Subantarctic Front Research Articles

Enhanced eddy activity along the Subantarctic Front under intensified westerly winds

Enhanced eddy activity along the Subantarctic Front under intensified westerly winds

Mesoscale eddies inhibit intensification of the Subantarctic Front under global warming

Abstract Oceanic mesoscale eddies are important dynamical processes in the Southern Ocean. Using high-resolution (~0.1○ for the ocean) Community Earth System Model (CESM-HR) simulations under a high-carbon emission scenario, we investigate the role of mesoscale eddies in regulating the response of the Subantarctic Front (SAF) to global warming. The CESM-HR simulates more realistic oceanic fronts and mesoscale eddies in the Southern Ocean than a coarse-resolution (~1○ for the ocean) CESM. Under global warming, the SAF is projected to intensify. The mean flow temperature advection intensifies the front, whereas the mesoscale-eddy-induced temperature advection and atmospheric dampening play primary (~67%) and secondary (~28%) roles in counteracting the effect of mean flow temperature advection. Our study suggests the importance of mesoscale eddies on inhibiting the SAF intensification under global warming and necessity of mesoscale-eddy-resolving simulations for faithful projection of future climate changes in the Southern Ocean.

Cross-frontal variability of phytoplankton productivity in the Indian sector of the Southern Ocean during austral summer of 2010–2018

Oceanic phytoplankton productivity, which regulates atmospheric CO2, is crucial for unraveling the complexities of the global carbon cycle. Despite its substantial contribution to the global carbon budget and its critical role in anthropogenic carbon sink, the Southern Ocean (SO) remains under-sampled due to logistical challenges. The present study attempts to elucidate the variability of water column primary production (PP) in the Indian Sector of the Southern Ocean (ISSO) by examining associated physicochemical parameters and physiological conditions of phytoplankton that drive this variability. The study revealed the nutrient limitation in the region north of the Subantarctic Front (SAF) and light limitation coupled with intense zooplankton grazing in the region south of the SAF. Coastal waters exhibit higher PP, characterized by the prevalence of large phytoplankton. The SAF displayed maximum productivity among the fronts, while the Polar Front 2 (PF-2) recorded the lowest. The water column PP varies from 27.01 to 960.69 mg C m−2 d−1 in the frontal region, while the coastal waters recorded productivity up to 1083.56 mg C m−2 d−1. Phytoplankton in the frontal regions indicated a stable surface abundance, except north of the Subtropical Front (STF), where the oligotrophic condition fosters the growth of picoplankton, subjected to high grazing by microzooplankton. Conversely, in the colder coastal waters, the phytoplankton experienced physiological acclimation. Model-based estimates of PP highlighted the efficacy of the Carbon-based Production Model (CbPM) in estimating net PP (NPP) in these polar waters, surpassing the Vertically Generalized Production Model (VGPM) and Eppley-VGPM. Notably, all model-based PP estimates significantly improved with in situ chlorophyll as input instead of satellite-retrieved chlorophyll. While the models performed well in the coastal water, their performance was suboptimal in the frontal region. This study advances our understanding of the intricate dynamics of phytoplankton productivity in the SO, offering valuable insights for future research endeavors.

Estimates of the Isotope Effect for Nitrate Assimilation in the Indian Sector of the Southern Ocean

AbstractThe Southern Ocean is a high‐nutrient, low‐chlorophyll (HNLC) region characterized by incomplete nitrate (NO3−) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the 14N‐ versus the 15N‐bearing form of NO3−, quantified as the NO3− assimilation isotope effect (15ε). Previous summertime estimates of the 15ε from HNLC regions range from 4 to 11‰. While culture work has shown that the 15ε varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the 15ε varies in the field. Here we estimate the 15ε from water‐column profile and surface‐water samples collected in the Indian sector of the Southern Ocean—the first leg of the Antarctic Circumnavigation Expedition (December 2016–January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the mid‐to‐late summer Southern Ocean, we estimate a higher 15ε (8.9 ± 0.6‰) for the northern Subantarctic Zone and a lower 15ε (5.4 ± 0.9‰) at and south of the Subantarctic Front. We interpret our data in the context of coincident measurements of phytoplankton community composition and estimates of iron and light stress. Similar to prior work, we find a significant, negative relationship between the 15ε and the average mixed‐layer photosynthetically active radiation flux of 30–100 μmol m−2 s−1, while above 100 μmol m−2 s−1, 15ε increases again. In addition, while we observe no robust relationship of the 15ε to iron availability or phytoplankton community, mixed‐layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude.

