Ocean microstructure measurements collected during three austral summers (2023-2025) along the Antarctic continental slope off Dronning Maud Land show enhanced subsurface mixing. Mean turbulent dissipation between 100 and 800 m depth is an order of magnitude higher than in the open ocean, with an extreme event reaching W at mid-depth. Elevated dissipation coincides with peaks in vertical velocity shear during periods of strong tidal acceleration associated with spring-tide flow reversals. Enhanced continental slope mixing drives a mean upward heat flux of 3 W into the base of cold surface waters, in agreement with independent estimates from an internal tide model. Combined with reanalysis data, the model suggests tidal mixing along the Antarctic continental slope could produce a circumpolar mean vertical heat flux of 9 W . This upward heat transport may warm the upper ocean and limit sea ice formation around Antarctica.

Global marine ecosystem models (MEMs) are increasingly being used for assessingclimate impacts at various spatial scales (global, regional, countries, etc.), but theiroutputs are influenced by uncertainties linked to the ocean models used as forcings.We run simulations following Track A of the Fisheries and Marine Ecosystem ModelIntercomparison Project 2.0 using the Dynamic Benthic Pelagic Model (DBPM). Wethen evaluated the impact of three sources of uncertainty influencing the accuracy ofsimulated fishing catches in the Southern Ocean (1961–2010): (1) coarseninghorizontal resolution of environmental forcings (from 0.25° to 1°), (2) restricting spatialdistribution of fishing effort using a sea ice mask, and (3) using regional versus globalfishing effort forcing data. Our results indicate that DBPM captured observed spatialcatch distributions, with minimal differences between forcings resolutions. DBPMoverestimated observed fishing catches to a similar degree in both horizontalresolutions. Temporal trends aligned more closely with observations prior to the 1990s,but catch overestimation grew larger thereafter. Overestimation appears to be linked toan unrealistic spatial distribution of effort by DBPM, which was improved by theinclusion of a sea ice mask, and to errors and biases in the underlying fishing effortforcing data, which continues to represent a major hurdle for model skill improvement.Our findings suggest that coarsening inputs from an eddy-permitting ocean model maybe an effective approach to improve MEM performance without increasingcomputational costs.

Ocean fronts are critical features that influence marine ecosystems and can affect climate at both regional and global scales. In many regions, fronts enhance vertical mixing and advection, increasing nutrient supply, which can stimulate primary production and modulate air–sea CO2 fluxes. However, a global perspective on the impacts of changing ocean fronts on primary production and air–sea CO2 exchange is still lacking. Here using satellite observations (2003–2024) and supplementary reanalysis data at higher latitudes (2003–2024), we identify areas with the richest frontal activity and the fastest-changing frontal properties. We find that 72

Theme 1 addresses the Southern Ocean’s (south of 30°S) critical role in regulating Earth’s climate through circulation patterns that mediate global exchanges of heat, carbon, freshwater, and nutrients. The region has absorbed over 70% of anthropogenic heat and has contributed to about 40% of the global ocean uptake of human-emitted carbon dioxide (CO₂) while also controlling ice shelf stability and sea level rise. However, fundamental gaps in year-round observations, particularly during austral autumn, winter, and spring, severely limit our understanding of these processes and their responses to rapid climate change. This white paper establishes key knowledge gaps and high-priority recommendations that are tractable within the InSync timeframe through coordinated program execution with strong engagement from national operators and funding agencies. Critical needs include seasonal observations of the marginal ice zone where carbon, heat and nutrient-rich waters upwell, year-round continental shelf measurements where dense water formation and ice-ocean interactions occur, standardized air-sea flux measurements, and strategic monitoring of regional choke points. Success requires international resource sharing, coordinated deployments, and sustained commitment to both process studies and long-term monitoring.

