Weddell Sea Coast Research Articles

The structure of sedimentary basins of Antarctica and a new three-layer sediment model

We use geophysical data together with a recent subglacial bedrock map (BEDMACHINE model) to obtain and investigate a new three-layer sediment model for Antarctica that locally improves the global sediment model. We provide a combined, continuous, sediment model for Antarctica and surrounding oceans by joining such improved continental sedimentary model with an existing global one (GlobSed). Our results reveal large differences between sedimentary basins for Antarctica due to their age and origin. The maximum thickness of sediments is reached under Filchner-Ronne Ice Shelf and off the Weddell Sea coast (10–12 km); further offshore, towards the ocean, the thickness of sediments drops to 4–5 km. We divide the sediment cover into three layers to distinguish material with different velocities. The lower sediment layer (deeper than 7 km) with high P-wave velocities (4.0–4.9 km/s) is found only for Lambert Rift and Filchner-Ronne basin. The middle layer (2–7 km) has large variations for different sedimentary basins: 3.5–3.7 km/s for Lambert Basin; 4.0–4.3 km/s for Ross, Byrd and Bentley basins; 3.3–4.0 km/s for Filchner-Ronne Basin. The upper sediment layer (0–2 km) has large velocity variations, from 2.0 km/s for Ross and Lambert basins (young sediments) to 4.7 km/s for Dronning Maud Land basins. We suggest that P-wave velocities larger than 4 km/s represent old, compacted sediments which belong to the Beacon Supergroup; about 3 km/s refer to Mesozoic (rifted?) sediments; and less than 3 km/s relate to young Cenozoic sediments. According to this criterion, Dronning Maud Land, Bentley and Byrd basins belong to the Beacon Supergroup, while more complex and thicker Ross, Lambert and Filchner-Ronne basins contain sediments from Beacon Supergroup in the middle or lower layer, respectively. Other sedimentary basins with more moderate velocities possibly belong to the East Antarctic Rift System which formed later during Gondwana breakup.

Testing for Dynamical Dependence: Application to the Surface Mass Balance Over Antarctica

AbstractDynamical dependence between key observables and the surface mass balance (SMB) over Antarctica is analyzed in two historical runs performed with the MPI‐ESM‐P and the CESM1‐CAM5 climate models. The approach used is a novel method allowing for evaluating the rate of information transfer between observables that goes beyond the classical correlation analysis and allows for directional characterization of dependence. It reveals that a large proportion of significant correlations do not lead to dependence. In addition, three coherent results concerning the dependence of SMB emerge from the analysis of both models: (i) The SMB over the Antarctic Plateau is mostly influenced by the surface temperature and sea ice concentration and not by large‐scale circulation changes; (ii) the SMB of the Weddell Sea and the Dronning Maud Land coasts are not influenced significantly by the surface temperature; and (iii) the Weddell Sea coast is not significantly influenced by the sea ice concentration.

Antarctic Thermocline Dynamics along a Narrow Shelf with Easterly Winds

AbstractDetermining the role of Southern Ocean warm intermediate water for driving melting of the Antarctic ice sheet is a major challenge in assessing future sea level rise. Analysis of 2859 CTD profiles obtained between 1977 and 2016 by ships and instrumented seals at the Weddell Sea continental slope reveals a seasonal rise of the Antarctic Slope Front thermocline by more than 100 m during the summer. The signal at Kapp Norvegia (17°W) corresponds with a seasonal warming downstream at the Filchner Trough (40°W), indicating that a coherent evolution of the slope front along the shelf break regulates the onshore flow of warm deep water. Climatological cross sections of the slope front hydrography show that downwelling of Antarctic Surface Water forms a secondary front above the warm deep water interface during summer. Enhanced baroclinic growth rates at this front suggest that the wind-driven suppression of the thermocline is partially compensated by a shallower eddy overturning cell when surface water is present. A simple model of the Weddell Gyre boundary current reveals that wintertime densification of surface waters is crucial for maintaining the deep thermocline along the eastern Weddell Sea coast. The sensitivity of the warm inflow to the cross-frontal density gradient implies a positive feedback with ice shelf melting that may lead to an abrupt transition into a high melting state once warm water rises over the shelf break depth. Despite its regional focus, this study highlights the role of upper ocean buoyancy fluxes for controlling the thermocline depth along seasonally ice-covered narrow shelf regions with cyclonic along-slope winds.

