At the edge of the Amazon River plume, stirring by the North Brazil Current (NBC) and its eddies creates sharp surface thermohaline gradients on horizontal scales of 𝒪 (1–100) km. This study provides a comprehensive picture of these gradients and fronts associated with the region's distinctive dynamics. Saildrone observations show that the plume amplifies density gradient variability at all scales from 1–100 km, with frontal sharpness up to 75×100×km. Density gradients are partially reinforced or compensated by temperature-salinity variations, with net frontogenesis observed in both regions. To expand in-situ observations, we use a 1 km
The warm Atlantic inflow across the Iceland-Faroe Ridge (IFR) is the strongest of the three branches carrying warm water to the Nordic Seas and further into the Arctic Ocean. This branch (IF-inflow) carries almost 50 % of the total Atlantic water inflow. After crossing the ridge, a large part of this water is converted to dense water that returns to the Atlantic as cold overflow, which contributes to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). A better understanding of the temperature and salinity variations of the IF-inflow is therefore important for assessing both regional and global climate. Based on satellite altimetry and drifter observations, we document the pathway of the Atlantic water through the Iceland Basin to the IFR and show how this pathway is affected by variable intrusion of the Subpolar Gyre into the basin with a narrower, faster and southward-shifted pathway during periods of strong Subpolar Gyre intrusion. The variable intrusion is furthermore shown to be the main cause of the long-term salinity variations upstream as well as downstream of the IFR while the temperature has increased due to the general warming of the oceans in addition to the variations caused by variable intrusion. As the Atlantic water crosses the IFR, it is cooled by at least 1 °C and freshened by at least 0.1 g kg−1. We show that this transformation mainly is caused by mixing with Arctic water masses, rather than by air-sea-interactions, and that most of the modification takes place upstream of the ridge crest. The Atlantic water changes character from an almost barotropic to a much more baroclinic flow over the ridge enabling water masses, otherwise constrained to follow isobaths, to cross the ridge. We find that the cooling and freshening of the Atlantic water across the IFR are relatively constant throughout the whole period from 1993 to 2023, except for the last few years. The freshening across the ridge implies that the salinity difference between IF-inflow water and the deep waters northeast of the ridge has been reduced by roughly 20 %–30 % after crossing the ridge, which weakens the potential for dense-water formation downstream. Updated transport estimates show a slight strengthening for this AMOC branch. From 1993 to 2023, the volume transport of the IF-inflow increased by 12±7 %, while the heat transport relative to 0 °C increased by 16±8 %.
The Arctic Ocean is undergoing rapid change, yet many CMIP-type climate models struggle to accurately represent its circulation and water masses. A key feature of the system is the topographically controlled boundary currents that transport warm, saline Atlantic Water northward at intermediate depths into the Atlantic Water layer. An important process affecting these boundary currents is the lateral flux of heat and salt driven by mesoscale eddies. Because the deformation radius is relatively small in the Arctic Ocean, numerical simulations require kilometer-scale resolution to fully capture eddy dynamics. Most of the current climate models, however, operate in a non-eddying regime, relying on mesoscale eddy parameterizations – typically combining isopycnal diffusion (Redi) with eddy-induced advection (Gent and McWilliams, GM). As horizontal resolution increases, future models will shift from an eddy-parameterized to eddy-permitting regime, entering a grey zone where eddies are only partially resolved and the role of GM parameterization becomes less straightforward. This study investigates the use of GM parameterization in eddy-permitting models, focusing on its effect on the northward transport of Atlantic Water in the Nordic Seas and Arctic Ocean. We conduct realistic simulations where we vary GM diffusivity strength and test two different GM scalings. These experiments are compared with a high-resolution reference simulation and observational data. Further, we show how the GM parameterization modulates the topographically steered boundary current through a reduction in baroclinicity of the large-scale circulation, how transport across Greenland–Scotland Ridge and Fram Strait is controlled by its strength; and how resolved and parameterized eddy heat fluxes contribute to the redistribution of heat. Our results suggest that mesoscale eddy buoyancy fluxes remain insufficiently resolved at eddy-permitting Arctic resolutions, supporting the continued use of GM-type parameterizations in the grey-zone regime. However, there are some obvious limitations with the tested GM formulations in our experiments that warrants further improvement of the GM scheme in an Arctic context.
