Η δυναμική της κυκλοφορίας του Αιγαίου πελάγους και της Λεβαντινής θάλασσας και η επίδραση της στις κλιματικές διακυμάνσεις

The eastern Mediterranean is known to have a complex thermohaline, wind, and heat/freshwater flux driven multi-scale circulation. This dissertation aims to investigate the dynamics of the Aegean Sea and the exchange with the Levantine basin at different temporal and spatial scales. Observational and modeling strategies were adopted to address questions of dynamics governing the regional circulation pattern and water mass formation process, as well as the climatic implications of the Aegean-Levantine Seas variability.Aiming at portraying the Aegean’s water mass structure and identifying Dense Water Formation processes, two winter cruises were conducted in 2005-2006, across the plateaus and depressions of the Aegean Sea. During the shipboard activities four Argo profiling floats were deployed, in order to monitor the Aegean’s deep layers. In addition to the observational methods, a hindcast simulation in the Aegean-Levantine basins for the years 1960-2000 was performed, using an eddy resolving ocean model (1o/30). The model incorporates a 6-hr atmospheric forcing and captures the observed variability of the 40-years.Observational results showed that the most prominent feature of the water mass distribution in the Aegean is a distinct “X-shape” of the Θ-S characteristics, suggesting a complicated coupling of the major sub-basins. The surface and deep waters are relatively decoupled with diverse origin characteristics, while the intermediate layers act as connectors of the main thermohaline cells. The central Aegean seems to play a key role due to formation processes of water masses with densities equal and/or higher than 29.2 kg.m-3, taking place in the sub-basin and dispersing in the north Aegean. The pre-EMT thermohaline pattern unveiled that the bottom density of the central basin was higher than that in the south Aegean and therefore the central Aegean possibly acted as a dense water reserve supply for the deeper part of the southern basin. On the other hand, the south Aegean appears greatly influenced by the eastern Mediterranean circulation and water mass distribution, especially under the Eastern Mediterranean Transient status. The Transitional Mediterranean Water monitored in the post-EMT period and characterized by low temperature at 14.2 oC, low salinity at 38.92 and low dissolved oxygen at 3.97 ml.l-1, with its core around 750 m and above the saline (39.06) Cretan Deep Water, altered significantly the south Aegean structure.Modeling results indicate the Eastern Mediterranean Transient as the most prominent climatic event of the period, with other weaker events