This study investigates wave-driven circulation and transport in a tropical barrier reef system in the western Caribbean Sea using a nested modeling framework. Realistic offshore wave conditions are downscaled with a basin-scale phase-averaged (SWAN) model and used to force a high-resolution, phase-resolving, non-hydrostatic model (SWASH). The phase-resolving simulations explicitly capture the nonlinear wave transformation, wave breaking, and the resulting setup and wave–current interactions from the fore-reef to the lagoon. The depth-averaged momentum balance analysis shows that cross- and alongshore terms are of similar magnitude and are primarily governed by the radiation stress gradients, pressure gradients, and bottom friction. Alongshore momentum and circulation arise from the combined effects of wave incidence angle, local changes in bathymetry alongshore, and spatial variations in setup. The circulation patterns are intrinsically two-dimensional, with energetic circulation cells and water exchanges with the open sea occurring through gaps in the reef crest. Transport timescale estimates show rapid flushing over the reef flat and much slower renewal in the lagoon, highlighting the critical role of geomorphic complexity and wave conditions in controlling water retention and potential exposure to stressors in reef systems.

The salinity structure in the Magdalena River estuary results from a massive river discharge and a micro-tidal regime over a narrow-deep channel. Both environmental forcings are analyzed here to assess their effects on the temporal and spatial variability of the estuarine system. We investigated long-term (seasonal to interannual) and short-term (diurnal) changes in salinity structure and circulation through in situ measurements and a 3D hydrodynamic model ensemble (21 model runs). The measurements cover low (below 10th percentile) and mean river discharge conditions. Results show that the Magdalena River estuary (MRE) behaves as a strongly stratified system during the dry season (February to April) and as a vertically homogeneous one during the wet and transitional seasons when the river plume exhibits a lift-off regime at the mouth. A river discharge threshold has been found for the estuary to lift-off regime transition, corresponding to the 30th percentile (5200 m3/s). At the seasonal scale, the salinity intrusion length in the MRE shows more reactivity to river discharge variability than other reported strongly stratified estuaries; it is reflected in the scaling factor n in the relationship L∼Q^-n . A n value of 3.9 is the best fit for the MRE; nevertheless, it follows the Schijf and Schönfeld theoretical model solution for a prismatic flat configuration. Micro tides modulate the salinity intrusion on the diurnal cycle with a more significant effect on salinity intrusion and water exchange than previously reported, particularly during low discharge conditions. The findings from measurements and model scenarios for the MRE can be broadly applied and stand for an exemplary tropical, micro-tidal, anthropogenic intervened system.

This study analyzes horizontal and vertical wind-driven circulation responses in small semienclosed bays, the associated offshore dynamic conditions, and the relative importance of each term in the momentum balance equations using a multiplatform observational system. The observational platform consists of three ADCPs and a land-based radar monitoring the velocity field within the bay and in the contiguous offshore area. The wind-driven patterns in the bay can switch from a barotropic cyclonic or anticyclonic circulation to a two-layer baroclinic mode response as a function of the wind regime (its direction and magnitude). For the baroclinic mode, the vertical location of the inflection point in the velocity profile can vary according to the proximity of the boundary current to the entrance of the bay. The influence of offshore combined meteorological and marine conditions on the inner-bay dynamics is evidenced under moderate to strong wind conditions and is almost nonexistent under negligible wind. The momentum balance analysis as well as the nondimensional numbers evidence the impact of wind stress, coastline shape, stratification, and the nonlinear advective terms. Advection can be at the same order of magnitude as pressure gradient, Coriolis, or wind stress terms and can be greater than the bottom stress terms. The nonlinear terms in the momentum equations are frequently neglected when analyzing wind-driven circulation by means of in situ data or analytical models.

