Southern Antarctic Circumpolar Current Research Articles

Baleen whales in the Scotia Sea during January and February 2003

Different species of baleen whales display distinct spatial distribution patterns in the Scotia Sea during the austral summer. Passive acoustic and visual surveys for baleen whales were conducted aboard the RRS James Clark Ross in the Scotia Sea and around South Georgia in January and February 2003. Identified calls from four species were recorded during the acoustic survey including southern right (Eubalaena australis), blue (Balaenoptera musculus), fin (B. physalus) and humpback whales (Megaptera novaeangliae). These acoustic data included up calls made by southern right whales, downswept D and tonal calls by blue whales, two possible types of fin whale downswept calls and humpback whale moans and grunts. Visual detections included southern right, fin, humpback and Antarctic minke whales (B. bonaerensis sp.). Most acoustic and visual detections occurred either around South Georgia (southern right and humpback whales) or south of the southern boundary of the Antarctic Circumpolar Current (ACC) and along the outer edge of the ice pack (southern right, blue, humpback and Antarctic minke whales). Fin whales were the exception, being the only species acoustically and visually detected primarily in the central Scotia Sea, along the southern ACC front. In addition to identifiable calls from these species, two types of probable baleen whale calls were detected: 50Hz upswept and pulsing calls. It is proposed that minke whales may produce the pulsing calls, based on their similarities with minke whale calls recorded in the North Atlantic Ocean. There was an overlap between locations of fin whale sightings and recordings and locations of 50Hz upswept calls in the central Scotia Sea, but these calls were most similar to calls attributed to blue whales in other parts of Antarctica. More study is required to determine if baleen whales produce these two call types, and if so, which species. The efficiency of acoustics and visual surveys varied by species, with blue whales being easier to detect using acoustics, Antarctic minke whales being best detected during visual surveys and other species falling in between these two extremes.

The Observed Seasonal Cycle of Macronutrients in Drake Passage: Relationship to Fronts and Utility as a Model Metric

AbstractThe Drake Passage Time‐series (DPT) is used to quantify the spatial and seasonal variability of historically undersampled, biogeochemically relevant properties across the Drake Passage. From 2004–2017, discrete ship‐based observations of surface macronutrients (silicate, nitrate, and phosphate), temperature, and salinity have been collected 5–8 times per year as part of the DPT program. Using the DPT and Antarctic Circumpolar Current (ACC) front locations derived from concurrent expendable bathythermograph data, the distinct physical and biogeochemical characteristics of ACC frontal zones are characterized. Biogeochemical‐Argo floats in the region confirm that the near‐surface sampling scheme of the DPT robustly captures mixed‐layer biogeochemistry. While macronutrient concentrations consistently increase toward the Antarctic continent, their meridional distribution, variability, and biogeochemical gradients are unique across physical ACC fronts, suggesting a combination of physical and biological processes controlling nutrient availability and nutrient front location. The Polar Front is associated with the northern expression of the Silicate Front, marking the biogeographically relevant location between silicate‐poor and silicate‐rich waters. South of the northern Silicate Front, the silicate‐to‐nitrate ratio increases, with the sharpest gradient in silicate associated with the Southern ACC Front (i.e., the southern expression of the Silicate Front). Nutrient cycling is an important control on variability in the surface ocean partial pressure of carbon dioxide (pCO2). The robust characterization of the spatiotemporal variability of nutrients presented here highlights the utility of biogeochemical time series for diagnosing and potentially reducing biases in modeling Southern Ocean pCO2 variability, and by inference, air‐sea CO2 flux.

