Abstract The sea level anomaly (SLA) has been accurately tracked by satellite altimetry, yet its barotropic and depth-integrated baroclinic components are routinely interpreted using theoretical or modeled vertical structures. In this study, we utilized manometric SLA from the Gravity Recovery and Climate Experiment (GRACE) and steric SLA from observational and reanalysis temperature/salinity databases to evaluate their roles in seasonal variations of geostrophic velocity anomalies and eddy kinetic energy (EKE) in the South China Sea (SCS). Through the empirical orthogonal function analysis, we found that the manometric component of geostrophic velocity anomalies is closely associated with the western boundary current, reflecting a barotropic response to seasonally reversed wind stress in summer and winter. The steric component, primarily driven by baroclinic instability, shapes two large cyclonic (anticyclonic) gyres (Luzon and Nansha Gyres) in the northeastern and southern SCS during summer (winter), as well as small mesoscale anomalies in the northwestern SCS during spring and autumn. The cross-correlation analysis demonstrates considerable influence of wind stress on the surface dynamics throughout SCS, while wind stress curl predominantly contributes to the gyres and dipole system off the Vietnamese coasts. Opposing covariances between manometric and steric EKE along the eastern deep and western shelf sides of the southwestern continental slope were investigated via vertical density, temperature, and salinity anomalies along three transects. These patterns arise from seasonally distinct horizontal and vertical mixing structure in upper-layer and near-bottom cross-shelf currents, providing observational evidence for significant interactions between baroclinic and barotropic instabilities in coastal regions. Significance Statement This study uses in situ, satellite, and reanalysis data to reveal different contributions of mass-induced and temperature-/salinity-induced sea level changes on seasonal ocean dynamics in the South China Sea. By decomposing geostrophic velocity anomalies and eddy kinetic energy, we distinguished their respective responses to the wind stress and wind stress curl in the deep and shelf ocean, revealing the wind–topography–flow structure near the shelf boundaries. Our studies provide an observational perspective on how barotropic and baroclinic processes respond separately to external forces such as wind and seafloor features, advancing our understanding of regional ocean circulation.

Abstract Climate models project a weakening of the Indonesian Throughflow (ITF) under future warming, but how this manifests in the vertical flow structure of individual passageways and its implications for inter‐basin freshwater transport remains uncertain. Using state‐of‐the‐art climate model simulations, we explore future volume and freshwater transport changes across all three inflow pathways. We find that Pacific wind stress curl changes induce a southward migration of equatorial currents that reduce surface transport through the Halmahera Sea. The projected weakening of the Atlantic Meridional Overturning Circulation generates Kelvin waves that propagate into the ITF region and weaken thermocline and intermediate flow through Makassar Strait and Maluku Sea. Despite decreasing volume transport, ITF freshwater transport is projected to increase due to enhanced regional precipitation. Our results highlight the remote and regional processes that will modulate ITF circulation under future warming.

Abstract The upper‐layer western boundary currents (WBCs) in the South China Sea (SCS) play a crucial role in regulating ocean dynamics, marine ecosystems, and regional climate. However, due to limited observations and coarse model resolution, changes in these currents on centennial timescales remain unclear. This study investigates trends in upper‐layer circulation in the SCS from 1951 to 2050 using outputs from an ensemble of 16 high‐resolution climate model members. Under global warming, the WBCs exhibit strengthening in both winter and summer, with a 7.7% increase (0.60 ± 0.47 Sv) in the winter Vietnam Coastal Current (VCC) and an 8.5% increase (1.10 ± 0.55 Sv) in the summer Vietnam Offshore Current (VOC) over the study period. Further analysis using a 1.5‐layer reduced‐gravity model suggests that enhanced stratification drives the intensification of the VCC in winter while the strengthening of the VOC in summer is primarily modulated by increased stratification and partially influenced by the intensified local wind stress curl. The results of this study may serve as a foundation for further research on the ecological effects caused by enhanced WBCs of the SCS.

