Variability Of Sea Ice Extent Research Articles

Holocene subsurface temperature variability in the eastern Antarctic continental margin

We reconstructed subsurface (∼45–200 m water depth) temperature variability in the eastern Antarctic continental margin during the late Holocene, using an archaeal lipid‐based temperature proxy (TEX86L). Our results reveal that subsurface temperature changes were probably positively coupled to the variability of warmer, nutrient‐rich Modified Circumpolar Deep Water (MCDW, deep water of the Antarctic circumpolar current) intrusion onto the continental shelf. The TEX86Lrecord, in combination with previously published climatic records, indicates that this coupling was probably related to the thermohaline circulation, seasonal variability in sea ice extent, sea temperature, and wind associated with high frequency climate dynamics at low‐latitudes such as internal El Niño Southern Oscillation (ENSO). This in turn suggests a linkage between centennial ENSO‐like variability at low‐latitudes and intrusion variability of MCDW into the eastern Antarctic continental shelf, which might have further impact on ice sheet evolution.

Future climate of the Bering and Chukchi Seas projected by global climate models

Atmosphere–Ocean General Circulation Models (AOGCMs) are a major tool used by scientists to study the complex interaction of processes that control climate and climate change. Projections from these models for the 21st century are the basis for the Fourth Assessment Report (AR4) produced by the Intergovernmental Panel on Climate Change (IPCC). Here, we use simulations from this set of climate models developed for the IPCC AR4 to provide a regional assessment of sea ice extent, sea surface temperature (SST), and surface air temperature (SAT) critical to future marine ecosystems in the Bering Sea and the Chukchi Sea. To reduce uncertainties associated with the model projections, a two-step model culling technique is applied based on comparison to 20th century observations. For the Chukchi Sea, data and model projections show major September sea ice extent reduction compared to the 20th century beginning now, with nearly sea ice free conditions before mid-century. Earlier sea ice loss continues throughout fall with major loss in December before the end of the 21st century. By 2050, for the eastern Bering Sea, spring sea ice extent (average of March to May) would be 58% of its recent values (1980–1999 mean). December will become increasingly sea ice free over the next 40 years. The Bering Sea will continue to show major interannual variability in sea ice extent and SST. The majority of models had no systematic bias in their 20th century simulated regional SAT, an indication that the models may provide considerable credibility for the Bering and the Chukchi Sea ecosystem projections. Largest air temperature increases are in fall (November to December) for both the Chukchi and the Bering Sea, with increases by 2050 of 3°C for the Bering Sea and increases in excess of 5°C for the Chukchi Sea.

Comparison of warm and cold years on the southeastern Bering Sea shelf and some implications for the ecosystem

The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice extent, temperature, and the distribution and abundance of species at multiple trophic levels over the past four decades. From 1972–2000, there was high interannual variability of areal extent of sea ice during spring (March–April). In 2000, this shifted to a 5-year (2001–2005) period of low ice extent during spring, which transitioned to a 4-year (2007–2010) period of extensive sea ice. High (low) areal extent of sea ice in spring was associated with cold (warm) water column temperatures for the following 6–7 months. The ocean currents also differed between warm and cold years. During cold years, the monthly-mean currents over the shelf were largely westward, while in warm years the direction of currents was more variable, with northward flow during December–February and relatively weak flow during the remainder of the year. The types and abundance of zooplankton differed sharply between warm and cold years. This was especially true during the prolonged warm period (2001–2005) and cold period (2007–2010), and was less evident during the years of high interannual variability. During the warm period, there was a lack of large copepods and euphausiids over the shelf; however, their populations rebounded during cold period. Small crustacean zooplankton taxa did not appear to vary between and warm and cold years. For both walleye pollock and Pacific cod, year-class strength (recruitment) was low during the prolonged warm period, but improved during the following cold period. Year-class strength did not appear to vary as a function of warm and cold years during the period of high year-to-year variability. Also, recruitment of arrowtooth flounder (a predator of pollock and cod) did not appear influenced by the warm or cold years. Finally, the distribution and relative abundance of fin whales appeared to differ in warm and cold years, with fewer whales on the southeastern, middle shelf during warm years.