The extreme wave height distribution in the Caribbean Sea is studied using a new method based on the maximum basin-wide aggregate of significant wave height, Hs, values per month. Besides, by means of the Self-Organizing Maps (SOM) technique, we identify coherent geographical regions with similar extreme wave height variability in the Caribbean Sea. Our findings revealed three primary regions: the eastern side with comparatively lower values, the central region with intermediate values, and the western side with the highest extreme wave heights. The study also examines the wind forcing conditions driving the spatial and temporal variability of the extreme waves, highlighting the influence of the low-pressure belt dynamics as well as the role played by the Caribbean Low-Level Jet (CLLJ) index, and the impact of cold fronts and hurricanes on extreme wave heights. Additionally, we explore the relationship between the extreme wave height distribution and climatic indices, such as the Atlantic Multidecadal Oscillation (AMO), the Atlantic Meridional Mode (AMM), the North Atlantic Oscillation (NAO) and the Oceanic Niño (ONI). The results reveal that the spatial distribution of extreme wave heights in the Caribbean Sea is mostly ruled by the influence of the CLLJ, with correlations close to 80%. In addition, significant correlations were observed between the extreme wave heights and the ENSO in the central Caribbean, as well as positive correlations between the extreme wave heights and NAO in the eastern part of the basin, and significant values of correlation with the negative phases of AMO and AMM in the whole basin. We show that, unlike conventional (or broadly used) methods deployed to identify extreme wave height, such as percentile 99th, Hs99, our methodology allows a further assessment of the wind and climate forcing conditions associated with the extreme wave events. Although, we acknowledge that the method here presented has limitations to capture extreme wave height outliers, it has the advantage of being used concomitant with the wind forcing to develop multivariate wave climate analysis at basin scale, and could be extended to a more local scale when studying coastal processes.

This study analyzed the hydrodynamic structure of the estuary and plume of a large, tropical, and strongly-stratified river, the Magdalena. The Magdalena River is the largest source of fresh water and sediment of the Caribbean Sea and drastically impacts the morphodynamics and ecosystems along the Colombian coast. The analyses are based on the results of a calibrated and validated numerical model. Three scenarios of low, mid, and high freshwater discharge were analyzed to consider the seasonal variability of the mixing processes. The relative importance of three mixing mechanisms in the system is investigated, and the estuary and plume are subdivided into sectors depending on the dominant hydrodynamic processes at each sector. The results show that mixing in this system is mainly controlled by turbulence at the pycnocline, especially at the lift-off point in the transition between the estuary and the plume's near-field. A hydraulic jump occurs in the transition between the near- and mid-fields of the plume, but the mixing produced here is comparatively low since the plume has been mostly diluted already in the estuary and the near-field. During low and mid discharge scenarios, saline intrusion in the river channel forms a strongly stratified estuary where bottom propagated turbulence generates instabilities producing minor mixing. Meanwhile, the saltwater is flushed out of the river during the high discharge scenario, and pycnocline instabilities produce mixing at least one order of magnitude larger than during low discharge conditions. Coriolis acceleration does not seem to be a relevant driver of the plume behavior, despite the larger extension of the plume during high discharges, given the low latitude of the system.

To assess the contribution of wind drag and Stokes drift on the near-surface circulation, a methodology to isolate the geostrophic surface current from high-frequency radar data is developed. The methodology performs a joint analysis utilizing wind field and in situ surface currents along with an unsupervised neuronal network. The isolation method seems robust in the light of comparisons with satellite altimeter data, presenting a similar time variability and providing more spatial detail of the currents in the coastal region. Results show that the wind-induced current is around 2.1% the wind speed and deflected from the wind direction in the range [18 & DEG;, 23 & DEG;], whereas classical literature suggests higher values. The wave-induced currents can represent more than 13% of the ageostrophic current component as function of the wind speed, suggesting that the Stokes drift needs to be analyzed as an independent term when studying surface sea currents in the coastal zones. The methodology and results presented here could be extended worldwide, as complementary information to improve satellite-derived surface currents in the coastal regions by including the local physical processes recorded by high-frequency radar systems. The assessment of the wave and wind-induced currents have important applications on Lagrangian transport studies.

