A Bay of Biscay model configuration is used as a test case to assess the data-based consistency of ensemble-based ocean model uncertainties of several types: [A] built-in stochastic parameterizations at regional ocean scales, [B] ocean model response to a global atmospheric model ensemble and [C] both A and B simultaneously. Ensembles of varying length were generated. In addition to a seasonal-range ensemble, three medium-range ensembles were carried out over successive overlapping segments permitting to compare consistency metrics for different lead times. The largest spread was obtained for the C case, although most of the model uncertainties were attributable to the stochastic ocean parameterizations in A. We addressed the question of which ensemble type and lead time was able to provide the most realistic model uncertainties given observations of SST, sea level, and Chlorophyll a, using a theoretical and diagnostic consistency analysis framework expanded from Vervatis et al. (2021a). In our results, consistency was satisfactory for the stochastic ensembles of types A and C, for the “aged” error cases (but only marginally with respect to the “young” error cases), and whenever physical and biogeochemical uncertainty processes were active in the region and could be detected by the observational networks, such as the onset of the spring shoaling of the thermocline and the phytoplankton abundance primary bloom. Sea level empirical consistency was improved when a wide range of low- to high-frequency errors were included in the signal of dynamic atmospheric process in the data and in the model inverse barometer. These findings provide additional insight that can help configure ensemble-based methods in academic studies and in operational ocean forecasting systems.

The production by biological and photochemical mechanisms of short-lived Volatile Organic Compounds (VOC) in the surface ocean is regulated by environmental parameters and nutrient abundance, and hence climate change. These gases then enter the atmosphere through the air–sea interface and contribute to photochemical pollution, affect the cloud properties, the radiative forcing and precipitation. Despite the improved understanding of the temporal and spatial distribution of marine trace gases of biogenic origin and their potential effects, further investigation is needed in different geographical regions and especially in polluted marine environments and populated coastal regions (Tinel et al., 2023). In this study we estimate the spatiotemporal distribution of seawater VOC concentrations in the climate sensitive geographical region of Eastern Mediterranean. State-of-the art empirical models linking remotely-sensed data of phytoplankton biomass (EU Copernicus Marine Environment Monitoring Service, CMEMS) and environmental parameters such as sea-surface temperature, and photosynthetically available radiation are used (Gali et al., 2018). Ocean-model data such as mixed layer depth, and euphotic zone are also exploited. The impact of the sea-to-air VOC emission fluxes on photochemistry, marine aerosols and cloud properties are assessed and quantified through advanced atmospheric simulations with the WRF-Chem atmospheric model coupled to chemistry and aerosols during typical conditions but also extreme events. ReferencesGali M., Levasseur, M., Devred, E., Simo, R. and Babin, M., Sea-surface dimethylsulfide (DMS) concentration from satellite data at global and regional scales, Biogeosciences, 15, 2018, pp. 3497-3519, https://bg.copernicus.org/articles/15/3497/2018 , doi:10.5194/bg-15-3497-2018.Tinel L., J. Abbatt, E. Saltzman, A. Engel, R. Fernandez, et al.. Impacts of ocean biogeochemistry on atmospheric chemistry. Elementa: Science of the Anthropocene, 2023, 11 (1), ff10.1525/elementa.2023.00032ff. ffhal-04221390f

Air–sea interactions play a pivotal role in shaping cyclone development and evolution. In this context, this study investigates the role of ocean optical properties and solar radiation penetration in modulating subsurface heat content and their subsequent influence on the intensity of Mediterranean cyclones. Using a regional coupled ocean–wave–atmosphere model, we conducted sensitivity experiments for Storm Daniel (2023) comparing two solar radiation penetration schemes in the ocean model component: one with a constant light attenuation depth and another with chlorophyll-dependent attenuation based on satellite estimates. Results show that the chlorophyll-driven radiative heating scheme consistently produces warmer sea surface temperatures (SSTs) prior to cyclone onset, leading to stronger cyclones characterized by deeper minimum mean sea-level pressure, intensified convective activity, and increased rainfall. However, post-storm SST cooling is also amplified due to stronger wind stress and vertical mixing, potentially influencing subsequent local atmospheric conditions. Overall, this work demonstrates that ocean bio-optical processes can meaningfully impact Mediterranean cyclone behavior, highlighting the importance of using appropriate underwater light attenuation schemes and ocean color remote sensing data in coupled models.

