocean

Abstract: A sea ice thickness profile, obtained in September 1996 from the Eurasian Basin of the Arctic Ocean between Fram Strait and the North Pole, was compared with a profile obtained in the same region in September-October 1976. A decline in mean ice draft of 43% was observed over the 20-year interval, in agreement with changes observed in other parts of the Arctic Ocean by Rothrock et al. [1999].

Sea ice as the glacial cycles' climate switch: Role of seasonal and orbital forcing

Abstract: A box model of the coupled ocean, atmosphere, sea ice, and land ice climate system is used to study glacial-interglacial oscillations under seasonally and orbitally varying solar forcing. The dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records, and to zeroth order, it does not depend on the seasonal and Milankovitch forcing. The sea ice controls, via its albedo and insulating effects, the atmospheric moisture fluxes and precipitation that enable the land ice sheet growth. This control and the rapid growth and melting of the sea ice allow the sea ice to rapidly switch the climate system from a growing ice sheet phase to a retreating ice sheet phase and to shape the oscillation's sawtooth structure. A specific physical mechanism is proposed by which the insolation changes act as a pacemaker, setting the phase of the oscillation by directly controlling summer melting of ice sheets. This mechanism is shown to induce deglaciations during periods of lower summer insolation. Superimposed on the 100 kyr are the linear Milankovitch-forced frequencies of 19, 23, and 41 kyr. The transition from 41 kyr glacial cycles to 100 kyr cycles one million years ago may be explained as being due to the activation of the sea ice switch at that time. This would be the case if sea ice extent was more limited during the warmer climate of the early Pleistocene.

Is There a Dominant Timescale of Natural Climate Variability in the Arctic

Abstract: A frequency-domain singular value decomposition performed jointly on century-long (1903–94) records of North Atlantic sector sea ice concentration and sea level pressure poleward of 40°N reveals that fluctuations on the interdecadal and quasi-decadal timescales account for a large fraction of the natural climate variability in the Arctic. Four dominant signals, with periods of about 6–7, 9–10, 16–20, and 30–50 yr, are isolated and analyzed. These signals account for about 60%–70% of the variance in their respective frequency bands. All of them appear in the monthly (year-round) data. However, the 9–10-yr oscillation especially stands out as a winter phenomenon. Ice variability in the Greenland, Barents, and Labrador Seas is then linked to coherent atmospheric variations and certain oceanic processes. The Greenland Sea ice variability is largely due to fluctuations in ice export through Fram Strait and to the local wind forcing during winter. It is proposed that variability in the Fram Strait ice ...

Radiocarbon Chronology of Ross Sea Drift, Eastern Taylor Valley, Antarctica: Evidence for a Grounded Ice Sheet in the Ross Sea at the Last Glacial Maximum

Abstract: More than 250 radiocarbon dates of lacustrine algae and marine shells afford a chronology for Ross Sea drift in eastern Taylor Valley Dates of algae that lived in ice‐dammed Glacial Lake Washburn

A millennium of variable ice flow recorded by the Ross Ice Shelf, Antarctica

Abstract: An enhanced composite Advanced Very High Resolution Radiometer (AVHRR) image is used to map flow stripes and rifts across the Ross Ice Shelf, Antarctica. The patterns of these flow-related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf. The most profound variations are visible in the track of rifts downstream of Crary Ice Rise, flow-stripe bends to the west of this ice rise and adjacent to Steershead ice rise, and changes in the northern margin of Ice Stream B. The track of rifts downstream of Crary Ice Rise indicates that the ice rise has existed for at least 700 years. The character of this track changes about 350 km downstream, indicating a rearrangement of flow patterns about 550 years ago. The large bulge in the flow stripes to the west of Crary Ice Rise is shown in detail, with bent flow stripes extending for several hundred kilometers along flow; this feature formed from the south, possibly due to a change in the discharge of Ice Stream A. The AVHRR image documents a complex history associated with the shutdown of Ice Stream C, with changes in the margins of Ice Stream C and the northern margin of Ice Stream B, and the grounding of Steershead ice rise with an associated bending and truncation of flow stripes. Landsat imagery shows a region that appears to be actively extending just downstream of the ice rise, as the shelf continues to respond to recent changes in ice-stream discharge. We present a four-stage flow history which accounts for the features preserved in the ice shelf.

