Search

Ocean eddies affect large‐scale circulation and induce a kinetic energy cascade through their non‐linear interactions. However, since global observations of eddy dynamics come from satellite altimetry maps that smooth eddies and distort their geometry, the strength of this cascade is underestimated. Here, we use deep learning to improve observational estimates of global surface geostrophic currents and explore the implications for the cascade. By synthesizing multi‐modal satellite observations of sea surface height (SSH) and temperature, we achieve up to a 30% improvement in spatial resolution over the community‐standard SSH product. This reveals numerous strongly interacting eddies that were previously obscured by smoothing. In many regions, these newly resolved eddies lead to nearly an order‐of‐magnitude increase in the upscale kinetic energy cascade that peaks in spring and is strong enough to drive the seasonality of large mesoscale eddies. Our study suggests that deep learning can be a powerful paradigm for satellite oceanography. Plain Language Summary: We developed a deep learning method to estimate global maps of surface ocean currents from satellite observations with significantly improved resolution and accuracy compared to existing methods. These maps dramatically improve our ability to observe eddy dynamics and the impact of eddies on the transfer of energy between scales in the ocean. Our study suggests that deep learning can be a powerful paradigm for satellite oceanography. Key Points: We develop the first deep learning global estimates of surface ocean currents from multi‐modal satellite observationsOur deep learning method is able to map surface currents with state‐of‐the‐art resolution and accuracyThe diagnosed kinetic energy cascade is an order of magnitude higher compared to conventional altimetry products [ABSTRACT FROM AUTHOR]

Harnessing ocean kinetic energy has emerged as a promising renewable energy solution in recent years. However, identifying optimal locations for extracting this energy remains a significant challenge. This study presents a novel scheme to estimate the total surface current (TSC) as permanent surface current by integrating geodetic data and in-situ measurements. The TSC is typically a combination of the geostrophic current, derived from dynamic topography, and the Ekman current. We utilize NOAA's Ekman current data to complement the geostrophic current and obtain the TSC. To further enhance the accuracy of the TSC estimates, we employ a 3DVAR data assimilation method, incorporating local current meter observations. The results are verified against two control current meter stations. The data-assimilation process resulted in an improvement of 4 to 15 cm/s in the precision of calculated TSC. Using the assimilated TSC data, we then assess the kinetic energy potential and identify six regions with the most significant promise for marine kinetic energy extraction. This innovative approach can assist researchers and policymakers in targeting the most suitable locations for harnessing renewable ocean energy. [ABSTRACT FROM AUTHOR]

The recurring occurrence of green tides as an ecological disaster has been reported annually in the Yellow Sea. While remote sensing technology effectively tracks the scale, extent, and duration of green tide outbreaks, there is limited research on the underlying driving mechanisms of green tide drift transport and the determination of the leeway coefficient. This study investigates the green tide transport mechanism and evaluates the feasibility of estimating the leeway coefficient by analyzing green tide drift velocities obtained from Geostationary Ocean Color Imager-II (GOCI-II) images using the maximum cross-correlation (MCC) technique and leeway method across various time intervals alongside ocean current and wind speed data. The results reveal the following: (1) Significant spatial variations in green tide movement, with a distinct boundary at 34°40′N. (2) Short-term green tide transport is primarily influenced by tidal forces, while wind and ocean currents, especially the combined Ekman and geostrophic current component, predominantly govern net transport. (3) Compared to 1, 3, and 7 h intervals, estimating the leeway coefficient with a 25 h interval is feasible for moderate-resolution geostationary images, yielding values consistent with previous studies. This study offers new insights into exploring the transport mechanisms of green tides through remote sensing-driven velocity. [ABSTRACT FROM AUTHOR]

We analyze the feasibility of so-called radial surface currents as derived from Sentinel-1 Wave mode Doppler shift observations to support the geodetic estimation of the mean dynamic topography (MDT). Broadly defined as subtracting the geoid from the mean sea surface, this separation problem suffers from data inhomogeneities and usually requires strong prior assumptions about the MDT's smoothness. Recent advancements in calibration and current retrieval methodology for Sentinel-1 data give reason to assess potential gains from this observation type, which can be linked to the gradient of the MDT under geostrophic approximation. Due to current lack of long time-series, we synthesize 10 years of observations from daily surface currents grids and include these in a parametric least-squares adjustment of the MDT. Our results utilizing the synthetic data in place of smoothness constraints are promising, showing 2 cm RMS agreement with a state-of-the-art MDT model. [ABSTRACT FROM AUTHOR]

By decomposing the total sea surface current into its geostrophic and ageostrophic components, we examined the contribution of each to the long-term variability of the total sea surface current. Our findings demonstrate the importance of geostrophic currents in Baltic Sea gyre formations. Additionally, ageostrophic currents contribute significantly to the flow across the region. Quantifying the difference between total sea surface current fields has revealed two dominant general sea surface circulation patterns in the Baltic Sea, whose characteristics correspond to the monthly mean climatology of sea surface current fields in May and December. Subsequently, a machine learning technique was employed to effectively detect the types of sea surface circulation patterns using wind vectors and sea level anomaly fields. This underscored the combined influence of sea level anomaly-driven and wind-driven components in shaping surface current vectors in the Baltic Sea, consistent with geostrophic and ageostrophic decompositions. [ABSTRACT FROM AUTHOR]

In order to address the issue of limited common coverage and high cost in mapping ocean surface vector current by two (or more) high-frequency surface wave radars, this paper proposes a single-station surface wave radar vector current inversion algorithm. The feasibility of this algorithm has been validated in the Taiwan Strait. Based on the ocean dynamic characteristics of the Taiwan Strait, the algorithm utilizes the radial current obtained from a high-frequency surface wave radar (HFSWR) in Fujian Province to invert the ocean surface vector current. The surface vector current can be decomposed into three primary dynamic components: tidal currents, wind-driven currents, and geostrophic currents. Firstly, tidal current forecasting models and Ekman and Stokes theories are used to calculate the tidal and wind-driven currents in the Taiwan Strait, respectively. Subsequently, the directions of geostrophic currents in the Taiwan Strait are determined with sea surface height data, and the magnitudes of the geostrophic currents are constrained using the radial current from the single HFSWR. Finally, the three components are added together to obtain the vector current. Comparative results demonstrate that the efficacy of the algorithm has been validated through field experiments (with two HFSWRs and two drifting buoys) conducted in the southwestern of the Taiwan Strait. Further research is needed on the applicability of this algorithm to other sea areas and monitoring systems. [ABSTRACT FROM AUTHOR]

The Barents Sea is one of the main pathways for warm and saline Atlantic Water (AW) entering the Arctic Ocean. It is an important region where water mass transformation and dense‐water production contribute to the Atlantic meridional overturning circulation. Here, we present data from three cruises and nine glider missions conducted between 2019 and 2022 in the northwestern Barents Sea, and compare them with historical data collected between 1950 and 2009. We present circulation pathways, hydrography and volume transports of Atlantic‐ and Arctic‐origin waters. Our observations show that 0.9 ± 0.1 Sv (1 Sv = 106 m3 s−1) of Atlantic‐origin water reaches the Polar Front (PF) region before splitting into several branches and eventually subducting beneath Polar Water (PW). The amount of Atlantic‐origin water stored in the Olga Basin north of the PF is controlled by the density difference between AW and PW, and reached a maximum in the 90s when PW was particularly fresh. In the recent period from 2019 to 2022, the inflow of AW into the Barents Sea freshened by up to 0.1 g kg−1 compared to previous decades. This led to a reduction in the production of dense water, an increased temperature gradient across the PF, and a reduced poleward transport of warm water. Plain Language Summary: Warm and salty water from the Atlantic Ocean flows through the Barents Sea on its path toward the Arctic Ocean. It undergoes cooling and freshening due to interactions with the surrounding water and the atmosphere, and eventually encounters much colder and fresher Polar Water, creating a distinct boundary known as the Polar Front. We conducted several research missions in the northwestern Barents Sea between 2019 and 2022 and compared the data to historical records collected between 1950 and 2009. The cooled Atlantic Water sinks beneath the Polar Water before continuing toward the Arctic Ocean. In recent decades, the temperature of the Atlantic Water inflow has increased. The warm Atlantic Water can be traced below the Polar Front, leading to an increase in the temperature of the deeper waters north of the front. However, the amount of Atlantic Water north of the front is regulated by the density difference between the Atlantic Water and Polar Water. In the recent period of 2019–2022, the Atlantic Water reaching the front has become less salty. As a result, there is a reduced flow of warm water moving northward beneath the Polar Water and a larger temperature difference across the Polar Front. Key Points: Circulation pathways and volume transport of Atlantic‐ and Arctic‐origin waters in the northwestern Barents Sea are detailedAt the Polar Front density differences between these waters create a geostrophic current that governs warm water flow into the Arctic domainOver recent decades Atlantic Water warmed and freshened, reducing its density difference with the Polar Water and inflow across the front [ABSTRACT FROM AUTHOR]

