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The dynamics of an oceanic storm track—where energy and enstrophy transfer between the mean flow and eddies—are investigated using observations from an eddy-rich region of the Antarctic Circumpolar Current downstream of the Shackleton Fracture Zone (SFZ) in Drake Passage. Four years of measurements by an array of current- and pressure-recording inverted echo sounders deployed between November 2007 and November 2011 are used to diagnose eddy–mean flow interactions and provide insight into physical mechanisms for these transfers. Averaged within the upper to mid-water column (400–1000-m depth) and over the 4-yr-record mean field, eddy potential energy is highest in the western part of the storm track and maximum eddy kinetic energy occurs farther away from the SFZ, shifting the proportion of eddy energies from to about 1 along the storm track. There are enhanced mean 3D wave activity fluxes immediately downstream of SFZ with strong horizontal flux vectors emanating northeast from this region. Similar patterns across composites of Polar Front and Subantarctic Front meander intrusions suggest the dynamics are set more so by the presence of the SFZ than by the eddy’s sign. A case study showing the evolution of a single eddy event, from 15 to 23 July 2010, highlights the storm-track dynamics in a series of snapshots. Consistently, explaining the eddy energetics pattern requires both horizontal and vertical components of W , implying the importance of barotropic and baroclinic processes and instabilities in controlling storm-track dynamics in Drake Passage.

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation, ventilates the abyssal ocean and sequesters heat and carbon on multidecadal to millennial timescales. AABW originates on the Antarctic continental shelf, where strong winter cooling and brine released during sea ice formation produce Dense Shelf Water, which sinks to the deep ocean. The salinity, density and volume of AABW have decreased over the last 50 years, with the most marked changes observed in the Ross Sea. These changes have been attributed to increased melting of the Antarctic Ice Sheet. Here we use in situ observations to document a recovery in the salinity, density and thickness (that is, depth range) of AABW formed in the Ross Sea, with properties in 2018–2019 similar to those observed in the 1990s. The recovery was caused by increased sea ice formation on the continental shelf. Increased sea ice formation was triggered by anomalous wind forcing associated with the unusual combination of positive Southern Annular Mode and extreme El Niño conditions between 2015 and 2018. Our study highlights the sensitivity of AABW formation to remote forcing and shows that climate anomalies can drive episodic increases in local sea ice formation that counter the tendency for increased ice-sheet melt to reduce AABW formation.Interacting atmospheric circulation patterns are responsible for a recent reversal of a decades-long decline in deepwater formation on the Antarctic shelf, according to an analysis of in situ and remote sensing data from the Ross Sea.

The East Antarctic Ice Sheet (EAIS) contains the vast majority of Earth’s glacier ice (~52 metres sea-level equivalent), but is often viewed as less vulnerable to global warming than the West Antarctic or Greenland ice sheets. However, some regions of the EAIS have lost mass over recent decades, prompting the need to re-evaluate its sensitivity to climate change. Here we review the EAIS’s response to past warm periods, synthesise current observations of change, and evaluate future projections. Some marine-based catchments that underwent significant mass loss during past warm periods are currently losing mass, but most projections indicate increased accumulation across the EAIS over the 21st Century, keeping the ice sheet broadly in balance. Beyond 2100, high emissions scenarios generate increased ice discharge and potentially several metres of sea-level rise within just a few centuries, but substantial mass loss could be averted if the Paris Agreement to limit warming below 2°C is satisfied.

An unusual double-thermostad warm-core ring of the Gulf Stream was discovered in the Slope Sea, south of Georges Bank, during the R/V Endeavor cruise 578 in May 2016. The ring’s stratification was peculiar as it included two thermostads at, respectively, 100–200 m (core T = 18.14°C, S = 36.52) and 250–500 m (core T = 16.70°C, S = 36.35). Extensive use of satellite data (SST imagery and SSH maps) allowed the life history of this ring to be reconstructed, with independent SST and SSH data mutually corroborating each other. The double-thermostad ring was formed by vertical alignment of two preexisting warm-core anticyclonic rings of the Gulf Stream. The first ring spawned by the Gulf Stream in February has cooled by ~2°C before merging in April with the second ring spawned by the Gulf Stream in March. During vertical alignment of these rings, the warmer ring overrode the colder ring, thereby forming the double-thermostad ring surveyed in May 2016. From ADCP sections through the ring, the upper and lower thermostads had different core relative vorticities of −0.65 f and −0.77 f , respectively, where f is the local Coriolis parameter. An in-depth literature survey has confirmed that this is the first report of a double-thermostad warm-core ring of the Gulf Stream and one of the best-documented cases of vertical alignment of two eddies ever observed in the World Ocean.