Real-time detection of airborne fluorescent bioparticles in Antarctica

Abstract. We demonstrate, for the first time, continuous real-time observations of airborne bio-fluorescent aerosols recorded at the British Antarctic Survey's Halley VI Research Station, located on the Brunt Ice Shelf close to the Weddell Sea coast (lat 75°34′59′′ S, long 26°10′0′′ W) during Antarctic summer, 2015. As part of the NERC MAC (Microphysics of Antarctic Clouds) aircraft aerosol cloud interaction project, observations with a real-time ultraviolet-light-induced fluorescence (UV-LIF) spectrometer were conducted to quantify airborne biological containing particle concentrations along with dust particles as a function of wind speed and direction over a 3-week period. Significant, intermittent enhancements of both non- and bio-fluorescent particles were observed to varying degrees in very specific wind directions and during strong wind events. Analysis of the particle UV-induced emission spectra, particle sizes and shapes recorded during these events suggest the majority of particles were likely a subset of dust with weak fluorescence emission responses. A minor fraction, however, were likely primary biological particles that were very strongly fluorescent, with a subset identified as likely being pollen based on comparison with laboratory data obtained using the same instrument. A strong correlation of bio-fluorescent particles with wind speed was observed in some, but not all, periods. Interestingly, the fraction of fluorescent particles to total particle concentration also increased significantly with wind speed during these events. The enhancement in concentrations of these particles could be interpreted as due to resuspension from the local ice surface but more likely due to emissions from distal sources within Antarctica as well as intercontinental transport. Likely distal sources identified by back trajectory analyses and dispersion modelling were the coastal ice margin zones in Halley Bay consisting of bird colonies with likely associated high bacterial activity together with contributions from exposed ice margin bacterial colonies but also long-range transport from the southern coasts of Argentina and Chile. Dispersion modelling also demonstrated emissions from shipping lanes, and therefore marine anthropogenic sources cannot be ruled out. Average total concentrations of total fluorescent aerosols were found to be 1.9 ± 2.6 L−1 over a 3-week period crossing over from November into December, but peak concentrations during intermittent enhancement events could be up to several tens per litre. While this short pilot study is not intended to be generally representative of Antarctic aerosol, it demonstrates the usefulness of the UV-LIF measurement technique for quantification of airborne bioaerosol concentrations and to understand their dispersion. The potential importance for microbial colonisation of Antarctica is highlighted.

Antarctic climate variability on regional and continental scales over the last 2000 years

Abstract. Climate trends in the Antarctic region remain poorly characterized, owing to the brevity and scarcity of direct climate observations and the large magnitude of interannual to decadal-scale climate variability. Here, within the framework of the PAGES Antarctica2k working group, we build an enlarged database of ice core water stable isotope records from Antarctica, consisting of 112 records. We produce both unweighted and weighted isotopic (δ18O) composites and temperature reconstructions since 0 CE, binned at 5- and 10-year resolution, for seven climatically distinct regions covering the Antarctic continent. Following earlier work of the Antarctica2k working group, we also produce composites and reconstructions for the broader regions of East Antarctica, West Antarctica and the whole continent. We use three methods for our temperature reconstructions: (i) a temperature scaling based on the δ18O–temperature relationship output from an ECHAM5-wiso model simulation nudged to ERA-Interim atmospheric reanalyses from 1979 to 2013, and adjusted for the West Antarctic Ice Sheet region to borehole temperature data, (ii) a temperature scaling of the isotopic normalized anomalies to the variance of the regional reanalysis temperature and (iii) a composite-plus-scaling approach used in a previous continent-scale reconstruction of Antarctic temperature since 1 CE but applied to the new Antarctic ice core database. Our new reconstructions confirm a significant cooling trend from 0 to 1900 CE across all Antarctic regions where records extend back into the 1st millennium, with the exception of the Wilkes Land coast and Weddell Sea coast regions. Within this long-term cooling trend from 0 to 1900 CE, we find that the warmest period occurs between 300 and 1000 CE, and the coldest interval occurs from 1200 to 1900 CE. Since 1900 CE, significant warming trends are identified for the West Antarctic Ice Sheet, the Dronning Maud Land coast and the Antarctic Peninsula regions, and these trends are robust across the distribution of records that contribute to the unweighted isotopic composites and also significant in the weighted temperature reconstructions. Only for the Antarctic Peninsula is this most recent century-scale trend unusual in the context of natural variability over the last 2000 years. However, projected warming of the Antarctic continent during the 21st century may soon see significant and unusual warming develop across other parts of the Antarctic continent. The extended Antarctica2k ice core isotope database developed by this working group opens up many avenues for developing a deeper understanding of the response of Antarctic climate to natural and anthropogenic climate forcings. The first long-term quantification of regional climate in Antarctica presented herein is a basis for data–model comparison and assessments of past, present and future driving factors of Antarctic climate.