taking place throughout the simulation period. The impact of the atmospheric versus lateral forcing on the buoyancy content of the Aegean-Levantine basins during the pre-EMT period, suggests preconditioning by surface fluxes mostly related to surface heat loss, and lateral fluxes mostly related to salt flux. While long-term trends of surface and lateral inputs are preconditioning the changes in the Aegean stratification, it is the extreme heat loss pulses, related to the variability of the wind field, that is controlling the formation processes by abruptly lowering the buoyancy content. Those events are possibly linked to an eastern Mediterranean bimodal atmospheric oscillation, with the anomalous surface heat fluxes shifting from the Levantine in the 1960s to the Aegean in the 1990s. During the EMT winters the central Aegean lower layers contain very dense waters, with σΘ larger than 29.3 kg.m-3. These waters form the core of the water mass outflowing in the Eastern Mediterranean, after being mixed with ambient waters along their southward flow. The outflowing layer is characterized by density of 29.21 kg.m-3. The deepest parts of the NW Levantine is initially filled with the new water mass, which later spreads to the SE parts of the basin, flowing over the Eastern Mediterranean Ridge.The results from this thesis could contribute to future scientific activities investigating the regional dynamics of the eastern Mediterranean Sea, and should serve as reference for observational and/or modeling based methods.Η ανατολική Μεσόγειος παρουσιάζει σύνθετη θερμοαλατική κυκλοφορία, λόγω της δράσης του ανεμολογικού πεδίου και των ατμοσφαιρικών ροών θερμότητας και νερού συνοπτικής, μέσης και μικρής κλίμακας. Αντικείμενο της διδακτορικής διατριβής είναι η μελέτη της δυναμικής του Αιγαίου Πελάγους και οι ανταλλαγές που παρατηρούνται με τη λεκάνη της Λεβαντίνης τα τελευταία 50 χρόνια. Για το σκοπό αυτό εφαρμόστηκαν μέθοδοι παρατήρησης μαζί με αριθμητικές προσομοιώσεις, προκειμένου να μελετήσουμε τις υπερετήσιες και δεκαετείς διακυμάνσεις της ανατολικής Μεσογείου.Στα πλαίσια προσδιορισμού της θερμοαλατικής δομής και της δημιουργίας βαθιών νερών στο Αιγαίο Πέλαγος, πραγματοποιήθηκαν δύο Ωκεανογραφικοί πλόες τους χειμώνες του 2005 και 2006. Κατά τη διάρκεια των ταξιδιών ποντίστηκαν τέσσερις αυτόνομοι πλωτήρες Argo, προκειμένου να καταγράψουν τις υπερετήσιες διακυμάνσεις των βαθύτερων στρωμάτων του Αιγαίου. Επιπροσθέτως, πραγματοποιήθηκαν αριθμητικές προσομοιώσεις για την περιοχή του Αιγαίου και της Λεβαντίνης για τη περίοδο 1960-2000, χρησιμοποιώντας ωκεανογραφικό μοντέλο υψηλής διακριτοποίησης (1ο/30). Το ωκεανογραφικό μοντέλο είναι συζευγμένο με ατμοσφαιρικό μοντέλο 6-ωρης ατμοσφαιρικής δράσης και αναπαράγει ικανοποιητικά τις υπερετήσιες διακυμάνσεις της περιόδου.