In the Indian sector of the Southern Ocean, 80°E marks an important transition in ocean circulation between the greater Prydz Bay gyre to the west and the Australian Antarctic gyre to the east. Here, the submarine Kerguelen Plateau impedes the eastward flow of the Antarctic Circumpolar Current (ACC), topographically steering the flow. Enhanced biological productivity associated with the southern plateau supports an important marine ecosystem with many foraging marine predators. We collate ship-based hydrographic data on the vertical structure of the upper water column near 80°E from eight voyages spanning 1994 to 2021, from 58°S towards the Antarctic continent. The study aims to investigate the mixed layer oceanography, the implications for nutrient supply from deep to near-surface waters, and associated biological production. Our results show that the major oceanographic fronts are constrained within the narrow Princess Elizabeth Trough, between the southern Kerguelen Plateau and the Antarctic slope. Therefore, the Southern Boundary and the Southern ACC Front (SACCF) are often co-located, albeit with some interannual variability, with the location of the SACCF ranging from roughly 63°S to 65°S. The average depth of the seasonal mixed layer ranges from 34-49 m, typically deepening from south to north, in association with longer time since sea-ice melt. Below the mixed layer, Winter Water (WW) characteristics also vary across the observed latitudinal range; typically the temperature and thickness of the WW layer are inversely related, with warmer WW layers being thinner. Subsurface nitrate concentrations range from 20-40 µM, while silicate concentrations reach 100 µM. Nutrient drawdown is calculated based on mean concentrations in the mixed layer and WW layer, with drawdown values at individual stations reaching nearly 12 µM and 60 µM for nitrate and silicate, respectively, and a positive correlation between the two. Nutrient drawdown was higher in association with longer time since sea-ice melt and with thinner WW layers, while higher nitrate-based production was associated with deeper mixed layers. Observed relationships between upper water column characteristics and biological processes are discussed in terms of likely nutrient supply mechanisms and seasonal patterns of utilization.

Circulation and water masses in the greater Prydz Bay region were surveyed in the austral summer 2021 (January-March) during the ‘Trends in Euphausiids off Mawson, Predators and Oceanography’ (TEMPO) experiment, and are described in this paper. The Southern Antarctic Circumpolar Current Front is found in the northern part of the survey area, generally near 63-64°S, whereas the Southern Boundary Front is located between 64 and 65.5°S. The westward flowing Antarctic Slope Front (ASF) is found in the southern part of the survey area near the continental slope on most transects. Highest concentrations of oxygen ( > 300 µ mol kg −1 ) are found in shelf waters at stations in Prydz Bay, south of 67°S along 75°E, whereas the lowest oxygen values are found in the Circumpolar Deep Water layer, with an average of roughly 215 µ mol kg −1 . North of the northern extension of the ASF, surface mixed layers are between 20 and 60 m deep. Mixed layers tend to deepen slightly in the northern part of the survey, generally increasing north of 64°S where the ocean has been ice-free the longest. We find evidence of upwelling of waters into the surface layers, based on temperature anomaly, particularly strong along 80°E. Enhanced variability of biogeochemical properties - nutrients, DIC, DO - in the AASW layer is driven by a combination of sea-ice and biological processes. Antarctic Bottom Water, defined as water with neutral density > 28.3 kg m -3 , was sampled at all the offshore full-depth stations, with a colder/fresher variety along western transects and a warmer/saltier variety in the east. Newly formed Antarctic Bottom Water – the coldest, freshest, and most recently ventilated – is mostly found in the deep ocean along 65°E, in the base of the Daly Canyon.

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.

The ocean is the main heat reservoir in Earth’s climate system, absorbing most of the top-of-the-atmosphere excess radiation. As the climate warms, anomalously warm and fresh ocean waters in the densest layers formed near Antarctica spread northward through the abyssal ocean, while successions of warming and cooling events are seen in the deep-ocean layers formed near Greenland. The abyssal warming and freshening expands the ocean volume and raises sea level. While temperature and salinity characteristics and large-scale circulation of upper 2000 m ocean waters are well monitored, the present ocean observing network is limited by sparse sampling of the deep ocean below 2000 m. Recently developed autonomous robotic platforms, Deep Argo floats, collect profiles from the surface to the seafloor. These instruments supplement satellite, Core Argo float, and ship-based observations to measure heat and freshwater content in the full ocean volume and close the sea level budget. Here, the value of Deep Argo and planned strategy to implement the global array are described. Additional objectives of Deep Argo may include dissolved oxygen measurements, and testing of ocean mixing and optical scattering sensors. The development of an emerging ocean bathymetry dataset using Deep Argo measurements is also described.

East Antarctic coastal polynyas—semi-permanent areas of ice-free ocean—are vital for sea ice production, Antarctic Bottom Water formation, and global heat redistribution. The Dibble Polynya has pronounced sea-ice production, yet the shelf water properties there and its offshore influence have been unclear. Here we show Dense Shelf Water outflow from the polynya that ventilates a lighter variety of local bottom water. Hydrographic records suggest Dense Shelf Water formation has persisted for 50 years, consistent with satellite-derived sea-ice production since the 1990s. Since the 2010s, the downstream cross-slope salinity gradient within bottom water has weakened and the Antarctic Slope Front has been modified, following a sharp decline in sea-ice production in the neighbouring Mertz Polynya after 2010 icescape changes. We infer that reduced denser Antarctic Bottom Water formation upstream weakens the slope current barrier, promoting outflow of not-so-dense shelf water from the Dibble Polynya that efficiently ventilates the abyssal basin. Recent Dense Shelf Water outflow from the Dibble Polynya in East Antarctica followed a reduction of Antarctic Bottom Water upstream, according to hydrographic observations.

Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot spots, characterized by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing a low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient–wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance.