Marine heatwaves (MHWs) pose significant threats to Mediterranean marine ecosystems and coastal economies, with frequency and severity projected to increase under future climate change. While Med-CORDEX fully-coupled Regional Climate System Models (RCSMs) offer enhanced resolution and improved representation of local processes relative to their parent Global Climate Models (GCMs), a systematic assessment of their added value for sea surface temperature (SST) and MHW properties has been lacking. This study quantifies the added value of Med-CORDEX RCSMs over the Mediterranean basin, evaluating their capacity to correct GCM biases and improve the representation of SST and MHW probability distributions. Results show that added value is scale-dependent and metric-specific. RCSMs generally improve the SST spatial pattern and the shape and upper tail of its temporal distribution, but mostly fail to correct Mediterranean basin-averaged errors in the mean, standard deviation, 90th percentile and linear trend. SST improvements are most consistent along coastlines and in semi-enclosed areas, where fine-scale ocean-atmosphere interactions and topographic constraints are best resolved by RCSMs. For MHW duration, downscaling provides consistent and spatially widespread improvements across nearly all models, driven by a better representation of short-lived events. For MHW intensity, added value is model-dependent and not systematic: while the majority of RCSMs improve this metric, some models exhibit deterioration linked to model-specific features. These results demonstrate that higher horizontal resolution is a necessary but not sufficient condition for improved MHW representation, and that simultaneous advances in other model components are required to fully exploit the potential of regional downscaling. In particular, within the Med-CORDEX ensemble, inter-model differences in MHW added value appear to be correlated with improvements in the thickness of the first layer.
Current and sea-level observations in the 2.4 km-long, 300 m-wide channel between Varna Lake and the Black Sea, conducted by three novel Hydromast stations with a 1 min resolution, revealed that moderate sub-hourly dynamics were occasionally interrupted by high-amplitude oscillations with a period of a few tens of minutes. The resonant excitation of the events was studied using barotropic 1D analytical and numerical methods. Basic features of high-intensity sub-hourly variations can be interpreted as linear barotropic long waves in a sea-channel-lake system with resonant and damped forcing. The observed 37 min oscillations, identified from the power spectra, spectrogram, and wavelet analysis, can be explained as the first mode of the channel-lake system. Another period of 19 min resembles the zero, quarter-wave mode of the channel and the second mode of the channel-lake system. During the measurement period, two high-amplitude events were highlighted. One of the events is interpreted as having been caused by a meteotsunami; moderate meteorological conditions prevailed; strong channel oscillations began abruptly and lasted for a few cycles. The second event occurred more than 5 h after landward winds up to 20 m s−1. The observed maximum changes in current and sea level – up to 0.8 m s−1 and 0.8 m, respectively, over 10 min – are harmful to ship navigation, harbor operations, and coastal management. Building on these observations, approaches for detecting and forecasting strong sub-hourly oscillations are discussed.
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× stronger inside the plume than outside, with differences reaching 100× at scales below 3 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 resolution CROCO (Coastal and Regional Ocean COmmunity model) simulation to assess the spatial distribution of surface fronts and their spatio-temporal variability. We characterize three distinct frontal regimes: (i) broken-up fronts parallel to shore occupy the plume core over the continental shelf, (ii) thin elongated fronts associated with NBC-plume interactions connect nearshore and offshore regions, and (iii) pools of anisotropic fronts driven by the seasonal mixed-layer cycle are present offshore. Salinity dominates density gradients throughout the year north of 15° N, whereas near-shore fronts exhibit seasonal shifts in temperature-salinity dominance linked to the Amazon discharge seasonality and NBC strength. Within the plume, freshwater filaments stirred by NBC rings systematically generate density-compensated fronts on their inner edge and reinforced fronts on their outer rim, a pattern with implications for energy cascades and tracer export.