High-resolution satellite observations are essential for studying fine-scale ocean processes. Yet, present satellite sea surface salinity (SSS) products remain too coarse to resolve many fine-scale structures. We investigate denoising diffusion models as a generative framework for SSS downscaling in a controlled proof-of-concept experiment based on GLORYS reanalysis fields. A multichannel diffusion prior is trained on
On the continental slope north of Svalbard, the boundary current carrying Atlantic Water (AW) loses heat as it flows eastward. This cooling cannot be fully attributed to atmospheric heat loss or turbulent mixing. Lateral exchange, potentially linked to mesoscale activity, has previously been proposed as a contributing factor, based on limited observations of eddies. Here, we analyse a year-long dataset of hydrography and velocity observations from two mooring arrays, supplemented by output from an eddy-resolving ocean model, to quantify the seasonal variability of eddy kinetic energy (EKE) and baroclinic and barotropic energy conversion rates over time-scales from days to months. Both EKE and conversion rates peak in autumn and winter, coinciding with the strongest boundary current and the warmest AW. Local EKE variability, however, is only weakly associated with conversion rates, suggesting advection from upstream generation sites or unresolved variability from limited measurements. Conversion is mainly baroclinic, through boundary current instability, providing conditions favourable for offshore propagation of warm-core eddies. Modelled conversion rates have a complex spatial structure with substantial values on the offshore, deeper side of the boundary current with comparable contributions from baroclinic and barotropic processes. Resulting mesoscale activity enhances lateral stirring and heat loss from the boundary current, particularly in winter and spring, contributing to the along-stream cooling of AW.
Global shelf seas have experienced unprecedented marine heatwaves (MHWs) in recent decades. Although MHWs have been extensively studied at the global scale, their regional variability and underlying mechanisms remain poorly understood, particularly in shelf seas influenced by multiple climate modes. Here, we examine MHW variability in the Northeastern Atlantic shelf using a correlation-based k-means clustering approach. Two distinct subregions with contrasting seasonal patterns are identified. In winter, the southern North Sea experiences increased MHW frequency, intensity, and duration. This enhancement is linked to a positive East Atlantic Pattern, which intensifies westerly winds and enhances warm Atlantic inflow through both atmospheric and oceanic pathways. In contrast, the northern North Sea shows enhanced MHW frequency and duration in summer, while MHW intensity weakens. This summer response is modulated by Atlantic Multidecadal Variability, with its positive phase strengthening Pacific–Atlantic connections via Rossby wave propagation, altering cloud cover and surface radiative forcing. A shallow mixed layer, enhanced stratification, and circulation-induced upwelling favor frequent and persistent but less intense summer MHWs. This north–south contrast demonstrates that different combinations of atmospheric and oceanic processes shape MHW variability across the shelf, providing a diagnostic and mechanistic framework for understanding regional MHW variability and its potential predictability.
The vast majority of studies examining the impact of freshwater from ice sheet melting on the Atlantic Meridional Overturning Circulation (AMOC) use climate models that cannot resolve mesoscale ocean processes and do not include an accurate spatio-temporal distribution of the freshwater forcing. These two factors critically affect the nature of the AMOC response. Our study partially fills that gap with a set of three hosing experiments using perpetual 1950 radiative forcing with the global configuration of the eddy-rich EC-Earth3P-VHR climate model. The model is forced for 21 years with a spatial and monthly distribution of Greenland meltwater fluxes derived from a product based on observations and model simulations. An annual average close to 0.04 SvWithin the first year, we observe a response of reduced salinity in the Greenland and Labrador currents. Since the beginning of the experiments, these boundary currents also experience an acceleration and cooling. The cooling arises because freshwater-induced stratification suppresses vertical mixing, reducing the entrainment of warmer subsurface waters into the surface layer of the boundary currents. The meltwater fluxes also lead to a rapid weakening of the AMOC at subpolar latitudes due to circulation adjustments forced by the changes in density gradients, with the salinity-driven density reduction outweighing the temperature-driven density increase. Around year 7, deep mixing in the Labrador Sea begins to weaken as freshwater anomalies accumulate through lateral exchanges with the boundary currents. This weakening in deep mixing affects the Deep Western Boundary Current (DWBC), which warms up at the OSNAP section, and sequentially further weakens the AMOC, resulting in an even stronger reduction that reaches also the subtropical latitudes. After the 21 simulated years, the AMOC has weakened by almost 3 Sv∼20 %Sv. In the context of the North Atlantic, a basin-wide decrease of 1.3 SvSv. This AMOC reduction is strong enough for some global climate impacts to emerge, such as a “bipolar seesaw” temperature response.
