Internal tides contribute significantly to ocean mixing and circulation, yet their dynamics and energetics in semi-enclosed seas, including the Mediterranean, remain poorly quantified. This study investigates internal tides in the Central Mediterranean Sea using a high-resolution (1 degrees/ 60) ROMS model, that agrees well with available observations. Total barotropic-to-baroclinic energy conversion is estimated at 147 MW with similar to 72% occurring in the Sicily Strait. The diurnal K-1 signal dominates from northwestern Sicily to the Adventure Bank, where slope-trapped Kelvin waves induce strong baroclinic shear, producing low Richardson numbers (Ri <025) and elevated bottom turbulent kinetic energy dissipation consistent with thermistor observations. Northeast of Pantelleria, the K-1 tide splits into two energy flux branches, while additional K-1 generation near thermistor station AT propagates energy northeastward toward the western Sicilian shelf. Although contributing only similar to 6% of total conversion, dominated by M-2 tides, overtides modulate conversion variability, while turbulent mixing remains elevated. Over the Malta Plateau, baroclinic conversion is predominantly diurnal, with flux pathways agreeing with recent observational studies. A theoretical framework decomposes baroclinic energy flux divergence into pressure work, advection, and diffusion; neglecting advection and diffusion leads to misrepresentation of local energy budgets. While these terms compensate in the Sicily Strait and across the domain, the balance breaks down over the Sicily subregion, the Malta Plateau and Messina Strait, resulting in enhanced mixing and residual circulation. By providing the first systematic energetic characterization of internal tides in this region, this study identifies key generation sites, flux pathways, and dissipation hotspots, offering a framework for understanding internal tide-driven mixing in other marginal seas worldwide.

This study presents a global assessment of the climatological seasonal variability of river discharge into the oceans, based on an expanded dataset comprising 958 gauging stations across 136 countries. Monthly discharges were compiled for 145 major rivers and tributaries, with a focus on improving the accuracy and spatial coverage of global freshwater flux estimates. Compared to previous datasets, this updated compilation includes a broader set of rivers, explicitly integrates tributary inflows, and quantifies both the absolute and relative seasonal amplitudes of discharge variability. The results reveal substantial differences among ocean basins. The Atlantic Ocean, although receiving the highest total runoff, shows relatively weak seasonal variability, with a coefficient of variation of CV = 12.6% due to asynchronous peak discharge from its major rivers (Amazon, Congo, Orinoco). In contrast, the Indian Ocean exhibits the most pronounced seasonal cycle (CV = 88.3%), driven by monsoonal rivers. The Pacific Ocean shows intermediate variability (CV = 62.1%), influenced by a combination of monsoon rains and snowmelt. At the river scale, Orinoco and Changjiang display high seasonal amplitudes, exceeding 89% of their mean flows, whereas more stable regimes are found in equatorial and temperate rivers like the Amazon and Saint Lawrence. In addition, the critical role of tributaries in altering discharge magnitude and seasonal variability is well established. This study provides high-resolution monthly discharge climatologies at global and basin scales, enhancing freshwater forcing in OGCMs. By improving the representation of land–ocean exchanges, it enables more accurate simulations of salinity, circulation, biogeochemical cycles, and climate-sensitive processes in coastal and open-ocean regions.

This study aimed to accurately simulate the main tidal characteristics in a regional domain featuring four open boundaries, with a primary focus on baroclinic tides. Such understanding is crucial for improving the representation of oceanic energy transfer and mixing processes in numerical models. To this end, the astronomical potential, load tide effects, and a wavelet-based analysis method were implemented in the three-dimensional ROMS model. The inclusion of the astronomical tidal and load tide aimed to enhance the accuracy of tidal simulations, while the wavelet method was employed to analyze the generation and propagation of internal tides from their source regions and to characterize their main features. Twin simulations with and without astronomical potential forcing were conducted to evaluate its influence on tidal elevations and currents. Model performance was assessed through comparison with tide gauge observations. Incorporating the potential forcing improves simulation accuracy, as the model fields successfully reproduced the main features of the barotropic tide and showed good agreement with observed amplitude and phase data. A complex principal component analysis was then applied to a matrix of normalized wavelet coefficients derived from the enhanced model outputs, enabling the characterization of horizontal modal propagation and vertical mode decomposition of both M2 and nonlinear M4 internal tides.

