Elina Tragou

The North Aegean Sea is one of the most interesting seas of the Mediterranean, being under the dominant impact of the Black Sea waters though the so-called Turkish Strait System (TSS – including the Dardanelles and Bosphorus Straits and the Marmara Sea). Moreover, it constitutes a potentially deep water formation site of the Eastern Mediterranean Sea along with the Adriatic Sea. Previous studies for the region focused – rightly – on the crucial role of low salinity Black Sea waters in controlling the overall thermohaline function and dynamics of the North Aegean. None of the previous modeling approaches studied the impact of tides in the mixing processes and the production of extremely dense water, especially during 1987, 1992 and 1993 when major deep water formation events took place in the region. In this work we examine the tidal impact via several long term simulations using a high resolution (1.0 km) ocean model covering the period from 1985 to 2013.The Regional Ocean Model Sys...

The coastal marine ecosystem of Saronikos Gulf, a busy Eastern Mediterranean embayment directly impacted by the greater metropolitan area of Greece’s capital, Athens, is examined through a series of state-of-the-art numerical models that address the hydrodynamics (Delft3D-FLOW), the wave regime (SWAN), the biogeochemistry, and pollution related to species of heavy metals and polyaromatic hydrocarbons (Delft3D-WAQ).The study so far has focused on calibrating model components and on reproducing the seasonal thermohaline conditions, known circulation patterns and the variability of biogeochemical constituents (chlorophyll-a, nutrients, dissolved and particulate matter) and pollutant concentrations, focusing on the vicinity of inner Saronikos.The annual cycle ‘Nov 2009 - Oct 2010’ is simulated, forced with atmospheric data from the ERA5 database. Three sets of open boundary conditions data are tested (Mediterranean Sea Physics Reanalysis dataset by Copernicus and two implementations of ...

Understanding the processes that control the buoyancy fluxes of the Aegean Sea is important for various reasons. First, the Aegean is directly connected with the Black Sea and acts as a buffer between two opposite thermohaline systems, a concentration and a dilution basin (the Mediterranean vs the Black Sea), receiving and filtering the variability and changes of a much broader geographical area. Second, the Aegean is capable to produce large amounts of very dense water, having temporarily been the major originator of Eastern Mediterranean Bottom Water. These processes are controlled by the buoyancy fluxes, both through oceanic advection and atmospheric exchanges. In this work we examine the characteristics and variability of heat, freshwater and the overall buoyancy air-sea fluxes, focusing on the potential role of the interaction with the Black Sea. A thirty-year-long simulation (1985-2015) of the whole Eastern Mediterranean/Black Sea system, forced by ERA-Interim data, was used t...

<p>Even before the introduction of the term “Marine Heat Wave” (MHW) and its statistical definition in global-scale studies, the scientific community had studied and recorded potentially harmful impacts of persistent conditions of warm surface layers and highly stratified water columns on the marine ecosystem. The main triggers for MHWs are yet not well understood and the current knowledge is mainly based on mass mortalities linked to temperature anomalies. EM-MHeatWaves is an interdisciplinary, collaborative, DAAD/IKYDA funded research project that investigates the atmospheric forcing, oceanic circulation and ecosystem response of MHWs in the Eastern Mediterranean Sea over the past 35 years. Two universities (Justus-Liebig-University Giessen, University of the Aegean) and one research center (Hellenic Centre for Marine Research) re-examine the definition of MHWs with emphasis on the Eastern Mediterranean by applying a holistic approach that includes reverse-engineering using model data and reanalysis covering the period 1985 to 2014. We focus on the Eastern Mediterranean because of the high sensitivity of the basin’s ecosystem to atmospheric and marine warming events, the invasion of tropical alien (Lessepsian) species, the characteristic oceanic circulation with the Eastern Mediterranean Transient events, the exchange with the Black Sea through the Turkish Strait System as well as the coastal upwelling areas. In order to study the spatiotemporal characteristics of Eastern Mediterranean MHWs we work towards a better understanding of the oceanographic processes as well as of the compounding character of the atmospheric contribution. Based on the response of marine biogeochemical cycles (depletion of subsurface oxygen levels, observed changes in the mixed layer and chlorophyll maxima depths, nutrient stoichiometries, carbon uptake and sequestration rates) and their impacts on ecosystems (i.e. shifts in planktonic and benthic community regimes, mass mortality events, disease outbreaks, etc.), triggered by the rise of ocean temperatures, we study the statistical characteristics of the oceanic temperatures and assess the corresponding ocean circulation, the synchronous and lagged contribution of the large scale atmospheric circulation. We further study the signature of these extreme Mediterranean MHW events in future projections from model runs with respect to duration, severity and spatial extent and compare them to reanalysis.    <br>EM-MHeatWaves aims at strengthening the partnership between the German and Greek institutions by conducting joint research at a high scientific level.</p>

