Weddell Sea Bottom Water Research Articles

Dissolved manganese in the Atlantic sector of the Southern Ocean

The first comprehensive dataset (492 samples) of dissolved Mn in the Southern Ocean shows extremely low values of 0.04 up to 0.64 nM in the surface waters and a subsurface maximum with an average concentration of 0.31 nM ( n=20; S.D.=0.08 nM). The low Mn in surface waters correlates well with the nutrients PO 4 and NO 3 and moderately well with Si(OH) 4 and fluorescence. Furthermore, elevated concentrations of Mn in the surface layer coincide with elevated Fe and light transmission and decreased export ( 234Th/ 238U deficiency) and fluorescence. It appears that Mn is a factor of importance in partly explaining the HNLC conditions in the Southern Ocean, in conjunction with significant controls by the combination of Fe limitation and light limitation. No input of Mn from the continental margins was observed. This is ascribed to the protruding continental ice sheet that covers the shelf and shuts down the usual biological production, microbial breakdown and sedimentary geochemical cycling. The low concentrations of Mn in the deep ocean basins (0.07–0.23 nM) were quite uniform, but some elevations were observed. The highest deep concentrations of Mn were observed at the Bouvet Triple Junction region and coincided with high concentrations of Fe and are deemed to be from hydrothermal input. The deep basins on both sides of the ridge were affected by this input. In the deep Weddell Basin the input of Weddell Sea Bottom Water appears to be the source of the slightly elevated concentrations of Mn in this water layer.

Modeling ice shelf water overflow and bottom water formation in the southern Weddell Sea

Descent of Ice Shelf Water (ISW) overflowed from the Filchner Depression in the southern Weddell Sea and consequent Weddell Sea Bottom Water (WSBW) production is investigated through high resolution numerical experiments. The model reproduces the part of ISW which interacts with the submarine ridge near 36°W. The baroclinic instability wave developed near the exit of the Filchner Depression leads to intermittent outflow of ISW, and the intermittent passage of ISW‐origin dense water induces 6‐day period oscillation of velocity at the east side of the ridge which coincides well with mooring data. The existence of the ridge induces two different mechanisms for cross‐isobath transport of the ISW‐origin dense water: a strong downslope current at the east side of the ridge and offshore transport by eddies, which are generated when a gravity current makes a detour around the tip of the ridge, over a wide region to the west of the ridge. In a sensitivity experiment where thermobaricity is removed from the equation of state, there is almost no transport of the ISW‐origin water below 3000 m depth. WSBW production rate as a result of descent of the ISW‐origin dense water in the model domain is about ∼0.15 Sv.

Distribution of barium in the Weddell Gyre: Impact of circulation and biogeochemical processes

The Southern Ocean data base of dissolved barium (Ba d) has been augmented with two densely sampled sections across the Weddell Gyre sampled by the icebreaker FS Polarstern during February and March 2005. Ba d was found to be relatively high in the surface layer as compared to the adjacent waters north (Antarctic Circumpolar Current) and east (Antarctic Zone of the Indian sector). Compared to the inflowing water into the Weddell Gyre and also to the surface water, the deep water is characterized by a significant Ba d enrichment. Depletion of Ba d in the surface layer is associated with well-known biogeochemical processes, but we speculate that also sea-ice formation leads to the extraction of Ba from solution via barite (BaSO 4) precipitation. The particulate Ba settles down the water column and redissolves in deeper water where undersaturation of barite is prevalent. In the bottom layer, enrichment of Ba d occurs, exhibited as a Ba d maximum, which is caused by Ba efflux from the sediments. In recently formed Weddell Sea Bottom Water, though, a Ba d minimum is observed, imposed by the shelf water component of bottom water, which has relatively low Ba d concentration. Like in other Southern Ocean regions, throughout the water column a strong correlation exists between Ba d and dissolved silicate, although the relationship is different from that in the Antarctic regions to the east. In a tentative budget, the deep water is found to be supplied with Ba d mainly by the surface layer, but also to some extent by the bottom layer. Weddell Sea Deep Water, enriched in Ba d, is transferred out of the Weddell Gyre into the ACC as Antarctic Bottom Water, making the gyre a source of Ba d to the deep and abyssal world oceans.

