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<p>The world’s oceans have been studied and monitored for many decades to enhance our understanding. In today’s world, with the explosion of new data provided by many different Earth observation sources and the availability of advanced computing infrastructures (cloud computing, HPC, IoT, Big Data), creating a digital representation of the ocean is becoming a reality. The EC recently funded the H2020 ILIAD project, which aims at establishing an interoperable, data-intensive Digital Twin of the Ocean (DTO). The ILIAD DTO will integrate real-time sensing of ocean variables, state-of-the-art high-resolution models, modern data analytics and digital infrastructures to create virtual representations of physical processes and understand their behaviour, anticipating and predicting their response to simulated events and future changes. ILIAD will enable an ecosystem of interoperable DTOs, integrating the plethora of existing EU Earth Observing and Modelling Digital Infrastructures. It will fuse a large volume of diverse data and will enhance ocean data infrastructures with additional observation technologies and citizen science.  ILIAD will provide a virtual environment representing the ocean, capable of running predictive management scenarios and will utilize Big Data analytics for forecasting of spatiotemporal events and pattern recognition. Several DT pilots will be undertaken in several key thematic areas such as offshore wind energy, wave and tidal energy, biodiversity assessments, marine pollution and more. </p> <p>The current work presents ongoing activities for a coastal, high-resolution Digital Twin pilot for Cretan Sea, to be demonstrated in the frame of ILIAD project. The pilot focuses on oil spill pollution monitoring and forecasting. The DT environment combines high-resolution forecasting services based on numerical weather (WRF), hydrodynamic (NEMO), sea state (WAVEWATCH III) and particle tracking models (MEDSLIK-II), enhanced by the integration of  Sentinel data and real-time observations from novel, low cost current and waves meters, drifting trackers, as well as citizen science. WRF model is applied for forecasting of meteorological variables at  ̴ 3 km resolution by dynamic downscaling of coarser resolution climatic modelling forecast data (NOAA’s GFS). This way, higher computational accuracy is achieved over Cretan Sea, thus revealing finer wind scales phenomena. The downscaled weather forecasting data are used to force NEMO and WAVEWATCH III, to obtain high-resolution forecasts of important marine parameters, such as sea currents, temperature, salinity and waves over a fine grid of   ̴ 1 km for the coastal area of Crete. For oil spills, the DT of Cretan Sea will integrate operational analysis of Sentine-1 images, triggering MEDSLIK-II oil spill model once an oil spill event or anomaly is identified. Adjusting forecasts to observations by reinitialising the model with updated observational patterns will contribute to the forecast error growth being implicitly accounted for and minimized. The pilot DT virtual environment will allow on-demand simulations of predictive scenarios of oil spill events and response strategies.  </p> <p>The aim of the Digital Twin is to aid the immediate response in case of accidental oil releases, minimize the damage and reduce the time for environmental recovery.</p> <p><strong>Acknowledgement:</strong> This research has received funding from the European Union’s H2020 RIA programme under GA No 101037643.</p>

<p>The ocean's turbidity and optical properties are determined by the interaction of sunlight radiation with suspended particles and dissolved matter of the water body's surface layers. Variations in the optical properties can affect the upper ocean's heat content, thus modifying the stratification and the mixed layer dynamics. These variations can be monitored using satellite products, along with in-situ observations, and their impact on ocean circulation can be analyzed through numerical modeling. For the oligotrophic Eastern Mediterranean, there is a gap of in-situ data used to evaluate remote sensing observations. Furthermore, this region receives significant atmospheric deposition of particulate inorganic matter through African dust, as well as from river discharges. These constituents' contribution in optical properties modulation is often considered negligible for oligotrophic regions, where the various parameters have been calculated based on chlorophyll variations. To fill this gap, in situ measurements of beam attenuation coefficient at 660 nm (c, in m<sup>-1</sup>)<sup>(1)</sup> provided by the Hellenic Centre for Marine Research (HCMR) were assessed, and a gridded dataset was constructed using Data-Interpolating Variational Analysis (DIVA), for the Aegean Sea, Eastern Mediterranean, for the years 1991-2019. The aim is to validate the accuracy of satellite products for this region using this dataset. Towards this goal, available satellite ocean color products of the ocean's inherent optical properties will be used to estimate c values, which will be compared to the in-situ dataset.</p><p>1. A. P. Karageorgis et al., Deep Sea Res. Part I 55, 177–202 (2008).</p>

<p>The thermohaline and dynamic characteristics of the upper ocean can be affected by the way the sunlight is absorbed and scattered on the surface layer. Changes in light penetration can be investigated through the turbidity of the layer, which is determined by a synthesis of terrestrial inputs (atmospheric and riverine), and the biological activity in a region, of both natural and anthropogenic origin. To examine the effects that turbidity variability has on the surface-layer characteristics of the ocean, a twin modeling sensitivity experiment was performed, using as a case study the Aegean Sea, NE Mediterranean. The Aegean Sea is an oligotrophic region with most nutrient inputs located in the northern coasts of the basin, creating a north-to-south chlorophyll-a gradient, with the highest concentrations on the North and lowest on the South. The first experiment corresponds to a very clear ocean, and the second incorporates the turbidity field, varying according to chlorophyll-a concentration.<br>The experiments were implemented using the NEMO models' ocean component (v3.6) for the region (34.05-41.16 °N, 22.29-28.98 °E) and for the period 1997-2001, discretized on a 1/36° Arakawa-C grid, with 75 partial step vertical levels. Atmospheric inputs are ERA-5 reanalysis products of the ECMWF service, whereas inputs for initial and boundary values have been derived from the Copernicus database. First, a two-band light penetration scheme was applied, using a Jerlov Type-I extinction depth at 23.0 m, representing the constant-low turbidity field. An RGB scheme was applied for the second experiment, using the multiyear monthly mean of the chlorophyll-a concentration variable derived from the ESA-CCI service. The sea surface variables' response is examined for the final year of the experiment. The results indicate that the RGB-scheme experiment estimates elevated sea surface salinity and temperature values, with the most significant difference in salinity located in the northern part of the basin, where there is a strong influence of the inflow of Black Sea Water from the Dardanelles Straits. Elevated eddy kinetic energy is observed in the gyres formed in the Cretan Sea.</p>