Antarctic Circumpolar Current Front Research Articles

Drivers of phytoplankton distribution, abundance and community composition off East Antarctica, from 55-80°E (CCAMLR Division 58.4.2 East)

Southern Ocean phytoplankton form the base of the Antarctic food web, influencing higher trophic levels through biomass and community structure. We examined phytoplankton distribution and abundance in the Indian Sector of the Southern Ocean during austral summer as part a multidisciplinary ecosystem survey: Trends in Euphausiids off Mawson, Predators and Oceanography (TEMPO, 2021). Sampling covered six meridional transects from 55-80°E, and from 62°S or 63°S to the ice edge. To determine phytoplankton groups, CHEMTAX analysis was undertaken on pigments measured using HPLC. Diatoms were the dominant component of phytoplankton communities, explaining 56% of variation in chlorophyll a (Chl a), with haptophytes also being a major component. Prior to sampling the sea ice had retreated in a south-westerly direction, leading to shorter ice-free periods in the west (< 44 days, ≤65°E) compared to east (> 44 days, ≥70°E), inducing a strong seasonal effect. The east was nutrient limited, indicated by low-iron forms of haptophytes, and higher silicate:nitrate drawdown ratios (5.1 east vs 4.3 west), pheophytin a (phaeo) concentrations (30.0 vs 18.4 mg m-2) and phaeo:Chl a ratios (1.06 vs 0.53). Biological influences were evident at northern stations between 75-80°E, where krill “super-swarms” and feeding whales were observed. Here, diatoms were depleted from surface waters likely due to krill grazing, as indicated by high phaeo:Chl a ratios (> 0.75), and continued presence of haptophytes, associated with inefficient filtering or selective grazing by krill. Oceanographic influences included deeper mixed layers reducing diatom biomass, and a bloom to the north of the southern Antarctic Circumpolar Current Front in the western survey area thought to be sinking as waters flowed from west to east. Haptophytes were influenced by the Antarctic Slope Front with high-iron forms prevalent to the south only, showing limited iron transfer from coastal waters. Cryptophytes were associated with meltwater, and greens (chlorophytes + prasinophytes) were prevalent below the mixed layer. The interplay of seasonal, biological and oceanographic influences on phytoplankton populations during TEMPO had parallels with processes observed in the BROKE and BROKE-West voyages conducted 25 and 15 years earlier, respectively. Our research consolidates understanding of the krill ecosystem to ensure sustainable management in East Antarctic waters.

Deep Winter Mixed Layer Anchored by the Meandering Antarctic Circumpolar Current: Cross-Basin Variations

Abstract A deep winter mixed layer forms north of the Antarctic Circumpolar Current (ACC) in the Indo-Pacific sectors, while the mixed layer depth (MLD) is shallow in the Atlantic. Using observations and a global atmospheric model, this study investigates the contribution of surface buoyancy flux and background stratification to interbasin MLD variations. The surface heat flux is decomposed into broad-scale and frontal-scale variations. At the broad scale, the meandering ACC path is accompanied by a zonal wavenumber-1 structure of sea surface temperature (SST) with a warmer Pacific than the Atlantic; under the prevailing westerly winds, this temperature contrast results in larger surface heat loss facilitating deeper MLD in the Indo-Pacific sectors than in the Atlantic. In the Indian sector, the intense ACC fronts strengthen surface heat loss compared to the Pacific. The surface freshwater flux pattern largely follows that of evaporation and reinforces the heat flux pattern, especially in the southeast Pacific. A diagnostic relationship is introduced to highlight the role of ACC’s sloping isopycnals in setting a weak submixed layer stratification north of ACC. This weak stratification varies in magnitude across basins. In the Atlantic and western Indian Oceans where the ACC is at a low latitude (∼45°S), solar heating, intrusions of subtropical gyres, and energetic mesoscale eddies together maintain relatively strong stratification. In the southeast Pacific, in comparison, the ACC reaches the southernmost latitude (56°S), far away from the subtropical front. This creates weaker stratification, allowing deep mixed layers to form, aided by surface buoyancy loss.