Abstract The Pacific South Equatorial Countercurrent (SECC) is a crucial but poorly characterized component of the equatorial current system. Here, we investigate its seasonal and interannual variability using Argo‐derived absolute geostrophic currents. During boreal spring, the SECC attains its widest meridional (5°S–13°S) but narrowest zonal (150°E−170°W) extent, with deepest penetration of over 300 m and a maximum transport of 12.8 Sv. In contrast, the SECC in summer weakens substantially as it shoals to depths less than 150 m, resulting in a transport of no more than 6.5 Sv. By autumn, the SECC extends eastward, reaching a peak zonal range (150°E−140°W). Subsequently, its western branch strengthens and deepens, while the eastern part retreats. These seasonal shifts are closely linked to the first‐mode baroclinic Rossby waves forced by remote wind stress curl anomalies, particularly over 180°–140°W. Contrasting responses are also observed during El Niño and La Niña events. The SECC expands horizontally but contracts vertically during El Niño, with positive velocity anomalies progressing from north to south; and vice versa during La Niña. Correspondingly, the SECC transport shows little difference between El Niño and La Niña during the developing phases, but by summer, during its decaying phase, the El Niño transport reaches nearly twice that of La Niña. Sensitivity experiments show that the wind stress curl anomalies east of 140°W primarily control the SECC on El Niño‐Southern Oscillation (ENSO) timescales between 180° and 140°W, while anomalies over 180°–140°W govern the SECC west of 180°.

Abstract Polynyas are within the sea ice cover, typically formed by wind‐driven sea ice divergence or upwelling of warm subsurface waters. They play a crucial role in ocean‐atmosphere interactions, climate regulation and marine ecosystems by substantially enhancing primary production. Open‐ocean polynyas in the Southern Ocean are rare and are typically associated with deep convection, which disrupts conventional circulation pathways and impacts regional heat and carbon budgets. The Cosmonauts Sea (30°E–60°E) is an exception, with open‐ocean polynyas forming annually. Using satellite‐derived sea ice observations, we examined the spatiotemporal variability of polynyas in this region over the past two decades. The Cosmonauts Sea polynya exhibited large spatial and interannual variability, with the largest event occurring in 2016 (139,000 km 2 ). An Argo float near the polynya recorded deep mixed layers (>400 m) and near‐complete erosion of stratification, and the presence of dense water. This event coincided with anomalously intense cyclonic wind stress curl due to synoptic scale storms and a prolonged positive Southern Annular Mode (SAM) phase (2014–2016), both generally associated with reduced sea ice concentrations. While the southward shift of the Antarctic Circumpolar Current (ACC) during 2015 acted as a preconditioning mechanism, bringing warmer water towards the polynya region and inducing upwelling by vortex stretching. Additionally, anomalously high shortwave radiative fluxes (∼+20 Wm −2 ) were observed in the summer preceding the 2016 event. The deep convective mixing observed during this event, together with the presence of dense water, indicates that the Cosmonauts Sea could be a potential dense water formation site.

The Canary Current upwelling system, located along the northwest African coast between approximately 10ºN and 35ºN, is among the most productive marine ecosystems globally, supporting a rich marine biodiversity. In the southern part (9ºN–18ºN), pronounced interannual variability in net primary production (NPP) is influenced by extreme warm and cold events, known as Dakar Niños and Niñas, respectively. In this study, we analyze the physical mechanisms driving the interannual variability of NPP from 2003 to 2023, using a combination of satellite observations, reanalysis data, and ocean model outputs. Our results indicate that the interannual NPP variability is closely linked to changes in sea surface temperature, with the most pronounced effects occurring during March-April-May, i.e., the main upwelling season. A total of six undocumented extreme coastal low NPP events are identified, nearly all of which are associated with Dakar Niños. These low NPP events are linked to both local and remote forcing. The local forcing is associated with fluctuations of alongshore winds and near-coastal wind stress curl. The remote forcing involves the propagations of coastal trapped waves emanating from the equator and the northern Gulf of Guinea. Part of the local and remote forcing can be associated with large-scale climate modes.Supplementary InformationThe online version contains supplementary material available at 10.1038/s41598-025-31860-y.

