The highly energetic East Australian Current (EAC) System is characterized by dynamic frontal zones that form between the EAC jet and its eddy field. Submesoscale processes at these fronts are critical for lateral and vertical mixing, heat and nutrient flux, and biomass increase and export, but are poorly characterized because in situ fine-scale observational data are lacking. Here, we quantify and contrast the kinematic properties of 13 fronts observed in different dynamical regions of the EAC System using high-resolution data from a dedicated shipboard experiment. We show evidence of strain-driven frontogenesis at most fronts, leading to subduction of colder surface waters rich in oxygen and Chl-a. However, strong surface strain or temperature gradients do not always lead to strong biomass subduction. EAC fronts are associated with relatively shallow subduction depths, likely due to strong stratification of surface waters, in contrast to deep-reaching fronts observed in dipole jets between counter-rotating eddies. We reveal that mesoscale context and frontal structure critically govern vertical exchange and vertical transport of biogeochemical tracers, with the greatest percentage of depth-integrated Chl-a below the euphotic layer (75%) in fronts at dipole jets, associated with downward transport of biomass due to ageostrophic motion. This work demonstrates the diversity of fronts in a dynamic, oligotrophic Western Boundary Current and highlights the essential role of frontal processes in nutrient cycling and carbon export. These results illustrate the need for high-resolution observations and models to accurately estimate biogeochemical processes and carbon fluxes across fronts in a Western Boundary Current System.

Bluebottles (Physalia spp.) are a floating siphonophore that inhabit coastal waters and can become stranded on beaches around the world. They inflict painful stings on beachgoers which can place significant strain on surf life saving personnel. With little capacity for active (directed) swimming, their movement and distribution is driven by physical atmospheric and oceanographic processes and other biological processes (e.g., reproduction). This study combines multiple long term and spatially extensive datasets documenting their occurrence along the east coast of Australia with high resolution oceanographic and atmospheric reanalysis products to investigate (1) how the occurrence of beached bluebottles relates to public safety concerns (i.e., sting rates) and (2) potential drivers of occurrence (in terms of beaching, stings per swimmers, or abundance of left-or right-handed bluebottles). We find that the risk of being stung increases rapidly as the number of beached bluebottles increases, suggesting that bluebottles on the beach is a good indicator that they are also in the water. Wind appears to be the primary driver of their occurrence, with onshore winds (approximate to 20-180 degrees) leading to the highest likelihood of occurrence, especially under stronger wind speeds (>= 8 m s-1) with patterns consistent with the theoretical and experimentally determined drift trajectories for bluebottles. Finally, we demonstrate some ability to predict occurrence and make recommendations for further development of a predictive model to assist stakeholders in mitigating the adverse effects of large swarms at beaches.

Abstract. The identification, tracking, and characterisation of ocean eddies using observational and numerical data is essential for understanding eddy dynamics and their global climate impacts. Eulerian (point-based) and Lagrangian (trajectory-based) schemes are widely used to detect and track ocean eddies. However, these methods typically do not provide information about the spatial structure of eddies or properties such as vorticity, deformation, and vertical tilt, especially when using sparse data. Here, we describe a new efficient and robust geometrical approach for mapping the three dimensional structure of ocean eddies by fitting (partial) velocity data to a simplified elliptical streamfunction model with a small number of parameters. The flexibility of the approach is demonstrated through three variants of the method adapted to different velocity sampling patterns: single and double sections (from ship transects or a numerical grid) and scattered data (e.g. from surface drifters). We validate and demonstrate these new geometric methods on idealised, axisymmetric and non-axisymmetric Gaussian eddies, as well as numerical and observational datasets. We conclude that elliptical streamfunction parametrisation offers a versatile and effective method for research into ocean eddy characteristics.