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 1/12° SSS, sea surface temperature (SST), and sea surface height (SSH) fields, and is then conditioned at inference time on a synthetically degraded coarse SSS observation (1/3°) together with high-resolution (1/12°) auxiliary SST and/or SSH. Conditioning is performed through pseudo-inverse guidance, which steers the generated samples toward states that are compatible with the coarse observation while remaining within the learned GLORYS-consistent multivariate distribution. We also test a gradient-enhancement procedure designed to increase contrast during inference. Experiments in the Gulf Stream region compare models conditioned on SST only, SSH only, and both variables. Validation over the year 2020 uses root-mean-square error (RMSE), structural similarity (SSIM), gradient distributions, and temporal Fourier spectra. In the present GLORYS configuration, conditioning on SST substantially improves accuracy relative to SSH alone; combining SST and SSH yields further gains, comparable to a strong convolutional baseline under RMSE/SSIM, while additionally providing an ensemble of plausible reconstructions. The gradient-enhanced sampler increases structural contrast but can risk amplifying part of the variability, illustrating a trade-off between pixel-wise accuracy and structural realism. Overall, the results support guided diffusion as a promising framework for SSS downscaling and uncertainty-aware reconstruction, while showing that transfer to real satellite SSS products will require product-aware observation operators, uncertainty weighting, and independent in-situ validation.
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.
A three-dimensional mooring array holding nearly 3000 high-resolution temperature sensors in 2500 m deep Mediterranean-Sea waters is used for a yearlong study on different sources of turbulent waterflows, which are vital for life. Although temperature differences are found never larger than 0.01 degrees C, daily, weekly, and seasonal variations are observed. With a delay of about a week, the deep-sea stratified turbulence tracks atmospheric disturbances, which are found 35 % more energetic in winter than in summer. About half the time, relatively warm stratified waters are moved to near the seafloor from 100's of meters higher levels. Combined internal-wave and sub-mesoscale eddy-induced motions lead to slantwise downward convective warm-water periods that are half an order of magnitude more turbulent than those induced via general geothermal heating from below, and about one order of magnitude more turbulent than those from open-ocean processes. The analysis estimates that eddy-induced stratified turbulence is likely more important for deep-sea life than rare, not observed, deep dense-water formation at the abyssal-plain mooring site.
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, τcrd50,ρB,Chla(t), explained 66 % of the variation of the measured critical shear stress for our data collected from three different sediment types (Mud, Sand and Mixed sediments). The modelled mean critical shear stress differed between sediment classes, with values of 0.49 N m−2 for Mud, 1.56 N m−2 for Sand, and 1.02 N m−2 for Mixed sediments. The variability in the critical shear stress around the mean values driven by a non-constant biological contribution was approximately 30 % for Mud and Sand, and approximately 50 % for Mixed sediments. Finally, we used a class-level map of the sea floor and the in situ grain size data to translate the wave model orbital velocities to near-bottom shear stresses. Based on the numerical model data, the critical shear stresses from the newly proposed model, τcr(d50,ρB,Chla(t)), were rarely exceeded based on only wave-induced motions in most of the model grid, but could, nonetheless, be exceeded to up around 10 % of the times in smaller areas. This study highlights the importance of incorporating both physical and biological factors – and their temporal dynamics – into sediment transport models to achieve reliable predictions of critical shear stresses and resuspension potential.
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 (SCHL), and sea surface wind (SSW) collectively, T-DINEOF reduces root mean square error (RMSE) by 12.9 %, mean absolute error (MAE) by 13.8 %, and mean absolute percentage error (MAPE) by 11.9 % compared to Multi-DINEOF. For each individual oceanic variable, T-DINEOF outperforms both Multi-DINEOF and the original DINEOF methods, reducing RMSE by 9.0 % and 14.7 %, MAE by 10.5 % and 14.6 %, and MAPE by 13.7 % and 13.4 % for SST; reducing RMSE by 9.3 % and 11.8 %, MAE by 9.9 % and 13.4 %, and MAPE by 8.3 % and 11.8 % for SCHL; and reducing RMSE by 16.6 % and 3.7 %, MAE by 16.8 % and 3.5 %, and MAPE by 16.4 % and 3.1 % for SSW. Additionally, T-DINEOF proves effective in regions with a high proportion of missing data and in cases of low data correlation.
