Alexandre Stegner

École Polytechnique · Laboratoire de Météorologie Dynamique (LMD)

Seasonal evolution of both surface signature and subsurface structure of a Mediterranean mesoscale anticyclones is assessed using the Coastal and Regional Ocean Community high‐resolution numerical model with realistic background stratification and fluxes. In good agreement with remote‐sensing and in‐situ observations, our numerical simulations capt...

Real-time reconstruction of ocean surface currents is a challenge due to the complex, non-linear dynamics of the ocean, the small number of in situ measurements, and the spatio-temporal heterogeneity of satellite altimetry observations. To address this challenge, we introduce HIRES-CURRENTS-Net, an operational real-time convolutional neural network...

Sensitivity of model grid horizontal and vertical resolution and forcing frequency to accurately reproduce mesoscale eddies in numerical simulations.

The mixed layer is the uppermost layer of the ocean, connecting the atmosphere to the subsurface ocean through atmospheric fluxes. It is subject to pronounced seasonal variations: it deepens in winter due to buoyancy loss and shallows in spring while heat flux increases and restratifies the water column. A mixed-layer depth (MLD) modulation over th...

Optimal ship routing systems require highly accurate oceanic data. Our technological innovation is based on the use of high-resolution currents derived from the fusion of various satellite observations by harnessing Artificial Intelligence methods. Today, routing strategies rely mainly on the outputs of operational oceanic models that cannot always...

Sea surface currents (SSC) derived from the sea surface height anomalies (SSHA) as measured by multi‐satellite altimeters are widely used for various applications including studies on ocean dynamics, marine ecology, and climate change. However, present SSC products estimated on the assumption of an idealized geostrophic balance is biased. To overco...

It is widely accepted that the signature of anticyclonic (cyclonic) eddies on the sea surface temperature corresponds to a warm (cold) core anomaly. Nevertheless, this statement has been put to question by recent regional studies showing the existence of inverse eddy SST anomalies: Cold Core anticyclones and, respectively, Warm Core cyclones. This...

The mixed layer is the uppermost layer of the ocean, driven by atmospheric fluxes. It follows a strong seasonal cycle, deepening in winter due to buoyancy loss, shoaling very close to the surface in summer with restratification. Recently several global and regional studies show a mixed layer depth (MLD) modulation by mesoscale eddies with the seaso...

Surface currents provided, in real time, by operational ocean models often differ from each other but also from satellite altimetry observations, especially in terms of mesoscale dynamics. Eddies, which play a dominant role on circulation at the regional scale, have a signature on both altimetry maps and satellite imagery, such as sea surface tempe...

It is widely accepted that the signature of anticyclonic (cyclonic) eddies on the sea surface temperature corresponds to a warm (cold) core anomaly. Nevertheless, this statement has been put to question by recent regional studies showing the existence of inverse eddy SST anomalies: cold-core anticyclones and respectively warm-core cyclones. This...

Statistics of anticyclonic eddy activity and eddy trajectories in the Levantine Basin over the 2000–2018 period are analyzed using the DYNED-Atlas database, which links automated mesoscale eddy detection by the Angular Momentum Eddy Detection and Tracking Algorithm (AMEDA) algorithm to in situ oceanographic observations. This easternmost region of...

We perform an Observing System Simulation Experiment that simulates the satellite sampling and the mapping procedure on the sea surface of the high‐resolution model CROCO‐MED60v40, to investigate the reliability and the accuracy of the eddy detection. The main result of this study is a strong cyclone‐anticyclone asymmetry of the eddy detection on t...

Using the 13 m diameter LEGI-Coriolis rotating platform, the evolution processes of a generated anticyclonic eddy throughout its lifecycle are analyzed. Experimental results have shown that the eddy lasted for approximately 3T0, where T0 is the rotating period of 90 s. After T = 0.3T0, the eddy enters its mature phase, whereby following this event,...

The Ras al Hadd oceanic dipole is a recurrent association of a cyclone (to the northeast) and of an anticyclone (to the southwest), which forms in summer and breaks up at the end of autumn. It lies near the Ras al Hadd cape, southeast of the Arabian peninsula. Its size is on the order of 100 km. Along the axis of this dipole flows an intense jet, t...

Statistics of anticyclone activity and trajectories in the southeastern Mediterranean sea over the period 2000–2018 is created using the DYNED atlas, which links the automated mesoscale eddy detection by the AMEDA algorithm with in situ oceanographic observations. This easternmost region of the Mediterranean sea, delimited by the Levantine coast an...

