Dissertation committee:
Α. Λασκαράτο, Αν. Καθηγητή Ε.Κ.Π.Α.(Επιβλέπων), Δ. Ασημακόπουλο, Καθηγητή Ε.Κ.Π.Α., Γ. Κάλλο, Καθηγητή Ε.Κ.Π.Α., Σ. Σοφιανό, Λέκτορα Ε.Κ.Π.Α., Κ. Ιακωβίδη, Αν. Καθηγητή Ε.Κ.Π.Α., Σ. Πούλος, Αν. Καθηγητή Ε.Κ.Π.Α., και Ι. Κρεστενίτη, Καθηγητή Α.Π.Θ.
Summary:
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.
Keywords:
Aegean-Levantine Seas, Climate variability, Eastern Mediterranean Transient (ΕΜΤ), Argo Floats, Numerical model