Abstract Subduction of water masses is crucial for thermocline ventilation and climate variability. Using four ocean reanalysis products, we identify a significant interannual see‐saw variability in subduction rates of the South Pacific western and eastern Subtropical Mode Water (SPWSTMW and SPESTMW), which is primarily driven by changes in the mixed layer depth (MLD). Composite analysis demonstrates that the El Niño–Southern Oscillation induced winds are largely responsible for variations in latent heat flux, which alter MLD and modulate the subduction rates. In addition, a west‐strong/east‐weak subduction rate pattern in 1963–1978 and a west‐weak/east‐strong subduction rate pattern in 1998–2011 are observed. This long‐term shift can be attributed to shoaling MLD in the SPWSTMW formation region and intensifying wind stress curl in the SPESTMW formation region, both of which are associated with the intensification and southward extension of the subtropical high.

Abstract Near-bottom currents collected over 1 year (August 2021–22) using a current- and pressure-recording inverted echo sounder (CPIES) at a depth of 1060 m showed fluctuations within a frequency band between 2 and 6.5 days near the southwestern slope of the Chukchi Abyssal Plain. The amplitude of the fluctuations was approximately 8 cm s −1 on average during the summer months and weakened to approximately 3 cm s −1 between February and June 2022. Similar fluctuations were reproduced by the data-assimilated Hybrid Coordinate Ocean Model (HYCOM), confirming that they were bottom intensified. Calculations of the bottom-trapping scale using HYCOM revealed that these fluctuations could be attributed to topographic Rossby waves (TRWs) with a length scale of approximately 50 km. The spatial distributions of TRWs in HYCOM and ray-tracing results suggest that TRWs likely propagated from the west-southwest. It is suggested that these TRWs were triggered by nonlocal wind stress curl (WSC), 220 km to the west along the continental slope, as the coherence in the TRW frequency band between the TRWs and WSC was significant. The weaker TRW signal from February to June 2022 was related to weaker WSC and higher sea ice concentration in the study area. The stronger TRWs from July to October occurred when the WSC was stronger and the sea ice concentration was lower in the study area. Our findings imply that changes in the Arctic WSC field or a longer sea ice–free season could trigger more energetic and frequent TRWs, observable down to 1000-m depth around the southwestern slope of the Chukchi Abyssal Plain.

Abstract From autumn to spring, the dynamics and thermodynamics of the anticyclonic eddies detached from the Loop Current in the Gulf of Mexico are actively influenced by the passage of cold fronts and accompanying strong northerly winds (locally called Nortes). In this work, the dynamical mechanisms of eddy–strong wind interactions are analyzed based on realistic, forced, eddy-permitting (∼3 km) numerical simulations of the NEMO ocean model that reproduce observations collected from an oceanographic cruise in November 2022 and satellite data. We focus the analysis on three interconnected aspects: (i) the upper-ocean rapid response to the passage of Norte events, where deepening of the mixed layer and upwelling velocities in the eddy interior are identified; (ii) the modulation of the vertical velocity patterns by the wind stress curl and the horizontal advection of vorticity due to Ekman transport, finding that the latter term is the dominant contributor under moderate winds, and similar contributions from both terms under strong wind forcing; and (iii) the rate of wind work and wind power input on the geostrophic flow over the anticyclone and its dependence on specific vortex and wind parameters. By contrasting simulations that use the absolute or relative wind in the wind stress parameterization, we also find that the eddy is either intensified or less damped in the absolute wind simulation. Furthermore, this is a dynamically consistent response identifiable in day-to-day variations. This analysis contributes to the understanding of key air–sea interactions occurring at the mesoscale in a highly energetic ocean region.

Abstract The interannual variations of wintertime mixed layer depth (MLD) in the northern South China Sea (SCS), a region characterized by the deepest MLD within the basin and subduction process, are elucidated based on the Simple Ocean Data Assimilation (SODA, version 2.2.4) reanalysis data between 1950 and 2010. Our results reveal that the wintertime MLD possesses two predominant modes of 2–4‐year and 7–8‐year, involving different dynamical processes. Responses of thermal structure to local air‐sea interface factors primarily explain the 2–4‐year period MLD variability, with sea surface net heat flux playing a more crucial role than wind stress curl. Whereas for the 7–8‐year period MLD variability, Luzon Strait Transport (LST), high‐dense inflow from the Pacific Ocean into the northern SCS, is the dominant driver, while local air‐sea interface factors play a minor role. The difference between surface and subsurface layers inflow (), which is appropriate to represent the LST impact on vertical density gradient, can effectively modulate the upper layer stratification intensity in the northern SCS. Thus, a stronger (weaker) conduces to establish a more unstable (stable) state therein, favorable to a vigorous (stagnant) vertical mixing and MLD deepening. Further analyses demonstrate the importance of both horizontal heat and salt advections related to LST in the northern SCS. That is to say, different from the 2–4‐year MLD variability, the salinity effect is of importance in driving this 7–8‐year MLD variability.