A seasonal cycle in the export of bottom water from the Weddell Sea

Dense water formed over the Antarctic continental shelf rapidly descends into the deep ocean where it spreads throughout the global ocean as Antarctic Bottom Water1, 2. The coldest and most voluminous component of this water mass is Weddell Sea bottom water1, 3, 4, 5, 6, 7. Here we present observations over eight years of the temperature and salinity stratification in the lowermost ocean southeast of the South Orkney Islands, marking the export of Weddell Sea bottom water. We observe a pronounced seasonal cycle in bottom temperatures, with a cold pulse in May/June and a warm one in October/November, but the timing of these phases varies each year. We detect the coldest bottom water in 1999 and 2002, whereas there was no cold phase in 2000. On the basis of current velocities and water mass characteristics, we infer that the pulses originate from the southwest Weddell Sea. We propose that the seasonal fluctuations of Weddell Sea bottom-water properties are governed by the seasonal cycle of the winds over the western margin of the Weddell Sea. Interannual fluctuations are linked to the variability of the wind-driven Weddell Sea gyre and hence to large-scale climate phenomena such as the Southern Annular Mode and El Nino/Southern Oscillation.

Data‐based estimation of anthropogenic carbon and acidification in the Weddell Sea on a decadal timescale

The amount of anthropogenic CO2 (Cant) that entered the Weddell Sea between 1992 and 2008 (Cant1992–2008) was assessed using the extended multiple linear regression (eMLR) method. In the Warm Deep Water (WDW) and the Weddell Sea Bottom Water (WSBW), Cant1992–2008 values were insignificant, whereas values as high as 8 μmol kg−1 were observed over the shelf. Cant1992–2008 concentrations in the surface layer varied with latitude between 2 and 11 μmol kg−1. Weak intrusion of anthropogenic CO2 into Weddell Sea Deep Water (WSDW) was demonstrated (Cant1992–2008 yields 1.5–2 μmol kg−1). That more Cant1992–2008 was found in the WSDW than in the WSBW is surprising, but can be explained by intense ventilation of the WSDW originating from east of the Weddell Gyre. The invasion of Cant1992–2008 provokes a shift in the equilibria of the carbonate system, resulting in acidification and reduction of CO32− concentration. The mean decrease of pH in the upper 200 m layer was 0.016. The largest decrease of calcite and aragonite saturation states was observed at the surface. This implies that surface waters might become undersaturated with respect to aragonite in the future while the underlying WDW is still saturated. Results of this analysis suggest that complete undersaturation of surface waters in the Weddell Sea will be reached after the 21st century.

On the temporal variability of the Weddell Sea Deep Water masses

The Weddell Sea is one of the key regions of the Southern Ocean with respect to climate as most of the Antarctic Bottom Water (AABW) that occupies the world ocean deepest layers is likely to originate from this region. This study applies the Optimum Multiparameter water mass analysis to the Weddell deep waters in order to investigate their distribution and variability. The dataset used is based on the WOCE repeat sections in the area (SR04 and A12) from 1984 to 1998. The mean water mass distribution is consistent with previous knowledge of the region, along with high interannual variability. Regarding the temporal variability, it seems that the years of maximum Weddell Sea Deep Water (WSDW) contribution correspond to the lowest levels of Weddell Sea Bottom Water (WSBW), and vice versa. In order to identify possible forcing mechanisms for such variability, the water mass temporal anomalies were compared with oceanic and atmospheric modes of variability in that region such as the Southern Annular Mode (SAM). An apparent correlation between the SAM index temporal gradients and WSBW anomalies indicate that the Weddell Sea export of dense waters to the world ocean may be linked to that index on several time scales.

Validation of three global ocean models in the Weddell Sea

We present a validation of three global z-level eddy-permitting/resolving ocean general circulation models against hydrographic observations and remote sensing sea ice data from the Weddell Sea. The Weddell Sea is a region in which complex processes such as water mass formation, sea ice formation and melt, and circulation under ice shelves take place. The representation of these processes is challenging even for the current generation of eddy-permitting ocean models. Simulating the hydrographic structure of this basin is a stringent test for models, notably so when considering the global influence of the regional processes. The performance of OCCAM (at two resolutions), ORCA025 and TPAC are tested regarding water mass properties, sea ice seasonality (OCCAM and ORCA025 only) and volume transport. OCCAM simulates the deep water masses reasonably well in both resolutions. The eddy-resolving run is not significantly better than the eddy-permitting simulation. ORCA025 and TPAC represent the surface layers and the Weddell Gyre circulation better but are generally too warm throughout the water column. All models underestimate Weddell Sea Bottom Water formation. Both OCCAM and ORCA025 struggle to correctly model the sea ice cover: OCCAM overestimates the summer ice extent while little multi-year sea ice remains in ORCA025. TPAC exhibits considerable drift in potential temperature during the model run. The choice of a model for a study has to be made carefully taking into account the model’s performance in the specific area, application and variable of interest. We identify several starting points for improving the models, namely model numerics and parameterisations of subgridscale processes, ensuring more accurate forcing datasets, correct initialisation, and ice–ocean interactions, all of which are likely to have larger benefits than simply increasing the horizontal resolution beyond eddy permitting.