ABSTRACT The identification and examination of 'complete' potential density overturns in CTD profiles, within the framework of SESAME project, is employed to assess vertical eddy diffusivities, mostly in the top 100 m of the water column, for a broad area covering the East Mediterranean, the Turkish Straits and the Black Sea. The implementation of this method shows that, mixing induced by mechanical turbulence is enhanced in frontal areas, in the proximity of straits and inside anticyclones; furthermore, that mechanical turbulence is insignificant, down to the scale of CTD resolution, within areas of double diffusive staircases, encountered in deep layers of the water column. Consequently, only laminar theories about double diffusion are applied for assessing diffusivities therein. Susceptibility to different types of double diffusion seems to be related to the interaction of different types of water masses.

ABSTRACT This work focuses on the direct measurement of the vertical flux of appendicularian houses in order to assess their importance as a component of vertical carbon flux in coastal areas. For this purpose, arrays of cylindrical sediment traps were deployed for 5 to 8 days at two depths in a coastal area of the northern Aegean Sea (inner Thermaikos Gulf) during spring. The data support the contention that resuspension was minimal. Fecal pellet (FP) production and grazing experiments with the dominant copepods (Acartia clausi) were conducted to provide additional information on the potential FP contribution to the total carbon flux. The magnitude of the vertical flux of particulate organic carbon (POC) ranged between 310 and 724 mg C m−2 day−1. The proportion of phytoplankton carbon in the POC vertical flux was up to 45 %. The contribution of zooplankton FPs to the total carbon never exceeded 5 %. On the contrary, appendicularian houses were an essential component of the biogenic carbon flux contributing up to 55.3 % of the total vertical carbon flux. Consequently, both phytoplankton and appendicularian houses contributed equally to the biogenic carbon flux exceeding 80 % of the total sinking POC. Taking into account the sinking speed of the particles and the environment in the area, all this carbon probably reaches the seafloor, thus indicating a strong pelagic–benthic coupling.

The recent post-Glacial Mediterranean sea-level rise converted mountain valleys to islandic embayments. These bays exhibit distinct hydrographic and biogeochemical characteristics, seasonally alternating thermohaline functioning and their coastal ecosystems support thriving coastal activities. The Kalloni Gulf, Lesvos island, Greece, constitutes a textbook example of the above type of embayments and -due to its proximity to the facilities of the University of the Aegean in the island’s capital, Mytilene- has become the focal point of the development of the prototype Coastal Environmental Observatory “AEGIS”. This laboratory has risen as the culmination of decade-long efforts of three Research Laboratories of the Department of Marine Sciences and includes both observational and forecasting components. The latter component comprises three levels of numerical modeling of the ocean circulation: The low-resolution (level 1) domain corresponds to the Central and North Aegean Sea, the inte...

The observed warming and salinification of the eastern Mediterranean Sea (EMed) during the last decades could impact the dense water formation (DWF) and consequently the thermohaline circulation of the basin. The main drivers of interannual DWF variability in the EMed are the atmospheric heat fluxes, the internal redistribution of salt, and the exchange of heat and salt at its straits. Extremely high salinity has been observed recently in the Levantine and Adriatic Seas due to the superposition of the basin’s decadal variability and long-term trend.In the Aegean Sea, a major DWF site of the EMed, record-high salinity is recorded in the upper and intermediate layers since 2018. Argo floats observations also show that the Aegean Sea is in the most prolonged state of increased DWF after the Eastern Mediterranean Transient (EMT) period.The causes of increased salinity and DWF in the Aegean Sea were investigated using: 1) in situ hydrographic observations, 2) satellite observations of se...