The Arctic Ocean is undergoing rapid change, with freshwater playing a central role in shaping stratification, vertical heat exchange, and sea-ice loss. Using long-term observations from the Nansen and Amundsen Basins Observational System (NABOS), we document a strong freshening event in the eastern Eurasian Basin between late 2015 and early 2017. During this period, salinity in the upper 175 m decreased by ∼ 0.5 psu, equivalent to an additional ∼ 0.6 m of freshwater, relative to the preceding (2013–2015) and following (2017–2018) years. The anomaly originated on the Kara Sea shelves in 2014–2015, when exceptional Yenisey and Ob discharge provided a combined freshwater surplus of ∼ 0.78 m, sufficient to explain the observed freshening. Trajectory analysis traced the freshwater anomaly to the Kara Sea, with transport times of 8–9 months to the shelf and 22–23 months to offshore. The resulting enhanced stratification suppressed upper-ocean currents by ∼ 22 % and vertical shear by ∼ 50 %. These changes enabled thicker sea ice to persist through the summers of 2016–2017, in contrast to near-ice-free conditions in adjacent years. While wind anomalies aided the retention of freshwater along the slope, anomalous river discharge was the dominant driver of the event. Overall, the 2015–2017 event demonstrates how episodic river discharge events can restructure upper-ocean stratification, reduce oceanic heat fluxes, and lead to delayed melt and increased summer sea ice, highlighting the sensitivity of upper-ocean processes and sea ice to episodic freshwater forcing in the Arctic.
The abyssal Ionian Sea is a deep region of interest for the entire ocean circulation of the Mediterranean Sea, since it plays an important role in the ventilation processes of the whole basin. Here we investigate spatial patterns of internal wave climate over the bottom of the Ionian sub-basin. To identify regional features of the internal wave field in terms of vertical shear and strain, we analyze LADCP and CTD profiles, measured across the basin in 2007, covering various seafloor morphologies (shelf, shelf break, and abyssal plain). By introducing broadband statistical quantities derived from the shear–strain variance partition, our results show that increasing seafloor roughness reduces the absolute values of shear-to-strain ratio, a pattern also influenced by correlations between slope and roughness. Roughness appears to constrain waves toward higher frequencies, with high shear-to-strain ratios associated with lower frequencies and flatter propagation angles, and low ratios linked to higher frequencies and steeper beams. Spectral analyses indicate that rougher regions enhance strain variance at small vertical scales while reducing shear variance at larger scales, leading to flatter shear spectra in the low-wavenumber band. Together, these findings suggest that roughness redistributes energy from large-scale (low-mode, low-vertical-wavenumber internal waves with vertical scales O(102–103 m)) toward small-scale (high-vertical-wavenumber internal waves with vertical scales O(10–101 m)), fundamentally altering the balance of internal wave energy across scales. These results provide useful knowledge for ad hoc finescale parameterization based on seafloor topography.