Revisiting circulation and water masses over the East Antarctic margin (80–150°E)

Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150°E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150°E), the poleward shift of the southern boundary of the ACC (50–120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1–1.6 °C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (∼0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10–20 μmol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112–140°E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 °C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m3) was found to the east of Mertz Polynya (142–148°E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin.

CO2 sink and source zones delimited by marine fronts in the Drake Passage

Net sea-air CO2 fluxes (FCO2) in the Drake Passage (DP) were studied at a climatological scale (1999–2019) using observations from the Surface Ocean CO2 Atlas (SOCAT) database. Based on the monthly climatological position of the main circumpolar fronts of the DP (the Subantarctic Front (SAF), the Polar Front (PF) and the Southern Antarctic Circumpolar Current Front (SACCF)) and the thermal and nonthermal contributions to FCO2, we present a regional subdivision into different regimes that provide new insights into the processes controlling these fluxes. Our results indicate that the region in the north of SAF (R1) behaves as an annual CO2 sink (-1.3 ± 1.0 mmol m−2 d−1); this sink weakens between SAF-PF (R2) and PF-SACCF (R3) and the region south of SACCF (R4) acts as an annual CO2 source (2.2 ± 3.3 mmol m−2 d−1). The annual mean CO2 uptake in DP is 1.3 ± 15.5 Tg C yr-1. Analysis of thermal (TE) and nonthermal (nonTE) effects on seasonal sea surface CO2 partial pressure (pCO2sw) variability indicates that DP is mainly dominated by nonTE. Results emphasize that carbon fluxes are driven by mesoscale and submesoscale processes north of the PF and by the upwelling of Upper Circumpolar Deep Waters in the Antarctic boundary of the DP, while seasonal patterns are mostly modulated by local factors such as nutrient availability, biological activity and ice cover.

Turbulence Across the Antarctic Circumpolar Current in the Indian Southern Ocean: Micro‐Temperature Measurements and Finescale Parameterizations

AbstractTurbulence structures across the Antarctic circumpolar current (ACC) in the Indian Ocean at around 50°E were revealed using microstructure measurements. Depth‐averaged turbulent energy dissipation rates (ε) in 320 m segments estimated using a conductivity‐temperature‐depth (CTD)‐attached micro‐temperature fast‐response thermistor (FP07) were well reproduced using existing finescale parameterizations excluding near‐surface and near‐bottom 160 m. The spatial variations of ε are explained by density stratification N2, bottom topographic roughness, and depth‐averaged current speed U. Diapycnal diffusivity Kρ(=0.2εN−2) is found proportional to the squared internal wave energy () and increases toward the bottom, especially over rough bottom topography. The decay scale height is >1000m and the turbulence remains large even in the upper pycnocline. Existing shear‐based finescale parameterizations were generally consistent with the FP07 measurements but tended to overestimate in the areas with large shear‐strain ratio Rω and low ε in the Continental Zone (CZ) south of the ACC fronts (58–65°S). This overestimation is improved by using the strain‐based finescale parameterization with a constant Rω = 3.

Composition patterns of surface mesozooplankton in the zonal fronts of Drake Passage

Abstract Zooplankton is the main food source for higher trophic levels in marine environments. In the Southern Ocean, the distribution of zooplankton is related to the physical gradient of the Antarctic Circumpolar Current (ACC) fronts. Our objective was to determinate the distribution of mesozooplankton in relation to the ACC fronts in the Drake Passage. Samples were collected with the Continuous Plankton Recorder in two transects. Mesozooplankton was associated with environmental variables. High mesozooplankton abundances were recorded in the Subantarctic Front in 2017 and in the Antarctic Zone in 2016. A total of 81 taxa and 23 species in 2016 and 31 in 2017 were identified. Copepoda was the most abundant group (89%), and Centropages brachiatus (3 872 ind. m−3) and Oithona spp. (2 916 ind. m−3) were the most abundant copepod taxa. Mesozooplankton abundance and composition were influenced by front variability. Taxa were contracted northward on 2016 and displaced southward in 2017 and linked to chlorophyll a (Chl a) values recorded in the coastal shelf of South America. Chl a values registered close to Antarctic Peninsula were not linked to mesozooplankton abundance. Changes in abundances of certain taxa may reflect distinct climate events. These changes may impact the availability of prey for higher trophic levels, either through displacement for food or food availability.