The Southern Annular Mode (SAM) influences Southern Hemisphere temperature and precipitation, ocean circulation, carbon cycling and the Antarctic cryosphere. In this Review, we examine the dynamics, projections and effects of the SAM, focusing on future implications for the Southern Ocean and Antarctica. The SAM is the leading mode of atmospheric variability in the Southern Hemisphere extratropics, associated with variations in the mid-latitude westerly jet strength and position. The SAM is primarily an internally driven atmospheric process, for which anomalies dissipate in 1–2 weeks; however, sustained SAM anomalies can also be forced by stratospheric processes and tropical Pacific variability. Ozone depletion during the 1970s–1990s contributed to large positive trends in austral summer. The SAM is now in its most positive mean state in over 1,000 years, and a year-round positive trend in the SAM is projected to continue throughout the twenty-first century in response to increasing greenhouse gases. Given the importance of SAM effects on Southern Ocean circulation, carbon cycling, and Antarctic ice mass balance for future climate and sea level rise projections, it is crucial that the effects of SAM are better modelled and understood, including accounting for the influence of the shifting seasonality of positive SAM trends and its increasing asymmetry. The Southern Annular Mode (SAM) has shifted towards its positive phase owing to ozone depletion and increasing greenhouse gas concentrations. This Review discusses the dynamics, trends and projections of the SAM and how these will affect southern high-latitude climate, including Southern Ocean circulation, carbon cycling and the Antarctic cryosphere.

Species distribution models (SDMs) quantify the relationship between species presence and environmental factors. They are often used to guide conservation management plans, but limited availability of environmental and biological data in undersampled regions, such as the Southern Ocean, represent an important challenge preventing us from accurately estimating species distributions. We used a weighted ensemble of 4 SDMs to predict crabeater seal Lobodon carcinophagus distribution in East Antarctica. We combined georeferenced occurrence records from multiple open-source databases to fit an SDM ensemble. Environmental data from satellites and a high-resolution sea ice-ocean model were used to fit SDMs. Outputs were compared to evaluate if predicted crabeater seal distributions were similar. Sea-ice-related variables and the distribution of Antarctic krill, the main prey of crabeater seals, were identified as key drivers of crabeater seal distribution. The inclusion of prey in our SDMs improved their performance, highlighting the importance of predator-prey relationships. We emphasise the importance of including a comprehensive suite of ecologically relevant environmental predictors in SDMs, as a reduced set may not capture key drivers of distribution. We reiterate that prior to estimating habitat distribution for any species, an evaluation of the ability of an ocean model to realistically reproduce observed past environmental conditions within the area of interest is necessary. These steps add rigour to SDM development and build confidence when using high-resolution coupled ocean models to predict the fate of top-level predators and inevitably the ecosystem as a whole.

Antarctic sea ice plays many crucial roles in the physical environments and ecosystems of Antarctica and the Southern Ocean. In this study, we synthesize the physical, biogeochemical, ecosystem, and societal impacts of summers with extreme low Antarctic sea-ice coverage. These extreme events result in the loss of multiyear landfast ice and changes in sea-ice seasonality. Following extreme low sea-ice events, we find surface warming of the Southern Ocean and changes to the formation rate of Antarctic Intermediate Water, likely affecting heat and carbon uptake. Ice-shelf calving is negatively correlated with sea-ice area, so that years with less sea ice show increased calving. Prolonged open water affects the magnitude and seasonality of surface-phytoplankton blooms. The impacts on higher trophic levels are species-specific and occur through habitat loss and changes to prey availability. Extreme sea-ice lows will adversely impact krill, a foundational prey species that relies on sea ice for nourishment and refuge. The loss of stable landfast ice in austral spring and summer hampers Antarctic operations and resupply missions. Understanding the full impacts of recent, and future, sea-ice extremes is of utmost importance and requires an enhanced observational network that spans the physical and ecological systems of Antarctica and the Southern Ocean.