Motivated by the recurrent formation of mesoscale anticyclones in the southeast of Crete, we investigated with a high resolution model the response of the ocean to orographic wind jets driven by the Cretean mountain range. As shown in the dynamical process study of Ioannou et al. (2020) which uses a simplified shallow-water model, we confirm here,...

The Agulhas leakage of Indian Ocean waters into the Atlantic has been shown to be a key link in global ocean circulation and climate; an increased understanding of this process is therefore of more than just of local interest. While knowledge of the Agulhas leakage has increased over the past 30 years, the precise mechanisms at play and water prope...

Motivated by the recurrent formation and intensification of the Ierapetra anticyclones in the southeast of Crete, we investigated with a reduced gravity model the response of the oceanic surface layer to a seasonal wind jet that varies slowly (over several weeks or months) and mimics the Etesian winds. Our study answers why the oceanic response to...

Mesoscale oceanic eddies have a visible signature on Sea Surface Temperature (SST) satellite images, portraying diverse patterns of coherent vortices, temperature gradients and swirling filaments. However, learning the regularities of such signatures defines a challenging pattern recognition task, due to their complex structure but also to the clou...

From 2015 to 2018 four field experiments (seven legs) were performed in the Western Mediterranean basin during winter or early spring. The main objectives were the assessment of high-resolution modeling and the observation of mesoscale structure and associated ageostrophic dynamics. Thanks to the intensive use of a towed vehicle undulating in the u...

Oceanic vortices are ubiquitous in the ocean. They dominate the sub-inertial energy spectrum, and their dynamics is key for the evolution of the water column properties. The merger of two like-signed coherent vortices, which ultimately results in the formation of a larger vortex, provides an efficient mechanism for the lateral mixing of water masse...

Until now, mesoscale oceanic eddies have been automatically detected through physical methods on satellite altimetry. Nevertheless, they often have a visible signature on Sea Surface Temperature (SST) satellite images, which have not been yet sufficiently exploited. We introduce a novel method that employs Deep Learning to detect eddy signatures on...

The transfer of Indian Ocean thermocline and intermediate waters into the South Atlantic via the Agulhas leakage is generally believed to be primarily accomplished through mesoscale eddy processes, essentially anticyclones known as Agulhas Rings. Here we take advantage of a recent eddy tracking algorithm and Argo float profiles to study the evoluti...

Mesoscale eddies, having a characteristic radius equal or larger than the local deformation radius, are generally considered to be geostrophic. Even if this is true for most of them, there are few cases where the ageostrophic velocity components induced by the local curvature of the streamlines are not negligible. In order to account for this ageos...

Motivated by the recurrent formation and intensification of the Ierapetra anticyclones in the southeast of Crete, we investigated with a reduced gravity model the response of the oceanic surface layer to a seasonal wind jet that varies slowly (over several weeks or months) and mimics the Etesian winds. Our study answers why the oceanic response to...

Deep-ocean islands have long been associated with the generation of oceanic eddies in their wakes, but their interaction with incoming eddies has seldom been considered. This study aimed to fill this gap, focusing in the Cabo Verde Archipelago between 2003-2014, an ideal location given the complex islands disposition and shape, and its exposure to...

From 2015 to 2018 four field experiments (7 legs) have been performed in the Western Mediterranean Basin during winter or early spring. The main objectives were the assessment of high-resolution modelling, the observation of mesoscale structure and associated ageostrophic dynamics. Thanks to the intensive use of a towed vehicle undulating in the up...

From 2015 to 2018 five field experiments (9 legs) have been performed in the Western Mediterranean Basin during winter or early spring. Thanks to the intensive use of a towed vehicle undulating in the upper oceanic layer between 0 and 400 meter depth (i.e. a Seasoar), a large amount of very high resolution hydrographic transects have been performed...

Mesoscale eddies, having a characteristic radius equal or larger than the local deformation radius, are generally considered to be geostrophic. Even if this is true for most of them, there are few cases where the ageostrophic velocity components induced by the local curvature of the streamlines are not negligible. In order to account for this ageos...

We investigate the linear stability of intense baroclinic anticyclones, with a particular focus on the centrifugal (inertial) instability. Various vertical and radial velocity profiles are studied. The vertical profiles are such that the velocity is maximum at the surface. These profiles correspond to oceanic eddies such as submesoscale mixed-layer...