Regional Antarctic snow accumulation over the past 1000 years

Abstract. Here we present Antarctic snow accumulation variability at the regional scale over the past 1000 years. A total of 79 ice core snow accumulation records were gathered and assigned to seven geographical regions, separating the high-accumulation coastal zones below 2000 m of elevation from the dry central Antarctic Plateau. The regional composites of annual snow accumulation were evaluated against modelled surface mass balance (SMB) from RACMO2.3p2 and precipitation from ERA-Interim reanalysis. With the exception of the Weddell Sea coast, the low-elevation composites capture the regional precipitation and SMB variability as defined by the models. The central Antarctic sites lack coherency and either do not represent regional precipitation or indicate the model inability to capture relevant precipitation processes in the cold, dry central plateau. Our results show that SMB for the total Antarctic Ice Sheet (including ice shelves) has increased at a rate of 7 ± 0.13 Gt decade−1 since 1800 AD, representing a net reduction in sea level of ∼ 0.02 mm decade−1 since 1800 and ∼ 0.04 mm decade−1 since 1900 AD. The largest contribution is from the Antarctic Peninsula (∼ 75 %) where the annual average SMB during the most recent decade (2001–2010) is 123 ± 44 Gt yr−1 higher than the annual average during the first decade of the 19th century. Only four ice core records cover the full 1000 years, and they suggest a decrease in snow accumulation during this period. However, our study emphasizes the importance of low-elevation coastal zones, which have been under-represented in previous investigations of temporal snow accumulation.

Wind‐driven spreading of fresh surface water beneath ice shelves in the Eastern Weddell Sea

AbstractSolar heated, fresh Antarctic Surface Water (ASW) is a permanent feature along the Eastern Weddell Sea (EWS) coast in summer down to a depth of roughly 200 m. Recently, ASW has been observed beneath the Fimbul Ice Shelf, suggesting that it might play an important role in basal melting. We propose that wind‐driven coastal downwelling is the main mechanism that spreads ASW beneath the ice shelf in this sector of Antarctica. We validate this hypothesis with observations, scaling analyses, and numerical modeling, along three principle lines: (i) data analyses of about 1500 salinity profiles collected by instrumented seals indicate that the observed freshening of the coastal water column is likely explained by the on‐shore Ekman transport and subsequent downwelling of ASW; (ii) an analytical model of the coastal momentum balance indicates that wind‐driven downwelling is capable of depressing the buoyant surface water to a depth similar to the ice shelf draft; and (iii) simulations from both idealized and regional eddy‐resolving numerical ice shelf/ocean models support our proposition. Our main conclusion is that wind‐driven spreading of ASW beneath the ice shelf occurs when downwelling exceeds the depth of the ice shelf base. Furthermore, our study adds to the understanding of the oceanic processes at the Antarctic Slope Front in the EWS, with possible implications for other sectors of Antarctica.