This study characterizes seasonal hydrography and mixing processes in Almirantazgo Fjord, a sensitive ecosystem in southern Chilean Patagonia. Although estuarine and tidal forcing conventionally explain fjord dynamics, wind stress effects remain less understood in this high-latitude region. The study analyses a comprehensive six-month dataset including a moored time-series of temperature, salinity, and dissolved oxygen, cross-fjord CTD transects, and hydrographic profiles derived from seal-deployed sensors. Observations indicate distinct seasonality, shifting from a stratified water column in summer – defined by low-salinity surface water from glacial melt – to a mixed winter state with significantly reduced vertical stability. The analysis identifies persistent, topographically channelled up-fjord winds as a primary physical driver. By applying the Wedderburn number (Wb) and mechanical energy balance calculations, we determined that strong wind stress perturbs the pycnocline (Wb > 1). During stratified summer periods, strong wind events (above the 90th percentile) generated wind power of the same order of magnitude as that of the estuarine circulation. Under such conditions, wind forcing amplifies vertical mixing, modulates the pressure gradient, and supports oxygenation in the upper and subsurface layers. First-order estimates indicate that the upper brackish layer is flushed in approximately one week, reflecting a dynamic surface exchange governed by the interplay of freshwater buoyancy and wind stress. These results indicate that wind constitutes a primary mechanism regulating the hydrographic structure and biogeochemical function of the Tierra del Fuego inner sea.
Understanding temporal variations in nearshore sea states is crucial, as they affect shoreline evolution and coastal hazard potential. Local sea state conditions are influenced by large-scale climate modes, yet the underlying mechanisms remain not fully understood. Previous studies have mainly established the climate–sea state links through correlation analyses or other statistical methods. This study investigates whether weather typing, a statistical downscaling method, can provide a physically interpretable link between climate modes and local wave and storm surge variability. The analysis was conducted at Hartlepool, UK, where 36 weather types were previously developed to assess the exposure to coastal hazards for a local nuclear power station. Six climate indices were examined, and we found that the North Atlantic Oscillation (NAO) and the Scandinavian pattern (SCAND) have significant correlations with local wave and storm surge variables. The analysis reveals that, in response to the phases of NAO or SCAND, storm surge distributions exhibit changes in the mean and standard deviation, peak wave period distributions shift between bimodal and near-unimodal shapes, and wind waves and swell show different dominant directions. Using weather types, these response patterns can be traced back to synoptic circulation conditions characterized by different prevailing winds, spatial patterns of storm activity, and local atmospheric pressure. NAO and SCAND modify the occurrence probabilities of these synoptic conditions, thereby providing a probabilistic link between large-scale climate modes and local sea states. This research demonstrates the potential of weather types to offer new perspectives on the impact of climate modes on local sea states.
Ocean bottom pressure (pb) is critical for monitoring and understanding ocean variability, yet global observations from GRACE and GRACE Follow-On suffer from limited spatiotemporal coverage. State estimation methods allow for the dynamical interpolation of sparse data by optimally combining observations with models. Here we examine the effects of assimilating GRACE data (local pb anomalies and global mean), along with other datasets, on state estimates produced by the project for Estimating the Circulation and Climate of the Ocean (ECCO). The ECCO optimization leads to large adjustments in pb fields at monthly and longer timescales. A substantial part of those adjustments is directly induced by GRACE constraints, with largest impacts occurring at high latitudes. Additionally, the mean ocean mass constraint is essential for mitigating large imbalances in freshwater fluxes derived from atmospheric reanalyses (used as prior forcing) and for producing a realistic barystatic sea level curve. Interpretation of remaining ECCO and GRACE differences highlights issues with non-oceanographic data signals. Our findings indicate that GRACE data contain information complementary to that available in other datasets, quantifying their value for determining pb
Since the 1990s, satellite observations have been providing reliable estimates of ocean surface state, including absolute dynamic topography (ADT), sea surface temperature (SST), and sea surface salinity (SSS) at sufficient space and time resolution to characterize ocean dynamics. Together with the extensive hydrographic dataset from Argo and ship-based hydrographic profiles, these measurements provide a comprehensive view of oceanic conditions. While ADT reflects full ocean dynamics, its steric component represents the integrated information for subsurface water properties. However, relating surface variables to subsurface profiles remains challenging because surface signatures are often non-linearly related to interior structures, and satellite data contain inherent non-steric signals. To address these limitations, we introduce TS-Cast, a novel uncertainty-aware deep neural network. Unlike direct regression models, TS-Cast is designed to adjust monthly climatological profiles as a physical prior and learns to dynamically adjust them. By using a 31 s sequence of satellite inputs (SST, SSS, and ADT) and quantifying prediction uncertainty, the model effectively captures the temporal variation of mesoscale dynamics. It was trained on approximately 155 000 Argo and ship-based thermohaline profiles in the northwestern Pacific. TS-Cast’s capability is demonstrated by comparisons with independent time-series data from moorings that measured temperature and salinity or vertical acoustic travel time. The network significantly improves upon the climatological baseline, achieving an overall Root Mean Square Error (RMSE) of <1° C for temperature and <0.1 psu for salinity in the upper 500 m depths at the Kuroshio Extension region. This performance is comparable to or surpasses that of data-assimilating numerical and statistical models, validating TS-Cast as a powerful tool for ocean monitoring. Critically, this framework reveals not only TS-Cast's high fidelity but also demonstrates that the limitations of the input satellite data fundamentally constrain its predictive skill.