Size-fractionated phytoplankton biomass and primary production in the eastern Indian sector of the Southern Ocean in the austral summer 2018/2019

The Southern Ocean significantly contributes to primary production in the global ocean. In this study, size-fractionated primary production and chlorophyll a (Chl a), which are not well elucidated, were determined in the eastern Indian sector in the austral summer of 2018–2019. The Southern Antarctic Circumpolar Current Front, Southern Boundary of the Antarctic Circumpolar Current (SB), and Antarctic Slope Front (ASF) were set as study areas, and sampling stations were divided into the three areas: to the north of the SB, the area between the SB and ASF, and to the south of the ASF. Primary production was 85.1–385.7 mg C m−2 day−1 north of the SB, 131.4–192.5 mg C m−2 day−1 between the SB and ASF, and 95.6–472.7 mg C m−2 day−1 south of the ASF. The Chl a biomass integrated within the euphotic zone in each area was 19.8–46.1 mg m−2, 10.3–19.9 mg m−2, and 17.0–77.9 mg m−2, respectively. Although no significant difference was observed in the primary production and Chl a biomass among the three areas, the biomass tended to be lower between the SB and ASF than in the other two areas. The large phytoplankton (>10 μm) significantly contributed to the primary production and the Chl a biomass at every station north of the SB, accounting for approximately 65–90%. In the other two areas, there were some stations where large phytoplankton accounted for 60–85%, and others where large phytoplankton did not dominate. The flow to the secondary production was one order of magnitude higher to the north of the SB than to the south of the SB. The time since sea-ice melt possibly plays an important role in determining the dominant phytoplankton size and hence size structure of phytoplankton in the survey area. The features in phytoplankton size structure observed north and south of the SB may not be unique to the areas. High phytoplankton biomass and primary production were also observed in the west of 130°E, which was similar to the results of the previous study. A time lag between the west and east surveys, about one month, may have caused the high-west and low-east patterns.

Biomass of Antarctic krill (Euphausia superba) in the eastern Indian sector of the Southern Ocean (80–150°E) in the 2018–19 austral summer

Biomass of Antarctic krill (Euphausia superba) in the eastern Indian sector of the Southern Ocean (80–150°E, corresponding to the longitudinal range of CCAMLR Division 58.4.1) in the austral summer of 2018–19 was estimated using data from a calibrated echosounder and biological sampling. This study was carried out as a part of the multidisciplinary ecosystem survey (KY1804) by Japanese research vessel Kaiyo-maru. The krill biomass was estimated using an up-to-date standard method of CCAMLR, the “swarm-based method”. Biomass of krill in the area had not been estimated since 1996 (BROKE survey). KY1804 covered a total of 4680 km of the surveyed distance in the surveyed area of 0.909 million km2. Krill were mainly distributed between the Southern Antarctic Circumpolar Current Front and the Antarctic Slope Front. The krill biomass during KY1804 was estimated as 4.325 million tonnes (CV = 17.0%), comparable with the 1996 BROKE estimate (4.83 million tonnes, CV = 17.0%); however, direct comparison is difficult because of differences in (1) biomass estimation methods; (2) timing of the surveys (KY1804 commenced about 40 days earlier in the year than BROKE); and (3) areal coverage, primarily because of different positions of the sea ice edge, especially in the western part of the area. There was no noticeable difference of krill biomass between day and night. This study provides a 2018–19 biomass estimate for Antarctic krill in the eastern Indian sector of the Southern Ocean, which is a basis for studies of this species as well as the Southern Ocean ecosystem.

Biological Uptake, Water Mass Mixing and Scavenging Limit the Transport of Manganese‐Rich Waters From the East Antarctic Shelf

AbstractManganese (Mn) is an essential element involved in photosynthesis, yet its concentrations in Southern Ocean open waters are very low, arising from biological uptake and limited external inputs. At southern latitudes, waters overlying the Antarctic shelf are expected to have much higher Mn concentrations due to their proximity to external sources. In this study, we investigated the potential export of Mn‐rich Antarctic shelf waters toward depleted open Southern Ocean waters. Our results showed that while high Mn concentrations were observed over the shelf, biological uptake decreased dissolved Mn concentrations in surface waters north of the South Antarctic Circumpolar Current Front (<0.1 nmol kg−1), limiting its export. Conversely, mixing between Mn‐rich Antarctic Bottom Waters and Mn‐depleted Lower Circumpolar Deep Waters combined with oxidative and scavenging processes led to a decrease in dissolved Mn concentrations within bottom waters, with distance from the coast. Particulate Mn concentrations also showed a decreasing trend with distance from the coast. A comparison with other Antarctic coastal regions suggests this bottom water Mn removal may be widespread and that East Antarctica may be characterized by lower Mn concentrations compared to other regions. Still, subsurface dissolved Mn maxima (0.3–0.6 nmol kg−1) represented a potential reservoir for surface waters. We hypothesize that these high subsurface values result from external sources near the shelf. Overall, these results suggest that the moderate lateral export of trace metal‐enriched waters contributes to the extremely low and potentially limiting Mn concentrations previously reported further north in this Southern Ocean region.

The Role of the Pacific‐Antarctic Ridge in Establishing the Northward Extent of Antarctic Sea‐Ice

AbstractMonitoring the Antarctic sea‐ice is essential for improving our knowledge of the Southern Ocean. We used satellite sea‐ice concentration data for the 2002–2020 period to retrieve the sea‐ice extent (SIE) and analyze its variability in the Pacific sector of the Southern Ocean. Results provide observational evidence of the recurring formation of a sea‐ice protrusion that extends to 60°S at 150°W during the winter season. Furthermore, we discuss how the Pacific‐Antarctic Ridge (PAR) influences this phenomenon. Our findings show that the northward deflection of the southern Antarctic Circumpolar Current front is driven by the PAR and is associated with the enhanced sea‐ice advance. The PAR also constrains eddy trajectories, limiting their interaction with the sea‐ice edge. These factors, within the 160°W–135°W sector, cause an average SIE increase of 61,000 km2 and 46,293 km2 per year more than the upstream and downstream areas, respectively.

Impact of the climate regime shift around 2000 on recruitment of Antarctic krill at the Antarctic Peninsula and South Georgia

We examined how the climate regime shift around 2000 affected Antarctic krill recruitment variability at the North Antarctic Peninsula (NAP) and South Georgia (SG) during the period of 1990s through to 2000s. We used sea surface temperature (SST) as a proxy of the monthly sea ice extent and related it to the recruitment index (proportion of 1-year-old krill) of each succeeding year for NAP and SG by examining the correlation of the time series before and after 2000. For NAP, recruitment index was negatively correlated with winter SST at NAP in the 1990s but with autumn SST in the 2000s, suggesting that good recruitment was supported by extensive winter sea-ice in the 1990s but by extensive autumn sea-ice in the 2000s. This was attributed to the regime shift featured by winter warming and autumn cooling after 2000. For SG, recruitment was negatively correlated with winter SST at NAP in the 1990s but positively correlated with autumn SST in the southern Scotia Sea (SSS) in the 2000s. According to particle tracking experiment from NAP to SG in relation to the sea-ice extent and position of SACCF (Southern Antarctic Circumpolar Current Front), less sea-ice extent in 2000s meant less larvae distributing as north as SACCF, resulting in much fewer larval being transported to SG. Hence, we hypothesized that the regime shift weakened the connection between NAP and SG, but instead SSS became the main driver of recruitment at SG. We presume the krill population demonstrated adaptability to the regime shift by offsetting the unfavorable winter condition with the favorable autumn condition at NAP and for SG, replacing the source of recruits with those closer to it.