Σύμφωνα με τις παρατηρήσεις των Ωκεανογραφικών σταθμών, οι οποίες προβάλλονται σε διάγραμμα Θ-S, η θερμοαλατική δομή παρουσιάζεται με τη μορφή ενός «X-σχήματος», υποδεικνύοντας τη σύζευξη των επιμέρους λεκανών του Αιγαίου. Τα επιφανειακά και βαθιά νερά είναι ασύζευκτα με θερμοαλατικά χαρακτηριστικά όμοια με των γειτονικών λεκανών, ενώ τα ενδιάμεσα στρώματα κλείνουν το κύριο θερμοαλατικό κύτταρο. Το κεντρικό Αιγαίο έχει κυρίαρχο ρόλο στη δημιουργία βαθιών νερών με χαρακτηριστικές πυκνότητες ίσες ή/και μεγαλύτερες των 29.2 kg.m-3, τα οποία συναντώνται και στο βόρειο Αιγαίο. Τα θερμοαλατικά χαρακτηριστικά της προ-ΕΜΤ περιόδου συνηγορούν στη σύζευξη κεντρικού και νότιου Αιγαίου, όπου είναι πιθανό τα βαθιά νερά του κεντρικού Αιγαίου να συναντώνται επίσης και στα βαθύτερα στρώματα του νότιου Αιγαίου. Tο νότιο Αιγαίο επηρεάζεται από την ευρύτερη κυκλοφορία της ανατολικής Μεσογείου. Η θαλάσσια μάζα TMW που παρατηρείται στην μετά-ΕΜΤ περίοδο, άλλαξε σημαντικά τη θερμοαλατική δομή του νότιου Αιγαίου. To TMW χαρακτηρίζεται από μικρή θερμοκρασία 14.2 oC, μικρή αλατότητα 38.92 και λίγο διαλυτό οξυγόνο 3.97 ml.l-1, με το θερμοαλατικό πυρήνα στα 750 m πάνω από το υψηλής αλατότητας (39.06) CDW.Η αριθμητική προσομοίωση 1960-2000 ανέδειξε το ΕΜΤ ως το χαρακτηριστικότερο γεγονός κλιματικής μεταβολής της συγκεκριμένης περιόδου, μαζί με άλλα μικρότερης έντασης. Το θερμοαλατικό περιεχόμενο των λεκανών στην προ-ΕΜΤ περίοδο προσδιορίζεται από τις επιφανειακές ατμοσφαιρικές ροές απώλειας θερμότητας και από τις πλευρικές ροές αύξησης αλατότητας. Τη μακρά περίοδο δημιουργίας κατάλληλων συνθηκών απώλειας πλευστότητας και μεταβολών στη στρωμάτωση του Αιγαίου, ακολουθούν έντονοι χειμώνες με σημαντικές απώλειες πλευστότητας στις υπό εξέταση λεκάνες. Η ατμοσφαιρική ανωμαλία της περιόδου 1960-2000, πιθανών συνδέεται με τη δημιουργία ενός δίπολου ροών απώλειας πλευστότητας πάνω από την ανατολική Μεσόγειο, με μετατόπιση της ανωμαλίας από τη Λεβαντίνη τη δεκαετία του ‘60 στο Αιγαίο τη δεκαετία του ‘90. Κατά τη διάρκεια του ΕΜΤ το κεντρικό Αιγαίο περιέχει πολύ πυκνά νερά σε σχετικά μικρά βάθη με πυκνότητες μεγαλύτερες των 29.3 kg.m-3. Τα νερά αυτά εκρέουν στην ανατολική Μεσόγειο, αφού πρώτα υφίστανται ισχυρή ανάμιξη. Το στρώμα εκροής πυκνών νερών του Αιγαίου κατά τη περίοδο του ΕΜΤ χαρακτηρίζεται από πυκνότητα 29.21 kg.m-3. Τα βαθύτερα στρώματα της νοτιοδυτικής Λεβαντίνης είναι αυτά που γεμίζουν πρώτα, ενώ η επίδραση του ΕΜΤ φτάνει έως και τη νοτιοανατολική Λεβαντίνη.Τα αποτελέσματα της διδακτορικής διατριβής μπορούν να συνεισφέρουν στη μελέτη της δυναμικής της ανατολικής Μεσογείου, καθώς επίσης και στο σχεδιασμό μελλοντικών πειραμάτων με παρατηρήσεις ή/και αριθμητικά μοντέλα

