Dynamical evolution of intense mesoscale Ierapetra eddies in connection with orographic wind forcing.

Directeurs.rices de thèses : Speich S. & Stegner A.

Date 2019-10-02
Diplôme IP Paris


Composition du jury

Andrea Doglioli ,Universite d’Aix-Marseille, (MIO),Rapporteur
Guillaume Roullet ,Université de Brest (UBO),(LOPS),Rapporteur
Isabelle Taupier-Letage,CNRS/Université de la Méditerranée/COM, (MIO),Examinatrice
Franscesco D’Ovidio, CNRS/UPMC ,( LOCEAN), Examinateur
Anestis Kalfas, Aristotle University of Thessaloniki, (LFMT), Examinateur
Alexandre Stegner, École Polytechnique (LMD), Directeur de thèse
Sabrina Speich,École Normale Supérieure (LMD),Directrice de thèse


Ierapetra eddy (IE) is one of the largest and longest-lived mesoscale eddies of the eastern Mediterranean Sea
(MED). Located in the South East corner of Crete, the IE recurrent formation has been attributed to the regional
Etesian wind forcing. Yet there are no many information on the eddy dynamical characteristics nor evidences that
the IE is wind-forced. In this study, a full characterization of the Ierapetra eddy dynamics is attempted. Firstly, by
using satellite observations of sea surface height and AMEDA eddy detection algorithm, the Ierapetra eddies are
followed over 22 years (1993-2014). Information on the eddy characteristics (such as radius, velocities, vorticity) and
the eddy trajectories are retrieved. The Ierapetra eddies are found to experience strong variability in their intensity
(Ro=0.07-0.27) while one year after formation they may re-intensify. This intensification process is firstly observed
here and can lead to an doubling of the eddy intensity in less than 3 months. For the Ierapetra eddies, we found
that the centrifugal acceleration terms cannot be omitted. An iterative method is optimized on idealized eddies to
recover with best accuracy the cyclogeostrophic balance. The importance of cyclogeostrophic corrections are
evaluated for the total MED. When taken into account, estimations on the IE core vorticity may reach values of
highly negative vorticity (0)/f=-1. Three specific oceanographic campaigns (EGYPT/ζ EGITTO, BOUM and
PROTEVS-PERLE) that provide information on measured eddy velocities are also presented. Comparison between
the observations and in-situ measurements demonstrate the importance of the cyclogeostrophic terms for the
correct estimation of the eddy velocities. Lastly, the Etesian wind forcing is investigated as the driving mechanism
for the IEs dynamics. An idealized climatological wind forcing shape that can represent the Etesian forcing regime is
build based on ALADIN reanalysis wind datasets. The transient effect of the idealized Etesian forcing is then
evaluated with a reduced gravity model. Various numerical simulations are performed in order to investigate the
isolated effect of such forcing to the ocean surface. Overall the results demonstrate the formation of a persistent
anticyclone. Both the wind forcing magnitude but also the wind forcing duration are key parameters that control the
formed eddy intensity. The eddy radius is mainly controlled by the spatial characteristics of the applied wind forcing.
Moreover, we recover the intensification process for pre-existing anticyclones in the presence of the shear wind. The
findings of this thesis suggest that the Ierapetra eddies are among the strongest mesoscale eddies of the
Mediterranean Sea while the regional wind forcing is sufficient to explain the observed eddy characteristics. An
asymmetric orographic wind jet or high γ values of a symmetric orographic forcing are the necessary preconditions
to favor the formation of strong anticyclones in the ocean.