Bottom water formation in the southern Weddell Sea and the influence of submarine ridges: Idealized numerical simulations

Cold Ice Shelf Water (ISW) flowing out of the Filchner Depression in the southern Weddell Sea provides a significant contribution to the production of Weddell Sea Bottom Water (WSBW) after mixing with the overlying Warm Deep Water (WDW). The characteristics of Filchner overflow including the plume path, thickness, transport, eddy formation and mixing processes are studied using the Finite Element Ocean circulation Model (FEOM) on a sigma grid with variable horizontal resolution. It is demonstrated that the cold dense water descends very fast on entering the continental slope. Its center is found below the 2500 m isobath after traveling only 200 km along the slope. Most mixing and entrainment also takes place within this short distance. Further downstream, the along-slope transport increases most significantly between the 2000 m and 3000 m isobaths. The conversion rate from ISW to newly formed WSBW (defined as a water mass with potential temperature below −0.8 °C) is found to be 2.2. The transport of WSBW does not show significant changes after 400 km downstream, but conversion between different water classes still remains active. Two ridges on the slope of the southern Weddell Sea influence the overflow by both steering and increasing mixing. The ridges split the path of the overflow, which leads to a higher transport between the 2000 m and 3000 m isobaths. Mixing between ISW and WDW is considerably increased by the ridges.

Evidence of deep- and bottom-water formation in the western Weddell Sea

Abstract During Ice Station POLarstern (ISPOL; R.V. Polarstern cruise ANT XXII/2, November 2004–January 2005), hydrographic and tracer observations were obtained in the western Weddell Sea while drifting closely in front of the Larsen Ice Shelf. These observations indicate recently formed Weddell Sea Bottom Water, which contains significant contributions of glacial melt water in its upper part, and High-Salinity Shelf Water in its lower layer. The formation of this bottom water cannot be related to the known sources in the south, the Filchner–Ronne Ice Shelf. We show that this bottom water is formed in the western Weddell Sea, most likely in interaction with the Larsen C Ice Shelf. By applying an Optimum Multiparameter Analysis (OMP) using temperature, salinity, and noble gas observations (helium isotopes and neon), we obtained mean glacial melt-water fractions of about 0.1% in the bottom water. On sections across the Weddell Gyre farther north, melt-water fractions are still on the order of 0.04%. Using chlorofluorocarbons (CFCs) as age tracers, we deduced a mean transit time between the western source and the bottom water found on the slope toward the north (9±3 years). This transit time is larger and the inferred transport rate is small in comparison to previous findings. But accounting for a loss of the initially formed bottom water volume due to mixing and renewal of Weddell Sea Deep Water, a formation rate of 1.1±0.5 Sv in the western Weddell Sea is plausible. This implies a basal melt rate of 35±19 Gt/year or 0.35±0.19 m/year at the Larsen Ice Shelf. This bottom water is shallow enough that it could leave the Weddell Basin through the gaps in the South Scotia Ridge to supply Antarctic Bottom Water. These findings emphasize the role of the western Weddell Sea in deep- and bottom-water formation, particularly in view of changing environmental conditions due to climate variability, which might induce enhanced melting or even decay of ice shelves.