Abstract The Miocene (∼23–5 Ma) experienced substantial paleogeographic changes, including the shoaling of the Panama Seaway and closure of the Tethys Seaway, which altered exchange pathways between the Pacific and Atlantic Oceans. Changes in continental configuration and topography likely also influenced global wind patterns. Here, we investigate how these changes affected surface wind‐driven gyre circulation and interbasin volume transport using 14 fully coupled climate model simulations of the early and middle Miocene. The North and South Atlantic gyres, along with the South Pacific gyre, are weaker in the Miocene simulations compared to pre‐industrial (PI), while the North Pacific gyres are stronger. These changes largely follow the wind stress curl and basin width changes. Westward flow through the Panama Seaway occurs only in early Miocene simulations when the Tethys Seaway is open and transports are strongly westward. As the Tethys transport declines, flow across the Panama Seaway gradually reverses from westward (into the Pacific) to eastward (into the Atlantic). In simulations with a closed Tethys Seaway, the Panama transport is consistently eastward. The Southern Hemisphere westerlies are weaker than PI in all simulations, contributing to a reduced Antarctic Circumpolar Current (ACC) in 11 of the 14 cases. In the remaining three, a stronger ACC is simulated, likely due to a combination of enhanced meridional density gradients and model‐dependent sensitivities. These findings highlight how changes in Miocene seaways and wind patterns reshaped ocean circulation, influencing interbasin exchange, thermohaline properties, and global climate.

Abstract We investigate key mechanisms driving the Atlantic extratropical-tropical teleconnection and associated Atlantic Intertropical Convergence Zone (ITCZ) shift in boreal summer under a strong external freshwater forcing using a coupled climate model. Our analysis reveals that the wind-evaporation-sea surface temperature (SST) feedback is not the primary mechanism. Instead, the southward advection of the upper extratropical North Atlantic signal by the North Atlantic subtropical gyre along a horseshoe pathway is a key mechanism for forming the horseshoe pattern of cold SST anomalies. Additionally, the weakening of the Atlantic Meridional Overturning Circulation changes the upper tropical North Atlantic western boundary current. This change is amplified by enhanced surface wind stress curl over the tropical North Atlantic, contributing to warmer tropical Atlantic subsurface thermocline temperature and SST in the tropical South Atlantic. The dipole Atlantic SST anomalies lead to the trade wind response and associated southward ITCZ shift over the tropical Atlantic.

ABSTRACT The variability of the Kuroshio Extension (KE) system, modulated by large-scale atmospheric forcing, critically influences North Pacific climate responses. However, despite the ongoing unprecedented northward KE shift, the low-frequency modulations on hydrography within the KE system and their impacts on the Kuroshio-Oyashio Confluence (KOC) region remain unclear. We analyzed hydrographic properties and mesoscale activities in the KE-KOC system and evaluated the effects of atmospheric forcing using satellite and reanalysis data from 1993 to 2023. Our findings indicate a poleward KE shift (0.45 ± 0.02°/decade) and increases in Sea Level Anomaly (SLA, 8.07 ± 0.30 cm/decade in the KE, 2.45 ± 0.27 cm/decade in the KOC) driven by negative wind stress curl (WSC) trends across the mid-latitude North Pacific. Following 2010, the KE system transitioned to a predominantly stable state, except for 2016–2017. The KOC region experienced enhanced eddy kinetic energy during convoluted KE meander phases (2021–2023), indicating baroclinic instability-driven energy transfer. The enhanced negative WSC anomalies north of 40°N and positive anomalies southward created a meridional dipole pattern in the eastern North Pacific after 2010. Accompanied by this shift, westward Rossby wave propagation was disrupted by some mesoscale signals, altering negative phase relationships between KE dynamics and climate indices to near-zero lags, resulting in the synchronized record-high SLA both in the KE and eastern North Pacific, coinciding with extreme negative Pacific Decadal Oscillation conditions during 2019–2023. These results reveal a new state of the North Pacific climate system and highlight the KE-KOC system’s accelerated adjustment to atmospheric forcing.