Some peculiarities of the near-surface air temperature change in the Antarctic Peninsula region

Current climate in the Antarctic Peninsula (AP) region can be interpreted as a continued warming, still exceeding climatic norms but without significant rise on the Antarctic Peninsula stations. Assessments of trends in the near-air temperatures are made by applying homogeneity procedure and series of monthly SAT normalizing by amplitudes. The most intensive increase in surface air temperature at the end of XX - beginning XXI century recorded at Vernadsky and Rothera stations at the west coast of the Antarctic Peninsula. Most of inhomogeneities are observed in coldest decade, 1950s; current period is characterized by smoothening in the near-surface temperatures (SAT) ranges especially in comparison to 1950s because of changes in atmospheric circulation. Normalized SAT series is comparable between winter and summer seasons at west coast of AP, including Vernadsky and Rothera stations. Spatial and temporal characteristics of near surface temperatures are described, with significant seasonal and topography-dependant variability. Significant differences exist between stations at the west coast and those lying at Weddell Sea coast, especially in summer. By the Vernadsky data, summer warming is mainly responsible for the recession of glaciation. Modern regional warming is associated with growing influence of El Niño-Southern Oscillation, especially after well-known climatic shift in the 1970s due to increased surface temperature of the equatorial Pacific. On the background the overall warming trend some intradecadal SAT fluctuations are registered being associated with El Niño. On the basis of the recent SAT variability at other stations and multi-years’ change in climate indexes it is concluded that no further warming can be expected in the near future.

Shackleton's emperor penguins

ABSTRACTThe loss of the expedition ship SY Endurance, and the subsequent dispersal of staff and crew, resulted in very little scientific information emerging from the Imperial Trans-Antarctic Expedition 1914–1917. Among surviving records were the charts and diaries maintained by the ship's master, F.A. Worsley. During the voyage in January 1915 along the ice cliffs of the Weddell Sea coast, Worsley recorded the ship's daily progress, soundings and trawling and dredging activities, and also daily encounters with seals, whales and seabirds, On 12 January he noted a group of fledgling emperor penguin chicks (Aptenodytes forsteri) on an ice foot, clearly a remnant of what was then only the third-known breeding colony of the species. Shackleton's first published account of the expedition mentioned the chicks only in a brief note (one that was omitted from later editions), and no further report covering Worsley's observations appeared in scientific literature. In consequence the discovery of the breeding colony and records of emperor penguin distribution along the Weddell Sea coast have since been overlooked by avian biologists, regrettably including the present author. This paper discusses the identity of the colony, Worsley's observations that foreshadowed the later discovery of more breeding concentrations along the coast, and a possible reason why colonies occur at points of particular glaciological disturbance.

Computer-based identification and tracking of Antarctic icebergs in SAR images

Icebergs play an important role in climate through the transfer of fresh water and heat between ice sheets and the oceans. Synthetic aperture radar (SAR) images offer a robust way of observing icebergs in the often dark and cloud-covered polar areas. A computer-based technique is proposed to identify icebergs at higher resolution than in the past, allowing the shape of small to medium icebergs to be retained and used for tracking between images acquired at different times. The identification consists of (i) automatic segmentation, followed by (ii) classification of these objects as icebergs or nonicebergs. The classification stage requires user intervention as the icebergs are not always a separable class from other objects, most notably sea ice. The icebergs are then matched between images using their size and shape similarities, and their position tracked. The system performance was assessed by applying the technique to three wintertime ERS-1 PRI images, around Kapp Norvegia (10°W), on the Eastern Weddell Sea coast. The results show both the identification and the tracking to be effective.

Foraging behaviour of Weddell seals, and its ecological implications

Time-depth recorder data of eight adult Weddell seals (Leptonychotes weddellii) provided simultaneous dive records over 8 days in the Drescher Inlet, eastern Weddell Sea coast, in February 1998. The seals primarily foraged within two depth layers, these being from the sea surface to 160 m where temperature and salinity varied considerably, and near the bottom from 340 to 450 m where temperature was lowest and salinity highest. While both pelagic and benthic diving occurred during daylight, the seals foraged almost exclusively in the upper water column at night. Trawling during daytime confirmed that Pleuragramma antarcticum were by far the most abundant fish both in the pelagial and close to the bottom. Pelagic night hauls at 110–170 m depth showed highly variable biomass of P.antarcticum. The temporal changes in the local abundance of P. antarcticum. particularly in the pelagial, may explain the trends in the seals’ pelagic and benthic foraging activities. This study describes the jaw movements of a hunting seal, which are presumably indicative of feeding events. Trophic links from the Weddell seal to fish, zooplankton and krill, Euphausia superba, are discussed.