This case study demonstrates how water transformation in a secluded bay can be investigated using a range of Lagrangian analysis methods that can be calculated with a mass-conserving Lagrangian trajectory model. The study focuses on analysing the water mass transformation and overturning circulation in the Gulf of Gabes. The gradual transformation of water masses flowing through the Gulf was analysed using model-simulated Lagrangian trajectories. It was found that the overturning circulation in the Gulf gradually deepens, although it is falsely exaggerated by up to 50 metres when computed as a simple longitude-depth Lagrangian stream function. The Lagrangian method enabled the determination of the spatial dependence of transit time. The analysis revealed that most of the water in the Gulf has a transit time short enough to adjust to seasonal variability. However, in the innermost part of the Gulf, there exists an anticyclonic vortex that tends to trap water on longer timescales, preventing it from adjusting to seasonal variability. The trajectories were computed using velocity and mass transport fields from a high-resolution (1/96 degrees) hydrodynamic ROMS model, which includes the relatively strong tides in this region of the Mediterranean.

The water mass transformation in the Gulf of Gabès and its associated overturning circulation is investigated. The strong all year evaporation in the Gulf leads to a salinification of the entering water masses, which hence return with a strong salinity increase. The heat transports in and out have a strong seasonal cycle, with approximately as much entering as exiting the Gulf in the yearly mean.The overturning circulation is calculated from Lagrangian trajectories, which makes it possible to follow in detail the water mass transformation from where the water enters in the north, following an anticlockwise circuit along the coast in the Gulf until it exits in the southeast. The densest and most saline water exits, however, in the deepest middle part of the Gulf, where it tends to mix with the slightly fresher Mediterranean Waters. The trajectories are computed with the velocity and mass transport fields from a high-resolution (1/96°) hydrodynamic model, which includes the strong tides in this part of the Mediterranean. Figure: Examples of trajectories entering and exiting the Gulf of Gabès. Colour for individual trajectories as a function of salinity.

Marine plastic pollution represents a major problem owing to its increasing presence in the environment, persistence and ability to spread in every compartment in the form of small plastic particles, namely microplastics (MPs). Studies concerning MPs abundance in the Mediterranean Sea are growing, but their occurrence in the Southern regions remains largely unexplored. In this study, distribution, abundance, size, and polymer type of microplastics were investigated in surface water samples collected with a Manta net (200 μm mesh size) and in 118 marine specimens of commercial interests, including fishes, crustaceans, and mollusks, during Spring and Autumn 2019 EU H2020 Claim Project sampling Campaigns in the Gulf of Gabes (Southern Mediterranean Sea). Laboratory characterization showed significant plastic pollution concentrations, with an average abundance of 312,887 and 77,110 items/km2 in surface water samples collected in Spring and Autumn, respectively. A 3D hydrodynamic and Tracking Model was used to identify dispersal and transport pathways of the floating plastics, reporting a seasonal variability observed in MPs distribution between I (Spring) and II Campaign (Autumn). Despite the high values of MPs abundance found in surface water samples, an overall low frequency of ingestion among studied species was observed, with a maximum value of 20% of individuals (in Scomber scombrus) found with ingested MPs. The present study contributes to expand our state of knowledge regarding MPs pollution level in water and biota samples collected in the Gulf of Gabes, an area of particular interest for its biological resources, but still little investigated.

The Mediterranean Sea has been described as one of the most affected areas by marine litter in the world. Although microplastics and their effects have been investigated in this area, most of the currently available studies have been limited to the northwestern part of the basin. This study constitutes a first attempt to determine the abundance, characteristics and composition of microplastics in near surface waters of the Gulf of Gabes (southern Mediterranean Sea, Tunisia). Samples were collected using a 200 mu m-mesh size trawl net along two transects. The study revealed an average concentration of 63,739 items/km(2) where fragments and films were the most frequent microplastics. Polyethylene, reformulated polyethylene and polypropylene were the most abundant plastics identified among the samples (86-100%). The influence of hydrodynamics on microplastics in the Gulf of Gabes was investigated through the use of a Lagrangian tracking model to simulate the dispersion of particles in water. Modelling results seem to be in agreement with the reported distribution and characteristics of microplastics in this area.

The circulation of central Mediterranean Sea is not yet well established despite the efforts that have been made. This mainly concerns the surface circulation along the Tunisian coasts which is still poorly understood.

The present study investigates the spatial distribution of the M-2 internal tide in the Strait of Sicily using a three-dimensional sigma coordinate model, the Regional Ocean Modeling System. Realistic topography and stratification from existing observational data were used. The numerical simulations show that there are three distinct sites of strong M-2 internal tide generation, namely the western sill of the Adventure Bank, northwest of Sicily and north of Pantelleria isle. The conversion rate of energy from the M-2 surface to internal tide integrated over the whole model domain amounts to 47.5 MW, 75% of which are found to be generated over the three prominent topographic features mentioned above. The depth-integrated baroclinic energy flux depicts the propagation away from these sites, particularly toward the north and southwest. The maximum flux amplitudes are found in the three main generation sites identified, with the largest value occurring at the narrowest passage through the western sill. A comparison between the model and currentmeter measurements at two stations shows reasonable agreement for different depths. The similarities in the spatial distribution of the baroclinic energy flux divergence and the barotropic forcing suggest that the generation of the M-2 internal tide in the Strait of Sicily results primarily from barotropic tidal flow over sloping topography.