Modeled methanesulfonic acid (MSA) deposition in Antarctica and its relationship to sea ice

[1] Methanesulfonic acid (MSA) has previously been measured in ice cores in Antarctica as a proxy for sea ice extent and Southern Hemisphere circulation. In a series of chemical transport model (GEOS-Chem) sensitivity experiments, we identify mechanisms that control the MSA concentrations recorded in ice cores. Sea ice is linked to MSA via dimethylsulfide (DMS), which is produced biologically in the surface ocean and known to be particularly concentrated in the sea ice zone. Given existing ocean surface DMS concentration data sets, the model does not demonstrate a strong relationship between sea ice and MSA deposition in Antarctica. The variability of DMS emissions associated with sea ice extent is small (11–30%) due to the small interannual variability of sea ice extent. Wind plays a role in the variability in DMS emissions, but its contribution relative to that of sea ice is strongly dependent on the assumed DMS concentrations in the sea ice zone. Atmospheric sulfur emitted as DMS from the sea ice undergoes net transport northward. Our model runs suggest that DMS emissions from the sea ice zone may account for 26–62% of MSA deposition at the Antarctic coast and 36–95% in inland Antarctica. Though our results are sensitive to model assumptions, it is clear that an improved understanding of both DMS concentrations and emissions from the sea ice zone are required to better assess the impact of sea ice variability on MSA deposition to Antarctica.

기후변화에 따른 북극해 빙해역 변화

This study examines changes in the Arctic sea ice associated with global warming by analyzing the climate coupled general circulation models (CGCMs) provided in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. We selected nine models for better performance under 20th century climate conditions based on two different criteria, and then estimated the changes in sea ice extent under global warming conditions. Under projected 21st century climate conditions, all models, with the exception of the GISS-AOM model, project a reduction in sea ice extent in all seasons. The mean reduction in summer (?63%) is almost four times larger than that in winter (?16%), resulting an enhancement of seasonal variations in sea ice extent. The difference between the models, however, becomes larger under the 21st century climate conditions than under 20th century conditions, thus limiting the reliability of sea-ice projections derived from the current CGCMs.

An assessment of the surface climate in the NCEP climate forecast system reanalysis

This paper analyzes surface climate variability in the climate forecast system reanalysis (CFSR) recently completed at the National Centers for Environmental Prediction (NCEP). The CFSR represents a new generation of reanalysis effort with first guess from a coupled atmosphere–ocean–sea ice–land forecast system. This study focuses on the analysis of climate variability for a set of surface variables including precipitation, surface air 2-m temperature (T2m), and surface heat fluxes. None of these quantities are assimilated directly and thus an assessment of their variability provides an independent measure of the accuracy. The CFSR is compared with observational estimates and three previous reanalyses (the NCEP/NCAR reanalysis or R1, the NCEP/DOE reanalysis or R2, and the ERA40 produced by the European Centre for Medium-Range Weather Forecasts). The CFSR has improved time-mean precipitation distribution over various regions compared to the three previous reanalyses, leading to a better representation of freshwater flux (evaporation minus precipitation). For interannual variability, the CFSR shows improved precipitation correlation with observations over the Indian Ocean, Maritime Continent, and western Pacific. The T2m of the CFSR is superior to R1 and R2 with more realistic interannual variability and long-term trend. On the other hand, the CFSR overestimates downward solar radiation flux over the tropical Western Hemisphere warm pool, consistent with a negative cloudiness bias and a positive sea surface temperature bias. Meanwhile, the evaporative latent heat flux in CFSR appears to be larger than other observational estimates over most of the globe. A few deficiencies in the long-term variations are identified in the CFSR. Firstly, dramatic changes are found around 1998–2001 in the global average of a number of variables, possibly related to the changes in the assimilated satellite observations. Secondly, the use of multiple streams for the CFSR induces spurious jumps in soil moisture between adjacent streams. Thirdly, there is an inconsistency in long-term sea ice extent variations over the Arctic regions between the CFSR and other observations with the CFSR showing smaller sea ice extent before 1997 and larger extent starting in 1997. These deficiencies may have impacts on the application of the CFSR for climate diagnoses and predictions. Relationships between surface heat fluxes and SST tendency and between SST and precipitation are analyzed and compared with observational estimates and other reanalyses. Global mean fields of surface heat and water fluxes together with radiation fluxes at the top of the atmosphere are documented and presented over the entire globe, and for the ocean and land separately.