A shift of El Niño onset location from eastern Pacific (EP) to western Pacific (WP) occurred around 1970. It was accompanied by a faster mean sea surface temperature (SST) warming in WP and a change of precursory SST and wind anomaly patterns. The eigenvalue analysis of a simple coupled model shows that an SST anomaly (SSTA) in WP may grow under the post-1970 mean condition but cannot under the pre-1970 mean condition. As a result, a warm SSTA appeared in WP accompanying to a preceding La Niña condition in EP after 1970, whereas such a warming was rarely seen before 1970. The preceding SSTA patterns led to distinctive zonal wind responses in EP, favoring El Niño onset in EP prior to 1970. For the post-1970 El Niño onset, an initial warming in WP was induced by anomalous downward solar radiation in association with atmospheric meridional overturning circulation or anomalous horizontal advection associated with thermocline induced eastward geostrophic currents. [ABSTRACT FROM AUTHOR]

We have analyzed the relationship between wind variability and sea level anomalies (SLA) on the Southwestern Atlantic Continental Shelf, focusing on sub-annual temporal scales. For this, we tested the capability of gridded altimetry to represent wind-driven SLA and compared results using an oceanographic model and tide gauge data. The present study used coherence analysis to analyze frequencies for which SLA and wind stress are coherent. The altimetry-SLA were found to have less energy below the three-month period compared to the model SLA. The coherence of along-shore wind stress and altimetry SLA was only significant for > 50 days (d), while the model SLA showed significant agreement in all periods considered, 20 d to annual. We further showed that geostrophic velocities on the continental shelf agreed significantly with SLA for > 50 d. As a result of an Empirical Orthogonal Function (EOF) analysis, we found that the second mode is highly coherent with the along-shore wind stress and accounts for 18.1% and 10.7% of variability in the model and altimetry sea level anomalies, respectively. [ABSTRACT FROM AUTHOR]

In the northwestern Barents Sea the warm and salty Atlantic Water meets the cold and fresh Polar Water, forming a distinct thermohaline front, the Barents Sea Polar Front. Here we present the structure of the front, its variability and associated mixing using observations from two cruises conducted in October 2020 and February 2021 during the Nansen Legacy project in the region between the Hopen Trench and the Olga Basin. Ocean stratification, currents and turbulence data were obtained during seven ship transects across the Polar Front near 77° N, 30° E. These transects are complemented by four missions using ocean gliders, one of which was equipped with microstructure sensors to measure turbulence. Across the front, we observe warm (> 1 °C) and salty (> 35.0 g kg -1) Atlantic Water intruding below the colder (< 0 °C) and fresher (< 34.6 g kg -1) Polar Water, setting up a baroclinic front and geostrophic currents reaching 25 cm s -1 , with estimated eastward transport of 0.2±0.6 Sv (1 Sv =1×106 m 3 s -1). We observe anomalous warm- and cold-water patches on the cold and warm side of the front, respectively, colocated with enhanced turbulence, where dissipation rates of turbulent kinetic energy range between 10-8 and 10-7 W kg -1. Short-term variability below the surface mixed layer arises from tidal currents and mesoscale eddies. While the effects of tidal currents are mainly confined to the bottom boundary layer, eddies induce significant shifts in the position of the front and alter the isopycnal slopes and the available potential energy of the front. Substantial water mass transformation is observed across the front, likely a result of eddy-driven isopycnal mixing. Despite the seasonal changes in the upper layers of the front (0–100 m) influenced by atmospheric forcing, sea ice formation and brine rejection, the position of the front beneath 100 m depth remained relatively unperturbed. [ABSTRACT FROM AUTHOR]

Based on satellite-observed and reanalysis data, this study investigates a thermal front east of the Gulf of Thailand (TFEGT) during the winter from 1982 to 2021. TFEGT exhibits distinct seasonal and interannual variation, emerging in December, peaking in February, gradually diminishing in March, and completely dissipating in April. Notably, the occurrence probability, area, and intensity of the thermal front are significantly higher in January and February compared with December and March. Through the application of a mixed temperature equation, we identify that geostrophic advection, driven by windinduced western boundary current in the South China Sea (SCS), plays a crucial role in the formation of the TFEGT. In winter, the prevailing northeast monsoon propels the western boundary current through wind stress curl, causing the southward transport of cold water from north to south. This cold water encountered warm water within the Gulf of Thailand (GoT), leading to the formation of TFEGT. Furthermore, the interannual variation of TFEGT is closely associated with the El Niño-Southern Oscillation (ENSO). In El Niño (La Niña) years, the northeast monsoon weakens (enhances), resulting in a weaker (stronger) western boundary current, ultimately influencing the weakening (enhancement) of TFEGT. [ABSTRACT FROM AUTHOR]

Submesoscale process is an important part in the kinetic energy cascade from large‐scale circulation to turbulent dissipation, and a key component of the global heat budget. Its dynamic response to weather event is an important process in forecasting marine bio‐chemical matter transport. So how will submesoscale instabilities response to tropical cyclones (TCs) is worth studying. Based on underwater glider observations and 1‐km high resolution numerical modeling, we investigated two TCs (Roke and Haitang)‐induced submesoscale baroclinic instabilities and their dynamic mechanisms in the Northern South China Sea. The TC Haitang induced significant surface cooling, mixed layer deepening, front sharpening, and enhanced the mixed layer baroclinic and symmetric instabilities. The submesoscale kinetic energy also enhanced sharply after TC Haitang, which was higher correlated with increased mesoscale strain rates. The submesoscale energetics analysis revealed that the enhanced frontal submesoscale kinetic energy after TC Haitang was mainly from potential energy release via baroclinic energy conversion. Four groups of sensitivity numerical experiments revealed that the turbulent heat buoyancy flux and the Ekman buoyancy flux contributed equally to the positive baroclinic energy conversion during the TC Haitang. This study helps us to understand the multiscale oceanic energy transfers and submesoscale air‐sea interaction processes. Plain Language Summary: The oceanic submesoscale process has scale ranges with 0.1–10 km in the horizontal, 0.01–1 km in the vertical, and hours‐days in time. The submesoscale process is a key component of the global energy balance and the heat budget, and can be generated by the atmosphere forcing, for example, cold weather, tropical cyclones (TCs). Based on 2‐km high resolution underwater glider and our set‐up 1‐km numerical modeling data, we found TC Haitang could trigger submesoscale symmetric instability and mixed layer baroclinic instabilities. The former instability gains kinetic energy from geostrophic currents, and the latter draw energy from the mesoscale flow potential energy in the northern South China Sea. The frontal submesoscale kinetic energies (KEs) enhanced sharply after TC Haitang. Based on the submesoscale kinetic energy budget analysis, we suggested that the enhanced frontal KEs after TC Haitang was transferred from potential energy via mixed layer baroclinic energy conversion. After conducting four groups of sensitivity numerical experiments, we found that the turbulent heat buoyancy flux and the Ekman buoyancy flux contributed to the strong vertical buoyancy fluxes during the TC Haitang. This study improves our understanding in submesoscale air‐sea interaction. Key Points: Both the submesoscale frontogenetic function and kinetic energy got enhanced after tropical cyclone (TC) HaitangThe enhanced submesoscale kinetic energy after TC Haitang was primarily dominated by the mixed layer baroclinic instabilityThe vertical buoyancy fluxes were mainly induced by turbulent heat and Ekman buoyancy fluxes during TC Haitang [ABSTRACT FROM AUTHOR]

The time series of near‐bottom temperatures collected from September 2018 until August 2020 from an array of three current‐ and pressure‐recording inverted echo sounders showed quasi‐seasonal and quasi‐monthly (∼28 days) variations at a depth of ∼1,300 m near the Chukchi slope in the western Arctic Ocean. They revealed an increase of ∼0.1°C during the winter‐spring period compared with the summer‐fall period. These variations were observed in the data‐assimilated Hybrid Coordinate Ocean Model (HYCOM) outputs near the observation site (correlation coefficient >0.7). They confirmed that variations in near‐bottom temperature are related to changes in the intensity of the Atlantic Water (AW) boundary current, concurrent with the deepening of the lower AW layer by approximately 50 m. The difference in sea surface height (SSH) between the Canada Basin and the Chukchi Shelf increased because of the negative wind stress curl (WSC) and retarded the AW boundary current according to the geostrophic effect. When the near‐bottom temperature increased during the winter‐spring period, the SSH in the Chukchi Shelf was lower than that in the summer‐fall period because of the less negative WSC. Quasi‐monthly variations were related to SSH on the Chukchi Shelf owing to the negative WSC. HYCOM outputs from 1994 to 2015 showed that the AW boundary current weakened more recently than in the past due to the increased melting of sea ice. The results imply that a longer sea‐ice‐free season in the Arctic amplifies changes in the AW boundary current and deep ocean temperature owing to increased atmospheric forcing. Plain Language Summary: Atlantic Water (AW) entering the Arctic Ocean from the Nordic Seas is considered a heat reservoir because it is warmer than the Arctic Ocean. The AW flows along the boundaries around the basin margins, known as the AW boundary current. The AW boundary current gradually deepened after entering and sat at approximately 200–800 m near the Chukchi slope. The AW boundary current exhibited quasi‐seasonal and quasi‐monthly (approximately 28 days) variations, confirmed using near‐bottom temperature data and data‐assimilated Hybrid Coordinate Ocean Model outputs. The variations are due to the geostrophic current caused by the pressure gradient due to the difference in the sea surface height (SSH) between the Chukchi Shelf and Canada Basin, which are located in the southwest and northeast of the Chukchi slope, respectively. Due to the difference in the SSH around the Chukchi slope, the geostrophic current is in the opposite direction to the AW boundary current and can suppress it. Sea surface height is affected by wind stress curl, and sea ice dampens atmospheric forcing. These findings imply that a longer sea‐ice‐free season in the Arctic can amplify changes in the AW boundary current. Key Points: The near‐bottom temperature observed at a depth of ∼1,300 m near the Chukchi slope showed quasi‐seasonal and quasi‐monthly variationsThe variations are related to the wind‐induced change in intensity of the Atlantic Water (AW) boundary current along the Chukchi slopeHYCOM outputs showed a more common occurrence of weaker AW boundary currents when sea ice melted compared to that in the past [ABSTRACT FROM AUTHOR]