We investigate the sensitivity of the sea surface circulation in the Sicily Channel to surface winds, using a 15-year long (1994-2008) air-sea coupled numerical simulation. Analysis is based on the clustering of six main wind regimes over the Sicily Channel domain. The analysis of the corresponding sea current clusters shows that sea circulation in this area is sensitive to surface wind patterns. This wind modulates the strength of the two main branches of the sea circulation in the Sicily Channel (i.e. the Atlantic Tunisian Current and the Atlantic Ionian Stream). The modulation of these two currents depends on the wind regime, and displays a strong seasonal variability. It is also shown that the sea circulation in the Sicily Channel is strongly controlled by the thermohaline circulation and the bathymetry (geostrophic current). However, the contribution to the total current of its ageostrophic component forced by the surface winds is significant, with a correlation coefficient varying from 0.3 to 0.7. (C) 2015 Elsevier Ltd. All rights reserved.

The purpose of this study is to study the medium-scale dynamics, as well as the dynamics of the sub-basin scale in the Central Mediterranean, and to elucidate the routes of the Atlantic waters in this region using a high-resolution numerical model of the eddy-resolving simplest equation. The seasonal variability of the two streams of modified Atlantic waters crossing the Sicilian Strait varies considerably. The main stream along the coast of Tunisia, giving rise to the Atlantic Tunisian current, is stronger than the Atlantic Ionian stream (AIP) from autumn to spring. The Atlantic Tunisian current, which, apparently, is present throughout the year, is characterized by high spatial and temporal variability from the simulation results. The high-resolution model is capable of well reproducing the flow and variability of AIP, including such associating characteristic structures as the Advent Benk Vortex, Moltis Channel Cross, Ionian Benc Vortex and the outflow to the northern Ionian Sea.

We have investigated the barotropic tides in the Tunisian shelf and the Strait of Sicily using the Regional Ocean Modelling System (ROMS) with very high-resolution. Model performance was evaluated with respect to tide gauge, satellite data, and current meter measurements. The model fields faithfully reproduced the major feature of the barotropic tidal currents and agreed well with existing tidal elevation and phase observations. General features for the various semidiurnal constituents are nearly similar to each other with maximum amplitude in the Gulf of Gabes. The larger tidal currents occur over the continental shelves. In the Adventure Bank, the current is essentially of diumal type whereas in the Gulf of Gabes it is of semidiurnal type.Tidal energy lost, which is primarily due to bottom stress dissipation, is predominantly in the Gulf of Gabes (similar to 61%), the Strait of Sicily, and the Strait of Messina. The forcing function for internal tides shows for both M, and K, constituents, significant spatial variability in the Strait of Sicily. This suggests that some internal tides will be generated in these regions and could thus explain the observed strong diumal internal waves in the Adventure Bank. (c) 2006 Elsevier B.V. All rights reserved.

Seasonal and inter-annual variations in surface freshwater fluxes in the Mediterranean Sea are examined. Evaporation and precipitation rates are estimated from ERA15, the re-analysis project carried out at the European Center for Medium-Range Weather Forecasts (ECMWF) for the period 1979-1993. A seasonal cycle of river runoff is computed from a recent historical data set. The climatological mean for precipitation (326 mm/yr) is comparable to previous estimates, whereas that for evaporation (920 mm/yr) is low compared to other independent estimates, but regions of high and low evaporation rates are correctly located. The budget reveals an annual mean freshwater deficit in the Mediterranean of 480 mm/yr, lower than previous estimates because of the lower evaporation rate estimated by ECMWF. Consequently, the flows through the straits of Gibraltar and Sicily, deduced from the freshwater budget, are found to be slightly low.Seasonal and inter-annual variabilities of ERA15 precipitation are consistent with those deduced from independent precipitation estimates obtained with SSM/I observations for the 1988-1993 period. ECMWF and satellite estimates both agree on the amplitude of the seasonal cycle. The seasonal cycle of the river runoff has a globally small contribution to the freshwater budget, but is significant in summer when the precipitation is nearly null. The variability of the freshwater flux for the Mediterranean as a whole shows a strong seasonal cycle (amplitude of 50 mm/month), which seems to be mainly controlled by evaporation. The inter-annual variability of the freshwater flux, however, appears to be governed mainly by precipitation. Its amplitude, which is of the order of 50 mm/yr on average but may reach nearly 150 mm/yr for a particular year, is considered to be large but nevertheless not sufficient to explain differences observed in the estimates of the climatological mean freshwater flux proposed by various authors. The freshwater deficit in the Mediterranean Sea has globally increased by nearly 50 mm, mainly because of a decrease in precipitation over this 15-yr period. (C) 2000 Elsevier Science B.V. All rights reserved.