Unprecedented low twentieth century winter sea ice extent in the Western Nordic Seas since A.D. 1200

We reconstructed decadal to centennial vari- ability of maximum sea ice extent in the Western Nordic Seas for A.D. 1200-1997 using a combination of a regional tree-ring chronology from the timberline area in Fennoscandia and d 18 O from the Lomonosovfonna ice core in Svalbard. The reconstruction successfully explained 59% of the variance in sea ice extent based on the calibration period 1864-1997. The significance of the reconstruction statistics (reduction of error, coefficient of efficiency) is computed for the first time against a realistic noise background. The twentieth century sustained the lowest sea ice extent values since A.D. 1200: low sea ice extent also occurred before (mid-seventeenth and mid- eighteenth centuries, early fifteenth and late thirteenth centuries), but these periods were in no case as persistent as in the twentieth century. Largest sea ice extent values occurred from the seventeenth to the nineteenth centuries, during the Little Ice Age (LIA), with relatively smaller sea ice-covered area during the sixteenth century. Mod- erate sea ice extent occurred during thirteenth-fifteenth centuries. Reconstructed sea ice extent variability is dominated by decadal oscillations, frequently associated with decadal components of the North Atlantic Oscilla- tion/Arctic Oscillation (NAO/AO), and multi-decadal lower frequency oscillations operating at *50-120 year. Sea ice extent and NAO showed a non-stationary rela- tionship during the observational period. The present low sea ice extent is unique over the last 800 years, and results from a decline started in late-nineteenth century after the LIA.

An analysis of the atmospheric processes driving the large‐scale winter sea ice variability in the Southern Ocean

The response of the sea ice in the Southern Ocean to the variability of the atmosphere on the period 1979–2004 is investigated using both model and observational data. On the one hand, our results show that in line with previous investigations, the classical modes of atmospheric variability do not explain a large part of the winter sea ice extent variability integrated over the entire Southern Ocean. On the other hand, the regression between the ice extent and the atmospheric pressure displays a pattern with low pressure areas over the South Atlantic, over the Indian Ocean, and on the southwestern part of the Pacific Ocean and a high pressure area over the Antarctic region extending toward both the southeastern Pacific and the region south of Australia. This pattern, which does not correlate significantly with any of the known modes of atmospheric variability, induces changes in the sea ice extent by influencing the air temperature, the ice production, and both the meridional and the zonal ice velocity. However, there is no clear link between the different centers of action of this pattern. Furthermore, the correlation of the sea ice extent between the different sectors is generally low. As a conclusion, we must consider that the sea ice extent in the Southern Ocean over the last 30 years does not behave as a single entity: Its variability is the result of the combination of regional sea ice changes. Each sector should thus be examined separately.