Periodic eddies are a type of eddy that occur almost annually in fixed timeframes with similar patterns and trajectories. Nearly every year from April to June, under the combined effect of the barotropic instability of the mean flow and wind work, a cyclonic eddy (the Taiwan Cyclonic Eddy, TCE) forms in the southwest of Taiwan, then propagates westward, and finally dissipates near the Dongsha Islands. TCE exerts a significant impact on the Kuroshio intrusion into the South China Sea (SCS) and water exchange. Based on multi‐year in situ and satellite observations, this study reveals the thermohaline structure and evolutionary process of the TCE. The evolutions of the three‐dimensional structures of temperature, salinity, and geostrophic velocity of the TCE are analyzed based on reconstructed data. The TCE shows important interannual variations associated with El Niño–Southern Oscillation (ENSO), and the relationship between ENSO and the TCE is modulated by the Pacific Decadal Oscillation (PDO). In the negative phase of the PDO, the intensity of the TCE is significantly correlated with the Niño‐3.4 index. In contrast, in the positive phase, the ENSO–TCE relationship becomes weak and non‐significant. Further investigations indicate that these differences are related to the establishment of the low‐latitude Pacific–East Asian Teleconnection, influencing local wind stress curl in the region. This offers a new perspective on understanding the interannual variation of periodic mesoscale eddies in the SCS. Plain Language Summary: The Taiwan Cyclonic Eddy (TCE) represents a significant periodic mesoscale eddy in the South China Sea (SCS), demonstrating annual occurrences and profound impacts on regional oceanography and climatological dynamics. This study expands on multi‐year in situ and satellite observations to elaborate the three‐dimensional thermohaline structure and dynamic behavior of the TCE, emphasizing its seasonal consistency and interannual variability influenced by major climatic oscillations such as El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The TCE's formation is primarily driven by the instability of the background current and significant wind forcing. Its interannual variation shows a non‐stationary relationship with ENSO, which is significantly modulated by the PDO. The study enhances our understanding of the interannual dynamics of the TCE by highlighting how shifts in large‐scale climatic conditions can directly alter the local structural and dynamic characteristics of the eddy. Understanding the TCE's formation and variation is vital as it affects water exchange in the SCS, and thus influences local marine ecosystems in the region. By studying this phenomenon, scientists can better predict the impacts of such eddies on marine and climatic environments. Key Points: The Taiwan Cyclonic Eddy (TCE) forms annually from April to June southwest of Taiwan due to current instability and wind forcingMulti‐year observations show the TCE has significant vertical extensions and temperature/salinity anomalies, impacting local conditionsThe TCE's evolution is tied to El Niño–Southern Oscillation and influenced by Pacific Decadal Oscillation phases, being stronger in negative phases due to enhanced teleconnections and wind adjustments [ABSTRACT FROM AUTHOR]

Exceptional precision and excellent resolution reconstruction of sea surface currents are beneficial for exploring complex oceanic dynamic processes. Normally, this required physical inversion models for global or regional oceans are constructed to reconstruct oceanic currents. These models are based on the analysis of sea surface geostrophic and Ekman currents derived from satellite observations of sea level and wind stress fields. Nevertheless, the presence of various typical dynamic processes in marine environments, such as mesoscale eddies and small-scale waves, continues to pose challenges in accurately reconstructing oceanic currents. Meanwhile, any product of surface current that neglects the contribution of wave motion would, at best, be incomplete. Therefore, in this paper, we introduce an accurate sea surface current product at a depth of 15 m, named GEST (Geostrophic-Ekman-Stokes-Tide). This product is produced by a multi-scale dynamics-informed neural network that learns the intricate representation of concealed characteristics in Ekman, geostrophic currents, wave-induced Stokes drift, and TPXO9 tidal currents. Its structure design is predicated upon the intricate coupling relationships between various ocean surface components and the veritable currents discerned by the deployment of drift buoys, with each ocean surface component correlating to discrete physical processes. Compared with the prevailing product, the GEST confers an elevation in precision by approximately 9.2 cm/s over the traditional multinomial fitting method, 10.4 cm/s beyond the OSCAR, and 8.81 cm/s surpassing GlobCurrent. [ABSTRACT FROM AUTHOR]

A new efficient analytical-numerical method is developed for solving a problem for the potential vorticity equation in the quasi-geostrophic approximation with allowance for vertical diffusion of mass and momentum. The method is used to analyze small perturbations of ocean currents of finite transverse scale with a general parabolic vertical profile of velocity. For the arising spectral non-self-adjoint problem, asymptotic expansions of the eigenfunctions and eigenvalues are constructed for small wave numbers and the existence of a countable set of complex eigenvalues with an unboundedly decreasing imaginary part is shown. On the integration interval , a system of three neighborhoods is introduced and a solution in each of them is constructed in the form of power series expansions, which are matched smoothly, so that the eigenfunctions and eigenvalues are efficiently calculated with high accuracy. For a varying wave number , the trajectories of complex eigenvalues are computed for various parameters of the problem and the existence of double eigenvalues is shown. The complex picture of instability developing in the simulated flow depending on physical parameters of the problem is briefly described. [ABSTRACT FROM AUTHOR]

Satellite altimetry offers a unique approach for direct sea surface current observation, but it is limited to measuring the surface‐constrained geostrophic component. Ageostrophic dynamics, prevalent at horizontal scales below 100 km and time scales below 10 days, are often underestimated by ocean reanalyzes employing data assimilation schemes. To address this limitation, we introduce a novel deep learning scheme, rooted in a variational data assimilation formulation with trainable observations and a priori terms, that harnesses the synergies between satellite‐derived sea surface observations, namely sea surface height (SSH) and sea surface temperature (SST), to enhance sea surface current reconstruction. Numerical experiments, conducted using realistic simulations, in a case study area of the Gulf Stream, demonstrate the potential of the proposed scheme to capture ageostrophic dynamics at time scales of 2.5–3.0 days and horizontal scales of 0.5°–0.7°. The analysis of diverse observation configurations, encompassing nadir along‐track altimetry, wide‐swath SWOT (Surface Water and Ocean Topography) altimetry, and SST data, highlights the pivotal role of SST features in retrieving a significant portion of the ageostrophic dynamics (approximately 47%). These findings underscore the potential of deep learning and 4DVarNet schemes in improving ocean reanalyzes and enhancing our understanding of ocean dynamics. Plain Language Summary: Satellite altimetry provides a unique means for direct observation of sea surface currents, but it is confined to the geostrophic component, limiting the recovery of a substantial portion of mesoscale sea surface currents in operational products. To address this limitation, we present a novel deep learning framework, rooted in a variational data assimilation paradigm, that unlocks new avenues for leveraging the synergistic relationships between satellite‐derived sea surface observations, namely sea surface height and sea surface temperature. This innovative scheme demonstrates its remarkable potential to enhance sea surface current reconstruction and recover a substantial portion of the elusive ageostrophic dynamics. Numerical experiments, employing realistic simulations, in a case study area along the Gulf Stream, underscore the efficacy of our proposed approach. These findings support the pivotal role of physics‐informed deep learning in maximizing the utilization of available multimodal observation data sets and numerical simulations to elucidate partially observed sea surface dynamics. Key Points: We present end‐to‐end deep learning schemes to improve the reconstruction of total sea surface currents from satellite‐derived observationsExperiments in a region of the Gulf Stream support the synergistic analysis of sea surface temperature and sea surface height dataThe strain of sea surface dynamics is a proxy of the uncertainty of the retrieved estimation [ABSTRACT FROM AUTHOR]