Abstract The East Australian Current (EAC) separates near 32°S, forming large, energetic eddies. While the seasonal cycle of the EAC system is well documented, the season‐specific long‐term trends in its separation latitude, eddy energetics, and their spatial heterogeneity remain poorly understood. Using 25‐year of satellite observations and high‐resolution model output, we find the significant southward shift in the EAC separation latitude is strongest in austral spring. Eddy kinetic energy (EKE) and mean‐to‐eddy energy conversion also show seasonal and spatial heterogeneity; decreasing strongly in the EAC retroflection during summer and increasing in the EAC southern extension during autumn. Barotropic instability dominates eddy generation year‐round in the retroflection, but only in autumn in the southern extension. EKE anomalies propagate southward with a 2–3 months lag. Our findings show that eddy energetics in the EAC system are characterized by spatial heterogeneity and seasonal dependence, accurately representing this spatiotemporal variability is essential for improving predictions.

There is a growing need for enhanced surface and subsurface coastal observations to enable integrated modelling and decision-support tools. Fishing vessels can serve as platforms for hosting and deploying an assortment of oceanographic instrumentation. Furthermore, many types of fishing gear already profile through the water column, presenting a unique subsurface data collection opportunity. Integrating ocean data collection with fishing vessel operations complements existing ocean observing networks by enabling the cost-effective acquisition of surface and subsurface ocean data, significantly expanding coverage in data-sparse regions. Counterintuitively, the shelf and coastal regions, where most fisheries operate, are among the most data scarce areas, especially for subsurface physical oceanographic data, which fishing vessels are so well suited to collect. These data can benefit fisheries science by coupling environmental information with catches, improving physical coastal and ocean models, and encouraging greater involvement of fishery stakeholders in the scientific process. This approach creates a win–win because the fishing industry can leverage the data to adopt innovative solutions to improve fisheries sustainability, profitability, and community resilience. To maximize these benefits and complement existing ocean observing networks, an emerging global network: the Fishing Vessel Ocean Observing Network (FVON), has been formed. FVON aims to foster the proliferation of collaborative cost-effective ocean data collection, democratize ocean observations, establish community standards and best practices, and facilitate observation uptake to improve ocean predictions while promoting sustainable fishing practices.

The eddy-mean flow interactions in western boundary currents are crucial for global ocean heat exchange and energy redistribution. However, to date, the eddy energetics in the Brazil Current (BC) system, one of the three main western boundary currents in the Southern Hemisphere, remains poorly understood. Here we examine the three-dimensional structure of the eddy-mean flow interactions and seasonal variability of energetics in the entire BC system. Results show that the BC jet features mixed barotropic-baroclinic instability along its path (25S-36S), with positive energy cascades enhancing eddy kinetic energy and potential energy. Strong eddy kinetic energy exists in the Brazil-Malvinas Confluence (BMC) located between 39S and 43S throughout the upper 1,000 m, which is dominated by baroclinic instability, with a clear subsurface maximum. Both the kinetic energy in the BC and the eddy kinetic energy in the BMC show distinct and vertically coherent seasonal variability with minima in winter, and the minimum eddy kinetic energy lags the mean kinetic energy by approximately 2 months. Barotropic and baroclinic conversions with comparable magnitude co-regulate EKE generation in the BMC, with energetic contributions differing by season and depth, advancing our understanding of eddy-mean flow interactions in the BC system.

Abstract. Understanding and predicting regional and coastal ocean dynamics requires the effective combination of numerical models and observations that resolve key processes at suitable time and space scales. Fine-scale oceanic features and the ocean's complex subsurface structure remain a significant source of uncertainty in coastal and regional models due to lack of observations at necessary scales. Recently available observation platforms provide an unprecedented view of the ocean's fine-scale structure both at the surface (the Surface Water and Ocean Topography satellite mission, SWOT) and below the surface (using data from the Fishing Vessel Observation Network, FVON). Here we use advanced data assimilation to demonstrate the impact of these novel observation types on dynamic ocean state estimates in an eddy-dominated western boundary current region, the East Australian Current. First we show that these newly available observations benefit from an updated data assimilation configuration. Using this improved configuration, we show that the inclusion of SWOT data improves model representation of both the ocean surface across scales and of subsurface temperature (as observed by FVON). Assimilating FVON data, which drastically increases subsurface information in the coastal and shelf region, significantly improves subsurface temperature representation. Using novel observations from SWOT and FVON in a realistic ocean model, we show enhanced representation of ocean structure, particularly below the surface, essential for improved ocean forecasts and projections.