Three-Dimensional Structure of Mesoscale Eddies and Their Impact on Diapycnal Mixing in a Standing Meander of the Antarctic Circumpolar Current

Mesoscale eddies are known to enhance diapycnal mixing in the ocean, yet direct observation of this effect remains a significant challenge, especially in the robust Antarctic Circumpolar Current (ACC). To quantify the diapycnal mixing induced by mesoscale eddies in the standing meander of the ACC, satellite altimeter and Argo profile data were combined to composite eddies, where the 1.6 m dynamic height contour was used for the first time instead of the climatological Northern Sub-Antarctic Front (SAFN) to define the northern boundary of the ACC to eliminate the influence of frontal shift. The 3D structures of the composite anticyclonic/cyclonic eddy (CAE/CCE) were obtained. Both the CAE and CCE were similar in shape to Taylor columns, from sea surface to the neutral surface of 28.085 kgm−3 (1689 ± 66 dbar) for the CAE, and from sea surface to 28.01 kgm−3 (1491 ± 202 dbar) for the CCE. On the same neutral surface, the diffusivity (κ) inside the CCE was one to two orders of magnitude higher than that inside the CAE. Vertically, the maximum influence depth of the CCE on κ reached 1200 dbar, while for the CAE, it reached 800 dbar, where κ exceeded O(10−4) m2s−1, and κ gradually decreased from these depths downwards.

Salinity Fronts in the South Atlantic

Monthly climatology data for salinity fronts in the South Atlantic have been created from satellite SMOS sea surface salinity (SSS) measurements taken from 2011–2019, processed at the Barcelona Expert Center of Remote Sensing (BEC), and provided as high-resolution (1/20°) daily SSS data. The SSS fronts have been identified with narrow zones of enhanced horizontal gradient magnitude (GM) of SSS, computed using the Belkin–O’Reilly algorithm (BOA). The SSS gradient fields generated by the BOA have been log-transformed to facilitate feature recognition. The log-transformation of SSS gradients markedly improved the visual contrast of gradient maps, which in turn allowed new features to be revealed and previously known features to be documented with a monthly temporal resolution and a mesoscale (~100 km) spatial resolution. Monthly climatologies were generated and analyzed for large-scale open-ocean SSS fronts and for low-salinity regions maintained by the Rio de la Plata discharge, Magellan Strait outflow, Congo River discharge, and Benguela Upwelling. A 2000 km-long triangular area between Africa and Brazil was found to be filled with regular quasi-meridional mesoscale striations that form a giant ripple field with a 100 km wave length. South of the Tropical Front, within the subtropical high-salinity pool, a trans-ocean quasi-zonal narrow linear belt of meridional SSS maximum (Smax) was documented. The meridional Smax belt shifts north–south seasonally while retaining its well-defined linear morphology, which is suggestive of a yet unidentified mechanism that maintains this feature. The Subtropical Frontal Zone (STFZ) consists of two tenuously connected fronts, western and eastern. The Brazil Current Front (BCF) extends SE between 40 and 45°S to join the subantarctic front (SAF). The STFZ trends NW–SE across the South Atlantic, seemingly merging with the SAF/BCF south of Africa to form a single front between 40 and 45°S. In the SW Atlantic, the Rio de la Plata plume migrates seasonally, expanding northward in winter (June–July) from 39°S into the South Brazilian Bight, up to Cabo Frio (23°S) and beyond. The inner Plata front moves in and out seasonally. Farther south, the Magellan Strait outflow expands northward in winter (June–July) from 53°S up to 39–40°S to nearly join the Plata outflow. In the SE Atlantic, the Congo River plume spreads radially from the river mouth, with the spreading direction varying seasonally. The plume is often bordered from the south by a quasi-zonal front along 6°S. The diluted Congo River water spreads southward seasonally down to the Angola–Benguela Front at 16°S. The Benguela Upwelling is delineated by a meridional front, which extends north alongshore up to 20°S, where the low-salinity Benguela Upwelling water forms a salinity front, which is separate from the thermal Angola–Benguela Front at 16°S. The high-salinity tropical water (“Angola water”) forms a wedge between the low-salinity waters of the Congo River outflow and Benguela Upwelling. This high-salinity wedge is bordered by salinity fronts that migrate north–south seasonally.