This paper presents the state-of-the-art research conducted as part of EN.I.R.I.S.S.T. regarding the emerging impact of shipping and transport to the environment. More specifically, it presents the digital platforms and services that are developed as part of the research infrastructure and are related to environmental sciences. The first is the “EcoMarpol Platform”, which caters for collecting and analyzing pollution-driven data as well as calculating the environmental imprint (atmospheric emissions, waste production, chemical pollution of marine waters and sediments) of maritime activities. This platform additionally includes the online monitoring of in-port emissions from shipping and pollution from ship-to-ship transfer operations or accidents and the environmental risk assessment management. The other platform is the “Passenger Sustainable Travel Platform”, as part of which a tool for calculating aircraft emissions (CO2 and air pollutants) will be developed and applied in the Gr...

<p>Atmospheric and marine heatwaves (AHW/MHW) have been observed around the world and are expected to increase in intensity and frequency under future climate change. Despite numerous studies that have examined AHW or MHW independently, only few regional studies investigated potential associations between these two types of extreme events. However, the co-occurrence of AHW and MHW could have broader and greater environmental, human, and economic impacts than an individual event, such as changes in species distributions, land and marine mass mortalities, or increased heat stress in coastal areas due to interactions between warm and moist air over the ocean. Based on research on AHW and MHW, we propose a comprehensive and globally applicable definition that relates the two extreme events and the two realms, and allows comparison with past and present concurrent and single events. Our definition is based on a conditional approach: We define a concurrent heatwave as an extreme event where sea surface temperature (SST) and 2 m air temperature (T<sub>air</sub>) exceed their daily 90th percentiles, based on a 30-year historical baseline period, for at least 5 and 3 consecutive days, respectively (Perkins & Alexander 2013; Hobday et al. 2016). Thereby, we account for a potential lagged relationship between the two extremes by calculating and choosing the lag that provides the maximum probability of observing a MHW and an AHW simultaneously or delayed. In this work, we show the results of the most common heatwave metrics, such as duration, frequency, intensity, and cumulative intensity, for concurrent and single heatwaves in the Mediterranean Sea, Western Australia, and the Northwest Atlantic. We use SSTs from Advanced Very High-Resolution Radiometer (AVHRR) satellite data (NOAA OISST V2) as well as T<sub>air</sub> from the ECMWF Reanalysis v5 (ERA5), both provided daily and globally on a high resolution (0.25°) for the period 1982 – 2022. In the Mediterranean Sea, we find concurrent heatwaves to be shorter and less frequent, but more intense and cumulatively intense than their single variants. For concurrent events, the MHW component (SST) is observed to be most intense in summer and spring, and the AHW component (T<sub>air</sub>) in fall and winter. Moreover, the MHW appears to determine the strength of the concurrent heatwave in that region.</p>