Transpressional deformation along the margin of Larsen Basin: new data from Pedersen Nunatak, Antarctic Peninsula

New structural data from the northern Antarctic Peninsula suggest that reverse faults and folds affecting the Pedersen Nunatak beds of the upper Mesozoic–Lower Cenozoic Larsen Basin succession were produced by transpressional forces acting parallel to the Weddell Sea coast of the Antarctic Peninsula during mid-Cretaceous compression of the Larsen Basin. At Pedersen Nunatak, Larsen Basin rocks are deformed into a series of synclines and anticlines that are cut by reverse faults.

Middle Cambrian rift-related volcanism in the Ellsworth Mountains, Antarctica: tectonic implications for the palaeo-Pacific margin of Gondwana

The Ellsworth Mountains of West Antarctica represent part of a displaced terrane once situated along the palaeo-Pacific margin of Gondwana, prior to supercontinent break-up, adjacent to South Africa and the Weddell Sea coast of East Antarctica. Middle Cambrian sedimentary rocks of the southern Ellsworth Mountains host locally thick volcanic and subvolcanic rocks forming five igneous centres. Geochemically, most of the igneous samples are mafic, with a subordinate suite of evolved types. The mafic suite is geochemically varied, ranging from MORB (mid-ocean ridge basalt)-like compositions to shoshonitic and lamprophyric (e.g. La N/Yb N = 0.95 to 15.2), with εNd i values ranging from +5.2 to −2.0, correlating with Ti/Y. They are interpreted as representing melts derived from more than one mantle source, with the MORB-like rocks being derived from a depleted mantle source, and the more enriched compositions representing partial melting of lithospheric mantle. Silicic rocks contain melt contributions from Late Proterozoic crust, which is inferred to form the basement of the Ellsworth Mountains. We interpret these igneous rocks as having been formed in a continental rift environment, with MORB-like basalts erupted near the rift axis, and melts from lithospheric mantle emplaced on the rift shoulder. Such an interpretation is consistent with the sedimentary host-rock palaeogeography and contemporaneous structures. This Middle Cambrian rift event is correlated spatially and temporally with rift-related sedimentary rocks in South Africa. It is currently unclear what rifted off the southern African–Weddell Sea sector of the Gondwana palaeo-Pacific margin at that time.

Synoptic forcing of wind and temperature in a large cirque 300 km from the coast of East Antarctica

Between 18 January 1988 and 3 June 1989, an automatic weather station recorded 13 different weather parameters every 3 h on a blue-ice area located in Scharffenbergbotnen, a large cirque in central Heimefrontfjella 300 km from the Weddell Sea coast. The first part of the paper reports on annual and monthly data regarding air temperature, air pressure, wind speed and wind direction, and a comparison is also made with corresponding data from the Neumayer and Halley stations. The second part deals mainly with winter (i.e. April–September) conditions in Scharffenbergbotnen. They seem, at least during 1988–89, to have been characterized by a large-scale (30–40 days) and, superimposed on the large-scale, a small-scale (3–4 days) co-variation of air temperature, air pressure and wind speed. The large-scale variation was earlier found to be synoptically forced. This paper shows that synoptic forcing exists also on smaller time scales. Pools of cold, stagnant air are regularly formed in the cirque only to be blown away by katabatic winds triggered by small variations in the synoptic pressure field. When this happens the air temperature increases by more than 20°C and the wind direction swings from east towards south-east. When low pressures dominate in the eastern part of the Weddell Sea, the katabatic winds become very strong, but weaker wind pulses also take place when the synoptic pressure gradient is directed towards the north-east. It therefore seems as if these very regular katabatic events are forced both by synoptic-scale pressure gradients and gradients due to the sloped inversion.