Meanders are significant features of the Antarctic Circumpolar Current in the Southern Ocean and sites of enhanced upwelling, cross-frontal tracer fluxes, and exchanges between the surface and deep ocean. They often overlap the locations of fronts and are linked to topographic features. While much is known about Southern Ocean fronts and how they are changing, the response of meanders to climate change is largely unexplored. In this study, we investigate the Campbell Plateau meander south of New Zealand. We apply a local gradient maxima method to satellite altimetry data to identify the position of the meander and estimate its width, geostrophic current speed, and associated trends from 1993 to 2020. We find that the position of the meander has been relatively fixed, except for a section downstream of the Plateau, which has shifted northward by about 0.4 degrees latitude per decade. The meander has become flatter at the western edge of the Plateau, but steeper at the eastern edge of the Plateau. Overall, the meander has widened by 2 km per decade and accelerated by 0.01 m s-1 per decade, particularly downstream of the Plateau. These findings are consistent with other work on standing meanders in the Southern Ocean. Increases in eddy kinetic energy and of the South Pacific Gyre index support our hypotheses that changes in the downstream jet stability and the South Pacific Gyre contribute to these observed trends. The impacts of these trends on cross-frontal transport remain to be evaluated. In the Southern Ocean, meanders are parts of the Antarctic Circumpolar Current (ACC) that deviate from the usual west-to-east flow by having a substantial north-south component, resulting in a wave-like appearance. Standing meanders are meanders that are stationary and do not move much over months and years. They are a special feature of the ACC and are fundamental for exchanges between the surface and deep ocean. Although changes in the ACC have been well studied, especially in the context of climate change, very little is known about how Southern Ocean meanders are changing. This study focuses on the Campbell Plateau meander south of New Zealand in the Southern Ocean. Using ocean sea surface height data from satellites, we analyze the monthly position of this meander, estimate its monthly width and speed, and quantify how these characteristics have changed over the 1993-2020 period. Upstream of the Campbell Plateau, the meander has undergone almost no changes in its position, width or speed. However, downstream of the Plateau, the meander has shifted northward, widened and accelerated. These trends are consistent with other observations in the Southern Ocean, and we discuss potential mechanisms to explain them. The position of the Campbell Plateau meander has remained stable for the past 30 years, apart from a section downstream shifting northward The amplitude of the Campbell Plateau meander has been decreasing (flatter) upstream of the Plateau and increasing (steeper) downstream The Campbell Plateau meander has been widening and accelerating over the 1993-2020 period, especially downstream of the Plateau

In recent years, the Southern Ocean has experienced extremely low sea ice cover in multiple summers. These low events were preceded by a multidecadal positive trend that culminated in record high ice coverage in 2014. This abrupt transition has led some authors to suggest that Antarctic sea ice has undergone a regime shift. In this study we analyze the satellite sea ice record and atmospheric reanalyses to assess the evidence for such a shift. We find that the standard deviation of the summer sea ice record has doubled from 0.31 million km2 in 1979-2006 to 0.76 million km2 for 2007-22. This increased variance is accompanied by a longer season-to-season sea ice memory. The atmosphere is the primary driver of Antarctic sea ice variability, but using a linear predictive model we show that sea ice changes cannot be explained by the atmosphere alone. Identifying whether a regime shift has occurred is difficult without a complete understanding of the physical mechanism of change. However, the statistical changes that we demonstrate (i.e., increased variance and autocorrelation, and a changed response to atmospheric forcing), as well as the increased spatial coherence noted by previous research, are indicators based on dynamical systems theory of an abrupt critical transition. Thus, our analysis is further evidence in support of a changed Antarctic sea ice system. SIGNIFICANCE STATEMENT: In recent years, there have been several summers with extremely low Antarctic sea ice cover, including consecutive record lows in February 2022 and February 2023. Since then, the 2023 winter has seen a remarkably low sea ice growth with an anomaly far below expected climatology. This has led researchers to question whether there has been a regime shift, and we assess the observational evidence for such a shift. In the last decade or so, the variability of summer sea ice has almost doubled, accompanied by a much longer sea ice memory from season to season. These statistical changes, as well an increased spatial coherence noted by other researchers, are consistent with theoretical indicators of a critical transition, or regime shift.