The Impact of Tides on the Bay of Biscay Dynamics

The impact of tides on the Bay of Biscay dynamics is investigated by means of an ocean model twin-experiment, consisted of two simulations with and without tidal forcing. The study is based on a high-resolution (1/36∘) regional configuration of NEMO (Nucleus for European Modelling of the Ocean) performing one-year simulations. The results highlight the imprint of tides on the thermohaline properties and circulation patterns in three distinct dynamical areas in the model domain: the abyssal plain, the Armorican shelf and the English Channel. When tides are activated, the bottom stress is increased in the shelf areas by about two orders of magnitude with respect to the open ocean, subsequently enhancing vertical mixing and weakening stratification in the bottom boundary layer. The most prominent feature reproduced only when tides are modelled, is the Ushant front near the entrance of the English Channel. Tides appear also to constrain the freshwater transport of rivers from the continental shelf to the open ocean. The spectral analysis revealed that the tidal forcing contributes to the SSH variance at high frequencies near the semidiurnal band and to the open ocean mesoscale and small-scale features in the presence of summer stratification pattern

Oil spill model uncertainty quantification using an atmospheric ensemble

We investigate the impact of atmospheric forcing uncertainties on the prediction of the dispersion of pollutants in the marine environment. Ensemble simulations consisting of 50 members were carried out using the ECMWF ensemble prediction system and the oil spill model MEDSLIK-II in the Aegean Sea. A deterministic control run using the unperturbed wind of the ECMWF high-resolution system served as reference for the oil spill prediction. We considered the oil spill rates and duration to be similar to major accidents of the past (e.g., the Prestige case) and we performed simulations for different seasons and oil spill types. Oil spill performance metrics and indices were introduced in the context of probabilistic hazard assessment. Results suggest that oil spill model uncertainties were sensitive to the atmospheric forcing uncertainties, especially to phase differences in the intensity and direction of the wind among members. An oil spill ensemble prediction system based on model uncertainty of the atmospheric forcing, shows great potential for predicting pathways of oil spill transport alongside a deterministic simulation, increasing the reliability of the model prediction and providing important information for the control and mitigation strategies in the event of an oil spill accident

Ensemble downscaling of a regional ocean model

International audienc

Mechanisms controlling the thermohaline circulation pattern variability in the Aegean–Levantine region. A hindcast simulation (1960–2000) with an eddy resolving model

A hindcast simulation in the Aegean–Levantine basins for the years 1960–2000 is performed, using an eddy resolving ocean model (1°/30). The model incorporates a 6-h atmospheric forcing provided by the ARPERA and captures the observed variability of the 40-years. The Eastern Mediterranean Transient (EMT) is the most prominent climatic event of the period. We investigate the impact of the atmospheric versus lateral forcing on the buoyancy content of the Aegean–Levantine basins. During the pre-EMT period, the basins’ buoyancy content is lowered by surface fluxes by about 1.5×10?8 m2 s?3 in the Aegean Sea, mostly related to surface heat loss, and lateral fluxes by about 0.9×10?8 m2 s?3, mostly related to salt flux, with the Levantine changes leading those of the Aegean. Furthermore, while long-term trends of surface and lateral inputs are preconditioning the changes in the Aegean stratification, it is the extreme heat loss pulses, related to the variability of the wind field that is controlling the formation processes by abruptly lowering the buoyancy content. Those events are possibly linked to an eastern Mediterranean bimodal atmospheric oscillation, with the anomalous surface heat fluxes shifting from the Levantine in the 1960s to the Aegean in the 1990s. The central Aegean due to its topography and thermohaline properties trigger events of excessive formation and producing the Aegean’s densest waters. During the EMT winters the central Aegean lower layers contain very dense waters, with ?? larger than 29.3 kg m?3. These waters form the core of the water mass outflowing in the Eastern Mediterranean, after being mixed with ambient waters along their southward flow. The outflowing layer is characterized by density of 29.21 kg m?3. The deepest parts of the NW Levantine is initially filled with the new water mass, which later spreads to the SE parts of the basin, flowing over the Eastern Mediterranean Ridge

Assessment of a regional physical–biogeochemical stochastic ocean model. Part 1: Ensemble generation

International audienceIn this article, Part 1 of a two-part series, we run and evaluate the skill of a regional physical–biogeochemical stochastic ocean model based on NEMO. The domain covers the Bay of Biscay at 1/36° resolution, as a case study for open-ocean and coastal shelf dynamics. We generate model ensembles based on assumptions about errors in the atmospheric forcing, the ocean model parameterizations and in the sources and sinks of the biogeochemical variables. The resulting errors are found to be mainly driven by the wind forcing uncertainties, with the rest of the perturbed forcing and parameters locally influencing the ensemble spread. Biogeochemical uncertainties arise from intrinsic ecosystem model errors and from errors in the physical state. Uncertainties in physical forcing and parameterization are found to have a larger impact on chlorophyll spread than uncertainties in ecosystem sources and sinks. The ensembles undergo quantitative verification with respect to observations, focusing on upper-ocean properties. Despite a tendency for ensembles to be generally under-dispersive, they appear to be reasonably consistent with respect to sea surface temperature data. The largest statistical sea-level biases are observed in coastal regions. These biases hint at the presence of high-frequency error sources currently unaccounted for, and suggest that the ensemble-based uncertainties are unfit to model error covariances for assimilation. Model ensembles for chlorophyll appear to be consistent with ocean colour data only at times. The stochastic model is qualitatively evaluated by analysing its ability at generating consistent multivariate incremental model corrections. Corrections to physical properties are associated with large-scale biases between model and data, with diverse characteristics in the open-ocean and the shelves. Mesoscale features imprint their signature on temperature and sea-level corrections, as well as on chlorophyll corrections due to the vertical velocities associated with vortices. Small scale local corrections are visible over the shelves. Chlorophyll information has measurable impact on physical variables