Observations of the Antarctic Circumpolar Current Over the Udintsev Fracture Zone, the Narrowest Choke Point in the Southern Ocean

AbstractAn up‐to‐date map of the Antarctic Circumpolar Current (ACC) fronts is constructed from the latest version of mean dynamic topography (MDT) from satellite altimetry and reveals the narrowest ACC width in the Udintsev Fracture Zone (UFZ), with the strongest concentration of the three major ACC fronts within a limited distance as short as 170 km, about 40% narrower than that at Drake Passage. At 144°W, at the entrance of the UFZ, which lies between the Pacific‐Antarctic Ridge (PAR) and its eastwardly offset segment (offset PAR segment), there is a triple confluence of the Subantarctic Front, Polar Front, and Southern ACC Front . Downstream of this longitude, the Subantarctic Front progressively meanders northward over the relatively shallow offset PAR segment before channeling through the Eltanin Fracture Zone, thus diverging from the Polar Front which proceeds through the UFZ. In situ observations from two recent cruises at 144°W confirm the satellite altimetry‐derived frontal circulation in the UFZ region and yield a baroclinic transport relative to the bottom of 113 × 106 m3/s, comparable to that through Drake Passage. The hydrographic sections show no Antarctic bottom water colder than 0.2 °C. Characteristics of major water masses are described, and the implications for their potential downstream modifications at Drake Passage are discussed in terms of the meridional overturning circulation across the ACC. Mesoscale eddy activity with periods shorter than 90 days is predominantly concentrated in the immediate downstream area of the offset PAR segment, suggesting a substantial poleward eddy heat flux there.

Fe sources and transport from the Antarctic Peninsula shelf to the southern Scotia Sea

The Antarctic Peninsula (AP) shelf is an important source of dissolved iron (Fe) to the upper ocean in the southern Scotia Sea, one of the most productive regions of the Southern Ocean. Here we present results from a four-year (2003-2006) numerical simulation using a regional coupled physical-biogeochemical model to assess the Fe sources and transport on the AP shelf and toward the southern Scotia Sea. The model was validated with a suite of data derived from in situ surveys and remote sensing. Model results indicate that sediments in the AP shelf and the South Orkney Plateau (SOP) provide the dominant source of Fe to the upper 500 m in the southern Scotia Sea. Additional Fe inputs to the region are associated with the Antarctic Circumpolar Current (ACC) and the northern limb of the Weddell Gyre, deep-ocean sediment sources, dust deposition, and icebergs. Fe on the AP shelf originates primarily from sediments on the relatively shallow inner shelf and is directly injected into the water column and subsequently transported toward Elephant Island by the confluent shelf currents. Off-shelf Fe export is primarily through entrainment of shelf waters by the ACC’s Southern Boundary frontal jet along the northern edge of the AP shelf, the Hesperides Trough, and the SOP shelf. About 70% of the off-shelf export takes place below the surface mixed layer, and is subsequently re-supplied to the euphotic zone through vertical mixing, mainly during austral fall and winter. The exported shelf-derived Fe is then advected downstream by the ACC and Weddell Gyre and spread over the southern and eastern Scotia Seas. Taken together, shelf Fe export witin top 500 m meets nearly all of the Fe demand of phytoplankton photosynthesis in the southern Scotia Sea. Waters with elevated Fe concentrations in the Scotia Sea are largely restricted to south of the Southern ACC Front.

Assessment of Finescale Parameterizations of Deep Ocean Mixing in the Presence of Geostrophic Current Shear: Results of Microstructure Measurements in the Antarctic Circumpolar Current Region

AbstractThe Southern Ocean is thought to be one of globally significant mixing hotspots. In this study, we carried out simultaneous measurements of microscale turbulence and finescale shear/strain in the south of Australia to assess the validity of the existing finescale parameterizations of deep ocean mixing in the Antarctic Circumpolar Current (ACC) region where geostrophic shear flows coexist with the background internal wavefield. It is found that turbulent dissipation rates are overall small but the internal wavefield is more energetic than the Garrett‐Munk (GM) wavefield. Finescale shear/strain ratio (Rω) well exceeds the GM value in the deep layer south of the Southern ACC Front, suggesting that the local internal wave spectra are significantly biased to lower frequencies. Through the comparison of the directly measured turbulent dissipation rates with those inferred from finescale parameterizations, we find that the Gregg‐Henyey‐Polzin and Ijichi‐Hibiya parameterizations, both of which take into account the distortion of the GM frequency spectrum in terms of Rω, can predict well the turbulent dissipation rates at most of the stations, whereas the shear‐based parameterization (the strain‐based parameterization) tends to overestimate (underestimate) the directly measured turbulent dissipation rates. However, at the observation stations located on the ACC jets, both the Gregg‐Henyey‐Polzin and Ijichi‐Hibiya parameterizations tend to overestimate the turbulent dissipation rates by a factor of ∼3. The most likely cause of the overestimates is spatial anisotropy of the internal wavefield associated with large‐amplitude monochromatic near‐inertial internal waves and/or internal lee waves.