A holistic review is given of the Southern Ocean dynamic system, in the context of the crucial role it plays in the global climate and the profound changes it is experiencing. The review focuses on connections between different components of the Southern Ocean dynamic system, drawing together contemporary perspectives from different research communities, with the objective of “closing loops” in our understanding of the complex network of feedbacks in the overall system. The review is targeted at researchers in Southern Ocean physical science with the ambition of broadening their knowledge beyond their specific field and facilitating better-informed interdisciplinary collaborations. For the purposes of this review, the Southern Ocean dynamic system is divided into four main components: large-scale circulation; cryosphere; turbulence; and gravity waves. Overviews are given of the key dynamical phenomena for each component, before describing the linkages between the components. The reviews are complemented by an overview of observed Southern Ocean trends and future climate projections. Priority research areas are identified to close remaining loops in our understanding of the Southern Ocean system.

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 genera-tion 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 identi-fied, 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 sur-face 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 gener-ation site. We find that about 79% (66%) of the waves in cyclonic eddies (the Subantarctic Front) are influenced by hori-zontal 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 en-ergy transfer (1.4 & PLUSMN; 1.0 x 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 & PLUSMN; 3.7 x 10-11 m2 s-3) is associated with waves in cyclonic eddies.

Southern Ocean organisms are uniquely adapted to the extreme environmental conditions that characterise this region, making them especially vulnerable to climate change. Alterations to the physical environment have already been linked to alterations in the structure and functioning of entire ecosystems, and ecological disruptions are expected to continue to occur. Although our understanding of the physical processes driving ecological change in the Southern Ocean has improved in recent years, significant knowledge gaps remain largely as a result of insufficient observational data being available. High resolution ocean models are an important tool that can help us overcome data scarcity. However, models generally contain biases that may affect their ability to accurately represent environmental conditions in the region of interest. Thus, their outputs must be evaluated to understand if and how model outputs can be used to answer questions about ecological impacts. Here, we examined the suitability of ACCESS-OM2-01, a high-resolution coupled ocean–sea ice model, for ecological applications. Our results highlight the heterogeneous nature of the mean state of the environmental variables examined and their trends across the Southern Ocean. Our assessment shows that the ACCESS-OM2-01 model simulated well the observed seasonal cycle and broad baseline climatological conditions of the mixed layer depth and sea ice variables for the Southern Ocean over the past 50 years. However, the model performance varies across regions and at seasonal time scales: the model simulated much deeper winter mixed layer depth in the Weddell Sea and within the Antarctic Circumpolar Current, and the climatological sea-ice concentration for summer was lower in the model compared to available observations. Finally, we provide detailed Python-based scripts with all the code and steps for this analysis, which can be used as reference material to reapply elsewhere and evaluate the suitability of other model outputs for ecological applications.

This dataset includes the post-processed data and a demo MATLAB script used for the paper titled "Global trends of fronts in a warming ocean and impacts on phytoplankton productivity" SST_FRONT_data.zip contains maps of sea surface temperature (SST) fronts detected by the Cayula and Cornillon single image edge detection algorithm over global ocean warming hotspot regions and covering the period 2003-2020. The original data was obtained from NASA OB.DAAC MODIS sea surface temperature (SST) product (MODIS Aqua Level 3 SST MID-IR 8 Day 4km Nighttime V2019.0: https://podaac.jpl.nasa.gov/dataset/MODIS_AQUA_L3_SST_MID-IR_8DAY_4KM_NIGHTTIME_V2019.0?ids=&values=&search=MODIS%20Aqua&provider=POCLOUD). Fdens_Ffreq_Fstre_example.mlx is a MATLAB live script showing how to compute metrics of fronts based on frontal maps: frontal frequency (Ffreq), frontal density (Fdens), and frontal strength (Fstre).