Wintertime thermal front west of Luzon Island (WTFWLI) plays a vital role in local marine biogeochemical environment, geostrophic thermal advection of Luzon cold eddy (LCE) was once recognized as the causation of WTFWLI. However, current investigation based on multi‐source data revealed poor spatial correspondence between WTFWL and LCE over an interannual scale during 1993–2022, highlighting the necessity of revisiting the mechanism of WTFWLI. Our analysis shows that formation of WTFWLI was mainly caused by a northward warm tongue caused by geostrophic thermal advection of a northward current west of Luzon Island. The northward warm tongue encounters cold water west of Luzon Island and forms the WTFWLI at the boundary of cold and warm water. Our study provides a new explanation for the formation mechanism of WTFWLI, which will help deepen people's understanding of thermal front dynamics. Plain Language Summary: Wintertime thermal front west of Luzon Island (WTFWLI) plays an important role in sea‐air interaction and marine ecosystem, previous studies pointed out that the formation of WTFWLI was related with the Luzon cold eddy (LCE). Warm (cold) water was located in the southeast (northwest) part of the LCE due to geostrophic thermal advection, then leading to the formation of WTFWL at the boundary of cold and warm water. However, current investigation shows that there was poor spatial correspondence between WTFWLI and LCE in winter from 1993 to 2022, which is inconsistent with previous conclusions. Further analysis shows that formation of WTFWLI was mainly caused by a northward warm tongue caused by geostrophic thermal advection of a northward current west of Luzon Island. The northward warm tongue encounters cold water west of Luzon Island and forms the WTFWLI at their boundary. This study provides a new insight into the formation mechanism of WTFWLI, which will raise people's awareness of thermal front dynamics. Key Points: A new insight was provided into the formation mechanism of thermal front west of Luzon Island in winterThe thermal front west of Luzon Island was mainly caused by the encounter of a northward warm tongue and cold waterThe northward warm tongue was caused by a northward current west of Luzon Island [ABSTRACT FROM AUTHOR]

For the past 20 years, Global Navigation Satellite System reflectometry (GNSS-R) technology has successfully shown its potential for remote sensing of the Earth's surface, including ocean and land surfaces. It is a multistatic radar that uses the GNSS signals reflected from the Earth's surface to extract land and ocean characteristics. Because of its numerous advantages such as low cost, multiple signal sources, and all-day/weather and high-spatiotemporal-resolution observations, this new technology has attracted the attention of many researchers. One of its most promising applications is GNSS-R ocean altimetry, which can complement existing techniques such as tide gauging and radar satellite altimetry. Since this technology for ocean altimetry was first proposed in 1993, increasing progress has been made including diverse methods for processing reflected signals (such as GNSS interferometric reflectometry, conventional GNSS-R, and interferometric GNSS-R), different instruments (such as an RHCP antenna with one geodetic receiver, a linearly polarized antenna, and a system of simultaneously used RHCP and LHCP antennas with a dedicated receiver), and different platform applications (such as ground-based, air-borne, or space-borne). The development of multi-mode and multi-frequency GNSS, especially for constructing the Chinese BeiDou Global Navigation Satellite System (BDS-3), has enabled more free signals to be used to further promote GNSS-R applications. The GNSS has evolved from its initial use of GPS L1 and L2 signals to include other GNSS bands and multi-GNSS signals. Using more advanced, multi-frequency, and multi-mode signals will bring new opportunities to develop GNSS-R technology. In this paper, studies of GNSS-R altimetry are reviewed from four perspectives: (1) classifications according to different data processing methods, (2) different platforms, (3) development of different receivers, and (4) our work. We overview the current status of GNSS-R altimetry and describe its fundamental principles, experiments, recent applications to ocean altimetry, and future directions. [ABSTRACT FROM AUTHOR]

This study presents a generalised characterisation of the surface circulation over the northern shelf of the Gulf of Cadiz, based on 4 years of high-resolution satellite altimetry data from Sentinel-3A and wind model data. The altimetry-based surface zonal currents, adjusted for bottom-drag and wind effects, are compared with a generic CMEMS product and validated against in-situ ADCP measurements. The proposed altimetry product demonstrates superior performance than the CMEMS product, accurately reflecting surface circulation direction compared to in-situ measurements (r = 0.77, RMSE = 0.10 m/s, bias = 0.01 m/s). The use of the bottom-drag and wind-corrected/uncorrected altimetry product for spatiotemporal analysis of the shelf circulation revealed the distinct contributions of wind-driven and geostrophic components in different basin sectors. The results show that over the western basin, positive (eastward) surface currents were predominantly driven by westerly winds, while only occasionally, westward flows coincided with easterly winds, suggesting a higher control of the geostrophic component over the westward flows. In contrast, over the eastern basin, both eastward and westward flows were found to be primarily driven by favourable winds. Additionally, the analysis of Absolute Dynamic Topography (ADT) values along the whole basin showed the presence of ADT gradients both along-shore and cross-shore over the shelf, contributing to geostrophic flows. Finally, the seasonal analysis showed that eastward circulation tends to dominate during the spring and summer months, related to the upwelling season in the Gulf of Cadiz and associated westerly winds. Westward flows prevail during the winter months, related to easterly winds and the rebalancing of the along-shore sea level gradient during relaxed upwelling conditions. The findings demonstrate a significant improvement in the use of satellite altimetry data to study complex oceanographic dynamics in coastal areas, where both spatial and temporal variability are high. Moreover, the similarity of our results to those obtained from in-situ systems supports the use of altimetry data and publicly available wind models to support oceanographic studies in remote or resource-limited areas. [ABSTRACT FROM AUTHOR]

Accurate prediction of ocean surface currents is important for marine safety, ship routing, tracking of pollutants and in coupled forecasting. Presently, velocity observations are not routinely assimilated in global ocean forecasting systems, largely due to the sparsity of the observation network. Several satellite missions are now being proposed with the capability to measure Total Surface Current Velocities (TSCV). If successful, these would substantially increase the coverage of ocean current observations and could improve accuracy of ocean current forecasts through data assimilation. In this paper, Observing System Simulation Experiments (OSSEs) are used to assess the impact of assimilating TSCV in the Met Office's global ocean forecasting system. Synthetic observations are generated from a high-resolution model run for all standard observation types (sea surface temperature, profiles of temperature and salinity, sea level anomaly and sea ice concentration) as well as TSCV observations from a Sea surface KInematics Multiscale monitoring (SKIM) like satellite. The assimilation of SKIM like TSCV observations is tested over an 11 month period. Preliminary experiments assimilating idealised single TSCV observations demonstrate that ageostrophic velocity corrections are not well retained in the model. We propose a method for improving ageostrophic currents through TSCV assimilation by initialising Near Inertial Oscillations with a rotated incremental analysis update (IAU) scheme. The OSSEs show that TSCV assimilation has the potential to significantly improve the prediction of velocities, particularly in the Western Boundary Currents, Antarctic Circumpolar Current and in the near surface equatorial currents. For global surface velocity the analysis root-mean-squareerrors (RMSEs) are reduced by 23% and there is a 4-day gain in forecast RMSE. There are some degradations to the subsurface in the tropics, generally in regions with complex vertical salinity structures. However, outside of the tropics, improvements are seen to velocities throughout the water column. Globally there are also improvements to temperature and sea surface height when TSCV are assimilated. The TSCV assimilation largely corrects the geostrophic ocean currents, but results using the rotated IAU method show that the energy at inertial frequencies can be improved with this method. Overall, the experiments demonstrate significant potential benefit of assimilating TSCV observations in a global ocean forecasting system. [ABSTRACT FROM AUTHOR]

The coastal ocean off western Patagonia is one of the main coastal regions with high freshwater inputs from rivers, rain, and glaciers in the Southern Hemisphere. This study conducts an analysis of the seasonal and interannual variations in sea surface salinity and meridional geostrophic transports, specifically focusing on the Cape Horn Current, using improved satellite-derived data of sea surface salinity (SSS) and geostrophic velocities spanning an ∼11-year period (September 2011–August 2022). Our results reveal a clear salinity minimum in a coastal band between 42–54°S associated with the highest freshwater content. The average geostrophic currents are stronger south of 49°S, in line with the location of the Cape Horn Current. The average salinity minimum tends to disappear south of 54°S, with salinity values increasing slightly southward. The seasonal cycle of salinity shows the most pronounced minimum in summer (∼33.2–33.4). The greatest variability in salinity (standard deviation of salinity fields) occurs in the southern region of the Cape Horn Current. Hovmöller plots reveal two cores of minimum salinity observed in spring and summer (∼33.3–33.4). The freshwater off the Gulf of Penas contributes to the northern core. The meridional geostrophic transport differs between the northern and southern sections, with transports predominantly towards the Equator (Pole) north (south) of about 47–48°S during spring–summer. There is a marked seasonal variability in the magnitude and northern limit of the southward-flowing Cape Horn Current, being extended further north during winter and with a maximum average magnitude during summer–fall (about − 2 × 10 4 m2 s−1). On the interannual scale, a major drop in surface salinity occurred off northern and central Patagonia during 2018–2019. Finally, a potential long-term freshening trend is observed in the coastal area off southern Patagonia (south of 52°S), although prolonged data records are essential to confirm this pattern. [ABSTRACT FROM AUTHOR]

We propose a method to reconstruct sea surface height anomalies (SSHA) from vector surface currents and winds. This analysis is motivated by the proposed satellite ODYSEA, which is a Doppler scatterometer that measures coincident surface vector winds and currents. If it is feasible to estimate SSHA from these measurements, then ODYSEA could provide collocated fields of SSHA, currents, and winds over a projected wide swath of ∼1700 km. The reconstruction also yields estimates of the low-frequency surface geostrophic, Ekman, irrotational, and nondivergent current components and a framework for separation of balanced and unbalanced motions. The reconstruction is based on a steady-state surface momentum budget including the Ekman drift, Coriolis acceleration, and horizontal advection. The horizontal SSHA gradient is obtained as a residual of these terms, and the unknown SSHA is solved for using a Helmholtz–Hodge decomposition given an imposed SSHA boundary condition. We develop the reconstruction using surface currents, winds, and SSHA off the U.S. West Coast from a 43-day coupled ROMS–WRF simulation. We also consider how simulated ODYSEA measurement and sampling errors and boundary condition uncertainties impact reconstruction accuracy. We find that temporal smoothing of the currents for periods of 150 h is necessary to mitigate large reconstruction errors associated with unbalanced near-inertial motions. For the most realistic case of projected ODYSEA measurement noise and temporal sampling, the reconstructed SSHA fields have an RMS error of 2.1 cm and a model skill (squared correlation) of 0.958 with 150-h resolution. We conclude that an accurate SSHA reconstruction is feasible using information measured by ODYSEA and external SSHA boundary conditions. [ABSTRACT FROM AUTHOR]