Ocean circulation and frontal structure near the southern Kerguelen Plateau: The physical context for the Kerguelen Axis ecosystem study

Oceanographic observations from ships, floats and remote sensing are used to describe the ocean circulation and frontal structure from the southern Kerguelen Plateau to the Antarctic margin. The flow of the Antarctic Circumpolar Current (ACC) is largely zonal upstream of the Kerguelen Plateau. The southern branch of the Polar Front and the northern branch of the southern ACC front (SACCF) are deflected to the north by the topography and cross the plateau through the Fawn Trough. The southern branch of the SACCF and the southern boundary of the ACC pass to the south of Banzare Bank, through the Princess Elizabeth Trough. These fronts turn sharply to the north on the eastern flank of the plateau, then retroflect to the south before continuing eastward. Satellite altimetry and float trajectories suggest the ACC regime to the east of the plateau is eddy-rich and characterised by extensive meandering of the ACC fronts, in contrast to the low eddy energy and zonal flow observed west of the plateau. The Antarctic Slope Current flows west along the upper continental slope, with hydrographic data indicating some of the flow turns offshore and feeds the Southern Boundary flow in the Western Boundary Current. Flow over the plateau itself is relatively quiescent and floats in this region have a long residence time (> 1 year). The regional circulation is closely associated with the distribution of biological productivity in surface waters, as revealed in satellite measurements of ocean colour, including high productivity in subpolar waters, along the eastern edge of the plateau and over the Antarctic continental shelf, moderate productivity over the plateau, and low productivity in ACC waters in the Fawn Trough and north of the SACCF.

Heat fluxes across the Antarctic Circumpolar Current in Drake Passage: Mean flow and eddy contributions

AbstractIn contrast to a long‐standing belief, observations in the Antarctic Circumpolar Current (ACC) show that mean velocity vectors rotate with depth, thus suggesting a possible importance of the time‐mean flow for the local poleward heat transport. The respective contributions of the eddy and mean flows to the heat flux across the ACC in Drake Passage (DP) are investigated using recently acquired and historical time series of velocity and temperature from a total of 24 current meter moorings and outputs of a high‐resolution (1/12°) model with realistic topography. Only 11 out of the 24 depth‐integrated eddy heat flux estimates are found to be significant, and they are poleward. Model depth‐integrated eddy heat fluxes have similar signs and amplitudes as the in situ estimates at the mooring sites. They are mostly poleward or nonsignificant, with amplitude decreasing to the south. The cross‐stream temperature fluxes caused by the mean flow at the moorings have a sign that varies with location and corresponds to the opposite of the vertical velocity estimates. The depth‐integrated temperature fluxes due to the mean flow in the model exhibit small spatial scales and are of opposite sign to the bottom vertical velocities. This suggests that the rotation of the mean velocity vectors with depth is mainly due to bottom topography. The rough hilly topography in DP likely promotes the small‐scale vertical velocities and temperature fluxes. Eddy heat fluxes and cross‐stream temperature fluxes are integrated over mass‐balanced regions defined by the model transport streamlines. The contribution of the mean flow to the ocean heat fluxes across the Southern ACC Front in DP (covering about 4% of the circumpolar longitudes) is about four times as large as the eddy heat flux contribution and the sum of the two represent on the order of 10% of the heat loss to the atmosphere south of 60°S.

Surface exchange between the Weddell and Scotia Seas

Within Drake Passage, the southern flank of the Antarctic Circumpolar Current (ACC) hosts the ventilation of deep water, the injection of Antarctic shelf waters and interactions between westward and eastward boundary currents. This exchange is explored through the trajectories of forty surface drifters released in January 2012 in the northwestern Weddell Sea. The drifters detail Lagrangian transport pathways between the eastern Antarctic Peninsula and sites of elevated chlorophyll in the Scotia Sea. ACC frontal currents, in particular the Southern ACC Front, act as dynamical transport barriers to the drifters and influence surface chlorophyll distributions, indicating that ACC fronts partition Weddell source waters in the Scotia Sea. Interannual fluctuations in surface chlorophyll in the south Scotia Sea and the northern Weddell Sea covary. This suggests that Scotia Sea ecosystem dynamics are linked to water properties injected from the tip of the Antarctic Peninsula and respond to Weddell Gyre circulation changes.

On the Relationship between Southern Ocean Overturning and ACC Transport

Abstract The eddy field in the Southern Ocean offsets the impact of strengthening winds on the meridional overturning circulation and Antarctic Circumpolar Current (ACC) transport. There is widespread belief that the sensitivities of the overturning and ACC transport are dynamically linked, with limitation of the ACC transport response implying limitation of the overturning response. Here, an idealized numerical model is employed to investigate the response of the large-scale circulation in the Southern Ocean to wind stress perturbations at eddy-permitting to eddy-resolving scales. Significant differences are observed between the sensitivities and the resolution dependence of the overturning and ACC transport, indicating that they are controlled by distinct dynamical mechanisms. The modeled overturning is significantly more sensitive to change than the ACC transport, with the possible implication that the Southern Ocean overturning may increase in response to future wind stress changes without measurable changes in the ACC transport. It is hypothesized that the dynamical distinction between the zonal and meridional transport sensitivities is derived from the depth dependence of the extent of cancellation between the Ekman and eddy-induced transports.

Frontal zones, temperature gradient and depth characterize the foraging habitat of king penguins at South Georgia

Investigating the responses of marine predators to oceanographic structures is of key importance for understanding their foraging behaviour and reproductive success. Using Global Positioning System (GPS) and Time-Depth-Temperature-Recorder (TDR) tags, we investigated how king penguins breeding at South Georgia explore their foraging area over the summer sea- son in both the horizontal and vertical dimensions. We determined how horizontal habitat use may relate to different Antarctic Circumpolar Current (ACC) frontal zones and associated thermal structuring of the water column. To study the penguins' use of the water column, we examined for- aging niches defined by temperature, temperature gradient and depth, and explored the impor- tance of these thermal properties for prey pursuit. King penguins foraged within the Polar Front (PF) and its southern edges during incubation, and the Antarctic Zone (AAZ) and Southern ACC Front (SACCF) during brooding. Foraging niches became more distinct with the advancing sum- mer season, defined by strong thermal gradients at shallow depths in the AAZ, and weak gradi- ents at greater depths in the SACCF. These niches indicate foraging in the sub-thermocline Winter Water (WW) in the AAZ, and in deep WW and Circumpolar Deep Water (CDW) at the SACCF. The influence of different ACC frontal zones in the area to the north of South Georgia appears to provide for a horizontally and vertically segregated environment. The presence of optional foraging areas and niches close to the colony clearly play an important role in these king penguins' foraging success.