Ocean fronts affect phytoplankton and higher trophic levels, including commercially important fisheries. As the oceans warm, uncertainty remains around the trends in fronts. Here we examine changes in sea surface temperature fronts (frequency, density, and intensity) and the concentration of chlorophyll, over recent satellite records (2003 – 2020) in ocean warming hotspots - areas that are warming faster than other parts of the ocean. Commonalities exist across hotspots with comparable dynamics. Most equatorial and subtropical gyre hotspots experienced a decline in frontal activity (frequency, density, strength) and chlorophyll concentration, while in high-latitude hotspots, frontal activity and chlorophyll concentration mostly increased. Continued warming may accentuate the impacts, changing both total biomass and the distribution of marine species. Areas with changing fronts and phytoplankton also correspond to areas of important global fish catch, highlighting the potential societal significance of these changes in the context of climate change.

Antarctic sea ice is a critical component of the climate system and a vital habitat for Southern Ocean ecosystems. Understanding the underlying physical processes and improving Antarctic sea ice prediction is of broad interest. Using the model data, we investigate sea ice and upper ocean predictability at interannual timescales in the Weddell Sea region. We find that oceanic predictability is largely confined to the Winter Water layer and responds to seasonal modifications of the water column, mainly driven by sea ice processes. Predictability depends not only on the depth of the Winter Water layer, but also on how strongly stratified its base is. Predictability is lost when warm Circumpolar Deep Water with no sea ice‐related memory entrains into the mixed layer. We show the strong dependence of sea ice predictability on the local upper ocean vertical structure, which suggests that both are likely to change in a warming climate.

Atlantic Water (AW), the main source of heat and salt for the Arctic Ocean, undergoes large transformations (cooling and freshening) north of Svalbard as it flows near the surface above the Yermak Plateau (YP). In September 2017, a SeaExplorer ocean glider deployed in the West Spitsbergen Current (WSC) and recovered north of Svalbard documented the circulation and properties of the AW crossing the YP. The glider sampled the different branches of the AW flowing into the Arctic around the YP: the WSC, the Svalbard Branch (SB), the Yermak Pass Branch, and the Yermak Branch. Unusual southerly winds prevailed in summer 2017 impacting AW circulation in the region. Cold and fresh lenses of shelf-origin waters detached from the slope in the WSC to reach their density level below the core of the AW. This resulted in cooling and freshening of the AW inflow from below. The eastward current associated with the SB was found to be weak at its usual location above the 400 m isobath, likely the result of the adjustment of the flow influenced by anomalous southerly wind conditions.

Extreme climate and weather events are unusual and rare events that often cause a lot of damage both to nature and to people. They take place in the air (storms, tornadoes, heavy rain, atmospheric rivers), in the ocean (storm surges, marine heatwaves), and on the land (wildfires, heatwaves, floods, droughts). Many weather and climate extremes happen naturally, even without climate change. But Earth’s changing climate does change where and how often some extreme events take place, and how strong those events are. What are extreme climate and weather events? Will new or stronger extreme events happen due to climate change? How is climate change impacting extreme events? These are the type of questions that our team of climate and earth scientists from around the world will answer in this article.

Major gaps exist in our understanding of the pathways between internal wave generation and breaking in the Southern Ocean, with important implications for the distribution of internal wave‐driven mixing, the sensitivity of ocean mixing rates and patterns to changes in the ocean environment, and the necessary ingredients of mixing parameterizations. Here we assess the dominant processes in internal wave evolution by characterizing wave and mesoscale flow scales based on full‐depth in situ measurements in a Southern Ocean mixing hot spot and a ray tracing calculation. The exercise highlights the importance of Antarctic Circumpolar Current jets as a dominant influence on internal wave life cycles through advection, the modification of wave characteristics via wave‐mean flow interactions, and the set‐up of critical layers for both upward‐ and downward‐propagating waves. Our findings suggest that it is important to represent mesoscale flow impacts in parameterizations of internal wave‐driven mixing in the Southern Ocean.