Characteristics and distribution patterns of snow and meteoric ice in the Weddell Sea and their contribution to the mass balance of sea ice

Abstract. Based on snow- and ice-thickness measurements at >11 000 points augmented by snow- and icecore studies during 4 expeditions from 1986 - 92 in the Weddell Sea, we describe characteristics and distribution patterns of snow and meteoric ice and assess their importance for the mass balance of sea ice. For first-year ice (FY) in the central and eastern Weddell Sea, mean snow depth amounts to 0.16 m (mean ice thickness 0.75 m) compared to 0.53 m (mean ice thickness 1.70 m) for second-year ice (SY) in the northwestern Weddell Sea. Ridged ice retains a thicker snow cover than level ice, with ice thickness and snow depth negatively correlated for the latter, most likely due to aeolian redistribution. During the different expeditions, 8, 15, 17 and 40% of all drill holes exhibited negative freeboard. As a result of flooding and brine seepage into the snow pack, snow salinities averaged 4‰. Through 18O measurements the distribution of meteoric ice (i.e. precipitation) in the sea-ice cover was assessed. Roughly 4% of the total ice thickness consist of meteoric ice (FY 3%, SY 5%). With a mean density of 290 kg/m3, the snow cover itself contributes 8% to total ice mass (7% FY, 11% SY). Analysis of ∆18O in snow indicates a local maximum in accumulation in the 65 to 75°S latitude zone. Hydrogen peroxide in the snow has proven useful as a temporal tracer and for identification of second-year floes. Drawing on accumulation data from stations at the Weddell Sea coast, it becomes clear that the onset of ice growth is important for the evolution of ice thickness and the interaction between ice and snow. Loss of snow to leads due to wind drift may be considerable, yet is reduced owing to metamorphic processes in the snow column. This is confirmed by a comparison of accumulation data from coastal stations and from snow depths over sea ice. Temporal and spatial accumulation patterns of snow are shown to be important in controlling the sea-ice cover evolution.

DIET OF WEDDELL SEALS LEPTONYCHOTES WEDDELLII AT VESTKAPP, EASTERN WEDDELL SEA (ANTARCTICA), IN RELATION TO LOCAL FOOD SUPPLY

AbstractThirteen Weddell seals (Leptonychotes weddellii) were collected at Vestkapp, eastern Weddell Sea coast, in austral spring 1986. All stomachs contained partially digested food. The mean wet weight of stomach contents was 7.5 kg, 3.3% of the. mean body weight of the collected seals. Twelve fish species and three cephalopod species were identified from 372 left otoliths and 25 lower beaks, representing 58.4% of 679 total prey items obtained. Composition by number of total prey was: Chionodraco myersi (15.8%), Trematomus eulepidotus (10.0%), Pagetopsis maculatus (9.7%), Racovitzia glacialis (9.6%) and Cryodraco antarcticus (4.1%). Otoliths of the seven other fish species and beaks of the three cephalopod species together represented 9.1% of total prey numbers. The pooled wet weights calculated from 13 prey species (regressions for two octopod species were not available) amounted to 43.5 kg food mass and represented 44.7% of the combined food mass in all stomachs. Composition by mass was: C. myersi (44.5%), T. eulepidotus (19.8%), squid Psychroteuthis glacialis (8.5%), P. maculates (7.9%), C. antarcticus (7.1%) and R. glacialis (6.2%). The remaining 7 fish species together represented 5.8% by mass. Temporal variation in food availability was apparent. Midwater fish Pleuragramma antarcticum was the staple food of Weddell seals from the same area during the 1985 summer, whereas it was absent in the samples taken in spring 1986. Estimates of fish biomass from net hauls demonstrate a highly variable availability of pelagic food resources for top predators in the Vestkapp area.