Surface Bacterioplankton Community Structure Crossing the Antarctic Circumpolar Current Fronts.

The Antarctic Circumpolar Current (ACC) is the major current in the Southern Ocean, isolating the warm stratified subtropical waters from the more homogeneous cold polar waters. The ACC flows from west to east around Antarctica and generates an overturning circulation by fostering deep-cold water upwelling and the formation of new water masses, thus affecting the Earth's heat balance and the global distribution of carbon. The ACC is characterized by several water mass boundaries or fronts, known as the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF), and South Antarctic Circumpolar Current Front (SACCF), identified by typical physical and chemical properties. While the physical characteristics of these fronts have been characterized, there is still poor information regarding the microbial diversity of this area. Here we present the surface water bacterioplankton community structure based on 16S rRNA sequencing from 13 stations sampled in 2017 between New Zealand to the Ross Sea crossing the ACC Fronts. Our results show a distinct succession in the dominant bacterial phylotypes present in the different water masses and suggest a strong role of sea surface temperatures and the availability of Carbon and Nitrogen in controlling community composition. This work represents an important baseline for future studies on the response of Southern Ocean epipelagic microbial communities to climate change.

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.

Variability of the Shelf Circulation Around South Georgia, Southern Ocean

AbstractA high‐resolution ocean model is used to characterize the variability of the shelf circulation and cross‐shelf transport around the South Georgia island (SG). The time‐mean shelf circulation consists of a counterclockwise flow with a net onshelf mass flow in the south and a net offshelf mass flow in the north. In the south, the cross‐shelf exchanges show a two‐layer structure with an offshelf flow below 350 m and onshelf flow above. In the north, the cross‐shelf exchanges show a three‐layer structure with the onshelf flow found only between 350 and 50 m. Correlation analysis shows that winds and the Southern Antarctic Circumpolar Current Front (SACCF) current modulate the variability of the shelf circulation and cross‐shelf transport. Local wind stress is significantly correlated with the coastal currents, mid‐shelf jet, and cross‐shelf transports in the upper layer, while the SACCF modulates the shelf and cross‐shelf transports in the southwestern shelf. Likewise, an Empirical Orthogonal Function analysis indicates that the first mode of shelf circulation variability is highly correlated with the SACCF, while the second mode is explained by the local wind stress and significantly correlated with the Antarctic Oscillation. The El Niño Southern Oscillation does not significantly contribute to the shelf circulation but is significantly correlated with the surface temperature variability. The atmospheric teleconnection drives changes in local heat flux, such that warm El Niño conditions over the equatorial Pacific are associated with a cooling of the SG waters. This superposes local signals onto temperature anomalies advected from upstream in the ACC found in previous studies.

Diapycnal and isopycnal mixing along the continental rise in the Australian–Antarctic Basin

Shipboard measurements of temperature, salinity and horizontal current velocity were used to estimate diapycnal and isopycnal diffusivities in the Australian sector of the Southern Ocean between 120∘E and 150∘E near 64∘S, along the climatological positions of the Southern Antarctic Circumpolar Current Front and the Southern Boundary. The diapycnal diffusivities estimated by the finescale parameterization on the neutral density surfaces of 1028.05, 1028.2, and 1028.3 kg m−3 are (9.5±6.8)×10−5, (10.7±2.6)×10−5, and (13.8±3.7)×10−5 m2s−1, respectively (mean ± one standard deviation). At these densities, salinity and temperature decreased with depth but salt fingering was unlikely. Using the water mass transformation framework and a box inverse model, the isopycnal diffusivities were estimated from the diapycnal diffusivities. Climatological estimates of the isopycnal diffusivities in the density ranges of Upper Circumpolar Deep Water, Lower Circumpolar Deep Water, and Antarctic Bottom Water were −30±116, 16±319, and −30±83 m2s−1, respectively. These diffusivities were likely underestimates by 10 to 20 m2s−1 because eddy transport could not be considered in the climatological data. The weak isopycnal diffusivities were not inconsistent with diffusivity suppressed by the frontal jets. The large uncertainties were caused by the uncertainty in the box inverse model and the fact that the size of the box was comparable to the advection speed times the diffusion time scale.