Mesoscale dynamics in the Mediterranean Sea have been investigated for years, and anticyclonic eddies are regularly observed features in the Algerian Basin. Here we used the AMEDA eddy detection algorithm to track and monitor a particular anticyclonic eddy from its birth to its death. The analysis of remote sensing data sets (AVISO and sea surface...

The Indo-Atlantic interocean exchanges achieved by Agulhas Rings are tightly linked to global ocean circulation and climate. Yet they are still poorly understood because they are difficult to identify and follow. We propose here an original assessment on Agulhas Rings, achieved by TOEddies, a new eddy identification and tracking algorithm that we a...

We investigate the linear stability of intense baroclinic anticyclones, with a particular focus on the centrifugal (inertial) instability. Various vertical and radial velocity profiles are studied. The vertical profiles are such that the velocity is maximum at the surface. These profiles correspond to oceanic eddies such as submesoscale mixed layer...

Automated 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 presents an optimized algorithm for detecting and tracking eddies from two-dimensional velocity fields. This eddy identification uses a hybrid methodology based on p...

This study investigates the linear and non-linear instability of a buoyant coastal current flowing along a sloping topography. In fact, the bathymetry strongly impacts the formation of meanders or eddies and leads to different dynamical regimes that can both enhance or prevent the cross-shore transport. We use the Regional Ocean Modeling System (RO...

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 Eddie...

This study investigates the linear and non-linear instability of a buoyant coastal current flowing along a sloping topography. In fact, the bathymetry strongly impacts the formation of meanders or eddies and leads to different dynamical regimes that can both enhance or prevent the cross-shore transport. We use the Regional Ocean Modeling System (RO...

Using in situ data and laboratory experiments, we show that the circulation of the Bransfield Current (BC) around the South Shetland Islands (SSI) may be characterized in terms of a propagating buoyant gravity current. First, we describe the SSI hydrography and some drifter trajectories, paying special attention to the recirculation of the BC at th...

We analyzed 20 years of AVISO data set to detect and characterize long-lived eddies, which stay coherent more than six months, in the Eastern Mediterranean Sea. In order to process the coarse gridded (1/8º) AVISO geostrophic velocity fields, we optimized a geometrical eddy detection algorithm. Our main contribution was to implement a new procedure...

This chapter presents the most recent laboratory and theoretical results on the various dynamical regimes which control an idealized oceanic Karman wake at the mesoscale and submesoscale. In order to classify the various dynamical regimes of the idealized island wake, we first need to identify the main dimensionless parameters which control the flo...

In situ measurements obtained during the 2010 COUPLING cruise were analyzed in order to fully char-acterize the velocity structure of the coastal Bransfield Current. An idealized two-layer shallow-water model was used to investigate the various instability processes of the realistic current along the coastal shelf. Par-ticularly studied is how the...

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 o...

The stability of axisymmetric vortices to inertial perturbations is investigated by means of linear stability analysis, taking into account stratification, vertical eddy viscosity, as well as finite depth of the flow. We consider different types of circular barotropic vortices in a linearly stratified shallow layer confined with rigid lids. For the...

Large-scale laboratory experiments were performed on the Coriolis rotating platform to study the stability of intense vortices in a thin stratified layer. A linear salt stratification was set in the upper layer on top of a thick barotropic layer, and a cylinder was towed in the upper layer to produce shallow cyclones and anticyclones of similar siz...

The stability of axisymmetric vortices to inertial perturbations is investigated by means of linear stability analysis, taking into account stratification, vertical eddy viscosity, as well as finite depth of the flow. We consider different types of circular barotropic vortices in a linearly stratified shallow layer confined with rigid lids. For the...

The stability of axisymmetric oceanic-like vortices to inertial perturbations is investigated by means of linear stability analysis, taking into account the thickness and the stratification of the thermocline, as well as the vertical eddy viscosity. The model considers different types of circular barotropic vortices in a linearly stratified shallow...

The 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 shelve...

The 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 shelve...

In the sub-mesoscale ocean vortices tend to be predominantly cyclonic [4]. Inertial instability (hereafter II), which is a centrifugal instability mechanism in the presence of the Coriolis force, is a destructive mechanism that acts only on anti-cyclones, and therefore is hypothesized to cause this asymmetry. Furthermore, since II is in fact the gr...