Early summer thermohaline characteristics and mixing in the western Weddell Sea

Abstract The Ice Station POLarstern (ISPOL) cruise revisited the western Weddell Sea in late 2004 and obtained a comprehensive set of conductivity–temperature–depth (CTD) data. This study describes the thermohaline structure and diapycnal mixing environment observed in 2004 and compares them with conditions observed more than a decade earlier. Hydrographic conditions on the central western Weddell Sea continental slope, off Larsen C Ice Shelf, in late winter/early spring of 2004/2005 can be described as a well-stratified environment with upper layers evidencing relict structures from intense winter near-surface vertical fluxes, an intermediate depth temperature maximum, and a cold near-bottom layer marked by patchy property distributions. A well-developed surface mixed layer, isolated from the underlying Warm Deep Water (WDW) by a pronounced pycnocline and characterized by lack of warming and by minimal sea-ice basal melting, supports the assumption that upper ocean winter conditions persisted during most of the ISPOL experiment. Much of the western Weddell Sea water column has remained essentially unchanged since 1992; however, significant differences were observed in two of the regional water masses. The first, Modified Weddell Deep Water (MWDW), comprises the permanent pycnocline and was less saline than a decade earlier, whereas Weddell Sea Bottom Water (WSBW) was horizontally patchier and colder. Near-bottom temperatures observed in 2004 were the coldest on record for the western Weddell Sea over the continental slope. Minimum temperatures were ∼0.4 and ∼0.3 °C colder than during 1992–1993, respectively. The 2004 near-bottom temperature/salinity characteristics revealed the presence of two different WSBW types, whereby a warm, fresh layer overlays a colder, saltier layer (both formed in the western Weddell Sea). The deeper layer may have formed locally as high salinity shelf water (HSSW) that flowed intermittently down the continental slope, which is consistent with the observed horizontal patchiness. The latter can be associated with the near-bottom variability found in Powell Basin with consequences for the deep water outflow from the Weddell Sea.

Ocean basins near the Scotia–Antarctic plate boundary: Influence of tectonics and paleoceanography on the Cenozoic deposits

The distribution of seismic units in deposits of the basins near the Antarctic–Scotia plate boundary is described based on the analysis of multichannel seismic reflection profiles. Five main seismic units are identified. The units are bounded by high-amplitude continuous reflectors, named a to d from top to bottom. The two older units are of different age and seismic facies in each basin and were generally deposited during active rifting and seafloor spreading. The three youngest units (3 to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The deposits are types of contourite drift that resulted from the interplay between the northeastward flow of Weddell Sea Bottom Water (WSBW) and the complex bathymetry in the northern Weddell Sea, and from the influence of the Antarctic Circumpolar Current and the WSBW in the Scotia Sea. A major paleoceanographic event was recorded by Reflector c, during the Middle Miocene, which represents the connection between the Scotia Sea and the Weddell Sea after the opening of Jane Basin. Unit 3 (tentatively dated ∼Middle to Late Miocene) shows the initial incursions of the WSBW into the Scotia Sea, which influenced a northward progradational pattern, in contrast to the underlying deposits. The age attributed to Reflector b is coincident with the end of spreading at the West Scotia Ridge (∼6.4 Ma). Unit 2 (dated ∼Late Miocene to Early Pliocene) includes abundant high-energy, sheeted deposits in the northern Weddell Sea, which may reflect a higher production of WSBW as a result of the advance of the West Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional paleoceanographic change. The timing of this event (∼3.5–3.8 Ma) coincides with the end of spreading at the Phoenix–Antarctic Ridge, but may be also correlated with global events such as initiation of the permanent Northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (dated ∼Late Pliocene to Recent) is characterized by abundant chaotic, high-energy sheeted deposits, in addition to a variety of contourites, which suggest intensified deep-water production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant wavy deposits and the development of internal unconformities, all of which attest to alternating periods of increased bottom current energy.

Interannual variations of water mass volumes in the Southern Ocean

Water mass volumes in the Southern Ocean are evaluated in a section between Antarctica and Tasmania for the period 1991–1996. Antarctic Bottom Water consisted of Weddell Sea Bottom Water and water formed along the Adélie Coast. The volume of the Adélie Coast component was found to decrease slightly over the observation period, while the volume of the Weddell Sea component remained unchanged. The volume of Circumpolar Deep Water increased slightly, with a pronounced trend toward Lower at the expense of Upper Circumpolar Deep Water. The volumes of Antarctic Surface Water, Western South Pacific Central Water, Subantarctic Mode Water, Antarctic Intermediate Water and Antarctic Bottom Water were smaller by up to one order of magnitude but their variations relative to their volumes were significant.