<p>The North Aegean Sea is a sub-basin of the Mediterranean which exhibits a range of oceanic processes at various scales. Due to the inflow of very light, mesotrophic Black-Sea waters it is the most productive region of the seas around the Hellenic Peninsula, although the regular seasonal coastal upwelling along its eastern shores does not contribute to its productivity. Despite the continuous buoyancy import by the Black Sea, the North Aegean hosts the densest waters of the Eastern Mediterranean. Finally, three semi-enclosed bays located in two islands of the North Aegean exhibit an alternating behavior as sources or sinks of buoyancy for the basin, while their productivity and natural beauty support a range of coastal activities. For the above reasons, the University of the Aegean has invested over several years in the development of a coastal oceanographic observatory (<em>AEGIS</em>), covering both the open North Aegean Sea and the three main bays of the islands of Lesvos and Lemnos. The Observatory consists of a numerical modeling component and an observational component.</p><p>The modeling component of the observatory consists of four coastal circulation models (for the three bays and the island of Lesvos) nested within a larger domain circulation model covering the whole Aegean Sea north of 37º N. Data assimilation, employing both satellite (sea-surface temperature and sea-level) and field data (employing mostly ARGO float observations) is used in the model of the extended domain (an implementation/configuration of the ROMS system), while the observations obtained in the coastal regions are currently used for coastal models’ (DELFT-3D FLOW and ROMS) validation. In addition to the above circulation models, SWAN is used to simulate the surface waves and DELFT-3D WAQ is being implemented to simulate the biochemical functioning at the various model domains.</p><p>The observational component at small geographical scales (in the Bays) comprise of continuous meteorological and oceanographic observations through an oceanographic mooring in the middle of the Bay of Kalloni, sea-level observations at the Bays of Kalloni and Gera, and High-Frequency radar observations of sea-surface currents and waves in a region east of Lemnos island, aiming to monitor the Black Sea outflow into the Aegean. The above continuous measurements are supplemented by periodic hydrographic and biogeochemical measurements in the three Bays, to validate the models and calibrate the <em>in-situ</em> continuous data. A recent addition to the <em>AEGIS</em>’s observational arsenal is an ocean glider aimed to capture the variability of the open North Aegean sea.</p><p>The <em>AEGIS</em> Observatory provides the necessary background to support strategic planning of human interventions at regional and local scales, such as Marine Spatial Planning or the construction of river dams affecting sensitive coastal basins. The implementation of the Coastal Laboratory has been supported by several projects, the most recent being the project “Coastal Environment Observatory and Risk Management in Island Regions <em>AEGIS</em>+” (MIS 5047038), implemented within the Operational Programme “Competitiveness, Enterpreneurship and Innovation” (NSRF 2014-2020), cofinanced by the Hellenic Government (Ministry of Development and Investments) and the European Union (European Regional Development Fund).</p>

Inter-basin water exchanges can be quite important in climatic-scale numerical studies simulating the circulation and hydrographic characteristics of neighboring oceanic basins connected through narrow straits. The crucial role of the interaction between the Mediterranean and the Black Seas is often overseen in simulations, which rely mostly on parameterizations to describe the exchange, essentially decoupling the two basins. In this study, the fully interconnected Eastern Mediterranean–Black Sea system is simulated for the historical period (1985–2015) using realistic boundary conditions (lateral, atmospheric and hydrological), with a hydrodynamic fully three-dimensional ocean modeling system. The setup of such a configuration is thoroughly described and the performance of the 30-year hindcast product is validated exhaustively against observations and model results, by evaluating the representation of surface fields, circulation, three-dimensional hydrographic characteristics, volu...

The internal variability of the thermohaline circulation of the Mediterranean Sea is examined under contrasting extreme thermal and mass atmospheric forcing conditions. Three millennium-long numerical simulation experiments were performed under: (a) the current climatology, (b) a strong buoyancy forcing (SBF) scenario due to cold and dry conditions resembling the Younger Dryas event, and (c) a weak buoyancy forcing (WBF) scenario due to S1a sapropel deposition-like conditions (warm and wet). To isolate the inherent variability of the system, independent of interannual atmospheric forcing variability, the latter was defined as a perpetual year pertinent to each experiment. Self-diagnosed heat and salt fluxes, consistent to sea-surface characteristics of the above periods, forced three millenium-long, relaxation-free numerical experiments. These simulations were preceded by initial spin-up periods. The inherent spatiotemporal variability of the Mediterranean Sea was analyzed using the...

Abstract The response of the Aegean Sea to seasonal forcing by the Etesian winds is investigated, based on Copernicus products for weather forecasting (winds) and satellite information (sea-surface temperature (SST) and chlorophyll-α (chl-a) concentration), as well as fisheries records from the Hellenic Statistical Authority. To that end, a site-specific response criterion for upwelling intensity has hereby been developed, based on the zonal SST gradient between eastern and western Aegean. Combined use of the above criterion with lagged cross-correlation between wind, zonal surface temperature gradient, surface chl-a concentration and local fish landings reveals a well-hidden (until now) signal of chl-a increase in response to intensification of wind forcing, as well as a relation between fish landings and upwelling events, challenging the conclusions of previous studies suggesting that the Aegean Sea is immune to summer coastal upwelling, due to the great depth of its nutricline.