Antarctic sea ice: a self-organizing map-based perspective

AbstractSelf-organizing maps (SOMs) provide a powerful, non-linear technique to optimally summarize a complex geophysical dataset using a user-selected number of ‘icons’ or SOM states, allowing rapid identification of preferred patterns, predictability of transitions, rates of transitions, and hysteresis in cycles. The use of SOMs is demonstrated here through application to a 24 year dataset (1973–96) of monthly Antarctic sea-ice edge positions. Variability in sea-ice extent, concentration and other physical characteristics is an important component of the Earth’s dynamic climate system, particularly in the Southern Hemisphere where annual changes in sea-ice extent (temporarily) double the size of the Antarctic cryosphere. SOM-based patterns concisely capture the spatial and temporal variability in these data, including the annual progression of expansion and retreat, a general eastward propagation of anomalies during the winter, and sub-annual variability in the rate of change in extent at different times of the year (e.g. retreat in January is faster than in November). There is also often a general seasonal hysteresis, i.e. monthly anomalies during cooling follow a different spatial path than during warming.

Marine pelagic ecosystems: the West Antarctic Peninsula

The marine ecosystem of the West Antarctic Peninsula (WAP) extends from the Bellingshausen Sea to the northern tip of the peninsula and from the mostly glaciated coast across the continental shelf to the shelf break in the west. The glacially sculpted coastline along the peninsula is highly convoluted and characterized by deep embayments that are often interconnected by channels that facilitate transport of heat and nutrients into the shelf domain. The ecosystem is divided into three subregions, the continental slope, shelf and coastal regions, each with unique ocean dynamics, water mass and biological distributions. The WAP shelf lies within the Antarctic Sea Ice Zone (SIZ) and like other SIZs, the WAP system is very productive, supporting large stocks of marine mammals, birds and the Antarctic krill, Euphausia superba. Ecosystem dynamics is dominated by the seasonal and interannual variation in sea ice extent and retreat. The Antarctic Peninsula is one among the most rapidly warming regions on Earth, having experienced a 2 degrees C increase in the annual mean temperature and a 6 degrees C rise in the mean winter temperature since 1950. Delivery of heat from the Antarctic Circumpolar Current has increased significantly in the past decade, sufficient to drive to a 0.6 degrees C warming of the upper 300 m of shelf water. In the past 50 years and continuing in the twenty-first century, the warm, moist maritime climate of the northern WAP has been migrating south, displacing the once dominant cold, dry continental Antarctic climate and causing multi-level responses in the marine ecosystem. Ecosystem responses to the regional warming include increased heat transport, decreased sea ice extent and duration, local declines in icedependent Adélie penguins, increase in ice-tolerant gentoo and chinstrap penguins, alterations in phytoplankton and zooplankton community composition and changes in krill recruitment, abundance and availability to predators. The climate/ecological gradients extending along the WAP and the presence of monitoring systems, field stations and long-term research programmes make the region an invaluable observatory of climate change and marine ecosystem response.

A model assessment of satellite observed trends in polar sea ice extents

For more than three decades now, satellite passive microwave observations have been used to monitor polar sea ice. Here we utilize sea ice extent trends determined from satellite data for both the Northern and Southern Hemispheres for the period 1972(73)–2004, and assess and interpret them using results from simulations by eleven climate models. In the Northern Hemisphere (NH), observations show a statistically significant decrease of sea ice extent and an acceleration of sea ice retreat during the past three decades. However, from the modeled natural variability of sea ice extents in control simulations, we conclude that the acceleration is not statistically significant and should not be extrapolated into the future. Most of the models, like the observations, show an absence of a prominent seasonal cycle in the trend values. Both observations and model simulations show that climate variability in sea ice extent in the Southern Hemisphere (SH) is much larger than in the NH and that the SH sea ice extent trends are not statistically significant.