The transport of the Weddell Gyre across the Prime Meridian

Abstract Current-meter observations from instruments moored from 1996 to 2001 are used to determine the structure and the volume transport of the Weddell Gyre on the Greenwich Meridian. The record length mean volume transport is 45 Sv to the east within the northern and 56 Sv to the west within the southern limb of the Weddell Gyre. Thus 11 Sv must leave the gyre circulation west of the Greenwich Meridian and enter it east of it. The deficit occurs with 8 Sv mainly in the Circumpolar Deep Water (CDW)/Warm Deep Water (WDW) layer. Surface waters and Weddell Sea Deep Water (WSDW) contribute 2 and 3 Sv, respectively. In contrast to this, 3 Sv more of Weddell Sea Bottom Water (WSBW) flow in the northern limb to the east, than in the southern limb to the west, which suggests a WSBW formation rate of about 3 Sv in the western Weddell Sea. In the Southern Boundary of the Antarctic Circumpolar Current (ACC) a volume transport of 35±16 Sv was found using geostrophic calculations adjusted with two measurements of the absolute velocity. The large heat content of the northern limb points to an extensive heat exchange between the Weddell Sea and the world ocean west of the Prime Meridian. Hence, the heat transport as well as the volume transport suggest that the water masses of the Weddell Gyre are not exclusively replaced in its eastern part, but that there is a significant exchange of water masses to the west of the Prime Meridian.

Miocene to Recent contourite drifts development in the northern Weddell Sea (Antarctica)

Multichannel and high-resolution seismic profiles complemented with swath bathymetry show a variety of contourite deposits in the northern Weddell Sea resulting from the interaction between bottom currents and the seafloor physiography. Seven types of contourite drifts are identified based on the seismic signature, reflector configuration and geometry of the depositional bodies. Giant elongated–mounded drifts are widespread in the area and associated with major channelized contour currents that flow at the base of large ridges. Thick basement/tectonic drifts result from the seafloor disruptions of the currents caused by the irregularities of the near-surface basement morphology. Sheeted drifts occur under the main core of the Weddell Gyre and also in areas of the abyssal plain away from the main flows. Various types of drifts in-fill depressions or are plastered against steep bathymetric ridges that intersect contour currents. The regional distribution of the drifts is mainly controlled by the physiography of the basin and the confined or unconfined nature of the bottom-current flows. The northern Weddell Sea is a region dominated by contourite processes and thus provides an area to compare contourite drifts with turbidite systems. The giant elongated–mounded drifts have a net asymmetry of the body, in contrast to turbidite channel–levee complexes that develop levees on each side of an axial turbidite channel. The basement/tectonic drifts prograde parallel to the main flow and are plastered following the irregularities of the basement unlike turbidite deposits. Other drifts, in contrast, show internal reflector characteristics similar to turbidite systems, such as the sheeted drifts. In these cases, however, the associations of turbidite and drift deposits are different. The giant elongate-mounded drifts are stacked along the margins and elongate or transverse drift sequences are observed in the basin centre of confined basins. In the unconfined setting, the drifts are normally asymmetric in relation to the marginal channel moats and sheeted drifts develop laterally from the ridges. In turbidite systems of confined or unconfined settings, generally symmetrical proximal channel–levee complexes evolve downstream to sheetlike basin plain sequences. Five main seismic units separated by regional unconformities are recognized above the oceanic basement. The age of the deposits is based on the magnetic anomalies of the oceanic crust and the overlying seismic sequences. The external geometry and acoustic character of the seismic units indicate strong bottom-current processes, except for the basal deposits attributed to the Early Miocene. The development of extensive drifts in the deposits of Unit 4 (∼Middle Miocene) shows the initial influence of the Weddell Sea Bottom Water (WSBW). The opening of the connection of Jane Basin with the Scotia Sea also is marked by a regional unconformity that records a reorganization of bottom flows. The two uppermost Units 1 and 2 (Late Miocene to Recent) indicate intensified bottom currents, which may reflect the increased production of WSBW. The evolution through time of the contourite deposits and the distribution of regional unconformities reflect the ice sheet dynamics that controlled the production of Antarctic Bottom Water (AABW).

