A hybrid identification and tracking of Lagrangian mesoscale eddies

International audienceAutomatic identification and tracking of mesoscale eddies are crucial in large oceanic observational and numerical model data. This work proposes a fully automated method that identifies and tracks from a single Lagrangian advection, all mesoscale eddies, without prior knowledge of their lifespans. The eddies' detection and tracking use a hybrid method based on geometrical properties of the evolving velocity along Lagrangian trajectories and a grid density-based clustering algorithm. The high eddies' monitoring capacity of the proposed method is demonstrated by automatically identifying and tracking these structures from two different datasets: satellite-derived surface geostrophic velocity fields and a two-dimensional fluid simulation. The proposed approach gives complete dynamical features and evolution of the detected mesoscale eddies by identifying their genesis event, monitoring their coherent core, and describing their splitting and vanishing image

How well mesoscale eddies are represented in oceanic models?

International audienceThe aim of this work is to assess the impact of data assimilation on the dynamics of mesoscale eddies. We do this based on the use of a recent mathematical method from nonlinear dynamics field that is capable of detecting Lagrangian eddies from birth to death. We apply this method on two composites time series of two configurations covering the whole globe, with and without assimilation, and carry on various statistical analysis to evaluate the impact of data assimilation on several properties of these Lagrangian mesoscale eddies

Life-time independent approach to Lagrangian vortices detection

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An Unsupervised Machine Learning Algorithm for the Identification and Tracking of Lagrangian Mesoscale Eddies

International audienceMesoscale eddies are the dominant mechanism of ocean heat transports from the tropics to the poles that help to maintain the extra-tropical climate. In addition to their role in moving heat poleward, mesoscale eddies affect a vertical transfer of heat in the ocean, largely moving heat upwards to partially compensate for the downward heat transport by time mean fields. Automation of the identification and tracking of mesoscale eddies are critical to global climate models that include ocean mesoscale eddies, and to understand the role of eddies in climate. This work proposes a fully automated method that identifies and tracks from a single Lagrangian advection, all mesoscale eddies from birth to death, without prior knowledge of their lifespans. The eddies' detection and tracking use an unsupervised machine learning algorithm, which consists of a hybrid method based on geometrical properties of the evolving velocity along Lagrangian trajectories and a grid density-based clustering algorithm. The high eddies' monitoring capacity of the proposed method is demonstrated by automatically identifying and tracking these structures from two different datasets: satellitederived surface geostrophic velocity fields and a two-dimensional fluid simulation. The proposed approach gives complete dynamical features and evolution of the detected mesoscale eddies by identifying their genesis event, monitoring their coherent core, and describing their splitting and vanishing image

Un Demi Siècle D’évaluation De L’activité De L’upwelling De La Côte Atlantique Marocaine

Les écosystèmes d'upwelling situés en bordures Est (EBUEs) sontparmi les écosystèmes les plus productifs au monde et leur réponse auchangement climatique d'une importance cruciale. La variabilité saisonnièreet décennale à long terme de l'activité d'upwelling de la partie nord du grandécosystème marin du Courant des Canaries (CCLME) a été étudiée dans cetravail sur une période de 53 ans (1967-2019) à deux stations situéesrespectivement au nord (31°N-10,5°O) et au sud (24,5°N-15,5°O) de la côteatlantique marocaine, en élaborant l’Indice d’Upwelling de Bakun (UI)mensuel. Les résultats obtenus montrent que l’upwelling présente unevariabilité saisonnière entre Cap Juby - Larache et une activité quasipermanente toute l’année entre Cap Juby - Cap Blanc. En relation avec lavariabilité décennale et dans la zone nord atlantique marocaine, les plus fortesactivités de l’upwelling ont été observées en saisons d’été, en particulier surla période 1998-2003 et son activité a légèrement dépassé la moyenne sur lespériodes 1972-1977, 1980-1984, 2004-2008. Les plus faibles activités del’upwelling dans cette zone ont été observés en saisons d’automne/hiver,notamment sur les périodes 1967-1970, 1995-1998 et 2000-2005. S’agissantde la zone sud atlantique marocaine, l’activité de l’upwelling a présenté uneforte activité de l’upwelling en été sur les périodes 1967-1980 et 2009-2019,avec des fluctuations annuelles à interannuelles variables entre ces deuxpériodes. Cette activité a été légèrement supérieure à la moyenne en été sur lapériode 1981-2007, sauf pour les années 1982-1983, 1988-1989, 1995-1997et 2004-2007 où il a été observé une relative tendance vers la baisse. De plus faibles activités de l’upwelling ont été observées dans cette zone sur la période2003-2010 en saisons d’automne/hiver. Eastern Boundary Upwelling Ecosystems (EBUEs) are among themost productive ecosystems in the world, meaning that their response toclimate change is of critical importance. In this work, we investigate the longterm seasonal and decadal variability of the upwelling activity along thenorthern part of the Canary Current Large Marine Ecosystem (CCLME) overa period of 53 years (1967-2019). Two stations were selected in this studyrepresentative of the upwelling activity in the moroccan atlantic coast, one inthe north (31° N-10.5°W) and the other in the south (24.5° N-15.5°W) usingthe monthly Bakun Upwelling Index (UI). The results shows a seasonalvariability in the northern region of morocco between Cap Juby - Larache andalmost permanent activity all year round in the southern region of moroccobetween Cap Juby - Cap Blanc. These results are consistent with previousstudies based on the seasonality of the upwelling regime in thisregion. Relatedto the decadal variability, the strongest activities of upwelling were observedduring summer seasons in the northern region, in particular over the periodv 1998-2003 and its activity slightly exceeded the average over the periods1972-1977, 1980-1984, 2004-2008. The weakest upwelling activities in thisregion were observed in the fall-winter seasons, particularly during the periods1967-1970, 1995-1998 and 2000-2005. In the southern region, the upwellingactivity showed strong upwelling activity in summer over the periods 1967-1980 and 2009-2019, with annual and interannual between these two periods.This activity was slightly above average in summer over the 1981-2007period, except for the years 1982-1983, 1988-1989, 1995-1997 and 2004-2007where a relative downward trend was observed. Lower upwelling activitieswere observed in this area over the 2003-2010 period in fall / winter seasons