A Model For Polar Continental Shelf-Water Mass Modification

The process by which water flows from the surface ocean to the deep ocean is crucial in determining the structure and composition of the world ocean (Killworth, 1983). An important element in this process is the mechanism by which surface water on the Antarctic continental shelves becomes dense enough to sink to the abyssal ocean. These shelves are active sites of surface water modification because of the intense cooling and salt rejection due to sea-ice formation which occurs there during the winter.Prior models of water mass modification on the Antarctic continental shelves have focused on either the wind-forced circulation (Killworth, 1973), or the density-forced circulation (Solomon, unpublished; Killworth, 1974). Solomon (unpublished) showed that the density-forced circulation alone is too weak to produce a flux of saline water to the deep sea which was consistent with observations. Killworth (1973) showed that the wind-forced circulation alone in the Weddell Sea was incapable of producing a large enough flux. In a complementary model of only the density-forced circulation, Killworth (1974) showed that the change in salinity of water parcels caused by regional density forcing was reasonable; but as in Solomon's (unpublished) model, the flux of modified water was too small. The result of these model studies was that either circulation alone, wind-forced or density-forced, is insufficient to produce the observed flux of saline shelf water. The conjecture based on the results of these studies was that, when combined, the density forcing determined the change of salinity in the shelf water and the wind-forced circulation determined the flux of the modified water.To determine whether the combination of wind stress and density forcing is sufficient to produce a flux and salinity change consistent with observations, a simple model which responds to both forcings was developed. The model is essentially that used by Killworth (1974) with the addition of the flow forced by wind stress. A different density forcing is used because observations of the sea-ice cover (Zwally and others, 1983) suggest that the regions of coastal open water, and hence freezing, are much more localized than used in prior density-forced models. The localization of the density forcing was parameterized as being a Gaussian function of distance from and along the coast. The model (Grumbine, unpublished) predicts the velocity and salinity in two layers of equal depth in a square region with a flat bottom, subject to wind and density forcing. The east, south, and west boundaries represent coasts of an embayment analogous to the Weddell Sea continental shelf, with the continental shelf break in the middle, north-south, of the region, and an open northern boundary. The salinity is initially constant in each layer. The velocity is initialized to the equilibrium value of the wind-forced circulation. Then the salinity forcing is started and the evolution of the salinity fields is computed using the wind-forced velocity field and a diagnostically computed density-forced velocity field.The model was run for three cases: wind stress forcing only, density forcing only, and wind stress and density forcing combined. The wind-forced circulation alone was incapable of producing a sufficient degree of water mass modification. With only density forcing, the change in salinity was consistent with observations, but the flux was too small. When the two forcings are combined, both the change in salinity and the flux of modified water are consistent with observations. The model is verified against the flux and composition values from observations by Foster and Carmack (1976) in the deep Weddell Sea. The observed flux value of 1 to 2.5 × 106 m3 s−1 corresponds to water with a mean salinity of 34.7‰. This is the salinity of the water which was able to reach the abyssal ocean off the Weddell Sea coast. Figure 1 gives the modelled seasonal cycle and annual mean of the equatorward flux of water which corresponds to a mean salinity of 34.7‰. The figure shows the flux for the last three years of a five- year run computed across the position of the continental slope. The mean annual flux of this modified water is 2.0 × 106 m3 s−1, compared to the observation of 1 to 2.5 × 106 m3 s−1. These results confirm the conjecture that the combination of wind and density forcing is sufficient to produce a degree of modification and flux of modified water consistent with observations.