Deep-water sedimentary environments of the northwestern Weddell Sea and South Sandwich Islands, Antarctica

Abstract Two transects have been sampled using short cores (multi and box), seabed photography, video sequences, and sediment profile images across the northwestern Weddell Sea and South Sandwich Forearc, Antarctica. A total of 12 core stations were examined for sediment structure, texture and composition to determine their depositional history. Four of the core stations from the Weddell Continental Slope, Abyssal Plain and South Sandwich Forearc and Trench were further examined for trace metal analysis (Ba, U, Mn and Pb) for sediment source and palaeoproductivity, and 210Pb for age profiles and depth of mixing by bioturbation. The 3055 m station at the base of the Weddell Continental Slope, recovered laminated muddy–silty contourites with a surface layer of hemipelagites with abundant ice-rafted debris, possibly originating from the 1995 Larsen A and 2002 Larsen B ice shelf disintegration. This is interpreted as a moderate-energy depositional environment influenced by Weddell Sea Bottom Water along-slope current, reworking sediment. Sediment sources to this station include the direct input of clastic material from ice with variation in sediment sources from in situ current winnowing and along-slope transport reflected in the Mn/Li and Ba/Li ratios. The 4760 m station on the Weddell Abyssal Plain recovered hemipelagites, interpreted as a low-energy depositional environment with sediment supplied both from direct ice-rafting and pelagic settling during productive seasonally ice-free periods. The 210Pb profiles indicate a sedimentation rate of 0.024 cm/year−1, with the 206/207Pb isotope ratios suggesting a principle sediment source from ice-rafting. The 2292 and 6326 m stations from the slope and floor of the South Sandwich Forearc recovered pelagic and turbiditic sediments. This is interpreted as a high-energy down-slope depositional environment, with high surface biogenic productivity and sediment supply leading to localised down-slope mass-movements on the slope and floor. High levels of Mn/Li and elevated U/Li ratios demonstrate a strong concentration of Mn in organic-poor sediment. The Mn redoxcline is present at 10 cm depth. The high Ba/Li (500 ppm) in the forearc floor core reflects some limited enhanced productivity. Source sediment for the forearc may, in part, be derived from the northwest Weddell Abyssal Plain, based on the 206/207Pb isotope ratios reflecting transport by Weddell Sea Bottom Water.

Methane dynamics in the Weddell Sea determined via stable isotope ratios and CFC‐11

The physical, chemical/biological processes that control the methane dynamics in the Weddell Sea are revealed by the distributions of methane (CH4), its stable carbon isotope ratio, δ13C‐CH4, and the conservative transient tracer, chlorofluorocarbon‐11 (CFC‐11, CCl3F). In general, a nearly linear correlation between CH4 and CFC‐11 concentrations was observed. Air‐sea exchange is the major source of methane to this region, and the distribution of methane is controlled mainly by mixing between surface water and methane‐poor Warm Deep Water. A significant influence of methane oxidation over the predominant two end‐member mixing was only found in the Weddell Sea Bottom Water (WSBW) of the deep central Weddell Basin, where the turnover time of methane appears to be about 20 years. Mixing also controls most of the δ13C‐CH4 distribution, but lighter than expected carbon isotopic ratios occur in the deep WSBW of the basin. From box model simulations, it appears that this “anomaly” is due to methane oxidation with a low kinetic isotope fractionation of about 1.004. The surface waters in the Weddell Sea and the Antarctic Circumpolar Current showed a general methane undersaturation of 6 to 25% with respect to the atmospheric mixing ratio. From this undersaturation and model‐derived air‐sea exchange rates, we estimate a net uptake of CH4 of roughly −0.5 μmol m−2 d−1 during austral autumn.

Decadal-scale variations of water mass properties in the deep Weddell Sea

Data from cruises between 1989 and 2003 with FS Polarstern were used to construct section-wide potential temperature and salinity time series of the main water masses in the Weddell Gyre. In tandem with these CTD data, two time series between 1989 and 1995 are presented from moored instruments in the central Weddell Sea. The regional and methodological consistency of the dataset allows us to quantify variations which are not visible in less homogeneous datasets. The data reveal significant temperature and salinity variations of the Warm Deep Water and the Weddell Sea Bottom Water on a decadal time scale. The longest time series were obtained at the prime meridian. Here warming is observed in the Warm Deep Water from 1992 to 1998 followed by cooling. In the Weddell Sea proper, measurements of instruments moored in the Weddell Sea Bottom Water layer recorded a temperature increase over 6 years at a rate of 0.01 °C a−1. After the mooring period, CTD casts in 1998 point to a weakening of the trend. The warming trend in the bottom water occurs over most of the Weddell Sea, as detected in the additional CTD surveys. The variations are close to the detection level in the voluminous Weddell Sea Deep Water. The initial warming trend of the Warm Deep Water is consistent with warming trends reported in literature of subsurface waters of the Antarctic Circumpolar Current. The reversal of the trend in the Weddell Sea seems to be related to variations of the atmospheric conditions which can affect both the intrusion of Circumpolar Deep Water from the north and the circulation of the Weddell Gyre. Because the Warm Deep Water is the major source water for the formation of deep and bottom water in the Weddell Sea, it is suggested that its increase in temperature and salinity is likely to at least partly cause the variations which were observed in the bottom water.

