Junde Li

Oceanic eddies play an important role in the transport, aggregation, and dispersion of heat, carbon, and nutrients, thereby regulating climate and marine ecosystems. In most western boundary current extensions (WBCEs), the integrated eddy kinetic energy (EKE) has increased significantly during the satellite era. However, the Kuroshio Extension exhibits pronounced spatial variations in its long‐term EKE trend, without a significant overall increase. Here, we show that EKE in the Kuroshio Extension exhibits a dipole trend pattern, with enhancement in the north and weakening in the south. This dipole trend pattern is driven by the increased stability of the Kuroshio Extension. As the Kuroshio Extension shifts northward due to increased stability, EKE generation via baroclinic energy conversion also shifts northward, whereas eddy activity decreases to the south owing to reduced baroclinic energy conversion. Furthermore, the increasing stability of the Kuroshio Extension is driven by a negative trend in wind stress curl anomalies across the North Pacific at corresponding latitudes, as quantitatively supported by a linear vorticity model. These findings highlight the distinct dynamics of EKE trends in the Kuroshio Extension in contrast to other WBCEs and provide insights for predicting future EKE changes.

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.

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.

The influence of the horizontal resolution on the structure of the Western Boundary Currents (WBCs) (e.g., the Kuroshio, Gulf Stream, Agulhas Current, the East Australian Current (EAC) and Brazil Current) is quantified by a new global gridded climatological dataset (named BOA_CLM) with different horizontal resolutions of 1°, 1/2°, 1/4°, 1/6°, 1/8° and 1/10°. As the resolution increases from 1° to 1/10°, the maximum frontal intensity in the upper 1950 dbar amplifies by 1.2-11.7 times, the penetration depth increases by 26.0%-144.8%, the surface width of the WBCs decreases by 23.3%-53.8% except for the EAC, and the strength of the geostrophic currents enhances by 1.0-6.6 times. At 1/10°, the detailed surface recirculation and the persistent fronts in the WBCs at 1000 dbar are evident, while they disappear at lower resolutions (1/4° to 1°). Furthermore, the geostrophic transports of the five typical sections in the WBCs all have a significant (R>0.86, P<0.03) quadratic relationship, while the potential vorticity remains largely unchanged by horizontal resolutions. From 1° to 1/10°, more mesoscale and even small-scale signals emerge, accompanied by steeper isopycnals from the surface to the deep ocean. As a result, the vertically averaged horizontal buoyancy gradient increases significantly (~ 70%), while the vertical buoyancy gradient (stratification) remains almost unchanged (< 1%). The study also shows that a resolution of 1/10° in the climatology is required to realistically resolve the detailed structure of the WBCs.

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.

Western Boundary Currents (WBCs) such as the East Australian Current (EAC) are vital for moving heat from low to high latitudes, controlling regional weather and global climate. Previous EAC System studies have provided a general overview of temperature variability and heat transport, but they lacked spatial and seasonal detail. Using a high‐resolution, 26‐year‐long ocean model simulation, we systematically characterize the seasonal variability and structure in key temperature metrics and heat transport in the EAC System. Our findings reveal a clear seasonal cycle with a poleward expansion of key variables in summer (mean and eddy kinetic energy, upper ocean heat content, heat transport) and contraction equatorward with reductions in winter. We show that the dynamical regime transition, from jet to eddy‐dominated at the EAC separation zone (S), modifies the net meridional heat transport. Upstream of separation, transport is poleward, with some zonal input of heat. Downstream of EAC separation, there is an increase in heat recirculation by mesoscale eddies combined with a smaller zonal export of heat. This study highlights the significance of resolving spatio‐temporal variability in WBC systems driven by mesoscale dynamics, which will have implications for the representation of critical dynamics in future climate models.

The barrier layer (BL) significantly impacts the upper ocean circulation and thermodynamic structure by inhibiting the heat and momentum exchange between the mixed layer (ML) and the subsurface layer. There exist sea surface temperature and salinity dipole modes in the tropical Indian Ocean, however, a BL dipole mode has not yet been identified. Using the latest observations and ocean reanalysis, here we show a robust BL dipole mode in the central and eastern equatorial Indian Ocean, which is highly correlated with the Indian Ocean Dipole (IOD) events. Composite analysis shows that the BL thickness anomalies peak in autumn and are much larger during positive IOD events than during negative IOD events. We show that a positive BL dipole phase is characterized by positive BL thickness anomalies in the central equatorial Indian Ocean and negative BL thickness anomalies in the eastern equatorial Indian Ocean, and vice versa for a negative BL dipole phase. During positive IOD events, negative surface salinity anomalies slightly affect the ML depth along the equatorial Indian Ocean. Positive subsurface temperature anomalies deepen the isothermal layer (IL) in the central equatorial Indian Ocean and strong negative subsurface temperature anomalies significantly lift the IL in the eastern equatorial Indian Ocean, controlling the BL thickness anomalies and forming a positive BL dipole pattern. This operates in an opposite direction during negative IOD events. Our study shows a close relationship between the BL dipole and the IOD and has far‐reaching implications for better understanding and predicting the IOD events.