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 com...

Abstract Successive CTD profiles of an Argo float trapped within a deep (~1250 m) sub-basin of the North Aegean Sea during 2014–2015 enabled identification of deep-water ventilation episodes and inference of a bulk (budget-based) vertical eddy diffusivity during the period that followed these events. Independent effective eddy diffusivity profiles were computed under the assumption that the turbulent exchange of heat, salt and buoyancy through each isobath below 400 m will balance the rate of change of the corresponding variable's content of the water volume within the basin below that depth. The initially estimated thermal, saline and density diffusivity profiles were significantly different from each other. Assuming turbulence was the dominant mixing process between the intermediate and deep water masses led to identification of conductivity sensor drift rate of 4.6 × 10−6 S m−1 day−1. After correcting the sensor's drift, bulk eddy diffusivities K ρ , K S and K T were found to span (2–30) × 10−3 m2 s−1, in close agreement below 400 m depth. These estimates are at least one order of magnitude higher than eddy diffusivities based on a finescale internal-wave strain parameterization. Enhanced boundary mixing and double diffusion are examined as candidates to explain the high effective diffusivity values. A hypsometric correction appears to provide sufficient diapycnal transport along the boundaries to reproduce bulk buoyancy transports.

<p>Even before the introduction of the term “Marine Heat Wave” (MHW) and its statistical definition in global-scale studies, the scientific community had studied and recorded potentially harmful impacts of persistent conditions of warm surface layers and highly stratified water columns on the marine ecosystem. The main triggers for MHWs are yet not well understood and the current knowledge is mainly based on mass mortalities linked to temperature anomalies. EM-MHeatWaves is an interdisciplinary, collaborative, DAAD/IKYDA funded research project that investigates the atmospheric forcing, oceanic circulation and ecosystem response of MHWs in the Eastern Mediterranean Sea over the past 35 years. Two universities (Justus-Liebig-University Giessen, University of the Aegean) and one research center (Hellenic Centre for Marine Research) re-examine the definition of MHWs with emphasis on the Eastern Mediterranean by applying a holistic approach that includes reverse-engineering using model data and reanalysis covering the period 1985 to 2014. We focus on the Eastern Mediterranean because of the high sensitivity of the basin’s ecosystem to atmospheric and marine warming events, the invasion of tropical alien (Lessepsian) species, the characteristic oceanic circulation with the Eastern Mediterranean Transient events, the exchange with the Black Sea through the Turkish Strait System as well as the coastal upwelling areas. In order to study the spatiotemporal characteristics of Eastern Mediterranean MHWs we work towards a better understanding of the oceanographic processes as well as of the compounding character of the atmospheric contribution. Based on the response of marine biogeochemical cycles (depletion of subsurface oxygen levels, observed changes in the mixed layer and chlorophyll maxima depths, nutrient stoichiometries, carbon uptake and sequestration rates) and their impacts on ecosystems (i.e. shifts in planktonic and benthic community regimes, mass mortality events, disease outbreaks, etc.), triggered by the rise of ocean temperatures, we study the statistical characteristics of the oceanic temperatures and assess the corresponding ocean circulation, the synchronous and lagged contribution of the large scale atmospheric circulation. We further study the signature of these extreme Mediterranean MHW events in future projections from model runs with respect to duration, severity and spatial extent and compare them to reanalysis.    <br>EM-MHeatWaves aims at strengthening the partnership between the German and Greek institutions by conducting joint research at a high scientific level.</p>

ABSTRACT The identification and examination of 'complete' potential density overturns in CTD profiles, within the framework of SESAME project, is employed to assess vertical eddy diffusivities, mostly in the top 100 m of the water column, for a broad area covering the East Mediterranean, the Turkish Straits and the Black Sea. The implementation of this method shows that, mixing induced by mechanical turbulence is enhanced in frontal areas, in the proximity of straits and inside anticyclones; furthermore, that mechanical turbulence is insignificant, down to the scale of CTD resolution, within areas of double diffusive staircases, encountered in deep layers of the water column. Consequently, only laminar theories about double diffusion are applied for assessing diffusivities therein. Susceptibility to different types of double diffusion seems to be related to the interaction of different types of water masses.