The upcoming Sentinel-3 Next Generation Topography (S3NG-T) mission, designed to succeed the current Sentinel-3 (S3) mission, will operate on the same ground tracks as the current S3 constellation to maximise continuity of measurements, but with a fixed 4 h temporal lag due to satellite design constraints. This configuration prevents the implementation of a classical near-simultaneous tandem phase, traditionally used for inter-mission cross-calibration, and raises concerns regarding the impact of short-term oceanic variability on continuity assessment. In this study, we evaluate the feasibility and expected performance of a 4 h delayed tandem phase for cross-calibrating S3 and S3NG-T. Using tandem datasets from Sentinel-3A/B and Jason-3/Sentinel-6 missions, combined with SWOT KaRIn observations, we develop a methodology to quantify the oceanic variability introduced by a 4 h delay and to evaluate its effect on the accuracy of inter-mission offset estimates. Results indicate that the classical tandem configuration achieves regional inter-mission Sea Level Anomaly (SLA) offset uncertainties of approximately 2 mm over a three-month period. In contrast, a 4 h delayed tandem phase increases this uncertainty to about 7 mm in the same period, but still performs significantly better than non-tandem scenarios. Extending the 4 h tandem phase to one year enables the detection of systematic instrumental errors of ±3.5 mm amplitude, sufficient to ensure continuity between S3 and S3NG-T. These findings demonstrate that, despite additional oceanic variability, a 4 h tandem configuration remains a viable and effective strategy for cross-calibration, especially when supported by improved environmental corrections and by extending the observation duration to a full year.
Extreme events potentially modify the physical and biogeochemical environment resulting in dramatic changes in phytoplankton community structure. In this study, the impact of ten well-identified storms on phytoplankton communities was explored in a productive temperate coastal ecosystem, the eastern English Channel (EEC). Our study targeted the summer season, which is typically characterised by low nutrient availability, yet transient phytoplankton blooms often occur. From 2012–2022, low-frequency (weekly to fortnightly) flow cytometry measurements of phytoplankton abundance were combined with high-frequency meteorological data (precipitation and wind) and river flow rates as a proxy for riverine influence. Storm impacts occurred in three distinct forms: high river inflow events, high wind stress-low inflow events, and low wind stress-low inflow events. Regardless of wind conditions, high inflow storms favoured diatoms, as nutrient-rich river plumes supported their growth. In contrast, low inflow events paired with strong winds enhanced vertical mixing and nutrient availability, favouring nanophytoplankton. Finally, weak winds and relatively low-nutrient conditions favoured picophytoplankton (Synechococcus and picoeukaryotes). Across years, storms repeatedly reset seasonal succession and maintained environmental heterogeneity, leading to transient monospecific peaks of phytoplankton. These findings highlight storms as recurrent structuring forces in the EEC, mediating nutrient availability and driving shifts in phytoplankton composition; as such, storm-driven dynamics might be an important input to prediction models for phytoplankton.
An asymmetric response of coastal currents to oscillating alongshore wind stress is observed over a coastal bank off the southern coast of Korea. Alongshore currents exhibit consistently larger variability in the western region than in the eastern region. Numerical experiments show that sea level reaches a maximum (minimum) in the western coastal region during westward (eastward) winds, leading to stronger cross-shore sea level gradients under both wind directions. Momentum balance analysis suggests that the alongshore pressure gradient force acts in the same direction as the wind stress in the western region but opposes the wind stress in the eastern region, resulting in a stronger current response in the western region. The asymmetry arises from spatial differences in mass convergence and divergence driven by spatially varying vertically integrated transport over the bank, which is subsequently governed by arrested topographic wave dynamics. This asymmetric mass redistribution leads to energy flux convergence in the western region. Although offshore currents and variations in the wind stress period and magnitude modulate coastal circulation, the qualitative asymmetry persists. These findings suggest that similar current asymmetries may occur in other coastal regions with bank-like geometry. Understanding such asymmetric current responses to wind stress is essential for assessing their potential ecological impacts in coastal bank regions.
Abstract. 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 and associated circulation fields.
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.