Fronts and habitat zones in the Scotia Sea

The fronts in the Antarctic Circumpolar Current (ACC) comprise the Southern ACC Front (SACCF), the Polar Front (PF) and Sub-Antarctic Front (SAF), which, together with the Southern Boundary (SB), separate zones with distinct water mass properties in the Southern Ocean. They are important for describing habitat ranges of oceanic species over the range of trophic levels and are also key habitat zones in their own right for a variety of species. Frontal positions were studied along a transect running S to N across the Scotia Sea during three hydrographic cruises in spring 2006, summer 2008 and autumn 2009. Subsurface data from these cruises and Argo data are here combined with Aviso satellite absolute dynamic topography data to locate these fronts, facilitating study of their ecological significance within the Scotia Sea. Northern and southern dynamic height limits are found for each front, as opposed to the more usual one dimensional line. Thus the SB was found between −132 to −116 dyn cm, the SACCF between −115 to −99 dyn cm and the PF between −71 to −45 dyn cm. Argo data allow the sensitivity of the dynamic heights found to be tested against seasonal, interannual and longitudinal effects and each is found to be small. Although the SAF lay beyond the northern extent of the transect we sampled along, Argo profiles allowed us to determine its limits as lying between −5 and +29 dyn cm. The dynamic height limits found are thus suitable for assessing the location, in relation to fronts, of any sample or tracked animal within this area. The chlorophyll distribution across the Scotia Sea is assessed relative to frontal positions and known iron sources. The physical conditions (sea surface temperature, sea ice and mixed layer depth) along the sampling transect are also presented, allowing the oceanographic conditions of the stations occupied across the Scotia Sea to be assessed.

Two repeat crossings of Drake Passage in austral summer 2006: Short-term variations and evidence for considerable ventilation of intermediate and deep waters

A high-resolution, full-depth, hydrographic section across Drake Passage with tracers and horizontal velocity measurements was repeated within three weeks in austral summer 2006. In-situ data (temperature, salinity, dissolved-oxygen and horizontal current velocities) are used to determine the location of the fronts and eddies of the Antarctic Circumpolar Current (ACC) inferred by Barré et al. (2011) from 7-day composite dynamic topography maps along the transect. Frontal displacements are about 40 km (2 stations) along the section. Considerable differences in properties between the two sections are observed throughout the whole water column with values as high as 0.2 °C in temperature, 0.01 in salinity, 0.03 kg m −3 in neutral density and 10 μmol kg −1 in dissolved-oxygen concentration found below a depth of 3000 m. Only part of the differences is attributable to frontal or eddy displacements along the section. The other part results from the spatial heterogeneity of water properties upstream the section and the funnelling of the flow due to the topographic constraints of the Shackleton Fracture Zone (SFZ). The considerable short-term differences in water properties in rather large-scale structures that cannot be accounted for by frontal motions along the section call for caution when interpreting differences in hydrographical properties obtained by oceanographic cruises that are years apart in terms of climatic signals. The two sections show efficient ventilation of Antarctic Intermediate Water (AAIW) by Winter Water (WW) in the Yaghan Basin, with an estimated eddy-enhanced subduction rate of over 170 m year −1. The SFZ constitutes a barrier, causing the two Southern ACC Fronts (SACCF) branches to separate by about 400 km and creating sheltered conditions in partial isolation from the ACC, while promoting an active recirculation region in the Ona Basin. This active recirculation, marked by cyclonic eddies carrying cold, fresh and oxygenated water from south of the Southern Boundary of the ACC, causes effective ventilation of the whole Circumpolar Deep Water (CDW) density range. The differences in small-scale structures between the two sections deserve further analyses.

Near-Surface Eddy Heat and Momentum Fluxes in the Antarctic Circumpolar Current in Drake Passage

Abstract The authors present new estimates of the eddy momentum and heat fluxes from repeated high-resolution upper-ocean velocity and temperature observations in Drake Passage and interpret their role in the regional Antarctic Circumpolar Current (ACC) momentum balance. The observations span 7 yr and are compared to eddy fluxes estimated from a 3-yr set of output archived from an eddy-resolving global Parallel Ocean Program (POP) numerical simulation. In both POP and the observations, the stream-averaged cross-stream eddy momentum fluxes correspond to forcing consistent with both a potential vorticity flux into the axis of the Subantarctic Front (SAF) and a sharpening of all three main ACC fronts through Drake Passage. Further, the POP analysis indicates that the mean momentum advection terms reflect the steering of the mean ACC fronts and are not fully balanced by the eddy momentum forcing, which instead impacts the strength and number of ACC fronts. The comparison between POP and observed eddy heat fluxes was less favorable partly because of model bias in the water mass stratification. Observed cross-stream eddy heat fluxes are generally surface intensified and poleward in the ACC fronts, with values up to approximately −290 ± 80 kW m−2 in the Polar and Southern ACC Fronts. Interfacial form stresses FT, derived from observed eddy heat fluxes in the SAF, show little depth dependence below the Ekman layer. Although FT appears to balance the surface wind stress directly, the estimated interfacial form stress divergence is only an order of magnitude greater than the eddy momentum forcing in the SAF. Thus, although the eddy momentum forcing is of secondary importance in the momentum balance, its effect is not entirely negligible.