A Model For Polar Continental Shelf-Water Mass Modification

The process by which water flows from the surface ocean to the deep ocean is crucial in determining the structure and composition of the world ocean (Killworth, 1983). An important element in this process is the mechanism by which surface water on the Antarctic continental shelves becomes dense enough to sink to the abyssal ocean. These shelves are active sites of surface water modification because of the intense cooling and salt rejection due to sea-ice formation which occurs there during the winter. Prior models of water mass modification on the Antarctic continental shelves have focused on either the wind-forced circulation (Killworth, 1973), or the density-forced circulation (Solomon, unpublished; Killworth, 1974). Solomon (unpublished) showed that the density-forced circulation alone is too weak to produce a flux of saline water to the deep sea which was consistent with observations. Killworth (1973) showed that the wind-forced circulation alone in the Weddell Sea was incapable of producing a large enough flux. In a complementary model of only the density-forced circulation, Killworth (1974) showed that the change in salinity of water parcels caused by regional density forcing was reasonable; but as in Solomon's (unpublished) model, the flux of modified water was too small. The result of these model studies was that either circulation alone, wind-forced or density-forced, is insufficient to produce the observed flux of saline shelf water. The conjecture based on the results of these studies was that, when combined, the density forcing determined the change of salinity in the shelf water and the wind-forced circulation determined the flux of the modified water. To determine whether the combination of wind stress and density forcing is sufficient to produce a flux and salinity change consistent with observations, a simple model which responds to both forcings was developed. The model is essentially that used by Killworth (1974) with the addition of the flow forced by wind stress. A different density forcing is used because observations of the sea-ice cover (Zwally and others, 1983) suggest that the regions of coastal open water, and hence freezing, are much more localized than used in prior density-forced models. The localization of the density forcing was parameterized as being a Gaussian function of distance from and along the coast. The model (Grumbine, unpublished) predicts the velocity and salinity in two layers of equal depth in a square region with a flat bottom, subject to wind and density forcing. The east, south, and west boundaries represent coasts of an embayment analogous to the Weddell Sea continental shelf, with the continental shelf break in the middle, north-south, of the region, and an open northern boundary. The salinity is initially constant in each layer. The velocity is initialized to the equilibrium value of the wind-forced circulation. Then the salinity forcing is started and the evolution of the salinity fields is computed using the wind-forced velocity field and a diagnostically computed density-forced velocity field. The model was run for three cases: wind stress forcing only, density forcing only, and wind stress and density forcing combined. The wind-forced circulation alone was incapable of producing a sufficient degree of water mass modification. With only density forcing, the change in salinity was consistent with observations, but the flux was too small. When the two forcings are combined, both the change in salinity and the flux of modified water are consistent with observations. The model is verified against the flux and composition values from observations by Foster and Carmack (1976) in the deep Weddell Sea. The observed flux value of 1 to 2.5 × 106 m3 s−1 corresponds to water with a mean salinity of 34.7‰. This is the salinity of the water which was able to reach the abyssal ocean off the Weddell Sea coast. Figure 1 gives the modelled seasonal cycle and annual mean of the equatorward flux of water which corresponds to a mean salinity of 34.7‰. The figure shows the flux for the last three years of a five- year run computed across the position of the continental slope. The mean annual flux of this modified water is 2.0 × 106 m3 s−1, compared to the observation of 1 to 2.5 × 106 m3 s−1. These results confirm the conjecture that the combination of wind and density forcing is sufficient to produce a degree of modification and flux of modified water consistent with observations.

200-Hundred Year Climate Record From Antarctic Peninsula

A stable isotope record extending back to 1795 is now available from Dolleman Island (70°35.2′S, 60°55.5′W), a small ice rise on the Weddell Sea coast of Antarctic Peninsula. An accurate chronology has been achieved by combined stratigraphic analysis of clear seasonal cycles in δ18O and excess SO4. Previous work (Peel and others, 1988) has shown that, since 1947, there is generally a satisfactory correlation between interannual variations in δ18O and air temperature (T) as recorded at weather stations in various parts of the region, suggesting that the derived δ18O/T ratio may be used to reconstruct air temperatures for the earlier period.Taken together with previously-reported data (Aristarain and others, 1986) for an ice core from James Ross Island it is now possible to propose a regional climatic signal for the Weddell Sea coastal sector of the region. The most striking feature is a broad maximum in δ18O for the mid-19th century, implying decadal average temperature at least as high as the present. This contrasts with available evidence from elsewhere in the southern hemisphere which suggest that this period was cooler than today. Tentative explanations for the anomaly are proposed based on evidence for a period (1974–80), where climatic shifts are clearly amplified in the isotopic records.

200-Hundred Year Climate Record From Antarctic Peninsula

A stable isotope record extending back to 1795 is now available from Dolleman Island (70°35.2′S, 60°55.5′W), a small ice rise on the Weddell Sea coast of Antarctic Peninsula. An accurate chronology has been achieved by combined stratigraphic analysis of clear seasonal cycles in δ18O and excess SO4. Previous work (Peel and others, 1988) has shown that, since 1947, there is generally a satisfactory correlation between interannual variations in δ18O and air temperature (T) as recorded at weather stations in various parts of the region, suggesting that the derived δ18O/T ratio may be used to reconstruct air temperatures for the earlier period. Taken together with previously-reported data (Aristarain and others, 1986) for an ice core from James Ross Island it is now possible to propose a regional climatic signal for the Weddell Sea coastal sector of the region. The most striking feature is a broad maximum in δ18O for the mid-19th century, implying decadal average temperature at least as high as the present. This contrasts with available evidence from elsewhere in the southern hemisphere which suggest that this period was cooler than today. Tentative explanations for the anomaly are proposed based on evidence for a period (1974–80), where climatic shifts are clearly amplified in the isotopic records.