<p>Mediterranean marine heat waves (MHW) can be defined as abrupt but prolonged, discrete and anomalously warm water events that last for five or more days and exceed temperatures warmer than the 99th percentile (Darmaraki et al. 2019). Like their atmospheric counterpart, Mediterranean MHW have already increased in intensity, frequency and duration - a trend projected to continue under anthropogenic climate change. Recent observations of MHW demonstrated a strong influence of these extreme climatic events on marine organisms, including mass mortalities and shifts in species ranges but also economic impacts on fisheries and aquaculture. MHW can be caused by a combination of atmospheric and oceanic processes and depend on the specific season and location of occurrence. However, the main triggers are generally still not well understood and the current knowledge is largely based on these reported regional impacts. This work focuses on historical (1985 – 2014) atmospheric and marine heat waves in a high resolution CMIP6 model as well as a fully three-dimensional oceanographic hindcast of the interconnected Eastern Mediterranean – Black Sea system. We detect the atmospheric and marine heatwaves and investigate the triggering, compound/concurrent effect of the atmosphere on marine heat waves in the Eastern Mediterranean. For the analysis of atmospheric heat waves, we follow the methodology of Kuglitsch et al. (2010). We use Eastern Mediterranean atmospheric model and ERA-Interim reanalysis to calculate daily maximum (TX) and minimum (TN) air temperatures as well as to set temperature thresholds to estimate the beginning and end of the heat wave events. We identify MHWs from daily sea surface temperatures, applying the approach of Darmaraki et al. (2019). Furthermore, we calculate the heat wave frequency, duration and intensity. The two pairs of datasets are then compared with respect to the spatio-temporal occurrence of heat waves in the atmosphere and ocean, in an effort to reveal feedbacks between the two spheres which would characterize the events as compound. Finally, we estimate a threshold at which an atmospheric heat wave triggers a marine heat wave, and thus a compound event.</p>

<p>Mediterranean marine heat waves (MHW) can be defined as abrupt but prolonged, discrete and anomalously warm water events that last for five or more days and exceed temperatures warmer than the 99th percentile (Darmaraki et al. 2019). Like their atmospheric counterpart, Mediterranean MHW have already increased in intensity, frequency and duration - a trend projected to continue under anthropogenic climate change. Recent observations of MHW demonstrated a strong influence of these extreme climatic events on marine organisms, including mass mortalities and shifts in species ranges but also economic impacts on fisheries and aquaculture. MHW can be caused by a combination of atmospheric and oceanic processes and depend on the specific season and location of occurrence. However, the main triggers are generally still not well understood and the current knowledge is largely based on these reported regional impacts. This work focuses on historical (1985 – 2014) atmospheric and marine heat waves in a high resolution CMIP6 model as well as a fully three-dimensional oceanographic hindcast of the interconnected Eastern Mediterranean – Black Sea system. We detect the atmospheric and marine heatwaves and investigate the triggering, compound/concurrent effect of the atmosphere on marine heat waves in the Eastern Mediterranean. For the analysis of atmospheric heat waves, we follow the methodology of Kuglitsch et al. (2010). We use Eastern Mediterranean atmospheric model and ERA-Interim reanalysis to calculate daily maximum (TX) and minimum (TN) air temperatures as well as to set temperature thresholds to estimate the beginning and end of the heat wave events. We identify MHWs from daily sea surface temperatures, applying the approach of Darmaraki et al. (2019). Furthermore, we calculate the heat wave frequency, duration and intensity. The two pairs of datasets are then compared with respect to the spatio-temporal occurrence of heat waves in the atmosphere and ocean, in an effort to reveal feedbacks between the two spheres which would characterize the events as compound. Finally, we estimate a threshold at which an atmospheric heat wave triggers a marine heat wave, and thus a compound event.</p>