The ocean regulates the Earth's climate by transporting heat from the equator to the poles. Here, we use satellite‐based sea surface observations of air‐sea heat fluxes and eddy detection to investigate the mesoscale heat transport. "Mesoscale" refers to both the Eulerian perspective as the spatio‐temporal scales of ∼100 km and ∼1 month, as well as the Lagrangian aspect as isolated vortices identified from the dynamic topography. Paradoxically, there are a considerable number of mesoscale eddies inconsistent between their surface thermal and dynamic signals, that is, cold‐core anticyclones and warm‐core cyclones are globally prevalent. On account of such inconsistency, we show that the mesoscale meridional heat transport carried by geostrophic components is 10 times larger than (and opposite in direction to) that of the wind‐driven Ekman components. An offset between SSH‐SST coherent and incoherent eddies in the Ekman heat transport is apparent, whereas the geostrophic heat transport is contained within coherent eddies. Plain Language Summary: Rotating water masses in the ocean, termed as mesoscale eddies, are dynamically important features because they can transport nutrients and displace heat in the Earth's climate system. In the same manner as the atmospheric weather systems that are associated with high or low pressures, mesoscale eddies are accompanied by sea level anomalies and therefore can be observed by satellite images. Conventionally, anticyclonic eddies detected as clockwise vortices in the Northern Hemisphere display warm temperature anomalies at the center. However, recent studies have found that the sea surface temperature signal of an eddy might be uncertain and even contradictory to their dynamical circulation patterns. In this study, we combine sea surface observations of temperature and velocity fields to determine the patterns of global ocean heat transport associated with mesoscale eddies. Key Points: The geostrophic component of meridional heat transport (MHT) at the mesoscale is 10 times larger than the Ekman componentSSH‐SST coherent eddies dominate the spatial patterns of MHT at the mesoscaleThough incomparable in magnitude with the large scale MHT, mesoscale eddies can still transport 30 TW of heat near sea surface [ABSTRACT FROM AUTHOR]

Realistic representation of monthly sea level anomalies in coastal regions has been a challenge for global ocean reanalyses. This is especially the case in coastal regions where sea levels are influenced by western boundary currents such as near the U.S. Atlantic Coast and the Gulf of Mexico. For these regions, most ocean reanalyses compare poorly to observations. Problems in reanalyses include errors in data assimilation and horizontal resolutions that are too coarse to simulate energetic currents like the Gulf Stream and Loop Current System. However, model capabilities are advancing with improved data assimilation and higher resolution. Here, we show that some current-generation ocean reanalyses produce monthly sea level anomalies with improved skill when compared to satellite altimetry observations of sea surface heights. Using tide gauge observations for coastal verification, we find the highest skill associated with the GLORYS12 and HYCOM ocean reanalyses. Both systems assimilate altimetry observations and have eddy-resolving horizontal resolutions (1/12°). We found less skill in three other ocean reanalyses (ACCESS-S2, ORAS5, and ORAP6) with coarser, though still eddy-permitting, resolutions (1/4°). The operational reanalysis from ECMWF (ORAS5) and their pilot reanalysis (ORAP6) provide an interesting comparison because the latter assimilates altimetry globally and with more weight, as well as assimilating ocean observations over continental shelves. We find these attributes associated with improved skill near many tide gauges. We also assessed an older reanalysis (CFSR), which has the lowest skill likely due to its lower resolution (1/2°) and lack of altimetry assimilation. ACCESS-S2 likewise does not assimilate altimetry, although its skill is much better than CFSR and only somewhat lower than ORAS5. Since coastal flooding is influenced by sea level anomalies, the recent development of skilful ocean reanalyses on monthly timescales may be useful for better understanding the physical processes associated with flood risks. [ABSTRACT FROM AUTHOR]

For the period of satellite altimetry observation 1993–2018, a comparison was made of estimates of the linear meridional shifts of the jet structure and variations in current intensity in the sector of the Antarctic Circumpolar Current (ACC) south of Africa (10° E–25° W), obtained on the basis of the parameters derived from the module of the absolute dynamic topography (ADT) gradient : directly , module of the velocity of geostrophic current |u| on the ocean surface, half of square of the ADT gradient , and specific kinetic energy . The analysis was carried out by a method developed earlier and refined in this paper using linear regression analysis. It is shown that qualitatively the characters of the meridional shifts (displacement of the curves of these parameters averaged over latitude and for each year) in latitude and the corresponding current intensity variations (i.e., variations in the parameters themselves) over 26-year observations, when calculated through each of the four parameters, turned out to be similar. The same similarity was obtained from the calculations with respect to the ADT scale. Meanwhile, the quantitative differences between calculations for different pairs of parameters can be significant even in the and pair. In quantitative terms, the absolute values of shifts of the jet structure and variations in the intensity of currents in the ACC band in the studied sector of the Southern Ocean generally increase from those linearly dependent on the ADT gradient module to quadratically dependent ones. [ABSTRACT FROM AUTHOR]

We present a study on the ocean surface currents reconstruction by merging Level-4 (L4, gap-free) altimeter-derived geostrophic currents and satellite sea surface temperature. Building upon past studies on the multi-variate reconstruction of geostrophic currents from satellite observations, we regionalized and optimized an algorithm to improve the altimeter-derived surface circulation estimates in the North Atlantic Ocean. A ten-year-long time series (2010–2019) is presented and validated by means of in situ observations. The newly optimized algorithm allowed us to improve the currents estimate along the main axis of the Gulf Stream and in correspondence of well-known upwelling areas in the North Eastern Atlantic, with percentage improvements of around 15% compared to standard operational altimetry products. [ABSTRACT FROM AUTHOR]

Satellite observation of sea surface height (SSH) may soon have sufficient accuracy and resolution to map geostrophic currents in Lake Superior. A dynamic atmosphere correction will be needed to remove SSH variance due to basinwide seiching. Here, the dynamics of rotating barotropic gravity modes are examined using numerical models and lake-level gauges. Gravity modes explain 94% of SSH variance in a general circulation model and evolve as forced, damped oscillators. These modes have significant SSH, but negligible kinetic energy (2 J m−2) and dissipation rates (0.01 mW m−2) relative to other motions in Lake Superior. Removing gravity modes from instantaneous SSH allows geostrophic currents to be accurately computed. Complex empirical orthogonal functions (CEOFs) from 50 years of data at eight lake-level gauges show patterns consistent with the first two gravity modes. The frequency spectra of these CEOFs are consistent with forced, damped oscillators with natural frequencies of 3.05 and 4.91 cycles per day (cpd) and decay time scales of 4.5 and 1.0 days. Modal amplitudes from the general circulation model and lake-level gauges are 80% coherent at 1 cpd, but only 50% coherent at 3 cpd, indicating that the atmospheric reanalysis used to force the general circulation model is not accurate at the high natural frequencies of the gravity modes. The results indicate that a dynamic atmosphere correction should combine modeled gravity modes below 1 cpd and observed mode-1 and mode-2 amplitudes (from lake-level gauges) at higher frequencies. An inverted barometer correction is also recommended to account for low-frequency atmospheric pressure gradients that do not project onto gravity modes. [ABSTRACT FROM AUTHOR]

The interannual variability of sea surface temperature (SST) in the South China Sea (SCS) exhibits two warming peaks around January and August in the subsequent year of El Niño. Results show that the second basin-scale SST warming peak is only evident following strong El Niño events, not regular events, which can be well identified by the Empirical Orthogonal Function (EOF) method. The heat budget analysis reveals that the anomalous Ekman heat advection (Eka) and geostrophic heat advection (Goa) contribute to the second warming feature, associated with the local dynamic response in the SCS, such as wind-driven Ekman downwelling, weakened Vietnam coastal upwelling, and abnormal anticyclonic geostrophic currents. The abnormal Eka and Goa are further attributed to the advanced abnormal equatorial easterlies breakout in the western Pacific, coinciding with the development of the western North Pacific anomalous anticyclone (WNPAC) in response to strong El Niño. This suggests that the second warming feature of the SCS SST is the footprint of strong El Niño events via the establishment of the WNPAC, which is tightly related to Indo-Pacific remote processes. [ABSTRACT FROM AUTHOR]