Testing early life connectivity using otolith chemistry and particle-tracking simulations

We measured the otolith chemistry of adult Scotia Sea icefish ( Chaenocephalus aceratus ), a species with a long pelagic larval phase, along the Antarctic Circumpolar Current (ACC) and compared the chemistry with simulated particle transport using a circulation model. Material laid down in otolith nuclei during early life showed (i) strong heterogeneity between the Antarctic Peninsula and South Georgia consistent with a population boundary, (ii) evidence of finer-scale heterogeneity between sampling areas on the Antarctic Peninsula, and (iii) similarity between the eastern and northern shelves of South Georgia, indicating a single, self-recruiting population there. Consistent with the otolith chemistry, simulations of the large-scale circulation predicted that particles released at depths of 100–300 m on the Antarctic Peninsula shelf during spring, corresponding to hatching of icefish larvae from benthic nests, are transported in the southern ACC, missing South Georgia but following trajectories along the southern Scotia Ridge instead. These results suggest that the timing of release and position of early life stages in the water column substantially influence the direction and extent of connectivity. Used in complement, the two techniques promise an innovative approach for generating and testing predictions to resolve early dispersal and connectivity of populations related to the physical circulation of oceanic systems.

Acoustic characterisation of the broad-scale distribution and abundance of Antarctic krill (Euphausia superba) off East Antarctica (30-80°E) in January-March 2006

Abstract A large-scale oceanographic survey (BROKE-West) was undertaken off East Antarctica in the austral summer of 2005/06. Throughout the survey, multi-frequency echosounder data and ancillary environmental data were collected to determine the distribution and abundance of Antarctic krill (Euphausia superba) and to explore its broad relationship with the bio-physical environment. The acoustic data were analysed using three different methods to provide measurements of krill abundance that can be set in context with previous studies. Based on the most recently developed acoustic method, the mean biomass-density of krill across the survey area (1.3 million km2) was estimated to be 24 g m−2. Total biomass was estimated to be 28.75 million tonnes (Mt) with a coefficient of variation (CV) of 16.18%. This biomass estimate has been used by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to update the precautionary catch limit for krill in this area (CCAMLR Division 58.4.2) from 0.450 to 2.645 Mt. Overall, krill were widely distributed at relatively low densities: 25% of the 2-km along-track echo-integration intervals were devoid of krill, 50% registered densities of 1 g m−2 of krill or less and 75% registered densities of 12 g m−2 or less. Mean densities were highest in the waters to the south of the Southern Boundary (SB) of the Antarctic Circumpolar Current (ACC), while the waters to the north of the Southern ACC Front (SACCF) were almost devoid of krill. Half of the cumulative krill density across the survey was found within 80 km of the 1000 m isobath (the shelf break), and 40% within 40 km. This was mostly due to particularly high densities (up to 4400 g m−2) around the shelf break on 3 of the 11 transects surveyed. The majority of acoustic krill detections were in the top 100 m of the water column, centred around 50 m depth.