Following on from a theoretical solution to the Kelvin–Helmholtz (KH) instability under submerged canopy-oscillatory flow environments a relationship between the incoming free surface wave frequency and KH frequency was developed. The KH frequencies ( ) are higher than those from the incoming wave expressed by . In addition, the analytical expression, along with the energy spectra analysis of the vertical turbulent flux of momentum ( ) for cases when the KH dominates vertical mixing, allowed us to observe the spectral shortcut mechanism. Based on comparisons between the KH time scale and the mean horizontal transport of mass time scale, it can be concluded that the instability has no effect on the wave-induced steady current process. The instability plays an important role on vertical mixing but not on the horizontal transport of mass.

A wave model based on an unstructured grid in the Mediterranean Sea is presented, which provides high-resolution in near-shore regions using the wave model WAVEWATCH III v6.07. The hindcast comprises hourly time series of integrated wave parameters and 2D directional spectra in selected locations, from January 1 " 1979 until December 31 " 2020, whereas a daily forecast simulation provides predictions for the following 5 days. The hindcast is validated against buoys and several satellite missions. Results show that the model provides a good performance for storm events and mean conditions in the Mediterranean Sea with normalized mean absolute error (NMAE) lower than 15% in 70% of the basin, spatial index of agreement (d(1)) above 0.6, and the model under/overestimations are below 35% compared with in-situ data. Next, a methodology for coastal risk assessment is developed, leveraging the detailed information provided by the model in shallow waters and relying on a Storm Power Index (SPI), Coastal Vulnerability Index (CVI) and Risk Index (RI). The methodology is applied to the coast of Liguria, Italy, as a region with a high social, economical and touristic value which has experienced catastrophic coastal flooding episodes in recent years. Results of risk assessment were presented for the storm event of October 2018, known to have caused severe damages in the Ligurian coastline. The results provide a SPI of 3 and 5 at the beginning and peak of the storm, respectively, leading to RI of 3-5 depending on the characteristics and socioeconomic importance of the coastal stretches. Therefore, it is concluded that the methodology estimates the risk in an efficient and adequate way for its implementation in an operational risk forecasting system.

The Mediterranean Sea is a prominent climate-change hot spot, with many socioeconomically vital coastal areas being the most vulnerable targets for maritime safety, diverse met-ocean hazards and marine pollution. Providing an unprecedented spatial and temporal resolution at wide coastal areas, high-frequency radars (HFRs) have been steadily gaining recognition as an effective land-based remote sensing technology for continuous monitoring of the surface circulation, increasingly waves and occasionally winds. HFR measurements have boosted the thorough scientific knowledge of coastal processes, also fostering a broad range of applications, which has promoted their integration in coastal ocean observing systems worldwide, with more than half of the European sites located in the Mediterranean coastal areas. In this work, we present a review of existing HFR data multidisciplinary science-based applications in the Mediterranean Sea, primarily focused on meeting end-user and science-driven requirements, addressing regional challenges in three main topics: (i) maritime safety, (ii) extreme hazards and (iii) environmental transport process. Additionally, the HFR observing and monitoring regional capabilities in the Mediterranean coastal areas required to underpin the underlying science and the further development of applications are also analyzed. The outcome of this assessment has allowed us to provide a set of recommendations for future improvement prospects to maximize the contribution to extending science-based HFR products into societally relevant downstream services to support blue growth in the Mediterranean coastal areas, helping to meet the UN's Decade of Ocean Science for Sustainable Development and the EU's Green Deal goals.

In this work, the backward wave breaking process by the presence of flow separation vortices under a solitary wave is studied. Based on a set of non-dimensional variables defined from the Buckingham Π theorem, a set of numerical experiments are performed in order to analyze the effect of varying the submerged obstacle geometry on the surrounding flow and free surface by using a RANS-VOF model. Model simulations are tested against available experiments showing a good performance of numerical results. The structure-submergence ratio and the structure-based Reynolds number modulate the type of breaking (collapsing-plunging). The structure aspect ratio defines the location and number of backward breaking points for rectangular structures and the breaking wave direction for triangular structures (either forward or backward). Moreover, when the flow separation vortex diameter is comparable to the local water depth a backward wave breaking process is originated. The vortex behaves as a rigid body that accelerates the flow at the upper side of the vortex, accumulating mass at the downstream side where a very large slope on the free surface makes the water fall backward due to gravity.