Ocean water clarity, influenced by marine chlorophyll concentration, significantly alters the distribution of shortwave radiation in the water column. This work aims to assess the effects of varying chlorophyll on the upper-ocean physical properties and their subsequent impact on the atmosphere, using a coupled ocean-atmosphere regional model for the Mediterranean and Black Seas. We performed 11-year (2011-2021) twin-simulation experiments based on different chlorophyll concentrations to estimate the penetration of solar radiation in the ocean. The first simulation used a monthly climatology field of chlorophyll concentrations derived from satellite observations, while in the second experiment, the chlorophyll concentration was kept constant at 0.05 mgm-3 $\mathrm{m}\mathrm{g}\ {\mathrm{m}}<^>{-3}$, representing clear water conditions. Results show that radiative heating driven by chlorophyll amplifies the seasonal cycle of temperature in the upper layers, leading to increased surface warming in summer and surface cooling in winter. Also, higher surface chlorophyll contributes to cooling in subsurface layers throughout the year due to its shading effect. The temperature response to chlorophyll variations is controlled by the mixed layer depth and a balance between (a) direct near-surface radiative heating due to the chlorophyll absorption and (b) indirect cooling resulting from vertical turbulent mixing processes with subsurface waters. The atmosphere moderates the seasonal sea surface temperature (SST) response caused by chlorophyll differential heating primarily through changes in latent heat flux. Ultimately, our simulations suggest that increased surface chlorophyll concentrations enhance the Mediterranean overturning circulation, highlighting the necessity of incorporating realistic optical forcing into regional climate modeling studies.

This study examines the impact of different turbidity products on the Aegean Sea surface physical characteristics, by performing twin-experiment simulations using a high-resolution regional ocean model. The turbidity products used include an in-situ based diffuse attenuation coefficient dataset at 490 nm (kd490, in m− 1) and a satellite derived kd490 product. Satellite turbidity products are broadly used in ocean simulations due to their spatiotemporal coverage and algorithm universality. Their validation and empirical components are trained mainly in phytoplankton driven regions and this may cause systematic differences in oligotrophic areas of variable optical properties’ composition. In the Aegean Sea, the in-situ based turbidity product accounts for the contribution of suspended particles in the solar heating profile, having further implications in the surface characteristics. The Aegean Sea upper-ocean thermohaline characteristics and general circulation patterns, reveal distinct differences between the twin-experiment simulations, showcasing mesoscale to locally induced impact of the turbidity variations. The turbidity impact on the air-sea interaction fluxes affects both thermodynamic processes i.e., solar radiation penetration and absorption in the water column, as well as dynamic processes i.e., momentum fluxes due to changes of the sea surface temperature and subsequently to the momentum drag coefficient. The Aegean Sea surface characteristics in the in-situ based turbidity product simulation, show a stronger decoupling between the North and the South Aegean Sea, when compared with the satellite derived turbidity product simulation. These results highlight the importance of incorporating more realistic turbidity products in ocean models, especially for optically complex regions such as the Aegean Sea.

The growing energy demand has intensified interest in marine renewable sources. This study assesses wave energy potential in the Mediterranean and Black Seas using a 15-year (2010–2024) wave hindcast with the WaveWatch III model. It examines the spatial and temporal distribution of significant wave height, wave energy period, and overall wave energy resources at a basin-wide scale. The findings identify the western Mediterranean—particularly the area west of Sardinia and from the Gulf of Lion to the Algerian coast— as having considerable wave energy potential, primarily due to the Mistral winds during winter. Other regions, such as the Alboran Sea, central-south Aegean Sea, and western Black Sea, show lower wave energy levels but remain promising for wave energy converter development due to moderate wave power variability over time. To assess the impact of wave-current interactions on wave power estimation, additional simulations were performed over a 5-year sub-period without current forcing. Results indicate that currents can reduce extreme wave power values in the most energetic areas by up to 5 kWm-1. This study establishes a baseline wave-only simulation to aid future research using a regional coupled ocean-atmosphere-wave system for more precise wave power assessments.

A CTD/ADCP/surface-drifter survey in fall 2004 reveals the behaviour of a mesoscale unidirectional flow coming from the Cretan sea in the south with depths ~1000 m and entering a channel-like area of the Cycladic shelf in the north, that forms a contraction which leads to a bottom elevation (sill depth ~100 m), and finally returning into the Cretan Sea in the lee-side of the sill. The flow decelerates/accelerates upstream/downstream of the sill. The along-stream density contours near the sill bottom are raised prior to reaching the sill, while they deepen in the lee side of it indicating supercriticality. The long-wavelength internal wave speeds with realistic stratification and no-rotation are higher than the section averaged flow speeds and indicate subcriticality. A key element in this apparent paradox is the large height of the sill that potentially increases the body (drag) force exerted on the flow by the sill while flow blocking is also observed upstream of the sill.

The contribution of surface and lateral forcing to the observed Arabian Gulf warming trend is studied based on the results of a high-resolution (1/100°, 60 vertical layers) MIT general circulation model (MITgcm) covering the period 1993–2021. The model validation against available observations reveals that the simulation satisfactorily reproduces the main features of the Arabian Gulf’s dynamics and their variability. We show that the heat content of the Arabian Gulf generally follows the reported variability of sea surface temperature, with significant increasing trends of 0.1 × 107 J m−3 and 0.2°C per decade. The interannual variability of the heat content is dominated by the surface heat fluxes, while the long-term warming of the basin is primarily driven by lateral fluxes. The analyses of the heat exchanges through the Strait of Hormuz indicate a pronounced upward trend in the transported heat toward the Arabian Gulf, which is associated with an increase in both the volume and temperature of the exchanged waters. Considering the inflow and outflow in the Strait separately, the temperature increase is more prominent in the inflowing waters; however, the dominant factor driving the rising trend in heat content exchanges is the increase in the volume of waters being exchanged. This implies that the observed warming of the Arabian Gulf during the investigated period is directly related to the acceleration of its overturning circulation.

Accurate modelling of air-sea processes is essential for reliable forecasts of Mediterranean tropical-like cyclones (also known as " Medicanes " ). Medicanes occasionally develop in the Mediterranean causing extreme weather conditions with catastrophic potential due to excessive precipitation, windstorms, and coastal flooding. In this work, we investigate how the complexity of ocean-wave-atmosphere coupling and model initialization affect the simulated track and intensity of the Medicane Ianos (2020). Results indicate that the model ' s initial conditions and the cyclone ' s development stage are the main drivers of track position errors, while ocean and wave feedback have a significant impact on the intensity and evolution of the cyclone. Compared with an atmosphereonly simulation, an atmosphere-ocean coupled system reproduces the cyclone ' s SST cooling effect (up to 3.7 degrees C), in agreement also with the satellite observations thus, reducing the cyclone intensity, as estimated by the minimum MSLP, the 10-m wind speed and the surface enthalpy flux. Adding a wave model to the coupled system, further increases the magnitude of ocean cooling (by about 1.2 degrees C), due to increased sea surface roughness leading to increased wind stress and enhanced upper ocean mixing. Overall, surface waves are shown to have competing effects on cyclone intensity i.e., negative feedback via increasing the surface momentum flux and positive feedback via increasing the enthalpy flux, the latter being more sensitive to surface roughness rather than to SST modifications brought by the wave coupled system. The turbulent air-sea fluxes under high winds, appear to be very sensitive to sea-state patterns resolved by the coupled models, highlighting the need to improve forecasting systems for extreme weather events in the Mediterranean.

The dynamics of fluid flows give rise to robust, persistent circulation features that underpin the flow and exert strong control over the advection of water masses, either enhancing it or suppressing it, collectively known as lagrangian coherent structures. Lagrangian approaches and metrics have been shown to be better suited than eulerian ones at locating and delineating such structures and capturing the effect they have on the formation and dispersion of water masses, particularly at the smaller scales. In this paper, we use the framework of lagrangian coherent structures to analyse the ocean velocity fields over a climatological year obtained from a high-resolution eddy-resolving model in order to investigate the lagrangian regimes that affect the motion, separation and mixing of water masses in the Mediterranean Sea. The lagrangian regimes that develop in each sub-basin over the course of the year are characterised and regions of persistent lagrangian activity and coherent structure formation and presence are identified. A quantitative picture of the seasonal variability of the lagrangian coherent structure-induced horizontal mixing and vortex formation is obtained.