Dispersion is a fundamental physical mechanism that shapes marine ecosystems through transporting and mixing heat, salt, and nutrients, as well as affecting the distribution and connectivity of organisms. Western Boundary Currents (WBC) are key to driving regional heat circulation, have immense economic and social value, and also have a significant impact on dispersion. Despite its importance, the understanding of dispersion in the East Australian Current (EAC) system, a dynamic WBC that is experiencing rapid changes, remains limited. Using 20 years of data collected as part of the Global Drifter Program, we conduct an in-depth analysis of dispersion characteristics in the EAC system. We show that both single (absolute) and pairwise (relative) dispersion metrics closely align with theoretical quasi- or surface quasi-geostrophic regimes based on both surface (undrogued drifter) and near surface (∼15 m; drogued drifter) observations, but absolute dispersion can be super-diffusive after the integral time scale, especially at the surface where wind and wave energy may play a role. Diffusivity is spatially heterogeneous, generally being higher where the current has a strong influence, but particularly so where the current separates into a vast eddy field in the Tasman Sea. Strengthening of the EAC jet and the eddy kinetic energy at its separation during austral summer can lead to an approximate doubling of the eddy diffusivity at 33° S and prolonged Richardson pairwise dispersion. This important baseline study demonstrates the influence that EAC dynamics can have on dispersion characteristics, and emphasizes the possible need for better model parameterizations that can account for anisotropic or spatially inhomogeneous dispersion prominent in dynamic WBC regions.

Subsurface marine heatwaves are newly recognized extreme ocean events with profound impacts on global marine ecosystems. Previous studies have linked their formation to downwelling favorable surface winds, warm ocean eddies, and planetary waves, yet whether other potential drivers of these heatwaves exist remains unclear. The tropical cyclones are well known to induce a pronounced warming in the subsurface. Here, we show that the induced warming can efficiently trigger subsurface marine heatwaves. It is shown from an ocean reanalysis that such warming reaches the heatwave threshold at least once along the tropical cyclone life cycle for 79.4% of the tropical cyclones. Tropical cyclone-induced heatwaves thus account for up to 40.0% of the heatwaves in cyclone-active regions. The area and intensity of tropical cyclone-induced heatwaves have increased over the past two decades, likely due to the steady warming of subsurface temperatures.

Ocean forecast models rely on observations to provide regular updates in order to correctly represent dynamic ocean circulation. This synthesis of observations and models is referred to as data assimilation. Since initial conditions dominate the quality of short-term ocean forecasts, accurate ocean state estimates, achieved through data assimilation, are key to improving prediction. Western boundary current (WBC) regions are particularly challenging to model and predict because they are highly variable. Understanding how specific observation types, platforms, locations, and observing frequencies impact model estimates is key to effective observing system design. The East Australian Current (EAC), the South Pacific’s WBC, is a relatively well-observed current system that allows us to study the impact of observations on prediction across different dynamical regimes, from where the current flows as a mostly coherent jet to the downstream eddy field. Here we present a review of the impact of observations on model estimates of the EAC using three different methods. Consistent results across the three approaches provide a comprehensive understanding of observation impact in this dynamic WBC. Observations made in regions of greater natural variability contribute most to constraining the model estimates, and subsurface observations have a high impact relative to the number of observations. Significantly, sampling the downstream eddy-rich region constrains the upstream circulation, whereas observing the upstream coherent jet provides less improvement to downstream eddy field estimates. Studies such as these provide powerful insights into both observing system design and modeling approaches that are vital for optimizing observation and prediction efforts.