Covariation of Sea ice and methanesulphonic acid in Wilhelm II Land, East Antarctica

AbstractSea ice plays an important role in ocean–atmosphere heat exchange, global albedo and the marine ecosystem. Knowledge of variation in Sea-ice extent is essential in order to understand past climates, and to model possible future climate Scenarios. This paper presents results from a Short firn core Spanning 15 years collected from near Mount Brown, Wilhelm II Land, East Antarctica. Variations of methanesulphonic acid (MSA) at Mount Brown were positively correlated with Sea-ice extent from the coastal region Surrounding Mount Brown (60–120˚ E) and from around the entire Antarctic coast (0–360˚ E). Previous results from Law Dome identified this MSA–sea-ice relationship and proposed it as an Antarctic Sea-ice proxy (Curran and others, 2003), with the Strongest results found for the local Law Dome region. Our data provide Supporting evidence for the Law Dome proxy (at another Site in East Antarctica), but a deeper Mount Brown ice core is required to confirm the Sea-ice decline Suggested by Curran and others (2003). Results also indicate that this deeper record may also provide a more circum-Antarctic Sea-ice proxy.

Two ice-core δ18O records from Svalbard illustrating climate and sea-ice variability over the last 400 years

Ice cores from the relatively low-lying ice caps in Svalbard have not been widely exploited in climatic studies owing to uncertainties about the effect of meltwater percolation. However, results from two new Svalbard ice cores, at Lomonosovfonna and Austfonna, have shown that with careful site selection, high-resolution sampling and multiple chemical analyses it is possible to recover ice cores from which part of the annual signals are preserved, despite the considerable meltwater percolation. The new Svalbard ice cores are positioned in different parts of Svalbard and cover the past 800 years. In this paper we focus on the last 400 years. The 6180 signals from the cores are qualitatively similar over most of the twentieth century, suggesting that they record the same atmospheric signal. Prior to AD 1920, the Austfonna ice core exhibits more negative 6180 values than Lomonosovfonna, although there are intermittent decadal-scale periods throughout the record with similar values. We suggest that the differences reflect the effect of the inversion layer during the winter. The pattern in the 6180 records is similar to the Longyearbyen airtemperature record, but on an annual level the correlation is low. The Austfonna record correlates well with the temperature record from the more distant and southwesterly located Jan Mayen. A comparison of the ice-core and sea-ice records from this period suggests that sea-ice extent and Austfonna 6180 are related over the past 400 years. This may reflect the position of the storm tracks and their direct influence on the relatively low-altitude Austfonna. Lomonosovfonna may be less sensitive to such changes and primarily record free atmospheric changes instead of variations in sea-ice extent, the latter is probably a result of its higher elevation.

Long‐term variation of surface phytoplankton chlorophyll a in the Southern Ocean during 1965–2002

The variation in the phytoplankton biomass over a decadal time scale, and its relationship with the Antarctic Circumpolar Wave (ACW) and climate change, has been poorly interpreted because of the limited satellite chlorophyll a (chl a) data compared with the physical parameters from satellite. We analyzed a long‐term chl a data set along the Japanese Antarctic Research Expedition (JARE) cruise tracks since 1965 to investigate inter‐annual variation of phytoplankton biomass. In the Southern Ocean, increasing trends of chl a and the spreading of higher chl a area to the north with 3–7 year cycles were found. Although relationships between the decadal change in chl a and climate change such as variation of sea ice extent and the El Niño are still obscure, large variation of primary production in proportion to the chl a is implied.

Annual changes in sea-ice, chlorophyll a, and primary production in the Ross Sea, Antarctica

