Unit:
Library of the School of ScienceDepartment of Physics
Author:
Mavropoulou Apostolia Maria
Dissertation committee:
Σαράντης Σοφιανός, Αναπληρωτής Καθηγητής,Τμήμα Φυσικής, ΕΚΠΑ
Ελισσάβετ Μποσιώλη, Επίκουρη Καθηγήτρια,Τμήμα Φυσικής, ΕΚΠΑ
Takamitsu Ito, Καθηγητής, Georgia Institute of Technology, Atlanta, USA
Έλενα Φλόκα, Καθηγήτρια, Τμήμα Φυσικής, ΕΚΠΑ
Χρήστος Τζάνης, Επίκουρος Καθηγητής,Τμήμα Φυσικής, ΕΚΠΑ
Ελένη Γιαννακάκη, Επίκουρη Καθηγήτρια,Τμήμα Φυσικής, ΕΚΠΑ
Διονύσιος Ραΐτσος-Εξαρχόπουλος, Επίκουρος Καθηγητής,Τμήμα Βιολογίας, ΕΚΠΑ
Original Title:
Η επίδραση των κλιματικών διακυμάνσεων και της φυσικής/ανθρωπογενούς εναπόθεσης στην κατανομή του οξυγόνου της Μεσογείου
Translated title:
The impact of climate variability and natural/anthropogenic depositions on the oxygen distribution of the Mediterranean Sea
Summary:
Dissolved oxygen is required by the majority of aquatic organisms for respiration and an oxygen deficiency in water poses a threat to our aquatic ecosystems. In a global environment with constantly changing climatic conditions, it is more than necessary to understand and analyze its variability. A higher dissolved oxygen level indicates a better water quality, but climate models predict a decrease in global ocean dissolved oxygen due to climate change. Both physical and biological processes such as warming-induced changes, the three-dimensional ocean circulation, and the supply of nutrients to the ocean interior, regulate the dissolved oxygen distribution and its variability. Understanding the mechanisms with which changes in the biological productivity and changes in the oceanic hydrographic characteristics and circulation, related to climate change and anthropogenic depositions, on time scales from months to centuries is the key to interpreting observed oceanic changes. The Mediterranean Sea is very interesting region for such a research effort because it combines strong forcing and enhanced response.
The current knowledge on the complexity of mechanisms involved is still insufficient in this basin. Hence, the scope of this dissertation is to investigate the variability of dissolved oxygen distribution in the Mediterranean Sea, including physical and biogeochemical processes. The complexity of the above mechanisms leads to a further investigation of the dissolved oxygen distribution using both in situ observations and numerical simulations. The analysis of in situ historical data is used to understand how the ocean is changing and the coupling of physical and biogeochemical models is the most effective tools to interpret the effects of each mechanism upon the dissolved oxygen.
The interannual variability and the mechanisms controlling the dissolved oxygen concentration in the Mediterranean Sea were investigated through generating gridded fields of dissolved oxygen, salinity and temperature. The Data-Interpolating Variational Analysis (DIVA) software was used to produce a gridded dataset for the time period 1960-2011. High oxygen concentrations for the upper and bottom layers, separated by an oxygen minimum zone at intermediate layers, are a typical structure of the dissolved oxygen in the Eastern and the Western Mediterranean sub-basins. Although an oxygen minimum zone is observed in both sub-basins, its vertical positions are different; in the Eastern Mediterranean is located between 600 and 1200 meters depth and in the Western Mediterranean between 400 and 600 meters. The vertical distribution of dissolved oxygen shows significant differences between the two sub-basins and their temporal evolution reveals large interannual to decadal variability. A negative correlation was observed between dissolved oxygen and surface temperature due to solubility changes over the whole period. However, in the intermediate and deep layers, the primary factors affecting the dissolved oxygen variability are the dynamical processes associated with the episodic events of deep-water formation. The positive correlation between the dissolved oxygen and thermohaline characteristics in the Eastern Mediterranean deep layers is an indication of the dynamical processes, which are involved in the dissolved oxygen interannual variability. The dissolved oxygen variability presents shifts with a multi-decadal signal, rather than trends as observed in the global ocean.
The dissolved oxygen in deep layers is strongly regulated by dynamical processes, such as overturning circulation. Thus, it is essential to examine separately this physical process in the basin, as little is known for the long-term variability of its strength. The interannual variability of the Mediterranean overturning circulation is investigated using a high-resolution (1/36°) ocean model (1958-2015). As the overturning circulation regulates the 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.
We know that nutrient deposition inputs from the atmosphere to the ocean are increased substantially by human activities and affect the marine biogeochemistry, but their impact on the oceanic oxygen cycle over the Mediterranean Sea, remains an open issue that is poorly understood. In the present work, an effort towards the investigation of the dependence of dissolved oxygen distribution on atmospheric nutrients deposition is also made. We performed twin sensitivity experiments focusing on natural (pre-industrial) and anthropogenic (industrial) atmospheric nutrient supply. The contribution of anthropogenic nutrient inputs in the Mediterranean Sea more than doubled and affects the spatial variability distribution of dissolved oxygen in a different way at each level of the water column, associated with the biochemical processes that take place in these depths (e.g., net primary production, zooplankton migration). The Mediterranean Sea remains oligotrophic, but there are different biases under the pre-industrial and present-day aerosol conditions.
Main subject category:
Science
Keywords:
Dissolved oxygen, Mediterranean Sea, Interannual variability, atmospheric deposition, biogeochemical numerical model
Number of references:
311
File:
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