&lt;p&gt;Marine chlorophyll concentration has an impact on turbidity affecting the upper-ocean properties and regulating the air-sea fluxes. This work aims at assessing the effect of turbidity, as estimated via surface chlorophyll, on the heat content and dynamics of the Mediterranean and Black Seas. We performed twin-simulation experiments using a regional configuration of the NEMO v4.2 ocean model comparing: 1) a run with climatological chlorophyll satellite data to estimate turbidity and shortwave penetration in the water column, with 2) a reference run of fixed turbidity (i.e., chlorophyll concentration fixed at 0.05 mg/m&lt;sup&gt;3&lt;/sup&gt;) corresponding to Yerlov type I clear waters. Interim results for long-term simulations (2008-2018) show that considering the effects of turbidity, as estimated from realistic surface chlorophyll concentrations, increases sea surface temperature, amplifies the seasonal cycle of temperature in the surface layer (0-20 m), and increases the annual heat loss in the Mediterranean Sea by about 1.5 W/m&lt;sup&gt;2&lt;/sup&gt;. The latter is explained because the surface warming during summer is more intense than the cooling observed during winter, with differences between the two experiments reaching up to 2 &amp;#176;C in some regions. The increasing turbidity also affects the subsurface layers (20-200 m), with cooler temperatures throughout the year due to less solar radiation penetrating the water column. Ongoing work is currently being undertaken to estimate the indirect atmospheric feedback due to turbidity changes, using a fully-coupled ocean-atmosphere system (NEMO-WRF).&lt;/p&gt;

Intense cyclones with tropical-like characteristics (also known as “medicanes”) occasionally develop in the Mediterranean. They can cause extreme weather phenomena with catastrophic potential due to excessive precipitation, windstorms, and coastal flooding. In this work, the impact of air–sea interactions on the track and intensity of a Mediterranean cyclone is evaluated using an atmosphere-only configuration (WRF) and a two-way coupled ocean–atmosphere configuration (NEMO-WRF). As a case study, we focus on a medicane that evolved over the central Mediterranean basin during 15–20 September 2020 (named “Ianos”), causing severe damage to western Greece. The atmosphere-only simulations were carried out using constant initial SST throughout the model integration, while in the coupling setup, the SST was consistent with the air–sea fluxes and updated every 6 min by the ocean model. The results from the two modeling approaches highlight the importance of air–sea feedbacks for predicting Mediterranean cyclone intensity, along with the forecast initialization time.

The diffuse attenuation coefficient is an indicator of light availability in the surface layer, and is used in a broad range of applications, including numerical simulations, for the parameterization of the light transmission in the water column. In this study, a new dataset of the diffuse attenuation coefficient for the Eastern Mediterranean Sea test case is developed using an existing optical dataset of 2614 beam attenuation coefficient profiles. This method introduces a way of overcoming the difficulty of measuring the diffuse attenuation coefficient in-situ by utilizing the most routinely measured variable, the beam attenuation coefficient. The proposed approach uses existing semi-analytical relationships and a neural network. The neural network, a multi-layer perceptron regression model, is trained and validated with a dataset of 29398 concurrent bio-optical in-situ measurements from the PROSOPE cruise and remotely sensed surface variables. The model is applied to the Eastern Mediterranean dataset and the results are interpolated into a gridded gap-free field, with a grid resolution of 0.0416° x 0.0416°, which is assessed and compared with a satellite-derived product, investigating their significant differences. The resulting field's mean value is slightly reduced with respect to the satellite product, showing regions of higher turbidity, with the most prominent located in the northern Aegean Sea in regions of excess colored dissolved organic matter and around mesoscale features and in the Cretan and Levantine Sea in regions of higher mesoscale activity.