Ice shelf water overflow and bottom water formation in the southern Weddell Sea

Cold shelf waters flowing out of the Filchner Depression in the southern Weddell Sea make a significant contribution to the production of Weddell Sea Bottom Water (WSBW), a precursor to Antarctic Bottom Water (AABW). We use all available current meter records from the region to calculate the flux of cold water (<−1.9°C) over the sill at the northern end of the Filchner Depression (1.6 ± 0.5 Sv), and to determine its fate. The estimated fluxes and mixing rates imply a rate of WSBW formation (referenced to −0.8°C) of 4.3 ± 1.4 Sv. We identify three pathways for the cold shelf waters to enter the deep Weddell Sea circulation. One path involves flow constrained to follow the shelf break. The other two paths are down the continental slope, resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36°W and 37°W. Mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m. Probably aided by thermobaricity, the plume water at this depth can flow at a speed approaching 1 m s−1, implying that the flow is occasionally supercritical. We postulate that such supercriticality acts to limit mixing between the plume and its environment. The transition from supercritical to slower, more uniform flow is associated with very efficient mixing, probably as a result of hydraulic jumps.

Seasonal ventilation of the cavity beneath Filchner‐Ronne Ice Shelf simulated with an isopycnic coordinate ocean model

The ocean cavity beneath Filchner‐Ronne Ice Shelf is observed to respond to the seasonal cycle of water mass production on the continental shelf of the southern Weddell Sea. Here we use a numerical model to investigate the propagation of newly formed shelf waters into the cavity. We find that the model reproduces the most distinctive features of the observed seasonality and offers a plausible explanation for those features. The most saline shelf waters are produced in the far west, where the inflow to the cavity peaks twice each year. The major peak occurs during the short period around midwinter when convection reaches full depth and the densest waters are generated. Once the surface density starts to decline, dynamic adjustment of the restratified water column leads to a gradual fall in the salinity at depth and a secondary peak in the inflow that occurs in summer at the western coast. Beneath the ice shelf the arrival of the wintertime inflow at the instrumented sites is accompanied by a rapid warming, while the slower decline in the inflow leads to a more gradual cooling. Water brought in by the secondary, summer peak flows mainly to the eastern parts of the cavity. Here the seasonality is suppressed because the new inflows mix with older waters that recirculate within a topographic depression. This pooling of waters in the east, where the primary outflow of Ice Shelf Water is generated, dampens the impact of seasonality on the local production of Weddell Sea Bottom Water.

Importance of the eastern Indian Ocean for the abyssal Pacific

The role of the eastern Indian Ocean in controlling the abyssal circulation in the Pacific is examined using a 1° × 1° world ocean model. Thanks to the improvement of bottom topography and the incorporation of a bottom boundary layer model, the bottom water formation off the Adélie Coast is produced. This bottom water is not only the source of the abyssal water in the eastern Indian but also has a great influence on the whole abyssal Pacific. The lower part of Circumpolar Deep Water (CDW), which enters the abyssal Pacific passing through south of New Zealand, changes its property through mixing with this bottom water flowing underneath CDW in the Australian‐Antarctic Basin and the South Australia Basin. The tracer experiments show that the water ventilated off the Adélie Coast occupies about 20% of the abyssal Pacific, whereas North Atlantic Deep Water (NADW) occupies about 50%, the Weddell Sea bottom water about 20%, and the Ross Sea bottom water about 10%. The flux of the ventilated water estimated by the tracer experiments is 17.1 Sv for NADW, 6.3 Sv from the Weddell Sea, 6.2 Sv from off the Adélie Coast, and 3.5 Sv from Ross Sea in this model. The total flux of 33.1 Sv is not far from the estimate from observations, though it may overestimate by 1–4 Sv. This does not directly indicate that the real world ocean should have the large amount of the bottom water sources in the Pacific‐Indian sector. However, the model offers a picture of the water properties consistent with the observation.