We investigated the seasonal variation in the current field in Harima-Nada using a high-resolution three-dimensional numerical model. The model showed sufficiently good performance at recreating its spatial and seasonal variation. Additionally, the model successfully reproduced the observed temperature and salinity patterns and values, as well as the observed surface current fields. The seasonal variation was consistent with previous studies, showing downwind circulation in winter induced by northwesterly winds and cyclonic circulation in summer induced by density-driven circulation. The water was well-mixed during winter and stratified in summer. During summer, a bottom cold water (BCW) dome develops beneath the thermocline in the central basin, where a surface cyclonic eddy occurs. Cold water from the previous winter is trapped at the bottom in topographic depressions after the onset of spring, forming BCW in the Harima-Nada (HNBCW). Meanwhile, pressure gradients associated with the density difference across the BCW drive geostrophic flow and form a cyclonic eddy at the surface above the HNBCW. HNBCW experiences a temperature increase of approximately 12.8 °C from spring to summer, which is associated with vertical diffusion and horizontal advection processes. Air–sea heat flux is the main factor influencing water temperature and its changes in the HNBCW. In addition, topographic features and river discharge factors affected water circulations as well as the formation and variation in the HNBCW. Harima-Nada is a small, shallow semi-enclosed coastal sea, and therefore any small change in these dynamic factors can affect water characteristics. [ABSTRACT FROM AUTHOR]

The impacts of large‐scale climate variability on the current system and meridional transport in the Cosmonauts Sea off East Antarctica are studied by employing the satellite‐observed absolute dynamic topography (ADT) data and reanalysis hydrographic data. Three currents converge in the Cosmonauts Sea: the Antarctic Circumpolar Current (ACC), the Weddell Gyre Eastern Branch (WGeb) and the Antarctic Slope Current (ASC) containing the Antarctic Slope Front (ASF) topographically locked in the slope region. Strong seasonal and interannual variations are associated with these currents. The ACC and ASC are both stronger in autumn and weaker in spring and summer, while the WGeb extends eastward in winter and retreats in summer. On the interannual timescale, the Southern Annular Mode (SAM) reinforces the westerlies during its positive phase, pushing the ACC southward and suppressing westward ASC, and vice versa. The results on related mechanisms suggest the significant barotropic modulations induced by the winds on ocean dynamics from both the sea surface and interior ocean. The westerlies could affect the meridional sea level gradient through Ekman transport convergence, inducing the geostrophic adjustment of currents. Moreover, the vorticity input from the wind field produces meridional water transport under the topographic and planetary β‐effects, potentially influencing the structure of the ASF and associated ASC. This frontal adjustment is superimposed on the background barotropic variations and causes strong local responses of the currents to the remote wind forcing, which indicates a propagation of variability signals from large‐scale barotropic circulations to frontal‐scale baroclinic currents in the ocean interior. Plain Language Summary: Seasonal and interannual variations in the large‐scale wind field can affect the strength and position of the current system in the region around the Cosmonauts Sea off East Antarctica. The Antarctic Circumpolar Current (ACC), the Weddell Gyre Eastern Branch (WGeb) and the Antarctic Slope Current (ASC) meet around the Cosmonauts Sea. The surface current velocities and variations based on satellite‐observed sea surface height (SSH) data are computed, and the potential forcing mechanisms are analyzed. The strengths of the ACC and ASC change similarly in different seasons but oppositely in different years, along with the changes in the wind belts over the Southern Ocean. The results from studies indicate that the wind field plays an important role in forcing water motions, not only at the sea surface to change the SSH but also at depth to influence the water mass structure. These processes could further regulate the ACC and the ASC, enabling the large‐scale wind field to force into the narrow current jets associated with water mass fronts and topographic slopes. Key Points: Three main currents converging in the Cosmonauts Sea form an integrated circulation system with interconnected variationsWinds exert critical barotropic modulations on water transport in both the surface and deep layersAdjustment of Antarctic Slope Front north of Cape Ann can be triggered by interior ocean transport variations driven by wind stress curl [ABSTRACT FROM AUTHOR]

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° 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. Plain Language Summary: 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. Key Points: The position of the Campbell Plateau meander has remained stable for the past 30 years, apart from a section downstream shifting northwardThe amplitude of the Campbell Plateau meander has been decreasing (flatter) upstream of the Plateau and increasing (steeper) downstreamThe Campbell Plateau meander has been widening and accelerating over the 1993–2020 period, especially downstream of the Plateau [ABSTRACT FROM AUTHOR]

The subpolar North Atlantic is known to have rich mesoscale and submesoscale variations; however, their spectral characteristics have not been documented in observations. This study documents the kinetic energy (KE) spectra using acoustic Doppler current profiler measurements that cover both the Iceland Basin and the Irminger Sea. The KE spectrum is partitioned into geostrophically balanced motions and unbalanced motions. The results reveal that balanced motions dominate the KE spectra. The unbalanced motions enhance in spring and fall to flatten the spectra and dominate small-scale (<50 km) energy, though uncertainty is high due to measurement noise and method assumptions. In addition, the dynamical framework that drives the balanced motions undergoes distinct seasonal shifts. In the spring and summer seasons of the Iceland Basin, as well as the summer season of the Irminger Sea, the wavenumber spectra of balanced motions exhibit a slope of approximately −3, consistent with the internal quasigeostrophic turbulence theory. Conversely, in the fall season of the Iceland Basin and the spring and fall seasons of the Irminger Sea, the wavenumber spectra of geostrophic balanced motions have a slope close to −2, consistent with surface quasigeostrophic turbulence theory. Additionally, we have found that the intensity of mesoscale eddies in the spring season can modulate both the slope and intensity of the wavenumber spectra of geostrophic balanced flows. [ABSTRACT FROM AUTHOR]

Global ocean surface currents estimated from satellite derived data based on a regular global grid are affected by the grid’s shape and placement. Due to different neighbourhood relationships, the rectangular lat/lon grids lose accuracy when interpolating and fitting elevation data. Hexagonal grids have shown to be advantageous due to their isotropic, uniform neighbourhood. Considering these merits, this paper aims to estimate global ocean surface current using a global isotropic hexagonal grid from satellite remote sensing data. First, gridded satellite altimeter data and wind data with different resolutions are interpolated into the centre of the global isotropic hexagonal grid. Then, geostrophic and Ekman currents components are estimated according to the Lagerlof Ocean currents theory. Finally, the inversion results are verified. By analyzing the results, we conclude that the ocean surface currents estimated based on the global isotropic hexagonal grid have considerable accuracy, with improvement over rectangular lat/lon grids.

An analysis of water exchange between the Bering (Pacific Ocean) and Chukchi (Arctic Ocean) seas in the summer was carried out using satellite data on sea level, geostrophic currents, and measurement data of water transport in the Bering Strait. It is shown that there is good agreement (r = 0.85, July–October 1997−2019) between the velocities of geostrophic currents (satellite data) and measurements of water transport (buoy station data) through the Bering Strait. It has been established that the temporal variability of water transport through the Bering Strait is determined by sea level variations in the southern part of the Chukchi Sea (66°–68° N, 170°–172° W). Strengthening of the eastern (western) winds is accompanied by a decrease (increase) in the sea level in the southern part of the Chukchi Sea and, as a result, an increase (decrease) in the flow of water through the Bering Strait. An increase (decrease) in the flow of the Yukon River is accompanied by a rise (decrease) in sea level and changes in water circulation in the northern Bering Sea and the southern Chukchi Sea. [ABSTRACT FROM AUTHOR]

The spatial distribution and seasonal variability of mesoscale eddies in the Sea of Japan have been investigated based on the regional database created from the AVISO Mesoscale Eddies Trajectory Atlas (1993–2020). The database contains information about the trajectories and parameters of mesoscale eddies in the Sea of Japan. The eddy detection method is based on the analysis of altimetric maps of absolute dynamic topography. A total of 578 eddies with a lifetime of more than 90 days have been identified (273 anticyclonic and 305 cyclonic). The average lifetime of eddies is 202 days for anticyclonic and 143 days for cyclonic and mean radius of 58 km for anticyclonic and 61 km for cyclonic. The mean speed of anticyclones and cyclones along their trajectories is 2.8 and 3.7 cm/s; the mean orbital velocities of geostrophic currents are 19.0 and 15.1 cm/s, respectively. The maximum number of cases of formation and destruction of anticyclones falls in July–September during the period with high values of water inflow through the Korea Strait. Most of the cyclonic eddies are generated between January and June and decay during the cold half of the year (October–March). A joint analysis of maps of the mean surface circulation in the Sea of Japan (satellite altimetry data) and the spatial distribution of mesoscale eddy shows that the stable eddies of the Sea of Japan are associated with the quasi-stationary meanders of the East Korea East Korea Warm Curent, Subpolar Front, and Tsushima current. The position of meanders is mainly determined by the interaction of the currents with the bottom topography. [ABSTRACT FROM AUTHOR]

Based on the relation between volume transport and sea level difference across the Korea/Tsushima Strait (KTS), in this study, we estimated the volume transport of the Tsushima Warm Current over the past four decades and examined its long-term trend using an ensemble empirical model decomposition (EEMD) method. The estimated geostrophic transports showed very good agreement with those measured by acoustic Doppler current profiler and those estimated from satellite altimetry-based sea level data. This corroborates the reliability of the long-term estimation. EEMD analysis revealed an acceleration in volume transport in the KTS, with a persistent increase in the trend rate occurring after the mid-1980s. Further analyses indicate that the trend shift in the KTS closely coincided with an upper-ocean warming trend in the East/Japan Sea (EJS) after the mid-1980s. Analyzing large-scale wind patterns suggests that long-term trends of wind stress curls over the North Pacific are likely a key driver of the trend shifts in the inflow through the KTS and the upper-ocean warming in the EJS over the past four decades. [ABSTRACT FROM AUTHOR]