Abstract The Bay of Bengal (BoB), where the Indian monsoon modulates air‐sea interactions, upper‐ocean dynamics, and biochemical processes, hosts the Sri Lanka Dome (SLD)—a biological hotspot characterized by unique thermohaline structures. Previous case studies observed summer chlorophyll‐a (chl) blooms near the SLD, attributing them to monsoon‐driven local wind stress curl (WSC) anomalies. However, these event‐based analyses left two critical gaps unresolved: (a) the existence of a concurrent negative chl anomaly core elsewhere in the BoB remains unconfirmed; and (b) the dynamics distinction and relationship between these potential cores. Using long‐term satellite observations (1997–2023), this study identifies a robust summer chl spatial contrast structure in the BoB, featuring an anomalous high‐concentration southern core (0.18 mg m −3 ) and a neglected low‐concentration northern core (−0.06 mg m −3 ). The southern core is sustained by enhanced local upwelling from WSC anomalies (accounting for 53%) and accompanied cyclonic mesoscale eddies. In contrast, the northern core's chl suppression is linked to remotely forced downwelling Rossby wave (accounting for 79%). These waves, triggered by Wyrtki Jet during boreal fall monsoon transition, arriving in the northern region after a 6‐month lag. Their downwelling effect deepens the thermocline, suppressing vertical nutrient flux—a process corroborated by Argo profiles showing a summer mixed layer shallower than the chl maximum depth. By quantifying monsoon forced local Ekman/eddy pumping and remotely forced Rossby wave, this work advances understanding of biophysical processes in the BoB, emphasizing the critical role of ocean dynamics to monsoon system in regulating low‐latitude marine productivity.

Abstract The partial pressure of carbon dioxide at the sea surface (pCO 2sea ) is a key component in the ocean carbon cycle, jointly influenced by the thermodynamic, dynamical, and biological processes. Among these, thermodynamic control generally induces a pronounced positive covariation between pCO 2sea and sea surface temperature (SST). However, using multiple observations, reanalysis data sets, and BIO‐ROMS model simulations, this study reveals an anomalous decoupling between pCO 2sea and SST in the northern Bay of Bengal (BoB) during Indian Ocean dipole (IOD) events, highlighting the importance of non‐thermal mechanisms in determining pCO 2sea interannual variability. During positive IOD events, anomalies in dissolved inorganic carbon (DIC) and sea surface salinity (SSS) jointly exert positive influences on pCO 2sea , though partially offset by the opposite SST effects. Specifically, IOD‐related negative SST anomalies in the equatorial eastern Indian Ocean trigger the Matsuno‐Gill atmospheric response, enhancing evaporation and suppressing precipitation over the northern BoB. This leads to positive freshwater flux anomalies that elevate both DIC and SSS, contributing to increased pCO 2sea . Simultaneously, positive wind stress curl anomalies in the northwestern BoB enhance cyclonic eddy and upwelling, bringing the colder, DIC‐rich subsurface water into the mixed layer. Overall, these processes result in surface cooling while further enriched DIC. Moreover, anomalous southerly wind in the southwestern BoB weakens the East India Coastal Current, facilitating anomalous transport of saline water that enhances positive SSS anomalies, thereby increasing pCO 2sea anomalies. Our findings underscore the complex interplay between thermodynamic and dynamical processes in shaping BoB carbon cycle variability under IOD influence.