Recent changes in Arctic Ocean sea ice motion associated with the North Atlantic Oscillation

Abstract: Examination of a new ice motion dataset of the Arctic Ocean over a recent eighteen year period (1978-1996) reveals patterns of variability that can be linked directly to the North Atlantic Oscillation. The intensity of the Icelandic Low, one of its centers of action, modulates the sea level pressure distribution over a broad region of the Arctic Ocean and the Greenland-Iceland-Norwegian and Barents Seas. Over the winters of 1988 through 1995, the Oscillation has remained in its positive phase contributing to coherent large-scale changes in the intensity and character of ice transport in the Arctic Ocean. The significant changes include: the weakening of the Beaufort Gyre; the increase in ice export through the Fram Strait; the increase in ice import from the Barents/Kara Seas; the enhanced eastward transport of sea ice from the Laptev Sea; the weakening of the Transpolar Drift Stream; and, the reduction in ice extent in the Nordic Seas. All of these changes affect the regional and total sea ice mass balance of the Arctic Ocean.

Influence of sea ice dynamics on habitat selection by polar bears

Abstract: Polar bears live in high-latitude environments characterized by cyclic variation in form and extent of sea ice. From 1991 to 1995, we used radio telemetry and monthly satellite images to compare patterns of ice selection by 110 female polar bears, relative to two geographic regions and four seasons. We hypothesized that extreme seasonal changes in ice characteristics in the Baffin Bay region, including a period of open water, may limit polar bear density despite supporting greater prey density than the Archipelago region, where ice is present year-round. Using cyclic time series analysis to model seasonal variation, we found differences in level, amplitude, and phase between sea ice characteristics and habitat selection by polar bears of the Arctic Archipelago and Baffin Bay regions. Polar bears not only followed seasonal changes, but they anticipated seasonal fluctuations, e.g., polar bears were found close to ice edges in spring in advance of the peak availability of edges. Also, seasonal selection of sea ice by polar bears was generally of a larger amplitude than cycles in ice and is best explained by intensive use of specific ice types in spring and summer, and sparse use during the remaining year. During spring and summer, Archipelago bears used landfast ice more intensively, whereas Baffin bears used moving ice, defined as thick first-year ice found in large floes. Both ice types likely represent areas where most seal pupping occurred in spring for each region. Bears from both regions selected first-year ice in winter when new ice was forming and multiyear ice in autumn when maximum ice melt had occurred. Overall, polar bear selection of ice habitat was similar between regions despite major differences in seasonal ice characteristics. Polar bear density may not directly relate to prey density, due to the limited ability of bears to track the extreme seasonal fluctuations in ice extent found in more productive environments.

Evidence from taylor valley for a grounded ice sheet in the ross sea, antarctica

Abstract: Four glacial drifts are exposed in eastern Taylor Valley and on Cape Bernacchi on the western side of McMurdo Sound, Ross Sea, Antarctica: Alpine I, Ross Sea, Wilson, and Bonney drifts. Bonney drif...

The southern ocean at the Last Glacial Maximum: A strong sink for atmospheric carbon dioxide

Abstract: Analysis of satellite ocean color, sea surface temperature, and sea ice cover data reveals consistent patterns between biological production, iron availability, and physical forcings in the Southern Ocean. The consistency of these patterns, in conjunction with information on physical conditions during the last glacial maximum (LGM), enables estimates of export production at the LGM. The LGM Southern Ocean experienced increased wind speeds, colder sea surface and atmospheric temperatures, increased deposition of atmospheric dust, and a greatly expanded winter sea ice cover. These variations had strong effects on Southern Ocean ecology and on air-sea fluxes of CO2. The seasonal ice zone (SIZ) was much larger at the LGM (30 million km2) than at present (19 million km2). The Antarctic Polar Front (PF) likely marked the northern boundary of this expanded SIZ throughout the Southern Ocean, as it does today in the Drake Passage region. A large northward shift in the position of the PF during glacial times is unlikely due to topographic constraints. North of the PF, the increased flux of aeolian dust during glacial times altered phytoplankton species composition and increased export production, and as a result this region was a stronger sink for atmospheric CO2 than in the modern ocean. South of the PF, interactions between the biota and sea ice strongly influence air-sea gas exchange over seasonal timescales. The combined influence of melting sea ice and increased aeolian dust flux (with its associated iron) increased both primary and export production by phytoplankton over daily-monthly timescales during austral spring/summer, resulting in a strong flux of CO2 into the ocean. Heavy ice cover would have minimized air-sea gas exchange over much of the rest of the year. Thus, an increased net flux of CO2 into the ocean is likely during glacial times, even in areas where annual primary production declined. We estimate that export production in the Southern Ocean as a whole was increased by 2.9-3.6 Gt C yr−1 at the LGM, relative to the modern era. Altered seasonal sea ice dynamics would further increase the net flux of CO2 into the ocean. Thus the Southern Ocean was a strong sink for atmospheric CO2 and contributed substantially to the lowering of atmospheric CO2 levels during the last ice age.