I review the history of ideas that have led to the establishment of the RAPID monitoring system for the Atlantic Meridional Overturning Circulation (AMOC) at 26.5° N. This history is closely connected to important events in my personal career. Starting from early and largely unsuccessful attempts at formulating a dynamically consistent force balance for the AMOC, I made theoretical progress by separately predicting the density at the eastern and western boundaries and by invoking exact geostrophic balance throughout, including the western boundary current. A remarkable confluence of individuals and ideas then enabled the establishment of the RAPID array, at its core based on monitoring boundary densities and on geostrophy. The RAPID results, such as the surprisingly large sub-seasonal variability, have encouraged AMOC monitoring approaches at other latitudes. I finish by pointing at two theoretical concepts—first, acknowledging the difference between convective mixing and sinking and, second, considering the advective rather than wave propagation of density perturbations in the deep western boundary current—that, together with continued observations and newly available global coupled simulations at very high resolution, should substantially improve our understanding of the causes of AMOC variability. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'. [ABSTRACT FROM AUTHOR]

In the northwestern Barents Sea the warm and salty Atlantic Water meets the cold and fresh Polar Water, forming a distinct thermohaline front (the Barents Sea Polar Front). Here we present the structure of the front, its variability and associated mixing using observations from two cruises conducted in October 2020 and February 2021 during the Nansen Legacy project, in the region between Hopen Trench and Olga Basin. Ocean stratification, currents, and turbulence data were obtained during seven ship transects across the Polar Front near 77° N, 30° E. These transects are complemented by four missions using ocean gliders, one of which was equipped with microstructure sensors to measure turbulence. Across the front, we observe warm (>2 °C) and salty (>34.8) Atlantic Water intruding below the colder (<0 °C) and fresher (<34.4) Polar Water, setting up a baroclinic front and geostrophic currents reaching 25 cm s-1, with estimated eastward transport of 0.3±0.2 Sv (1 Sv = 1×106 m³ s-1). We observe anomalous warm and cold-water patches on the cold and warm side of the front, respectively, collocated with enhanced turbulence, where dissipation rates of turbulent kinetic energy range between 10-8 and 10-7 W kg-1. Short-term variability below the surface mixed layer arises from tidal currents and mesoscale eddies. While the effects of tidal currents are mainly confined to the bottom boundary layer, eddies induce significant shifts in the position of the front, and alter the isopycnal slopes and the available potential energy of the front. Substantial water mass transformation is observed across the front, likely a result of eddy-driven isopycnal mixing. Despite the seasonal changes in the upper layers of the front (0–100 m) influenced by atmospheric forcing, sea ice formation, and brine rejection, the position of the front beneath 100 m depth remained relatively unperturbed. [ABSTRACT FROM AUTHOR]

In the northwestern Barents Sea the warm and salty Atlantic Water meets the cold and fresh Polar Water, forming a distinct thermohaline front (the Barents Sea Polar Front). Here we present the structure of the front, its variability and associated mixing using observations from two cruises conducted in October 2020 and February 2021 during the Nansen Legacy project, in the region between Hopen Trench and Olga Basin. Ocean stratification, currents, and turbulence data were obtained during seven ship transects across the Polar Front near 77° N, 30° E. These transects are complemented by four missions using ocean gliders, one of which was equipped with microstructure sensors to measure turbulence. Across the front, we observe warm (>2 °C) and salty (>34.8) Atlantic Water intruding below the colder (<0 °C) and fresher (<34.4) Polar Water, setting up a baroclinic front and geostrophic currents reaching 25 cm s-1, with estimated eastward transport of 0.3±0.2 Sv (1 Sv = 1×106 m3 s-1). We observe anomalous warm and cold-water patches on the cold and warm side of the front, respectively, collocated with enhanced turbulence, where dissipation rates of turbulent kinetic energy range between 10-8 and 10-7 W kg-1. Short-term variability below the surface mixed layer arises from tidal currents and mesoscale eddies. While the effects of tidal currents are mainly confined to the bottom boundary layer, eddies induce significant shifts in the position of the front, and alter the isopycnal slopes and the available potential energy of the front. Substantial water mass transformation is observed across the front, likely a result of eddy-driven isopycnal mixing. Despite the seasonal changes in the upper layers of the front (0–100 m) influenced by atmospheric forcing, sea ice formation, and brine rejection, the position of the front beneath 100 m depth remained relatively unperturbed. [ABSTRACT FROM AUTHOR]

In this study, we investigate the record-breaking intensification and abrupt weakening of the East Korea Warm Current (EKWC) in the summer of 2021. We analyzed the ocean data assimilation products resolving this event to examine the association between the abrupt changes in the EKWC and various oceanic/atmospheric factors. The results indicate that during the summer of 2021, the EKWC extended northward beyond its climatology, reaching up to 40°N with the maximum speed of 1.16 m s-1 on August 1. In mid-August, the EKWC underwent a rapid weakening, returning to its climatological level. We could attribute the temporal variability in the anomalous EKWC in 2021 to the distinct temporal variability in the dynamic height anomalies between coastal and offshore regions. The offshore variability in the dynamic height anomaly, which is related to warm eddy variability, led to an anomalously increased EKWC velocity (up to 0.59 m s-1) during the EKWC peak velocity period in 2021. However, anomalous coastal downwelling induced by a weak northerly wind anomaly decelerated the EKWC by -0.06 m s-1 in the same period. In mid-August, a typhoon-related northerly wind induced a sudden rise in the coastal dynamic height anomaly, resulting in a rapid weakening of the EKWC. Our findings suggest that changes in geostrophic current related to warm eddies and typhoons have substantially contributed to the temporal variability in the EKWC, improving our understanding of the temporal variability in the western boundary currents. [ABSTRACT FROM AUTHOR]

For the period of satellite altimetry observation 1993–2018, a comparison was made of estimates of the linear meridional shifts of the jet structure and variations in current intensity in the sector of the Antarctic Circumpolar Current (ACC) south of Africa (10° E–25° W), obtained on the basis of the parameters derived from the module of the absolute dynamic topography (ADT) gradient : directly , module of the velocity of geostrophic current |u| on the ocean surface, half of square of the ADT gradient , and specific kinetic energy . The analysis was carried out by a method developed earlier and refined in this paper using linear regression analysis. It is shown that qualitatively the characters of the meridional shifts (displacement of the curves of these parameters averaged over latitude and for each year) in latitude and the corresponding current intensity variations (i.e., variations in the parameters themselves) over 26-year observations, when calculated through each of the four parameters, turned out to be similar. The same similarity was obtained from the calculations with respect to the ADT scale. Meanwhile, the quantitative differences between calculations for different pairs of parameters can be significant even in the and pair. In quantitative terms, the absolute values of shifts of the jet structure and variations in the intensity of currents in the ACC band in the studied sector of the Southern Ocean generally increase from those linearly dependent on the ADT gradient module to quadratically dependent ones. [ABSTRACT FROM AUTHOR]

The offshore transport of Greenland coastal waters influenced by freshwater input from ice sheet melting during summer plays an important role in ocean circulation and biological processes in the Labrador Sea. Many previous studies over the last decade have investigated shelfbreak transport processes in the region, primarily using ocean model simulations. Here, we use 27 years of surface geostrophic velocity observations from satellite altimetry, modified to include Ekman dynamics based on atmospheric reanalysis, and virtual particle releases to investigate seasonal and interannual variability in transport of coastal water in the Labrador Sea. Two sets of tracking experiments were pursued, one using geostrophic velocities only, and another using total velocities including the wind effect. Our analysis revealed substantial seasonal variability, even when only geostrophic velocities were considered. Water from coastal southwest Greenland is generally transported northward into Baffin Bay, although westward transport off the west Greenland shelf increases in fall and winter due to winds. Westward offshore transport is increased for water from southeast Greenland so that, in some years, water originating near the east Greenland coast during summer can be transported into the central Labrador Sea and the convection region. When wind forcing is considered, long-term trends suggest decreasing transport of Greenland coastal water during the melting season toward Baffin Bay, and increasing transport into the interior of the Labrador Sea for water originating from southeast Greenland during summer, where it could potentially influence water column stability. Future studies using higher-resolution velocity observations are needed to capture the role of submesoscale variability in transport pathways in the Labrador Sea. [ABSTRACT FROM AUTHOR]