This study investigates the dynamics and alteration of a low-density filament driven by freshwater-induced buoyancy embedded within a tidal mixing front, focusing on its spatial and temporal evolution in the near-surface layer (0.1–10 m) of the water column. A high-resolution, multi-sensor dataset, consisting of surface drifters, a drifting sensor chain, and an autonomous surface vehicle equipped with an Acoustic Doppler Current Profiler, temperature, and conductivity sensors, was used to observe patterns of divergence, vorticity, and vertical velocities. The measurements resolved three phases of the filament occurring on length scales of O(0.1–2 km)<1 m, (II) the ongoing filamentolysis in the lower NSL induced filamentogenesis above 0.6 m, (III) restratification of the upper 0.6 m. Vertical velocities ranged between
Sediment resuspension, driven by wind-wave-induced shear stress, is a key process influencing coastal water quality, biogeochemical cycles, and the transport of pollutants and organisms. The critical shear stress, τcr, is a central parameter in sediment transport models, since initiation of motion can occur when wave-induced shear stress exceeds the critical value. In this study, we implemented a high-resolution (20 m) spectral wave model to simulate near-bottom orbital velocities across the complex archipelago of southwestern Finland. We then used laboratory measurements from in situ sediment samples to determine a model for the critical shear stress that accounts for physical properties using the median grain size and the dry bulk density, and the time-varying biological variation using chlorophyll a. Our proposed model,
Satellite-derived oceanic data are frequently affected by cloud cover, resulting in spatiotemporal gaps. The Multi-DINEOF method is widely used to reconstruct multiple oceanic variables. However, Multi-DINEOF essentially remains a matrix-based DINEOF approach and does not fully leverage the correlations among multiple variables. To address this limitation, this study proposes the T-DINEOF model, aiming to improve the accuracy of reconstructing multiple oceanic variables simultaneously. When applied to sea surface temperature (SST), sea surface chlorophyll a
Satellite and reanalysis data sets are analyzed to explain sea level changes in the tropical North Atlantic margin off northwest Africa. The study domain sea level was rising as far back as 1986 and a pause in sea level rise (hiatus) began around 2010 and stopped in 2019. Characteristics of sea level anomaly and its drivers during a period of rise (1996–2004) and the hiatus period (2010–2018) are analyzed and compared. Results show that the most effective cause of domain-wide sea level rise during the period of rise is seawater expansion owing to changes in density structure (steric expansion), with almost equal contribution from temperature-driven (thermosteric) expansion and salinity-driven (halosteric) expansion. The cause of the domain-wide pause in sea level rise is a large thermosteric contraction that counteracted halosteric expansion and mass accumulation. Multidecadal sea level increase, defined here as the difference between the mean sea level during the period of rise and the hiatus period, is owing to steric expansion, vertical land motion, and mass accumulation, which contributed 56 %, 24 %, and 16 %, respectively. There are, however, regional differences in the patterns of multidecadal steric and mass adjustment. In the northern subdomain where ocean processes predominate mass-driven sea level variability, the steric adjustment is dominated by halosteric expansion, whereas in the southern subdomain where atmosphere-ocean processes predominate mass-driven sea level variability, the steric adjustment is dominated by thermosteric expansion. The accumulation of low-salinity water in the northern subdomain and precipitation in the southern subdomain appears to be associated with a mutual adjustment of vertical and horizontal velocity distribution inside the domain and west of it in the area of the Guinea Dome, a permanent upwelling region where isotherms are displaced upwards. The low-salinity water influx to the northern subdomain is linked to changes in the southward-flowing Canary Current. A probable hypothesis inferred from correlation and potential vorticity analysis is that the Canary Current source region was freshened by currents that supply water to the region via two pathways: an open ocean path that is consistent with the Azores Current, and a Western Europe coastal ocean path that is consistent with the Portugal Current and Portugal Coastal Current system. The results obtained highlight a multidecadal linkage between sea level anomalies in the eastern tropical North Atlantic margin and salinity anomalies elsewhere in the North Atlantic.