Assessment of a regional physical-biogeochemical stochastic ocean model. Part 1: Ensemble generation

In this article, Part 1 of a two-part series, we run and evaluate the skill of a regional physical-biogeochemical stochastic ocean model based on NEMO. The domain covers the Bay of Biscay at 1/36 degrees resolution, as a case study for open-ocean and coastal shelf dynamics. We generate model ensembles based on assumptions about errors in the atmospheric forcing, the ocean model parameterizations and in the sources and sinks of the biogeochemical variables. The resulting errors are found to be mainly driven by the wind forcing uncertainties, with the rest of the perturbed forcing and parameters locally influencing the ensemble spread. Biogeochemical uncertainties arise from intrinsic ecosystem model errors and from errors in the physical state. Uncertainties in physical forcing and parameterization are found to have a larger impact on chlorophyll spread than uncertainties in ecosystem sources and sinks. The ensembles undergo quantitative verification with respect to observations, focusing on upper-ocean properties. Despite a tendency for ensembles to be generally under-dispersive, they appear to be reasonably consistent with respect to sea surface temperature data. The largest statistical sea-level biases are observed in coastal regions. These biases hint at the presence of high-frequency error sources currently unaccounted for, and suggest that the ensemble-based uncertainties are unfit to model error covariances for assimilation. Model ensembles for chlorophyll appear to be consistent with ocean colour data only at times. The stochastic model is qualitatively evaluated by analysing its ability at generating consistent multivariate incremental model corrections. Corrections to physical properties are associated with large-scale biases between model and data, with diverse characteristics in the open-ocean and the shelves. Mesoscale features imprint their signature on temperature and sea-level corrections, as well as on chlorophyll corrections due to the vertical velocities associated with vortices. Small scale local corrections are visible over the shelves. Chlorophyll information has measurable impact on physical variables

An Integrated Weather and Sea State Forecasting System for the Arabian Peninsula (WASSF)

Saudi Aramco is the oil industry of the Kingdom of Saudi Arabia with several activities related to the environment. In order to optimize daily operations and minimize environmental risks a forecasting system has been employed and setup in operations. The objectives of the system include prevention and mitigation of environmental problems, as well as early warning of local conditions associated with extreme weather events. The management and operations part is related to early warning of weather and dust storms that affect operations of various facilities, whereas the environmental part is mainly focused on air quality and desert dust levels in the atmosphere

Assessing impacts of observations on ocean circulation models with examples from coastal, shelf, and marginal seas

Ocean observing systems in coastal, shelf and marginal seas collect diverse oceanographic information supporting a wide range of socioeconomic needs, but observations are necessarily sparse in space and/or time due to practical limitations. Ocean analysis and forecast systems capitalize on such observations, producing data-constrained, four-dimensional oceanographic fields. Here we review efforts to quantify the impact of ocean observations, observing platforms, and networks of platforms on model products of the physical ocean state in coastal regions. Quantitative assessment must consider a variety of issues including observation operators that sample models, error of representativeness, and correlated uncertainty in observations. Observing System Experiments, Observing System Simulation Experiments, representer functions and array modes, observation impacts, and algorithms based on artificial intelligence all offer methods to evaluate data-based model performance improvements according to metrics that characterize oceanographic features of local interest. Applications from globally distributed coastal ocean modeling systems document broad adoption of quantitative methods, generally meaningful reductions in model-data discrepancies from observation assimilation, and support for assimilation of complementary data sets, including subsurface in situ observation platforms, across diverse coastal environments