The weather and climate greatly affect socioeconomic activities on multiple temporal and spatial scales. From a climate perspective, atmospheric and ocean characteristics have determined the life, evolution, and prosperity of humans and other species in different areas of the world. On smaller scales, the atmospheric and sea conditions affect various sectors such as civil protection, food security, communications, transportation, and insurance. It becomes evident that weather and ocean forecasting is high-value information highlighting the need for state-of-the-art forecasting systems to be adopted. This importance has been acknowledged by the authorities of Saudi Arabia entrusting the National Center for Meteorology (NCM) to provide high-quality weather and climate analytics. This led to the development of a numerical weather prediction (NWP) system. The new system includes weather, wave, and ocean circulation components and has been operational since 2020 enhancing the national capabilities in NWP. Within this article, a description of the system and its performance is discussed alongside future goals.

The interannual variability of the Mediterranean overturning circulation is investigated using a high-resolution (1/36°) ocean model. As the overturning circulation regulates the replenishment and ventilation of the deep layers, we study the spatiotemporal scales of the maximum value of the overturning streamfunction over three main sub-basins of dense water formation (Aegean Sea, Adriatic, and the northwestern Mediterranean). The variability of the zonal overturning is also discussed. The spectrum analysis shows that the overturning variability has its largest signal on annual timescales in all sub-basins, explained by perpetual winter formation. On shorter frequencies (decadal) there are marked differences observed, due to regional processes of the overturning cells, led by buoyancy flux long-term variability in each sub-basin. The decomposition of the total overturning circulation into barotropic, geostrophic shear, and Ekman components revealed weakening and strengthening for the Aegean and Adriatic Sea total overturning, respectively, with opposite trends for the barotropic and geostrophic shear components. The simultaneous contribution of the Ekman and geostrophic component to the total overturning differentiates the variability of zonal overturning circulation from the local meridional overturning circulation of the three sub-basins. The cross spectra between the maximum overturning value and the buoyancy fluxes also revealed that the system keeps the “memory” of this forcing and shows annual variability.

Research infrastructures have been established throughout Europe in order to create robust organizations that will facilitate and enhance research and innovation processes and will advance society with innovative products and services. The Hellenic Integrated Marine Observing, Forecasting and Technology System (component of HIMIOFoTS RI) has been implemented in the framework of the National Roadmap for Research Infrastructures to form a large-scale infrastructure for the marine environment in Greece. It links together ocean observing and forecasting systems, coastal zone monitoring and management practices, as well as ocean engineering testing facilities. The overarching framework of the system supports the coordination of five organizations with expertise in the field of marine science and technology, the central management of research activities, and the common development of services and products. It comprises facilities and resources while it provides open access to research communities (academia, industry) to support the scientific advancements and innovation in their fields. The Hellenic Marine Observing, Forecasting and Technology System was further enhanced during its implementation through significant upgrades and developments in order to extend its observing capacity and the forecasting and technological abilities, while advancing the provided services and products.

An extensive collection of light transmission/beam attenuation coefficient vertical profiles (LT/cp), particulate matter concentration (PMC) and particulate organic carbon concentration (POC), total particle volume concentration (VC), and particle median diameter (D50) vertical profiles were assembled in order to provide an overview of particulate matter dynamics in the Aegean Sea, as well as a first time assessment of suspended particle size properties. A typical beam cp vertical profile is composed of a relatively more turbid surface nepheloid layer (SNL) associated with enhanced primary productivity and atmospheric/fluvial inorganic/organic particle inputs, more transparent mid-waters with the sporadic occurrence of intermediate nepheloid layers (INL), and occasionally a more turbid bottom nepheloid layer (BNL) attributed to seabed sediment resuspension. The Aegean Sea is characterized by very low cp values, apart from river discharge coastal regions of the North Aegean Sea. In a north-south direction, cp decreases, and the water column becomes progressively more transparent, highlighting the transition from meso/oligo- to ultra-oligotrophic conditions prevailing in the Aegean Sea. There is a clear differentiation of particle abundance between “wet” and “dry” seasons, with the former showing enhanced cp, PMC, POC, and VC values. Particle size, studied within the range 1.25–250 μm, exhibits relatively small variability, with D50 varying between 81 and 105 μm, with a general increase over depth which is more pronounced in the South Aegean Sea, most likely associated with the variable abundance of Transparent Exopolymer Particles (TEP). Deep basins of the North Aegean Sea host permanent/semi-permanent INLs at mid-depths; their presence is attributed to internal wave activity. Good correlations of cp:PMC and cp:POC allow for the conversion of cp data to PMC and POC values, in support of modeling and remote sensing applications.