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.

Hydrography of the Southern Ocean fronts – The zooplankton link: A detailed study in the Indian Ocean sector

Despite its importance, zooplankton diversity, biomass and abundance are still poorly known in the remote Indian sector of the Southern Ocean (SO). Therefore, systematic observations of zooplankton and hydrography have been made in the Indian sector of the SO, with a focus on Copepoda. We collected the data along 57° E transect during austral summer and examined various oceanographic locations in the central Indian Ocean sector of the SO lying in the east of the Kerguelen Plateau. The contribution of copepod abundance was substantial (85 %) to the total zooplankton biomass across the entire study region. We observed surface stratification in the Subtropical Front (STF) and Subantarctic Front (SAF); on the other hand, subsurface stratification was encountered in the Polar Front (PF) and South of PF (S-PF). As a result, nutrients were lower in warmer waters than in colder waters. On the other hand, zooplankton abundance was higher in colder waters than in warmer waters. The highest zooplankton grazing impact was recorded in the PF zone, where zooplankton removed ∼ 81 % of the daily phytoplankton production, while north of the PF zooplankton grazing was generally low. Four copepod species were shown to be highly connected to oceanic fronts. The vertical distribution of zooplankton abundance and species composition was chiefly driven by physical and biological conditions in the Indian sector of the SO. As a result, our findings indicate that physical processes play an important role in organizing plankton diversity, which ultimately determines the fate of the foodweb structure and the biological carbon pump in the studied area.

Trans-equatorial migration links oceanic frontal habitats across the Pacific Ocean: year-round movements and foraging activity of a small gadfly petrel

Gadfly petrels are among the widest-ranging birds and inhabit oceanic regions beyond the legislative protection of national jurisdictions (the High Seas). Detailed information on breeding phenology, at-sea distributions, and habitat requirements is crucial for understanding threats and designing conservation measures for this highly threatened group. We tracked 10 Stejneger’s petrels Pterodroma longirostris, endemic to Isla Alejandro Selkirk, Juan Fernández Islands in the southeast Pacific Ocean, with geolocator-immersion loggers over two years to examine year-round movements, phenology, habitat use, and activity patterns. Birds conducted round-trip trans-equatorial migrations of 54,725 km to the northwest Pacific Ocean between Hawaii and Japan. Across the boreal summer, birds followed the c. 1000 km northward movement of the North Pacific Transition Zone Chlorophyll Front, before their return migration which took a long detour south toward New Zealand before heading east at 40–50°S, presumably benefitting from Antarctic circumpolar winds. To our knowledge, a comparable triangular migration is unique among seabirds. During the pre-laying exodus, birds traveled southwest to the Sub-Antarctic Front, and unlike congeners, there was no evidence of sexual segregation. Foraging areas during incubation were similar to pre-laying, with trips lasting 13 d and taking birds up to 4810 km southwest of the colony. Petrels spent > 75% of their time flying during breeding and migration, yet flight activity was substantially lower during non-breeding, presumably due to flight feather molt. Birds spent 87% of their time at sea within the High Seas and their apparent preference for oceanic frontal regions demonstrates the importance of protecting these remote habitats.

Sea Ice‐Driven Variability in the Pacific Subantarctic Mode Water Formation Regions

AbstractSubantarctic Mode Water (SAMW) forms north of the Subantarctic Front, in regions of deep winter mixed layers, and is important to the absorption and storage of anthropogenic CO2 and heat. Two SAMW pools exist in the Pacific, a lighter Central mode (CPSAMW), and a denser Southeast mode (SEPSAMW). Both have experienced significant interannual variability in thickness and properties in recent years. We compute mixed layer temperature and salinity budgets for the two SAMW formation regions, to determine the relative contribution of processes driving variability in the properties of mixed layers that subduct to form SAMW. The dominant drivers of temperature and salinity variability are shown to be surface fluxes, horizontal advection, and entrainment of deeper water. Salt advection into each SAMW formation region is found to be strongly correlated with changes in sea ice area in the northern Ross Sea, with lags of up to 2 years. Further correlation is found between meridional salt advection in the southeast Pacific formation regions, and sea ice area in the northern Amundsen/Bellingshausen seas, suggesting that freshwater derived from sea ice melt reaches the SEPSAMW formation region within 6 months. In 2016, strong advective freshening of the SEPSAMW formation region, linked to increased winter sea ice in the Amundsen/Bellingshausen seas, led to anomalously fresh mixed layers. However, a regime change in Antarctic sea ice in 2016 resulted in a subsequent lack of the usual advective freshening in the SEPSAMW formation region, driving increased salinity of the mixed layer the following year.

Deep-sea response to interglacial-glacial variability on the South Australian margin over the last 94 ka

The continuous record offered by deep-sea sediments has been extensively used to constrain shifting continental and oceanographic conditions. Yet, past fluctuations in deep-sea benthic conditions and bottom-currents are in numerous parts of the globe scarcely documented, one such example being the South Australian margin. Indeed, though variations in surface water masses and continental aeolian dust and river outflow are well documented in the area, little is known about benthic environments and their dynamics during the last interglacial-glacial cycle. We focus here on benthic foraminiferal assemblages sampled from a sediment core recovered at 2420 m depth from a small plateau south of Kangaroo Island within the underwater Murray Canyons Group (South Australian margin). Benthic foraminiferal assemblages show a distinct separation between interglacial and glacial periods over the last 94 ka, and indicate that the benthic environment was well-ventilated and oligotrophic during glacial periods, whilst being rather marked by reduced oxygenation associated to higher food input during the Holocene and Marine Isotope Stage 5a-c. We demonstrate that autochtonous deep-sea benthic foraminiferal communities neither respond to changes in the Murray River's discharges, nor do they follow variations in aeolian dust input from South Australia. Instead, the deep-sea and the terrestrial realm appear decoupled. Moreover, our observations suggest that bottom-water slope currents were stronger during the Holocene and Marine Isotope Stage 5a-c. We propose that this strengthening was triggered by an intensification of the poleward-circulating deep eastern boundary current transporting carbon-rich Indian Deep Water. In contrast, glacial seafloor conditions, especially during the Last Glacial Maximum, may reflect a greater influence and a shoaling of oxygen-rich Antarctic Bottom Water of South Australia. This bottom-water shift would follow the northward displacement of the Subtropical and Subantarctic Fronts and coincide with a withering influence of the Leeuwin Current within surface waters.

Observed amplification of the seasonal CO2 cycle at the Southern Ocean Time Series

The Subantarctic Zone, the circumpolar region of the Southern Ocean between the Subtropical and Subantarctic fronts, plays an important role in air-sea CO2 exchange, the storage of anthropogenic CO2, and the ventilation of the lower thermocline. Here we use a time series from moored platforms deployed between 2011 and 2021 as part of the Southern Ocean Time Series (SOTS) observatory to investigate the seasonality and interannual variability of upper ocean hydrography and seawater CO2 partial pressure (pCO2). The region is a net sink for atmospheric CO2 over the nearly 10-year record, with trends revealing that the ocean pCO2 may be increasing slightly faster than the atmosphere, suggesting that oceanic as well as anthropogenic atmospheric forcing contributes to the decadal change, which includes a decline in pH on the order of 0.003 yr−1. The observations also show an amplification of the seasonal cycle in pCO2, potentially linked to changes in mixed layer depth and biological productivity.

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.

Upper-ocean temperature characteristics in the subantarctic southeastern Pacific based on biomarker reconstructions

