Alexandre Stegner

Shelf Impact on Buoyant Coastal Current Instabilities

AbstractThe impact of shelf slope on the linear stability of buoyant coastal currents and on the nonlinear formation of coastal meanders and eddies is investigated. The authors consider a simplified two-layer stratification in cylindrical geometry where a buoyant surface current flows along the coast above a denser water, with a flat bottom or steep shelves. Simulations were performed using the Nucleus for European Modelling of the Ocean (NEMO) ocean global circulation model. The initial state of these simulations was defined according to laboratory experiments performed in the same configuration. Comparisons between laboratory and numerical results highlight the role of momentum diffusion and of the initial perturbations amplitude. The authors’ results confirm that the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the current) is the relevant parameter to quantify the shelf impact on the linear and nonlinear dynamics of the surface current. When the evolution of the b...

Long‐lived mesoscale eddies in the eastern Mediterranean Sea: Analysis of 20 years of AVISO geostrophic velocities

We analyzed 20 years of AVISO data set to detect and characterize long-lived eddies, which stay coherent more than 6 months, in the eastern Mediterranean Sea. In order to process the coarse gridded (1=8) AVISO geostrophic velocity fields, we optimized a geometrical eddy detection algorithm. Our main contribution was to implement a new procedure based on the computation of the Local and Normalized Angular Momentum (LNAM) to identify the positions of the eddy centers and to follow their Lagrangian tra-jectories. We verify on two mesoscale anticyclones, sampled during the EGYPT campaign in 2006, that our methodology provides a correct estimation of the eddy centers and their characteristic radius corresponding to the maximal tangential velocity. Our analysis reveals the dominance of anticyclones among the long-lived eddies. This cyclone-anticyclone asymmetry appears to be much more pronounced in eastern Mediter-ranean Sea than in the global ocean. Then we focus our study on the formation areas of long-lived eddies. We confirm that the generations of the Ierapetra and the Pelops anticyclones are recurrent and correlated to the Etesian wind forcing. We also provide some evidence that the smaller cyclonic eddies formed at the southwest of Crete may also be induced by the same wind forcing. On the other hand, the generation of long-lived eddies along the Libyo-Egyptian coast are not correlated to the local wind-stress curl but surprisingly , their initial formation points follow the Herodotus Trough bathymetry. Moreover, we identify a new formation area, not discussed before, along the curved shelf off Benghazi.

Evolution of an oceanic anticyclone in the lee of Madeira Island: In situ and remote sensing survey

Island wakes are areas of a strong eddy activity influencing the availability and transport of organic matter in the ocean which, in turn impact biological productivity. Despite this, eddy formation in the lee of North Atlantic tropical islands is scarcely documented, except for the Canary Islands. Moreover, the occurrence of anticyclones leeward of Madeira has seldom been detected. During the summer of 2011, a multiplatform approach, combining satellite data with in situ measurements, was used to study an anticyclonic eddy generated in the lee of the Madeira Island. The main objective was to confirm recent numerical evidence suggesting that orographically perturbed winds can induce anticyclonic eddies leeward of Madeira, particularly during summer months. The high resolution sampling of the eddys interior shows a strong downwelling of ≈100 m of the isopycnal layer below the mixed layer, typical of intrathermocline eddies. The 25 km radius of this anticyclonic structure exceeds the local deformation radius by a factor of 2. The vortex Rossby number remained moderate (Ro = 0.26) even if the relative core vorticity reached a finite value (ζ/f = −0.7). The occurrence of strong trade winds (10–15 m s−1) prior to the detection of the first surface eddy signatures (July 2011) concurrent with opposite flowing geostrophic currents, shows that the orographic wind forcing is the main mechanism for generating this mesoscale long-lived eddy. After leaving the shelter of the island, the eddy traveled northwesterly following a perpendicular net Ekman transport pathway at a speed of 5 km/d, for at least 2 months. An interaction with a cyclonic partner generated in the area helped precipitate the northward trajectory. This study presents the first clear evidence of a wind-induced mesoscale anticyclone in the lee of Madeira.

Angular Momentum Eddy Detection and Tracking Algorithm (AMEDA) and Its Application to Coastal Eddy Formation

AbstractAutomated methods are important for the identification of mesoscale eddies in the large volume of oceanic data provided by altimetric measurements and numerical simulations. This paper pres...

Dynamical Evolution of Intense Ierapetra Eddies on a 22 Year Long Period

Considered as wind forced, the recurrent formation of Ierapetra eddy affects the Eastern Mediterranean Sea circulation. Even though this large, coherent and long-lived anticyclone has been extensively studied, there are only few quantitative information on its dynamical characteristics. The main goal of this study is to quantify the Ierapetra Eddies (IEs) intensity and examine their seasonal and interannual variability over a 22 year period (1993-2014). We choose the automatic eddy detection algorithm AMEDA to estimate the main IEs dynamical parameters such as their size, their intensities and their lifetimes. Applied to daily AVISO altimetric products, the AMEDA algorithm allows a full characterization providing additional information on vortex velocity profiles as well as on merging and splitting events. Among the years of observations, the IEs Rossby number experience a strong variability and could vary by a factor 4 (Ro=0.07-0.27). This is mainly due to the eddy velocity variations rather than size variations. Moreover, we found that after their formation IEs could re-intensify. This intensification process may lead to a doubling of the vortex intensity in less than four months. That extra input of energy coincides with the Etesian winds period. Such high intensities are not expected from large-scale anticyclones and require cyclogeostrophic corrections. Considering this ageostrophic part, the maximum values of the core vorticity were derived and we found that the IEs might sometimes exhibit a negative potential vorticity core. Evidences on the eddy intensity from two oceanographic campaigns suggest that the IEs are probably more intense than we even estimate.