ABSTRACT Ocean mesoscale spiral eddies is a phenomenon that came apparent in the last 50 years but until today there are many questions yet to be answered about their formation, distribution and correlation to the dynamical processes on the sea surface. Main objective of the present paper is to provide an extensive analysis on the occurrence and statistics of smallmesoscale eddies over the Aegean Sea using synthetic aperture radar (SAR) images. The study area is characterized from unique hydro-dynamical and topographical conditions that give another aspect on the phenomenon. Present study based on 169 medium resolution (WSM) ENVISAT ASAR images acquired in 2011. As a result of the analysis 192 eddies formations were detected. The majority of those eddies were visualized due to the presence of surfactant films (black eddies) on sea surface and majority of them were cyclonically rotating. The diameter of the observed formations of eddies was within 1 to 16 km. The detected eddies were classified by categories depending on their shape and their generation mechanism. Seasonal and spatial distribution is presented, in order to understand their variability compared with the upper surface circulation. The value of the baroclinic Rossby radius of deformation was used for the discrimination of wind driven or geostrophic balanced spiral eddies. Though most of the observed formations seem to be wind driven, an important correlation with the upper circulation of the Aegean Sea is shown.

A buoyancy flux across the sea surface between the outcropping ofisopycnals must be balanced by a subsurface diapycnal buoyancy flux. If this flux were only advective, its derivative with respect to buoyancy would provide a direct estimate of the buildup of volume between isopycnals ...

The deep basins of the North Aegean are filled with locally formed very dense water, despite the very high stratification of the upper water column resulting from the outflow of very light, brackish water from the Black Sea into the Mediterranean. Furthermore, the Aegean is a source of deep and bottom water, which occasionally overflows into the Eastern Mediterranean. The interaction between the North and South Aegean, and the relative importance of the basins as dense water formation sites, is still unknown. In this study we attempt an assessment of the exchange between the North and South Aegean through buoyancy budget estimations. This will provide a first-order estimate of the North Aegean contribution to the interannual variability of the Eastern Mediterranean Deep Waters, as well as a better understanding of the thermohaline circulation within the Aegean Sea. Our budget estimation consists of comparing the buoyancy change of the deep basins between successive CTD campaigns with the time integrated buoyancy flux through the surface (considering the Dardanelles outflow as a surface flux). Surface buoyancy exchanges are estimated using heat and freshwater data from the ECMWF dataset. For the buoyancy forcing through the Dardanelles we exploit the output of a state-of-the-art hydraulic model simulating the exchange flow through the Turkish Straits system, developed and fine-tuned in the framework of a bilateral Greek-Ukrainian project. For the buoyancy content estimation of the deep basins we isolate periods where there are enough CTD observations covering most of the North and South Aegean. Considering one box per basin, the results suggest that the North Aegean behaves as a dilution basin throughout the year, constituting a source of buoyancy for the South Aegean. However, this contradicts the fact that the North Aegean contains by far the densest waters. This paradox can be explained by introducing two-layer boxes, using the vertical buoyancy flux through the interface as a tuning parameter. The results shed light into the functioning of the Aegean Sea; when there is considerable buoyancy loss through the surface of the North Aegean there can be export of dense waters to the South. During those incidents large quantities of surface waters are advected from the South to the North, in order for the North Aegean to remain a source of buoyancy for the South, thus altering the thermohaline balance of the Aegean and accelerating its conveyor belt.

This work is an effort to identify significant dense water formation events over the Aegean Sea for the period 1949-present. The Aegean Sea is one of the major dense wa-ter formation sites of the Mediterranean Sea. The interannual variability of its dense-water production appears to be remarkably large, ranging from being the major con-tributor for the bottom layers of the Eastern Mediterranean during the early nineties, to an insignificant contributor since then. Here, our aim is to determine the characteristic time scales of this variability. Hydrographic observations in the Aegean Sea have been very sporadic before 1986, thus hindering any direct assessments of deep water ven-tilation. However, there are data sets extending back to 1949, which may provide us with indirect evidence for conditions favorable to formation. In the Aegean Sea, ma-jor buoyancy forcing terms are the atmospheric exchanges and the Dardanelles buoy-ancy input. Regarding the air-sea interaction we exploit th...