A Fourier approach to Lagrangian vortex detection

International audienceWe study the transport properties of coherent vortices over a finite time duration. Here we reveal that such vortices can be identified based on frequency-domain representation of Lagrangian trajectories. We use Fourier analysis to convert particles' trajectories from their time domain to a presentation in the frequency domain. We then identify and extract coherent vortices as material surfaces along which particles' trajectories share similar frequencies. Our method identifies all coherent vortices in an automatic manner, showing high vortices' monitoring capacity. We illustrate our new method by identifying and extracting Lagrangian coherent vortices in different two-and three-dimensional flows

Surface Mixing and Biological Activity in The North African Upwelling

International audienceNear-shore water along The North African margin represent one of the world's major upwelling regions. It is characterized by a high productivity of plankton associated with large commercial fisheries. Being characterized by physical structures of oceanic front such as eddies and filaments that influence the biological productivity, the study of these physical oceanic structures in connection with the chlorophyll has a fundamental importance for understanding how the plankton distributions is controlled. The aim of this work is to study the horizontal stirring and mixing in the upwelling area using attracting/repelling Lagrangian coherent structures (LCS) obtained as subset of hyperstreamline of the Cauchy-Green strain tensor, whom the normal repulsion rate is larger than tangential stretch over backward/forward time interval, and their link to the chlorophyll fronts concentrations, based on 10 years satellite data. These LCS move with the flow as material line, thus the horizontal mixing is calculated from the intersection of theses LCS with the finite time lyapunov exponent (FTLE) map. The temporal variability of surface stirring is compared to the fronts chlorophyll concentration, showing similar seasonal variations, nearly coincident maxima and minima, leading to a global positive correlation. This contradicts a recent work on the same region, which found negative correlation. However this work was based on Finite Size Lyapunov Exponent, (FSLE) whose output is a plot of scalar distributions. FSLE can only provide LCS for sharp enough ridges of nearly constant height. In the other hand, the method we use here (variational theory of hyperbolic LCS) yields analytical solutions of LCS as material lines that extremize the normal repulsion/attraction overall nearby material lines

Robust Detection of the North-West African Upwelling From SST Images

International audienceAnalysis and study of coastal upwelling using sea surface temperature (SST) satellite images is a common procedure because of its coast effectiveness (economic, time, frequency, and manpower). Developing on the Ekman theory, we propose a robust method to identify the upwelling regions along the northwest African margin. The proposed method comes to overcome the issues encountered in a recent method devoted for the same purpose and for the same upwelling system. Afterward, we show how our method can serve as a framework to study and monitor the spatio-temporal variability of the upwelling phenomenon in the studied region

Surface mixing and biological activity in the North-West African upwelling

International audienceNear-shore water along the NorthWest African margin is one of the world's major upwelling regions. It is associated with physical structures of oceanic fronts which influence the biological productivity. The study of these coherent structures in connection with chlorophyll concentration data is of fundamental importance for understanding the spatial distributions of the plankton. In this work, we study the horizontal stirring and mixing in different upwelling areas using Lagrangian coherent structures (LCSs). These LCSs are calculated using the recent geodesic theory of LCSs. We use these LCSs to study the link between the chlorophyll fronts concentrations and surface mixing, based on 10 years of satellite data. These LCSs move with the flow as material lines, thus the horizontal mixing is calculated from the intersection of these LCSs with the finite time Lyapunov exponents (FTLEs) maps. We compare our results with those of a recent study conducted over the same area, but based on Finite Size Lyapunov Exponents (FSLEs) whose output is a plot of scalar distributions. We discuss the differences between FSLE and geodesic theory of LCS. The latter yields analytical solutions of LCSs, while FSLEs can only provide LCSs for sharp enough ridges of nearly constant height