Abstract. As remnants of living organisms, alkenones and isoprenoid glycerol dialkyl glycerol tetraether lipids (isoGDGTs) are widely used biomarkers for determining ocean water temperatures from the past. The organisms that these proxy carriers stem from are influenced by a number of environmental parameters, such as water depth, nutrient availability, light conditions, or seasonality, which all may significantly bias the calibration to ambient water temperatures. Reliable temperature determinations thus remain challenging, especially in higher latitudes and for undersampled regions. We analyzed 33 sediment surface samples from the southern Chilean continental margin and the Drake Passage for alkenones and isoGDGTs and compared the results with gridded instrumental reference data from the World Ocean Atlas 2005 (WOA05) and previously published data from an extended study area covering the central and western South Pacific towards the Aotearoa / New Zealand continental margin. We show that for alkenone-derived sea surface temperatures (SSTs), the widely used global core-top calibration of Müller et al. (1998) yields the smallest deviation of the WOA05-based SSTs. On the contrary, the calibration of Sikes et al. (1997), determined for higher latitudes and summer SSTs, overestimates modern WOA05-based SSTs in both the annual mean and summer. Our alkenone SSTs show a slight seasonal shift of ∼ 1 ∘C at the southern Chilean margin and up to ∼ 2 ∘C in the Drake Passage towards austral summer SSTs. Samples in the central South Pacific, on the other hand, reflect an annual mean signal. We show that for isoGDGT-based temperatures, the subsurface calibration of Kim et al. (2012a) best reflects temperatures from the WOA05 in areas north of the Subantarctic Front (SAF). Temperatures south of the SAF are, in contrast, significantly overestimated by up to 14 ∘C, irrespective of the applied calibration. In addition, we used the GDGT [2]/[3] ratios, which give an indication of the production depth of the isoGDGTs and/or potential influences from land. Our samples reflect a subsurface (0–200 m water depth) rather than a surface (0–50 m water depth) signal in the entire study area and show a correlation with the monthly dust distribution in the South Pacific, indicating terrigenous influences. The overestimation of isoGDGT surface and subsurface temperatures south of the SAF highlights the need for a reassessment of existing calibrations in the polar Southern Ocean. Therefore, we suggest a modified Southern Ocean tetraether index (TEX86)-based calibration for surface and subsurface temperatures, which shows a lower temperature sensitivity and yields principally lower absolute temperatures, which align more closely with WOA05-derived values and also OH–isoGDGT-derived temperatures.

Observations of Internal Wave Interactions in a Southern Ocean Standing Meander

Abstract We characterize the internal wave field at a standing meander of the Antarctic Circumpolar Current (ACC) where strong winds, bathymetry, and a strong eddy field combine to form a dynamic environment for the generation and dissipation of internal waves. We use Electromagnetic Autonomous Profiling Explorer float data spanning 0–1600 m depth collected from a meander near the Macquarie Ridge, south of Australia. Of the 112 internal waves identified, 69% are associated with upward energy propagation. Most of the upward propagating waves (35%) are found near the Polar Front and are likely generated by mean flow–topography interactions. Generation by wind forcing at the sea surface is likely responsible for more than 40% of the downward propagating waves. Our results highlight advection of the waves and wave–mean flow interactions within the ACC as the dominant processes affecting the wave dynamics. The larger dissipation time scales of the waves compared to advection suggests they are likely to dissipate away from the generation site. We find that about 79% (66%) of the waves in cyclonic eddies (the Subantarctic Front) are influenced by horizontal strain, whereas 92% of the waves in the slower Polar Front are influenced by the relative vorticity of the background flow. There is energy exchange between internal waves and the mean flow, in both directions. The mean energy transfer (1.4 ± 1.0 × 10−11 m2 s−3) is from the mean flow to the waves in all dynamic regions except in anticyclonic eddies. The strongest energy exchange (5.0 ± 3.7 × 10−11 m2 s−3) is associated with waves in cyclonic eddies.

Plankton diversity and dynamics in the upper surface of the Indian sector of the Southern Ocean ecosystem and biogeochemical processes