Due to the semi-enclosed nature of the Mediterranean Sea, natural disasters and anthropogenic activities impose stronger pressures on its coastal ecosystems than in any other sea of the world. With the aim of responding adequately to science priorities and societal challenges, littoral waters must be effectively monitored with high-frequency radar (HFR) systems. This land-based remote sensing technology can provide, in near-real time, fine-resolution maps of the surface circulation over broad coastal areas, along with reliable directional wave and wind information. The main goal of this work is to showcase the current status of the Mediterranean HFR network and the future roadmap for orchestrated actions. Ongoing collaborative efforts and recent progress of this regional alliance are not only described but also connected with other European initiatives and global frameworks, highlighting the advantages of this cost-effective instrument for the multi-parameter monitoring of the sea state. Coordinated endeavors between HFR operators from different multi-disciplinary institutions are mandatory to reach a mature stage at both national and regional levels, striving to do the following: (i) harmonize deployment and maintenance practices; (ii) standardize data, metadata, and quality control procedures; (iii) centralize data management, visualization, and access platforms; and (iv) develop practical applications of societal benefit that can be used for strategic planning and informed decision-making in the Mediterranean marine environment. Such fit-for-purpose applications can serve for search and rescue operations, safe vessel navigation, tracking of marine pollutants, the monitoring of extreme events, the investigation of transport processes, and the connectivity between offshore waters and coastal ecosystems. Finally, future prospects within the Mediterranean framework are discussed along with a wealth of socioeconomic, technical, and scientific challenges to be faced during the implementation of this integrated HFR regional network.

Mean wave energy flux (hereinafter WEF) is assessed in the Caribbean Sea from a 60-year (1958–2017) wave hindcast. We use a novel approach, based on neural networks, to identify coherent regions of WEF and their association with different climate patterns. This method allows for a better evaluation of the underlying dynamics behind seasonal and inter-annual WEF variability, including the effect induced by the latitudinal migration of the Intertropical Convergence Zone (ITCZ) and the influence of El Niño-Southern Oscillation (ENSO) events. Results show regional differences in WEF variability likely due to both intensification and migration of the ITCZ. WEF exhibits a strong semi-seasonal signal in areas of the continental shelf, with maxima reached in January and June, in agreement with the known sea surface temperature and sea-level pressure variability patterns. At larger scales, WEF shows a significant correlation with the Oceanic Niño Index (ONI, which is the primary index for tracking the ocean part of ENSO climate pattern), depicting positive values in the central and western sides of the basin and negative ones at the eastern side.

<p>By applying an unsupervised neuronal network (NN) to sea velocity <span>profiles and wind data, it was possible to determine the main wind-driven circulation patterns in the Toulon bay. In addition, the NN outputs were utilized to perform a conditional averaging to High-Frequency radar surface current data (HFR) and the atmospheric AROME model, in order to understand the connectivity between the inner Toulon bay circulation features and the offshore marine-atmospheric conditions. For instance, upwelling scenarios are observed under strong westerly winds, whereas the downwelling is present under easterly wind conditions. Additionally, a barotropic system is observed when weak-mid wind blows for long time periods, and first baroclinic modes occur under strong wind events. Up to date, few studies have presented a clear connectivity between semi-enclosed bays and the offshore conditions, particularly in </span><span>the </span><span>north</span><span>western</span><span> Mediterranean Sea. Thus, this methodology presents great advantages when trying to study the interaction between semi-enclosed bays and the open sea by means of a combination of several in situ meteo-marine information.</span></p>