Spatial and temporal variation and distribution of mesozooplankton in the Drake Passage sampled with the continuous Plankton Recorder

Zooplankton is the main link between primary producers and higher trophic levels in marine foods webs. In the Southern Ocean, zooplankton could be key in identifying environmental changes, given rapid warming registered in the area in the last decades. The Drake Passage is a choke point of oceanographic processes, since it is the narrowest and most dynamic path of the Antarctic Circumpolar Current (ACC). The objective of this study was to determine the horizontal spatial variations of mesozooplankton in the Drake Passage relative to Southern Ocean oceanographic fronts and zones. Three transects were sampled using a CPR in the Drake Passage during the austral summer of 2009 and 2010. Temperature and salinity were measured at 1-min intervals with a thermosalinograph and Chlorophyll a data extracted from NASA's ocean color web. Organisms were counted and identified to the lowest level, resulting in 53 taxa, with 24 identified to species, eight to genus and 21 to a higher taxonomic level. Four fronts (Subantarctic Front - SAF, Polar Front - PF, South of ACC Front - SACCF and Peninsula Front -PF*) delimiting five zones (Subantarctic Zone - SAZ, Polar Front Zone - PFZ, Antarctic Zone - AZ, Southern Zone - SZ and Transitional Water Zone with Bellingshausen Sea Influence TBW) were registered according to temperature values. Higher mesozooplankton abundances were recorded in the PFZ (412.0 ± 27.9 ind. m−3) in 2009 and in the AZ (128 ± 12.3 ind. m−3) during 2010. Copepoda were the most abundant group (82%), followed by crustacean eggs (4.3%) and Larvacea (4.0%). Within Copepoda, the most abundant taxa were Calanidae copepodites (33.5 ± 53.4 ind. m−3), Oithona spp. (25.4 ± 27.1 ind. m−3) and Calanus simillimus (19.4 ± 26.1 ind. m−3). Differences in community structure were found between zones with C. simillimus and Thysanoessa macrura contributing most to the dissimilarities between the SAZ and the PFZ, Ctenocalanus citer between the PFZ and AZ, Euphausia superba between zones south of the Polar Front and Salpa thompsoni between the SZ and TBW. Some species or taxa are related to or: indicators of a particular zone. The PFZ was the most diverse and had higher mesozooplankton abundances compared to the other zones. Changes in composition are perceptible compared to previous studies, with a decrease of larger organisms and richness, lack of larger copepods, decrease of krill and increase of S. thompsoni abundances.

Shoaling of abyssal ventilation in the Eastern Indian Sector of the Southern Ocean

Antarctic Bottom Water formation, which ventilates the abyssal layers of the Southern Ocean, is an integral component of the global ocean meridional overturning circulation. Considering evident freshening and density decreases in the source waters, widespread warming in the Southern Ocean suggests a weakening in the Antarctic Bottom Water supply. We demonstrate that the weakening is robust based on water mass warming in the deep and abyssal layers of the Australian-Antarctic Basin, which remained after removing the southward shift effect of the Southern Antarctic Circumpolar Current Front. Moreover, a decrease in apparent oxygen utilisation and reduced warming in the intermediate density layer below Circumpolar Deep Water extended further from the Australian-Antarctic Basin to the South Australian Basin. We suggest that a concurrent weakening in the densest portion and strengthening in the less dense portion shape the multi-basin change in the meridional overturning circulation that originates from the Southern Ocean.