Abstract The observed Atlantic Niño/Niña displays robust variations at decadal timescale (decadal ATL), besides the well‐known interannual variability. The underlying mechanisms, however, remain largely elusive. Analyzing observations and model outputs, we find the decadal ATL originates in the South Atlantic. During its positive phase, the cold tongue warming, triggering atmospheric Rossby wave train, weakens the St. Helena anticyclone, which enhances wind and cools sea surface temperature over the Southwestern Atlantic, leading to the positive phase of the South Atlantic Ocean Dipole. Meanwhile, the weakened anticyclone reduces the transport of the subtropical cell, suppressing the equatorial upwelling, which amplifies the initial cold tongue warming. The phase shift of the decadal ATL is attributed to an eastward propagation of thermocline displacements at 3°S–15°S, induced by a propagation of local wind stress curl anomalies in response to combined effects of the equatorial and mid‐latitude air‐sea coupling.

Abstract The Gulf of Mexico (GoM) coast has experienced an acceleration of sea‐level rise between about 2010 and 2020, garnering notable attention from both the scientific and coastal communities. This study investigates the underlying causes of this acceleration by comparing high‐resolution (HR) and low‐resolution (LR) ensembles of multi‐year prediction simulations and historical climate simulations. The findings demonstrate that HR outperforms LR in predicting this acceleration, although they perform comparable prediction skill caused by external forcings. As the acceleration was driven by internal dynamics rather than external climate forcings, improved prediction skill in HR is attributed to its enhanced ability to capture internal variability. Further analysis reveals a strong link between GoM sea‐level variability and a dipole‐like wind stress curl anomaly straddling the region around Cuba, generating Ekman pumping and suction, and triggering remote changes in GoM sea‐level rise through Rossby wave propagation. HR effectively captures this process likely due to its improved prediction of the multi‐year Atlantic Meridional Mode.

Marine heatwaves (MHWs) are extreme sea surface temperature (SST) phenomena that can profoundly impact marine ecosystems but remain largely unexplored in the Arabian Gulf. We use a high resolution satellite SST and atmospheric reanalysis datasets to perform an extensive study of spatial and temporal variations of MHWs and their potential driving factors. Significant positive trends in MHW days, duration, and frequency are observed, particularly in the southeastern Gulf and the Sea of Oman during the summer. The Gulf experienced an increase of 0.6 MHW days/yr with an increasing intensity of 0.05^{circ }C/yr in summer while summer SST also shows an increase of 0.07^{circ }C/yr, indicating a notable warming trend. The first two EOF modes explain 80% of MHW variability, showing a Gulf-wide pattern in the first mode (62%) and contrasting Gulf versus Sea of Oman behavior in the second (18%), reflecting different underlying physical processes. Weakened Shamal winds and strengthened Kaus winds are observed during major MHW years, suggesting their potential influence on surface warming. MHWs often coincide with negative wind stress curl in upwelling zones and reduced latent heat loss, indicating weakened ocean cooling. While positive mean sea level (MSL) pressure anomalies are frequently observed, their inconsistency suggests that multiple ocean–atmosphere processes contribute to MHW development. Understanding these trends helps predict future extremes and mitigate their impacts, contributing to global efforts to protect marine ecosystems and coastal communities in a warming world.

A persistent high-chlorophyll turbid zone off the west coast of Hainan Island, China.

Abstract This study analyzes the effects of interannual variation in surface heat flux on the variability of the upper layer circulation in the East Sea (Sea of Japan) via a series of numerical experiments. Comparing the intrinsic variability of the upper layer circulation, the interannual variation in the surface heat flux amplifies the variability in the Yamato Basin, but not in the Ulleung Basin. The variability in the water temperature in the northern region is highly correlated with the variability in the surface heat flux with a 1-month time lag. Under strong cooling conditions, the cyclonic gyre and convection in the northern region are intensified, and the expansion of the cold water area facilitates a more pronounced southward intrusion compared to weak cooling conditions. However, the interannual variation in surface heat flux does not significantly impact the northernmost latitude of the western boundary current (i.e., the East Korea Warm Current). Instead, the northernmost latitude of the East Korea Warm Current is influenced primarily by the magnitude of the strong positive winter wind stress curl in the northern region, which greatly affects the southward flow of cold water along the Korean coast. An increased (decreased) volume transport or lateral heat flux through Korea/Tsushima Strait promotes the eastward (westward) propagation of eddies distributed in the Yamato Basin, thereby affecting the meandering pattern of the Tsushima Warm Current. This study is expected to enhance understanding of the respective contributions of interannual variation in individual external forcing factors to the variability of the upper layer circulation in the East Sea.