Abstract A satellite-based study was conducted to quantify the seasonal-to-interannual changes in phytoplankton production in the southwestern Ross Sea between 1997 and 2001. The size of the Ross Sea polynya from 1992 to 2001 was related to the climate state as expressed by the Multivariate ENSO Index (MEI). These data show that 76% of the variation in sea-ice extent between 1992 and 2000 can be explained by the MEI, with the El Nino year of 1997–1998 exhibiting the greatest sea-ice cover and the La Nina year of 1999–2000 the least. Whether the relationship between ice area and the MEI is caused by interannual changes in wind patterns or changes in the movement of circumpolar deep water onto the Ross Sea continental shelf has yet to be determined. The year 2000–2001 was excluded from the climate analysis, due to the effects of the enormous B-15 iceberg, which inhibited the normal advection pattern of sea-ice out of the Ross Sea, resulting in higher than normal sea-ice conditions. Phytoplankton blooms were markedly less extensive in the heavy sea-ice years of 1997–1998 and 2000–2001 and were significantly delayed in their development, with concentrations of Chl a peaking 2 months later (February) than in reduced sea-ice years (December). Annual production during the 1997–1998 El Nino (27.4 Tg C) was similar to that of the iceberg impacted year 2000–2001 (28.3 Tg C). These annual rates were approximately 40% below values estimated between November 1998 and March 2000 (47.5 Tg C) when open-water area during spring and summer was much greater. This decreased production during heavy sea-ice years is more a function of the delay in the start of the bloom than in the reduction of open water area.

Relationship between the Decrease of Sea Ice and Early Summer Atmospheric Circulation in and around the Sea of Okhotsk

The variability in sea ice extent in the Sea of Okhotsk during the sea ice disappearance period and its response to the atmospheric circulation in early summer are investigated. Significant correlations were found between the disappearance in sea ice in May and the geopotential heights in and around the northern part of the Sea of Okhotsk in June. A disappearance in sea ice is defined as the difference in the sea ice area between the end of the current month and the end of the previous month. Thus, large sea ice disappearance years and small disappearance years are selected from a time series of sea ice disappearance for the month of May. Composite maps of the geopotential heights in the northern part of East Asia are investigated annually. In small sea ice disappearance years, anticyclones and ridges of the geopotential heights are prominent in and around the Sea of Okhotsk in June. Conversely in large sea ice disappearance years, surface cyclones and troughs are dominant in and around the Sea of Okhotsk. A strengthening of the Okhotsk anticyclone is presumed to be related to variability in the decrease in sea ice in the Sea of Okhotsk.

Arctic sea ice, cloud, water, and lead classification using neural networks and 1.6-μm data

Polar sea ice plays a critical role in regulating the global climate. Seasonal variation in sea ice extent, however, coupled with the difficulties associated with in situ observations of polar sea ice, makes remote sensing the only practical way to estimate this important climatic variable on the space and time scales required. Unfortunately, accurate retrieval of sea ice extent from satellite data is a difficult task. Sea ice and high cold clouds have similar visible reflectance, but some other types of clouds can appear darker than sea ice. Moreover, strong atmospheric inversions and isothermal structures, both common in winter at some polar locations, further complicate the classification. This paper uses a combination of feed-forward neural networks and 1.6-/spl mu/m data from the new Chinese Fengyun-1C satellite to mitigate these difficulties. The 1.6-/spl mu/m data are especially useful for detecting illuminated water clouds in polar regions because 1) at 1.6 /spl mu/m, the reflectance of water droplets is significantly higher than that of snow or ice and 2) 1.6-/spl mu/m data are unaffected by atmospheric inversions. Validation data confirm the accuracy of the new classification technique. Application to other sensors with 1.6-/spl mu/m capabilities also is discussed.

Analysis of seasonal cycles in climatic trends with application to satellite observations of sea ice extent

We present a new technique to study the seasonal cycle of climatic trends in the expected value, variance, and other moments of the statistical distribution. The basis of the technique is multiple linear regression, but with periodic basis functions. The technique allows us to provide comprehensive information on statistical parameters of climate for every day of an observational period. Using daily data, the technique has no problems caused by different lengths of months or the leap‐year cycle. Without needing to assume the stationarity of contemporary climate, the technique allows the study of statistical parameters of climatic records of arbitrary length. We illustrate the technique with applications to trends in the satellite observed variations of sea ice extent in the Northern and Southern Hemispheres. We show that a significant part of the variability in hemispheric sea ice extents for the period 1978–1999 is related to linear trends.

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.