Contrasting life-history traits of two toothfish (Dissostichus spp.) species at their range edge around the South Sandwich Islands

The South Sandwich Islands (SSI), a chain of volcanic islands in the Atlantic sector of the Southern Ocean, are home to two large notothenoid species: the Patagonian toothfish Dissostichus eleginoides and the Antarctic toothfish Dissostichus mawsoni. Both species support valuable fisheries throughout the Southern Ocean under management of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). The SSI region, which is located south of the Southern Antarctic Circumpolar Current Front, has a diverse and distinct biodiversity and it represents a range edge for the distribution of both toothfish species. In this paper we have updated and expanded previous biological analyses with recent data, explored the stock hypotheses and links of these species to other regions, and investigated the role of the SSI archipelago in the life cycles of both toothfish species, where they overlap in their distribution. We conclude that Patagonian toothfish around the SSI are linked to the adjacent South Georgia population, but have some unique characteristics, including faster growth and better somatic condition, possibly reflecting ‘Bergmann's rule’ which states that body size increases with decreasing temperature and increasing latitude. By comparison, the Antarctic toothfish at the SSI appear to be the northern extent of a larger stock connecting further south towards the Antarctic continent. Finally, we consider the relative importance of the SSI in the life cycle of both species, including in the context of climatic changes to this region.

Turbulent Mixing and Lee‐Wave Radiation in Drake Passage: Sensitivity to Topography

AbstractRadiation and breaking of internal lee waves are thought to play a significant role in the energy and heat budget of the Southern Ocean. Open questions remain, however, regarding the amount of energy converted from the deep flows of the Antarctic Circumpolar Current (ACC) into lee waves and how much of this energy dissipates locally. This study estimated the linear lee‐wave energy radiation using a unique 4‐year time series of stratification and near‐bottom currents from an array of Current and Pressure measuring Inverted Echo Sounders (CPIES) spanning Drake Passage. Lee‐wave energy was calculated from two 2D anisotropic and one 1D isotropic abyssal hill topographies. Lee‐wave energy radiation from all three topographies was largest in the Polar Front Zone associated with strong deep meandering of the ACC fronts. Both baroclinic and barotropic instabilities appeared to modulate the conversion to lee waves in the Polar Front Zone. Fine structure temperature, salinity, and velocity profiles at the CPIES locations were used to estimate turbulent dissipation due to breaking internal waves by employing a finescale parameterization. High dissipation near the bottom was consistent with upward‐propagating, high‐frequency lee waves as found by earlier studies. In contrast to idealized numerical predictions of 50% local dissipation of lee‐wave energy, this study found less than 10% dissipated locally similar to some other studies. Improving the representation of the abyssal hills by accounting for anisotropy did not reduce the discrepancy between radiated lee‐wave energy and local dissipation. Instead, alternative fates must be considered for the excess radiated lee‐wave energy.

Changes of Circumpolar Deep Water between 2006 and 2020 in the south-west Indian Ocean, East Antarctica

The climatological distribution of circumpolar deep water (CDW) and its changes in austral summer between 2006 and 2020 are documented using historical hydrographic data and recent measurements in the south-west Indian Ocean, East Antarctica. The CDW is primarily distributed in the northeast of the Cooperation Sea, north of the southern Antarctic Circumpolar Current Front (sACCF), as shown by the CDW thickness (on average > 800 m) and mean depth (maxima >900 m). The CDW layer becomes thinner to the south of sACCF and is less than 100 m thick near the Southern Boundary (SB). The CDW distribution shows significant changes between years. Its extension reached further south in 2020 than in 2006, and there was a positive thickness anomaly with a magnitude of 50–200 m. There was also evidence of warmer (0.1–0.2 °C) and more saline (0.005) CDW in 2020 than 2006 along 70°E and between 62.8° and 63.7°S. Those warm and saline signals extended further south. Warmer and more saline water occupied the subsurface layers between 35° and 69°E at 63.6°S and 64°S, and occurred to the east of 55°E. The region between 55° and 65°E is a major entrance for the CDW poleward extension. Furthermore, we found that the changes of CDW between 2006 and 2020 were dominated by the sACCF shift due to enhanced northwest summer winds. The wind anomalies raised the sea surface height by the strengthening northward Ekman transport. Thus the ACC shifted southward (∼120 km in average), carrying more CDW poleward. A tongue-shaped southward shift of sACCF generated anomalous anticyclonic eddies, advecting CDW southward as far as 66° in 2020.