Ocean forecast models rely on observations to provide regular updates in order to correctly represent dynamic ocean circulation. This synthesis of observations and models is referred to as data assimilation. Since initial conditions dominate the quality of short-term ocean forecasts, accurate ocean state estimates, achieved through data assimilation, are key to improving prediction. Western boundary current (WBC) regions are particularly challenging to model and predict because they are highly variable. Understanding how specific observation types, platforms, locations, and observing frequencies impact model estimates is key to effective observing system design. The East Australian Current (EAC), the South Pacific's WBC, is a relatively well-observed current system that allows us to study the impact of observations on prediction across different dynamical regimes, from where the current flows as a mostly coherent jet to the downstream eddy field. Here we present a review of the impact of observations on model estimates of the EAC using three different methods. Consistent results across the three approaches provide a comprehensive understanding of observation impact in this dynamic WBC. Observations made in regions of greater natural variability contribute most to constraining the model estimates, and subsurface observations have a high impact relative to the number of observations. Significantly, sampling the downstream eddy-rich region constrains the upstream circulation, whereas observing the upstream coherent jet provides less improvement to downstream eddy field estimates. Studies such as these provide powerful insights into both observing system design and modeling approaches that are vital for optimizing observation and prediction efforts.

Marine plastic pollution poses a global environmental challenge, understanding dispersal patterns at management-relevant scales is required to inform effective actions. We combine high-resolution oceanographic modelling and particle tracking with empirical data from Australia's largest marine debris database to investigate plastic debris transport across southeastern Australia. Our results challenge the assumption that marine debris primarily originates from distant sources, revealing that source regions are predominantly confined within a 15-km coastal band, with 50% of the debris modelled only spending 8 days at sea, and originating (on average) 20-km away. A comparison of empirically-informed simulations with uniform seeding scenarios shows significant differences in the number and intensity of identified marine source hotspots of plastic debris across the coastal band. These findings emphasize the importance of local management strategies and demonstrate how integrating observational data with oceanographic modelling can enhance our understanding of marine debris dynamics, ultimately supporting more targeted pollution reduction efforts.

Knowledge of the three-dimensional structure and variability of ocean temperature is critical for understanding ocean circulation, heat uptake, marine extremes, and the abundance and distribution of marine life. While satellite technology offers near-global coverage of surface ocean temperatures, subsurface observations represent a big gap in the coastal ocean record. Here we present the first results from FishSOOP (Fisheries Ships of Opportunity), Australia’s pilot program that uses commercial fishing gear to collect subsurface ocean data. Since early 2023, temperature and pressure data have been collected through the FishSOOP project across the Australian continental shelf and upper-slope waters. These new data provide insights into the development of marine heatwaves throughout the water column and new understanding of how the East Australian Current interacts with shelf water to produce nonuniform temperature changes. Comparison with the South East Australian Coastal Ocean Forecasting System (SEA-COFS) model indicates potential for improving forecasts of upper ocean heat content and subsurface temperatures by filling large gaps in observational data coverage. FishSOOP already provides a step change in the amount of open access temperature data available as well as ocean information critical to marine industries for operational decision-making, showing the value of using fishing vessels to observe challenging western boundary current regions.

Marine heatwaves (MHWs) pose a significant threat to marine ecosystems and economies. Predicting MHWs is essential for mitigating their impact, but remains a challenge. Despite considerable progress having been made in understanding the regional-scale drivers of MHWs, a significant knowledge gap remains when it comes to understanding the synoptic-scale processes associated with these events. In this study, we used self-organising maps to identify the synoptic-scale atmospheric and oceanic patterns associated with MHWs identified in four sub-regions of the Tasman Sea between 1985 and 2014. Our results reveal patterns associated with recurring, as well as distinct extreme warming events. We show that anomalous atmospheric influence is consistently present during MHWs and that the two most recurring patterns are linked to a La Niña climate phase. Distinct synoptic air-sea patterns are also identified in the 1997/98 El Niño event. Furthermore, we identify a ‘reservoir’ of warm subsurface temperatures from 2000-2014, during which MHW frequency increased two-fold. Importantly, we have identified patterns of persistent anomalous conditions before the onset of MHWs with timescales on the order of days for atmospheric conditions and weeks to months for oceanic conditions, providing valuable insight into MHW predictors. These findings highlight the importance of understanding synoptic-scale drivers of MHWs and timescales of recurring patterns for MHW prediction. The temporal variability observed in the lead-up to MHWs underscores the potential significance of factors such as surface-layer temperature and sea-level anomalies in capturing longer-term warming trends, likely influenced by sustained atmospheric stress and oceanic dynamics, whilst atmospheric conditions at onset precipitate the transition to the extreme warming thresholds.