Abstract. The Mediterranean Sea is a prominent climate change hot spot, being their socio-economically vital coastal areas the most vulnerable targets for maritime safety, diverse met-ocean hazards and marine pollution. Providing an unprecedented spatial and temporal resolution at wide coastal areas, High-frequency radars (HFRs) have been steadily gaining recognition as an effective land-based remote sensing technology for a continuous monitoring of the surface circulation, increasingly waves and occasionally winds. HFR measurements have boosted the thorough scientific knowledge of coastal processes, also fostering a broad range of applications, which has promoted their integration in the Coastal Ocean Observing Systems worldwide, with more than half of the European sites located in the Mediterranean coastal areas. In this work, we present a review of existing HFR data multidisciplinary science-based applications in the Mediterranean Sea, primarily focused on meeting end-users and science-driven requirements, addressing regional challenges in three main topics: i) maritime safety; ii) extreme hazards; iii) environmental transport process. Additionally, the HFR observing and monitoring regional capabilities in the Mediterranean region required to underpin the underlying science and the further development of applications are also analyzed. The outcome of this assessment has allowed us to finally provide a set of recommendations for the future improvement prospects to maximize the contribution in extending the science-based HFR products into societal relevant downstream services to support the blue growth in the Mediterranean coastal areas, helping to meet the UN’s Decade of Ocean Science for Sustainable Development and the EU’s Green Deal goals.

Abstract. Due to the semi-enclosed nature of the Mediterranean Sea, natural disasters and anthropogenic activities impose stronger pressures on its coastal ecosystems than in any other sea of the world. With the aim of responding adequately to science priorities and societal challenges, littoral waters must be effectively monitored with High-Frequency radar (HFR) systems. This land-based remote sensing technology can provide, in near real-time, fine-resolution maps of the surface circulation over broad coastal areas, along with reliable directional wave and wind information. The main goal of this work is to showcase the current status of the Mediterranean HFR network and the future roadmap for orchestrated actions. Ongoing collaborative efforts and recent progress of this regional alliance are not only described but also connected with other European initiatives and global frameworks, highlighting the advantages of this cost-effective instrument for the multi-parameter monitoring of the sea state. Coordinated endeavours between HFR operators from different multi-disciplinary institutions are mandatory to reach a mature stage at both national and regional levels, striving to: i) harmonize deployment and maintenance practices; ii) standardize data, metadata and quality control procedures; iii) centralize data management, visualization and access platforms; iv) develop practical applications of societal benefit, that can be used for strategic planning and informed decision-making in the Mediterranean marine environment. Such fit-for-purpose applications can serve for search and rescue operations, safe vessel navigation, tracking of marine pollutants, the monitoring of extreme events or the investigation of transport processes and the connectivity between offshore waters and coastal ecosystems. Finally, future prospects within the Mediterranean framework are discussed along with a wealth of socio-economic, technical and scientific challenges to be faced during the implementation of this integrated HFR regional network.

The variability of La Guajira upwelling system, in the south-central Caribbean Sea, is strongly influenced by the intensity and location of the atmospheric Caribbean Low-Level Jet (CLLJ), a near-surface branch of the easterlies, as well as by the regional ocean circulation. During favorable conditions (i.e., strong easterlies blowing almost parallel to the coast), upwelling is enhanced and a large amount of primary productivity occurs in La Guajira area. In contrast, during relatively mild wind conditions, the CLLJ is misaligned to the coast and the Caribbean Counter Current (CCC, locally also known as the Darien Current), which forms as a branch from the Panama-Colombia Gyre, flows northeastward over the continental shelf advecting waters from the southwestern Caribbean basin toward La Guajira. The CCC has a clear signature at the surface layer that extends from the Darien Gulf toward La Guajira peninsula during mild wind periods, while disappears during the months of strong winds. The direction and the magnitude of the easterlies, and more specifically of the CLLJ, control the position and pathway of the CCC, which extends more than 900 km in the southern Caribbean Sea during May, June, August, September, and October. The high concentration of chlorophyll-a at the sea surface evidenced by satellite-based color images is semi-seasonally modulated by the CLLJ, which during its relaxation phase allows the irruption of the CCC toward the east up to La Guajira.