In the Southern Ocean (SO) marine ecosystem, the interplay between CO2 sink and climate change is closely linked to the shifting plankton communities, leading to potentially wide-ranging biogeochemical effects. Within the Indian sector of the Southern Ocean (ISSO), the phytoplankton community was predominantly composed of Fragilariopsis spp., followed by Chaetoceros sp., while the zooplankton consisted mainly of Pleuromamma spp. and Oithona similis. Plankton diversity is collectively influenced by variables such as temperature, photosynthetically available radiation (PAR), and macronutrient availability. During the austral summer of 2019, significant latitudinal variations in phytoplankton were observed at major fronts in the region, including the Subtropical Front (STF), Sub Antarctic Front (SAF), Polar Front (PF), and South of Polar Front (SPF). At the STF, flagellates dominated (87%), while their presence decreased to 12% at the SPF, while diatoms progressively increased from the STF (<1%) to the SPF (87%) through the SAF and PF. Chlorophyll a (Chl-a) concentrations at the surface were highest in the PF (0.62 mg m−3), followed by the SPF (0.56 mg m−3), SAF (0.51 mg m−3), and STF (0.28 mg m−3). The integrated Chl-a values across the water column were higher at the SPF (21.49 mg m−2) than at the STF (14.73 mg m−2). The dominant diatom species at the surface along the fronts, except for STF, was Fragilariopsis spp. (∼5 × 105 Cells L−1), followed by Chaetoceros sp. (∼1 × 105 Cells L−1), and Coscinodiscus sp. (∼5 × 104 Cells L−1). Conversely, at the STF, the most abundant flagellate species were Gyrodinium spp. (∼2.4 × 104 Cells L−1), followed by Protoperidinium sp. (∼2 × 104 Cells L−1), and Amphidinium sp. (∼1 × 104 Cells L−1). Zooplankton biovolume in the upper 200 m ranged from 0.71 to 10 ml 100 m−3, with relatively higher biovolume and numerical abundance found in the SAF compared to the PF. The higher Chl-a concentration in the PF and lower zooplankton biovolume could have significant negative consequences for higher trophic levels. PAR showed higher values up to 40 m than observed depths along the fronts. Additionally, variations in salinity and temperature were recorded, with a significant decline in temperature from the STF to the SPF. Nitrate (NO3−), phosphate (PO43−), and silicate (SiO44−) concentrations were higher at the SPF than at lower latitudes. Overall, these findings indicate that the plankton community composition in the study region is influenced by a diverse set of governing variables over time, highlighting the significant implications for the regional biogeochemical cycle.

Structure and Temporal Variability of the Northern Branch of the Antarctic Circumpolar Current in the Drake Passage

The Antarctic Circumpolar Current plays the key role in the circulation of the Southern Ocean and affects the distribution of heat by the ocean on the global scale. The study of the dynamics and structure of this current becomes especially important in a changing climate. The current is well revealed by satellite altimetry data, which makes possible to study temporal and spatial variability of its structure at different scales. In these studies, the methods for determining the position of individual fronts of the Antarctic Circumpolar Current based on satellite altimetry data become especially important. In this work, we compare various approaches for detection of front locations. The structure of the northernmost branch of the Antarctic Circumpolar Current, the Subantarctic Front, and its spatial and temporal variability was studied based on satellite altimetry data from 1993 to 2020 and the results of a hydrographical section occupied by the R/V “Akademik Mstislav Keldysh” in the northern part of the Drake Passage in February 2020. We selected a 350 km long part of the TOPEX/Poseidon and Jason-1,2,3 satellite altimeter track from Tierra del Fuego to the south for the analysis of the dynamics of the front. Criteria for determining the position of the northern and main branches of the Subantarctic Front are presented based on satellite altimetry data. A long-term shift of the position of the fronts relative to the previously accepted levels of absolute dynamic topography has been found. It was found that the accuracy of determining the position of fronts using fixed values of dynamic topography may decrease with time, in particular due to changes in the mean sea level. A statistically significant long-term trend of sea level rise in the region of the Subantarctic Front was found. This trend is 4 mm/yr for the northern branch and 2.5 mm/yr for the main branch.

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.

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.

The Relationship Between Nitrate and Potential Density in the Ocean South of 30°S

AbstractAs a main source of nitrate for the pycnocline in the global ocean, the ocean south of 30°S plays a pivotal role in the global marine biogeochemical cycle. Nitrate in the pycnocline often shows a more stable relationship with density than with depth. Hence, it is important to understand the nitrate structure and its relationship with density in the ocean south of 30°S. In the present study, the nitrate‐density relationship and the mechanisms for its variability are studied based on the data from more than 60,000 profiles. Through analysis, we find that nitrate is approximately linearly correlated with potential density in the nitracline, except for the western boundary current zones. The upper bounds of the nitracline depend on the mixed layer and euphotic layer depth. The lower bounds are related to the depth of maximum nitrate, which mainly vary meridionally with sharp changes near the Subantarctic Front. The diapycnal gradients of nitrate also show meridional variability, with large magnitudes corresponding to thick Subantarctic Mode Water, low diapycnal mixing in the nitracline, and high biological uptake in the euphotic layer. The nitrate‐density relationship can be applied to predict nitrate concentrations based on the observed temperature and salinity profiles and support some detailed studies on the physical‐biogeochemical interactions. Two examples show that the predicted nitrate data could help better resolve the effects of ocean circulations and mesoscale eddies on nitrate than the existing observed nitrate profiles.