The North Pacific Intermediate Water (NPIW) plays an important role in regulating thermohaline structure and biogeochemical distributions in the North Pacific. However, limited continuous observations have hindered our understanding of its short-term variability and structural response to mesoscale processes. Based on 1 year mooring observations from three sites (M1–M3) in the western North Pacific, this study investigates the intraseasonal structural variability of the NPIW and its modulation by mesoscale eddies. The NPIW thickness exhibits pronounced intraseasonal variability, with a dominant period of approximately 60–80 d that is coherent among the three mooring sites. Unlike previous studies that mainly focused on temperature and salinity anomalies, this study introduces NPIW thickness as a structural diagnostic parameter to characterize the vertical compression and expansion of the intermediate layer associated with mesoscale variability. The results reveal a clear inverse relationship between NPIW thickness and salinity, indicating that anticyclonic conditions are generally associated with thinner and more saline intermediate layers, whereas cyclonic conditions correspond to thicker and relatively fresher layers. Spatial composite analyses further show that thickness and salinity responses exhibit clear regional differences, with stronger variability near the western boundary, likely related to complex water-mass redistribution and possible mixing processes. These findings provide quantitative observational evidence for intraseasonal variability in NPIW thickness and highlight its usefulness as a diagnostic indicator for mesoscale–intermediate water interactions in the North Pacific.
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 large seasonal increases in marine photosynthetic organisms – i.e., phytoplankton blooms – are a ubiquitous oceanic phenomenon that contributes to the removal of carbon dioxide from the atmosphere and supports the growth of larger marine organisms. The underlying mechanisms controlling the intensity and timing of these blooms have been proposed to be dominated by vertical transport and mixing processes that are enhanced at fine-scale frontal and filamental circulations, commonly known as submesoscale currents. Here we show that the winter blooms characteristic of the ultra-oligotrophic waters of the Eastern Mediterranean Sea, which manifest as a seasonal increase in satellite-derived levels of surface chlorophyll, are intensified by enhanced horizontal stirring induced by the submesoscale currents. Using ocean color remote sensing data and high-resolution numerical simulations, we demonstrate that the intensification of submesoscale currents in winter efficiently connect the coastal waters and the ultra-oligotrophic open-sea waters, thereby enriching the latter with chlorophyll-rich waters. A climatological chlorophyll time series comparison between two different regions equidistant to the Nile River Delta indicates that this submesoscale horizontal stirring mechanism accounts for the ∼ 24.8 % larger wintertime increase in surface chlorophyll observed downstream of the Nile Delta. These results shed new light on the processes governing phytoplankton bloom intensity and emphasize the important role of submesoscale horizontal stirring in modulating the marine ecosystem.
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.
Abstract. Recent advancements in remote sensing systems, combined with new machine-learning model-fitting algorithms, have enabled the estimation of seawater carbon dioxide partial pressure (pCO2,sw) and pH (pHT,is) in the waters around the Canary Islands (13–19° W; 27–30° N). Continuous time-series data collected from moored buoys and Voluntary Observing Ships (VOS) between 2019 and 2024 were used to train and validate the models, providing a robust observational basis for satellite-derived estimates. Among all models tested, bootstrap aggregation (bagging) performed best, achieving an RMSE of 2.0 µatm (R2>0.99) for pCO2,sw and 0.002 for pHT,is. Multilinear regression (MLR), neural networks (NN) and categorical boosting (CatBoost) also showed good predictive skill, with RMSE values between 5.4 and 10 µatm for pCO2,sw (360–481 µatm) and 0.004–0.008 for pHT,is (7.97–8.07). Using the most reliable model, we identified an increasing trend in pCO2,sw of 3.51±0.31 µatm yr−1, exceeding the atmospheric CO2 growth rate (2.3 µatm yr−1), alongside an acidification trend of −0.003 ± 0.001 yr−1. Over the 2019–2024 period, rising atmospheric CO2 and increasing sea surface temperatures (reaching up to 0.2 °C yr−1 during the unprecedented 2023 marine heatwave) likely contributed to these trends. The Canary Islands region shifted from a weak CO2 source (0.90 Tg CO2 yr−1) in 2019 to 4.5 Tg CO2 yr−1 in 2024. After 2022, eastern sites that previously acted as annual CO2 sinks became net sources.
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.
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.