Isak (aged 8, Queensland, Australia) asks Prof. Moninya Roughan how strong ocean boundary currents form.

Western boundary currents (WBCs) play a crucial role in global ocean circulation, regulating climate, influencing weather patterns, driving marine ecosystems, and transporting heat, momentum, and biogeochemical properties across ocean basins. Despite their importance, their strong variability and deep structures make them challenging to observe. Here, we synthesize the physical properties of the five major subtropical WBCs and highlight the need for improved and sustained observations. We present dynamically driven priorities for observation, emphasizing novel and cost-effective methods. Advances in satellite altimetry, autonomous vehicles, and ship-based measurements have enhanced monitoring efforts, but gaps remain, particularly in subsurface observations and cross-system comparisons. Emerging technologies such as the fishing vessel observation network and uncrewed surface vehicles provide new opportunities for broad-scale, high-frequency data collection. Modified Argo float deployments (more frequent profiling) and repeat glider missions offer improved resolution of eddy structures and upper-ocean heat content estimates. We emphasize the need for consistent observational strategies across WBC systems to enable direct comparisons and improve predictive modeling. Integrating satellite data with in situ observations and high-resolution models is essential for refining estimates of WBC variability, heat transport, and climate-driven changes. A coordinated, multi-platform approach for observation and analysis is critical to understanding WBC dynamics and their long-term impacts on regional and global climate.

The East Australian Current (EAC) exhibits significant variability across a wide range of spatial and temporal scales, from mesoscale eddies and meanders to seasonal, interannual, and long-term fluctuations in its intensity, pathway, and influence on the continental shelf circulation. Understanding and monitoring this variability is crucial, as the EAC plays an important role in controlling shelf dynamics, regional circulation, coastal weather, and global climate patterns. As such, two high-frequency (HF) coastal radar systems have been deployed on the eastern coast of Australia to measure surface currents upstream and downstream of the East Australian Current (EAC) separation point. The multiyear radar dataset (spanning 4–8 years) is presented here, and its use is demonstrated to assess the spatial and temporal variability in the EAC and the adjacent continental shelf circulation, ranging from seasonal to interannual scales. The dataset is gap-filled using a 2dVar approach (after rigorous comparison with the traditional unweighted least-squares (LS) fit method). Additionally, we explore the representation of the depth variability in the observations by comparing the data with surface Lagrangian drifter velocities (with and without depth drogues). The multiyear radar-derived surface current dataset, which was validated using short-term drifter and long-term current meter observations, revealed that the local upstream circulation is strongly dominated by the EAC's annual cycle, peaking in the austral summer. The analysis using 8 years of upstream data revealed the period of the EAC intensification at around 3–5 years. The interannual variability in the poleward transport downstream was driven by the intrinsic variability in the jet. This dataset which continues to be collected, complemented by numerical simulations and in situ measurements, will provide a comprehensive view of the EAC's variability and its impact on the broader regional circulation dynamics that can be used for a range of dynamical investigations. The datasets are freely available at https://doi.org/10.5281/zenodo.13984639 (Tran, 2024a).

Ocean boundary currents are complex and highly variable systems that play key roles in connecting the open and coastal ocean through cross-slope circulation and upwelling of nutrient-rich water. The structure, strength, and variability of boundary currents are associated with a broad range of spatial and temporal scales. For that reason, long-term boundary current monitoring is challenging and requires the use of complementary observing platforms and sensors coupled with numerical simulations. The Ocean Observations Physics and Climate Panel Boundary Systems Task Team recently held a virtual dialogue series to discuss six mature boundary current monitoring systems. The goal of the series was to examine strategies for developing a conceptual design for sustained observing activities applicable to a wide range of boundary current systems. This article provides a brief overview of the six systems, including users and the observational and modeling components needed to achieve scientific, operational, and societal goals. Ocean observing best practices and recommendations are shared to provide guidance for the coordination and sustainability of observing systems at ocean boundaries and to strengthen and integrate partnerships across and within the global observing networks.