This study outlines the development and testing of a Digital Twin Ocean (DTO) framework, aimed at improving coastal ocean forecasts through the use of autonomous underwater gliders. A fleet of gliders were deployed in the western English Channel during August-September 2024 to collect measurements of temperature, salinity, chlorophyll and oxygen, aiming to track the movement of the harmful algal bloom Karenia mikimotoi. Measurements were assimilated into a very high resolution (1.5 km) numerical forecast model, with an implementation of biogeochemistry data assimilation for this purpose. The model forecast was then used by a probabilistic uncertainty model to plan a series of waypoints to navigate the glider fleet towards features of interest. By utilising a continuous feedback loop of measurement, prediction, guidance, and refinement a system with real time coupling between the real ocean environment and its digital counterpart has been established.Building upon a prior pilot study of Ford et al.2022), this work improves every element of the system to address several limitations of the prior configuration. Whilst a bloom was present in the wider area, measurements and modeling suggest it didn't enter the glider operation zone. Despite this and other operational challenges the mission clearly demonstrates the benefits of such a system. The ability to simultaneously track multiple features of interest, namely chlorophyll maxima and oxygen minima, would not have been possible with a single glider resulting in significant benefits to the system. Furthermore, the improvement to biogeochemical forecasting has been demonstrated through a series of post mission experiments, highlighting the advantages of high temporal resolution observations and increased spatial resolution of the model.
Marine heatwaves (MHWs) are defined as prolonged periods of anomalously high ocean temperatures. These events can have severe impacts on marine ecosystems and, if they occur at the surface, can feed back on the atmosphere, changing inland air temperatures and precipitation.We use a comprehensive set of model, reanalysis, and observational datasets to investigate recent changes in North Sea MHWs. All datasets show a significant warming trend, accompanied by a marked increase in the frequency of MHWs. In contrast, the maximum intensity of MHWs has decreased in many regions of the North Sea, including the German Bight. If the linear trend in temperature is removed, only a few MHWs have been detected after 2019, suggesting natural variability has damped the effect of the long-term warming.While distinct weather patterns are associated with the onset of MHWs, their occurrence alone is not sufficient to trigger them. As the heat content is an integrated quantity, the ocean temperature at the beginning of the season (ocean preconditioning) is a key factor, in addition to prevailing weather patterns during the season. As a consequence, only in winter we find a significant dependency of MHWs on established climate indices. In our study region, MHWs result from a combination of short-term weather-related variability and longer-term seasonal to decadal variability.Furthermore, we find that the evolution of the surface temperature in the German Bight is largely determined by local atmospheric conditions rather than remote variability in the Atlantic. Although the inflow of warm water through the English Channel is important, it is the atmosphere that controls its volume transport and temperature. Whether the atmospheric conditions themselves are linked to remote variability in the Atlantic Ocean remains to be studied.
We used a high-resolution (1