The circulation and water masses of the Antarctic shelf and continental slope between 30 and

Abstract The circulation and water masses from the Antarctic continental shelf to 62 ∘ S between 30 and 80 ∘ E are described using hydrographic data collected on seven hydrographic sections during the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) experiment. The eastern limb of the Weddell Gyre dominates circulation between 30 and 40 ∘ E , and is significantly cooler and fresher than the region to the east. The Antarctic Circumpolar Current (ACC) extends from the north into the survey region east of 40 ∘ E , reaching as far south as 65 . 5 ∘ S at 60 ∘ E . This results in increasing observed maximum temperature and salinities progressively towards the east, peaking at 80 ∘ E due to the intrusion of the southern ACC Front (sACCF) to 63 ∘ S . This southward extension is steered by the southern end of Kerguelen Plateau, causing a horizontal shear of over 0.15 m s - 1 between the eastward ACC and westward-flowing Antarctic Slope Current (ASC). The ASC is observed at all six meridional sections immediately north of the shelf break. It is strongly barotropic and transports a total of 15.8 ± 7.4 Sv westwards, while the bottom referenced baroclinic component only contributes 1.3 ± 0.3 Sv . At each section this current intensifies to a narrow westward ‘jet’ with absolute velocities up to 0.3 m s - 1 over the steepest shelf slope gradients. At 70 ∘ E a ‘V’ shape is observed in the ASF. This, and the nearby presence of denser shelf water and ice-shelf water, is characteristic of Antarctic Bottom Water (AABW) formation, but no new AABW is found on this section. Instead, significantly warmer, saltier and less oxygenated AABW to the east and newly formed AABW high on the continental slope immediately to the west suggest a formation region just west of 70 ∘ E . This newly formed AABW progressively becomes warmer and saltier west of 60 ∘ E and is observed extending offshore and moving westward below eastward flowing water masses. ACC frontal positions are found to be 1–2° farther north in the survey region than suggested by historical climatology.

Direct observations of the ACC transport across the Kerguelen Plateau

Major pathways and transport of the Antarctic Circumpolar Current (ACC) crossing the Kerguelen Plateau were directly observed during the 2009 Track cruise. The net eastward transport to the south of the Heard/McDonald Islands is estimated as 56 Sv (1 Sv = 106 m3 s−1), 43 Sv of which is tightly channelled into the Fawn Trough that appears as a predominant cross‐plateau gateway of circumpolar flow associated with the Southern ACC Front (SACCF). There are also two secondary passages, with one (6 Sv) being attached to the nearshore slope just south of the Heard/McDonald Islands and the other (7 Sv) passing through the northern Princess Elizabeth Trough. With an additional 2 Sv inferred just south of the Kerguelen Islands, the transport across the entire plateau amounts to 58 Sv, accounting for ∼40% of the total ACC transport transiting through the region, 147–152 Sv, quantities consistent with other independent estimates in the Indian sector of the Southern Ocean.

Antarctic Circumpolar Current frontal system in the South Atlantic: Monitoring using merged Argo and animal‐borne sensor data

We describe large‐scale features of the Antarctic Circumpolar Current (ACC) in the Atlantic part of the Southern Ocean by merging Argo data and data obtained by novel animal‐borne CTD sensors. Twenty one of these CTD‐Satellite Relay Data Loggers (CTD‐SRDLs) were attached to Southern elephant seals (Mirounga leonina) on South Georgia. The merged data yield unified gridded hydrogaphic fields with high temporal and spatial resolution, enabling the determination of features absent in each of the data sets separately. The structure and variability of the frontal field revealed by this data set were compared with those in daily quarter‐degree, optimally interpolated sea surface temperature fields and fields of weekly gridded sea level anomaly. In general, the frontal positions derived using our data set are in agreement with previous work, especially where the pathways are constrained by topography, e.g., at the North Scotia Ridge and the South Scotia Ridge. However, with the improved temporal and spacial resolution provided by the CTD‐SRDLs, we were able to observe some novel features. All frontal positions are more variable than previously indicated across the Scotia Sea and west of the Mid‐Atlantic Ridge on seasonal time scales. The merged data set shows the temporal variability of the Southern ACC Front (SACCF) north of South Georgia and in its position east of the island, where the SACCF lies further north than has been suggested in previous work. In addition, the Subantarctic Front crosses the Mid‐Atlantic Ridge about 400 km further north when compared to previous work.