The characteristics of cold core eddies and their effects on the hydrodynamics and biological production in the Andaman seas were studied using in-situ and satellite observations. The existence and spatial extent of the eddy are revealed by the precise structure and patterns of the temperature-salinity (T–S) profiles, nutrients, and chl a. A good illustration of this is the anomaly in sea surface height (SSHA). The Cyclonic Eddies (CE1 and CE2) are monitored using the Okubo-Weiss parameter (− 2 × 10–11/s2) calculated from the satellite SSHA and a geostrophic current centered at 8°N and 92°E and 13°N and 93°E, respectively. At the eastern flank of CE1, measurements are taken in-situ along 8°N and 92.5°–93.5°E. Vertical currents measured with Acoustic Doppler Current Profiles (ADCP) at 0.3 m/s show northward flow along the track. A significant northward flow (0.3 m/s) can be seen in the vertical currents recorded with ADCP, yet a weak southern flow can be seen over the western margin. In addition, the SSHA acquired from altimetry demonstrates the spatial extent, supporting the occurrence of cyclonic eddies. The vertical shear in the horizontal flow is the main contributor to baroclinic instability (Ri 0.0001) in the water column, according to an analysis of the factors that could result in the formation of an eddy. The T–S profiles show that the area contains Bay of Bengal (BoB) water, and there are semi-annual Rossby waves there as well. However, the wind stress curl did not provide a reliable indication of divergence in the region. The region's biological production (chl a) and nutrient distribution (NO2, NO3, PO4, and SiO4) were impacted by the eddy. CE2 is connected to convective mixing processes that take place along the northwest coast of the Andaman Islands as a result of the primarily cold, dry continental air flowing from the north east. [ABSTRACT FROM AUTHOR]

Surface currents in oceanic environment are of vital importance from economical, biological and environmental aspects. Modelling ocean currents has generally been performed using numerical ocean circulation models as a solution to initial-boundary value problems in oceanic domain. Due to lack of knowledge about model parameters as well as initial and boundary values, they need to be externally calibrated for accurate local and regional applications. In this study, an alternative approach is proposed to incorporate spaceborne geodetic observations as well as hydrographic data to estimate the total surface current in the Persian Gulf and the Oman Sea. Being the data-driven approach, the method is comparable to numerical ocean models and regionally it is more accurate and simpler in application. The proposed method focuses on the computation of dynamic topography (DT) by least squares variance component estimation combining two different schemes. They are (1) DT estimation via direct observations of sea surface height from satellite altimetry and (2) steric and non-steric modeling of sea level anomaly using temperature and salinity data for the steric component; and Gravity Recovery and Climate Experiment observations for the non-steric component. Ultimately, the total surface current is obtained by computing the horizontal gradient of DT using geostrophic equation and adding the components of the Ekman current. Moreover, the estimated total surface current is further improved by assimilating with in situ current meter data using 3D-Variational data assimilation method and it is validated against two control stations. This assimilation leads to improvement of about 3 to 15 cm/s in total surface current computed using geostrophic equation and Ekman current. Besides, to illustrate the significance of the proposed approach, the estimated total surface current is externally validated and compared with the output of Copernicus Marine Environment Monitoring Service (CMEMS), as a numerical ocean model developed for oceanographic applications. Our comparison reveals that the proposed method is more accurate and reliable than CMEMS products. As for the circulation and current pattern, the estimated surface velocities reveal the existence of eddies in the region of the Persian Gulf and the Oman Sea, indicating the occurrence of cyclonic and anti-cyclonic circulations. Moreover, they elucidate that the velocities are lower in spring and summer and higher in autumn and winter. [ABSTRACT FROM AUTHOR]

Long-term features of the vertical and horizontal structure of water temperature field in Lake Sevan are discussed. It is shown that the climate warming has led to an increase in epilimnion temperature in Bol'shoi Sevan in July by 2.0‒3.0°C. The increase in water temperature in hypolimnion was not greater than 1.1°C. In autumn (October), the epilimnion became 1.2°C warmer, while hypolimnion temperature practically has not changed on the average over years. Temperature fields were used to calculate the density currents in summer and autumn periods. A dominating cyclonic water circulation was revealed all over the lake, confirmed by chlorophyll distribution by satellite image data. In the case of large horizontal gradients of water density, the flow velocity can reach 50 cm/s. Autonomous buoy stations revealed a wide range of water temperature variations due to internal waves of different nature. The reversible vertical mixing of water mass by internal waves plays an important role in the distribution of nutrients and plankton within the water mass. The water level rise by ~3 m, unlike it drop by 1981 by 18.48, has not caused any significant changes in lake hydrological regime. [ABSTRACT FROM AUTHOR]

The Great Whirl (GW) and the Socotra Gyre (SoG), two prominent anticyclonic eddies in the western Arabian Sea, exhibit strong dynamic interactions. This study reports a case of the merging of the GW and the SoG recorded by Argo floats in September 2019. Combined with satellite observations and a state‐of‐the‐art ocean reanalysis, we show that the merging process was first detected at the subsurface layer (∼150 m depth) rather than the surface. As the original water inside the GW is cooler than the SoG, the merging created a baroclinic structure between the eddies. The density gradients associated with the baroclinic structure drive strong subsurface geostrophic currents following the thermal wind relationship, leading to the fast merging at 100–200 m depth. Energy analysis shows that the predominant energy source for the merged eddy was the barotropic and baroclinic instability. The dissipative processes caused the rapid decay of the merged eddy. The merging process induced submesoscale activities and promoted ocean vertical exchanges south of Socotra Island. Plain Language Summary: The Great Whirl (GW) and the Socotra Gyre (SoG) are two large clockwise eddies in the western Arabian Sea. Ocean observations caught the merging process of these two eddies in September 2019. The Argo floats detected the process first at the subsurface layer. Ocean reanalysis reveals that the water inside the GW was cooler than the SoG, which led to the density difference that enhanced the subsurface flow. The subsurface flow transported the warm and salty water from the SoG faster than the surface in the interior of the eddy. During the merging process, the smaller‐scale motions led to increased surface chlorophyll concentrations in the region, with replenished nutrients and dissolved oxygen. Key Points: The merging process of the Great Whirl (GW) and the Socotra Gyre (SoG) was faster at the subsurface, resulting from density gradientsThe barotropic and baroclinic instability provided the predominant energy for the mergingThe merging process stimulated submesoscale activities and promoted ocean vertical exchanges [ABSTRACT FROM AUTHOR]

Among the global mapping of lee wave generation, a missing piece exists in the northwestern Pacific Ocean (NPO), which features complex topographies and energetic circulations. This study applies Bell's theory to estimate and map internal lee waves generated by geostrophic flows in the NPO using Mercator Ocean reanalysis data and the full topographic spectra obtained from the latest synthetic bathymetry product. Unlike the dominant contributions from abyssal hills in the Southern Ocean, multiple topographies, including ridges, rises, and continental margins, result in an inhomogeneous lee wave generation with multiple hotspots in the NPO. The generation rate is generally higher in the Philippine basin and lower in the central Pacific seamounts. Over ridges, the rough topography creates a high potential for triggering lee waves. Over rises and continental margins, the stronger currents at the shallow depths are favorable for lee wave generation. In the Kuroshio extension region, the rough topography and strong currents cause the strongest lee wave generation, with an energy flux reaching 100 mW m−2. By mean–eddy decomposition, it is found that the lee wave hotspots contributed by mean flow are concentrated in specific regions, while those by geostrophic eddies are widely distributed. Geostrophic eddies are the primary contributor to lee wave generation, which account for 74.6% of the total energy transferred from geostrophic flow to lee waves. This study also reveals that tides suppress the lee wave generation by 14%, and geostrophic flow can cause an asymmetric generation of internal tides. [ABSTRACT FROM AUTHOR]

The East Kamchatka and East Sakhalin Currents (EKC and ESC) are the western boundary currents of the subarctic North Pacific and Okhotsk Sea. Variability in the EKC and ESC velocities could exert a substantial effect on ecosystems and fish stocks in the southwestern Bering Sea and Okhotsk Sea. Using satellite-derived data (sea surface heights, geostrophic current velocities, and sea surface temperatures, 2002–2020), we demonstrate that changes in zonal wind generate sea level variations on the shelf in the southwestern Bering Sea over a period of 18–29 days and with an amplitude of 5–20 cm. The ebb/flood events on the shelf lead to changes in the velocity, direction, and position of the EKC. The sea level anomalies propagate along the western Kamchatka, northern Kuril Islands and the northern and western Okhotsk Sea and result in the variability of geostrophic current velocities in the ESC zone. The strengthening (weakening) of ESC leads to an increase (a decrease) in SST in the southern part of the Okhotsk Sea by 1–3 °C. In the northwestern Okhotsk Sea, in addition to wind-induced variability, there are temporary changes in the geostrophic currents with a period of 14 days caused by fortnightly tides. [ABSTRACT FROM AUTHOR]

This paper reviews the recent progress in our estimation of ocean dynamic topography and the derived surface geostrophic currents, mainly based on multiple nadir radar altimeter missions. These altimetric observations provide the cornerstone of our ocean circulation observing system from space. The largest signal in sea surface topography is from the mean surface dominated by the marine geoid, and we will discuss recent progress in observing the mean ocean circulation from altimetry, once the geoid and other corrections have been estimated and removed. We then address the recent advances in our observations of the large-scale and mesoscale ocean circulation from space, and the particular challenges and opportunities for new observations in the polar regions. The active research in the ocean barotropic tides and internal tidal circulation is also presented. The paper also addresses how our networks of global multi-satellite and in situ observations are being combined and assimilated to characterize the four-dimensional ocean circulation, for climate research and ocean forecasting systems. For the future of ocean circulation from space, the need for continuity of our current observing system is crucial, and we discuss the exciting enhancement to come with global wide-swath altimetry, the extension into the coastal and high-latitude regions, and proposals for direct total surface current satellites in the 2030 period. [ABSTRACT FROM AUTHOR]