This paper examines how satellite altimeter and scatterometer measurements could be jointly used in a numerical ocean model in an attempt to simulate a realistic ocean circulation. The aim of the study is to determine quantitatively how sensitive the efficiency of the assimilation process may be to the variability of the wind stress curl and to evaluate the ability of the Topex/Poseidon altimetric data to correct for the sampling of scatterometer wind data that is to be provided by the forthcoming ERS 1 and NSCAT satellite missions. The model is quasigeostrophic, eddy-resolving and multi-layered, and is applied to the prognostic description of the mid-latitude ocean circulation in a schematic box ocean. Satellite altimeter data are simulated from model runs (forced by ECMWF winds) under the conditions of the Topex/Poseidon mission and are assimilated into the model by using a Newtonian relaxation or nudging technique. Satellite scatterometer wind data are simulated from ECMWF daily analyses of the wind stress over the North Atlantic. The use of realistic winds varying over a large range of temporal and spatial scales clearly has important dynamical consequences for the resulting ocean circulation but also militates against the efficiency of the altimeter data assimilation. The insertion of the altimeter data into the model is still able to constrain strongly the slow baroclinic circulation. However, the high frequency forcing generates barotropic Rossby waves that dominate the instantaneous flow throughout the basin and prevent complete convergence in the barotropic mode. Nevertheless, by smoothing out this fast time variability it is demonstrated that the assimilation of the altimeter data is still efficient in driving the larger time scales of the model ocean circulation. When performing altimeter data assimilation into a model domain forced by scatterometer wind data, the efficiency of the assimilation process is only slightly diminished but remains globally equivalent to the true wind forcing situation. Different ways of assimilating altimeter data were tested; they revealed that the use of the sea-level instead of the vorticity is more favorable; this result was not observed when model forcing did not incorporate such variability, i. e. when it was schematized by large scale constant wind stress curl (Verron, 199 1).

The present study is an attempt to evaluate the capacity of scatterometer wind data to be provided by the forthcoming European Remote Sensing 1 (ERS 1) and NASA Scatterometer (NSCAT) satellite missions, to successfully force basin scale ocean circulations. The mid‐latitude ocean is simulated with a three‐layer, quasi‐geostrophic, eddy‐resolving model in a square box geometry. The variable wind stress, which is used to simulate scatterometer winds, is derived from European Centre for Medium‐Range Weather Forecast (ECMWF) daily analyses of the wind stress over the North Atlantic. Its curl is applied as a forcing at every grid point of the model to drive a circulation of reference. The model is forced in real time under the swath with the curl derived from the raw scatterometer winds, with a condition of persistence that keeps the forcing constant between two consecutive passages of the satellite. The winds of ERS 1 (at 3‐ and 35‐day periods) produce circulations which globally resemble that of the reference experiment. A 3‐day repeat orbit appears to be the most suitable for producing raw winds capable of realistically driving a numerical ocean model. NSCAT (3‐day period) reproduces the main features of the reference large scale circulation with more success than ERS 1, and it is clear that NSCAT raw winds are far more suitable for driving an ocean Eddy‐Resolving General Circulation Model (EGCM) than ERS 1 winds because of their denser spatial coverage. ERS 1 and NSCAT raw wind stress curl fields are then objectively analyzed to produce smoothed “interpolated” wind stress curl fields having continuous values at every model grid point. Simulations with ERS 1 and NSCAT interpolated forcings show an improvement in the structure of the mean flow, but discrepancies in the energetics lead us to conclude that our interpolated winds are not significantly better than the raw winds. A «combined» forcing is constructed consisting of daily maps containing NSCAT raw data under the swaths and the corresponding objectively analyzed field outside the swaths. The circulation driven by the combined forcing reproduces remarkably well the main features and the energetics of the reference circulation. The patchy resolution of the combined forcing appeared to have little effect on the results of the simulations. Our conclusion is that, within the framework of the present study, scatterometer winds (and particularly those of NSCAT) seem appropriate for providing a forcing function to mid‐latitude ocean models, and for that purpose, these satellite data may not require prior assimilation in meteorological models, despite their patchiness and irregular resolution.

The horizontal ocean circulation generated by vertical convection is investigated analytically. The stratification is parameterized by a two-layer ocean and attention is focused on the spinup phase when the phenomena can be considered as linear. It is found that the response of the ocean to a two-dimensional idealized thermohaline forcing is baroclinic. A cyclonic gyre is generated in the upper layer, an anticyclonic gyre in the lower layer. Variation of the Coriolis parameter with latitude causes the center of the gyre to drift westward at the speed of long baroclinic Rossby waves.