Western Antarctic Peninsula physical oceanography and spatio–temporal variability

Abstract This study focuses on 12 years of physical oceanography data, collected during the Palmer, Antarctica, Long-Term Ecological Research program (PAL LTER) over the continental margin of the western Antarctic Peninsula (WAP). The dataset offers the most long-lived consistent CTD-gridded observations of Antarctic waters collected anywhere in the Southern Ocean. The physical characteristics, water column structure and spatio–temporal variability of the various properties are examined for physically consistent and ecologically important patterns and modes of variability. Unique findings of note include: (1) The average annual ocean heat flux (to the atmosphere) over the continental shelf shows a decreasing trend through time averaging 0.6 W m−2 yr−1, with an annual average ocean heat flux of ∼19 W m−2. The ocean heat content over the shelf shows a linearly increasing trend of 2.6×107 J m−2yr−1, due predominantly to increased upwelling of warm Upper Circumpolar Deep Water (UCDW) onto the shelf with a small contribution due to a slight warming of UCDW (but over longer time scales (50 yr), the warming of UCDW dominates), (2) optimal multi-annual average vertical turbulent diffusivity coefficient (kz) is ∼8.5×10−5 m2s−1, determined by inversion considering warming of trapped remnant winter mixed layer water, (3) the water masses in the grid are well separated according to bathymetrically controlled features, dividing the sample domain into 3 sub-regions: slope, shelf and coastal waters; (4) the Antarctic Circumpolar Current (ACC) was always present along the shelf-break (consistent with the Orsi et al. [1995. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Research I 42 (5), 641–673.] climatology) where UCDW shows its farthest southern extent and forms the Southern ACC Front (SACCF). The spatio–temporal variability of the delivery and distribution of ocean heat is dictated by the dynamics that are consistent with changes in the state of ENSO (La Nina drives enhanced upwelling in this region) and in the strength of the Southern Annular Mode (SAM; +SAM drives a local response similar to that of La Nina). The large 1997–1998 El Nino, followed by the transition to the strong La Nina of 1998–1999 (amplified by a large +SAM) introduced a regime shift on the shelf, resulting in the elimination of ∼0.5 m of sea ice melt (presumably from the loss of sea ice being grown). 2002 was an anomalous year coinciding with an extraordinary storm forcing driving a 4.5σ increase in the heat content on the shelf. These jumps coincide with considerable changes in sea ice distribution as well. Pure UCDW on the shelf is primarily restricted to the deep canyons, with occasional appearances on the shelf floor near the middle of the grid. Anomalies in summer sea surface temperatures reflect wind strength (stronger winds mixing more cold winter water to the surface, with cooler SST; light winds, the opposite).

Deep western boundary current and southern frontal systems of the Antarctic Circumpolar Current southeast of the Kerguelen Plateau

The structures of the deep western boundary current and southern frontal systems of the Antarctic Circumpolar Current (ACC) southeast of the Kerguelen Plateau were investigated. Top‐to‐bottom hydrography was conducted around 58°S, and two underway observations were made around 56°S and 61°S. Middepth floats were also used to describe the subsurface flow and temperature fields. Along the 58°S section, a cold dome was located just off the plateau, and northwestward and southeastward flows were found in its western and eastern sides. The observed flow structure was highly barotropic. The maximum volume transport of the bottom water was 22 Sv equatorward, including a possible cyclonic recirculation of 10 Sv. The transport of the deep recirculation can be explained by the potential vorticity homogenization in the closed isopachs formed by the bottom slope off the plateau and the overlying ACC flowing southeastward through the Fawn Trough. The spatial scale of the cold dome, which showed a signature of the southern ACC front, was around 100 km at 58°S, and the similar cold domes were also found around 56°S and 61°S, along with a signature of the southern boundary of ACC around 61°S. The sharp excursion of the southern ACC front was also confirmed by the middepth float trajectories and the distribution of temperature maximum. This indicates the equatorward excursion up to 54°S, with the warmer ACC water just east of the southern frontal systems.