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

El Aouni, Anass

Vidard, Arthur

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HAL Id: hal-03482822https://hal.inria.fr/hal-03482822Submitted on 16 Dec 2021HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestinée au dépôt et à la diffusion de documentsscientifiques de niveau recherche, publiés ou non,émanant des établissements d’enseignement et derecherche français ou étrangers, des laboratoirespublics ou privés.How well mesoscale eddies are represented in oceanicmodels?Anass El Aouni, Arthur VidardTo cite this version:Anass El Aouni, Arthur Vidard. How well mesoscale eddies are represented in oceanic models?. 2021- ECMWF/OceanPredict workshop on Advances in Ocean Data Assimilation, May 2021, ShinfieldPark, United Kingdom. ￿hal-03482822￿HOW WELL MESOSCALE EDDIES ARE REPRESENTED IN OCEANIC MODELS?{ ANASS EL AOUNI AND ARTHUR VIDARD } UNIV. GRENOBLE ALPES, INRIA, CNRS, GRENOBLE INP, LJK, 38000 GRENOBLE, FRANCEOBJECTIVESThe aim of this work is to assess the impact of data assimilation on thedynamics of mesoscale eddies. We do this based on the use of a recentmathematical method from nonlinear dynamics field that is capable of de-tecting Lagrangian eddies from birth to death. We apply this method ontwo composites time series of two configurations covering the whole globe,with and without assimilation, and carry on various statistical analysis toevaluate the impact of data assimilation on several properties of these La-grangian mesoscale eddies.RESULTS 1Figure 1: Density distribution of Lagrangian mesoscale eddies here they gain their coherency and form solid cores: gf8l (upper), ORAS5 (bottom), from left to right:1975∼1985, 1993∼1999, 2010∼2015.REFERENCES[1] Anass El Aouni. A hybrid identification and tracking of lagrangian mesoscaleeddies. Physics of Fluids, 33(3):036604, 2021.SUMMARYIn the light of evaluating the impact of data assimilation procedure on eddies activity, we made use of a recent method from nonlinear dynamics field, together with two historical time series of ocean current derived from ocean modelwith and without assimilation, then carried on several statistical analysis of various physical properties of Lagrangian eddies over the globe.RESULT 3Figure 2: (Upper panel) monthly eddies activity in oras5 and gf8l. (Upper panel) Meridional climatology of the eddies activity for the three periods of time.MATERIALS & METHODS1 DATATwo sets of velocity field covering three different periods of time are used inthis study, presenting the first level of oceanic current of an oceanic modelwith and without assimilation. The first set consists of the Ocean ReAnal-ysis System 5 (ORAS5) products, developed at ECMWF based on NEMOat 1/4◦ resolution and NEMOVAR-3DFGAT scheme, while the second one(gf8l) represents the same configuration, but without assimilation proce-dure.2 Eddy detection methodThe method we use here relies on the analysis of the Lagrangian geometri-cal properties of velocity fields and the use of a grid density-based cluster-ing algorithm [1]. The method can be summarized in the three followingsteps:• (i) It starts by segmenting Lagrangian trajectories into rota-tional (s(tβ , tα,x0), with sgn(θ(tβ , tα,x0)) such that ∀ θ(t) ∈θ(tβ , tα,x0), θ(t) > 0 or ∀ θ(t) ∈ θ(tβ , tα,x0), θ(t) < 0 ) and non-rotational segments based on the geometrical properties of theirevolving velocities.θ(ti+1, ti,x0) = cos−1 ~v(x0, ti) · ~v(x0, ti+1)|~v(x0, ti)| |~v(x0, ti+1)|, i ∈ [α, β[ (1)• (ii) It maps the initial and final position of the extracted rotationalsegments.{xi(tiα),xi(tiβ )}ni=0={Φtiαt0 (xi(t0)),Φtiβt0 (xi(t0))}ni=0,{tiα}ni=0 ∈ [t0, tf [,{tiβ}ni=0∈]t0, tf ](2)• (iii) It uses a grid density-based clustering algorithm to group theminto different classes characterized by different transport properties,which represent the coherent cores of the eddies.Njεα ({xi(tiα )}ni=1,pj)={pj ,pk∈{xi(tiα )}ni=1:d(pj ,pk)≤ε}Njεβ ({xi(tiβ )}ni=1,pj)={pj ,pk∈{xi(tiβ )}ni=1:d(pj ,pk)≤ε}(3)where Njε is a function that associates to each particle pj a given den-sity based on its neighboring. d(pj , pk) denotes the Euclidean distancebetween particle pj and pk. ε is the minimum Euclidean distance forneighboring. We define a coherent eddy as a time-evolving domain:E =⋃t∈[tα,tβ ]E(t) ⊂ R2 × [tα, tβ ], (4)in a way that E is a set of adjacent particles {xj}mj=0 characterizedby rotational segments that co-exist in the dense regions G(tα) ={Njεα}mj=0 and G(tβ) ={Njεβ}mj=0respectively at tα and tβ :E = G(tα) ∩G(tβ) (5)RESULTS 2Figure.1 shows the statistical distribution of the genesis of Lagrangian ed-dies in both datasets and over different period of time, highlighting bothvariation of eddies activity both between period of times and between con-figurations.Figure.3 shows Hovmoller diagrams at 1◦/30-day spatio-temporal resolu-tion of the eddies intensity, where the main patterns can be synthesizedinto regions of permanent and seasonal eddies activities.Figure 3: A hovmoller plot displaying meridional-temporal variation of eddies ac-tivity in gf8l (upper panel) and oras5 (bottom).Figure.2 shows both the monthly variation of the number of eddies in bothconfigurations and over three different periods of time, characterized bya considerable year-to-year variability and oscillations between periodsstrong and weaker eddies intensities, with clear anomalies such as 1977,1980 and 1995. In the same Figure, we show the meridional climatology ofthe eddies activity, in which we clearly observe the impact of the assimila-tion on the eddies dynamics.<R> <L> <Td>Period gf8l oras5 gf8l oras5 gf8l oras51975∼1985 78 79 123 121 191 1941993∼1999 79 80 120 117 200 2002010∼2015 79 79 121 114 197 201Table 1: Physical properties of Lagrangian eddies: (Td(km): traveled distance; L(day):lifetime; R(km): diameter)Transport (Sv)Period gf8l oras51975∼1985 54.22 57.571993∼1999 54.55 57.422010∼2015 52.90 60.59Table 2: Climatology of coherent water transport (Sv) achieved by Lagrangian ed-dies

How well mesoscale eddies are represented in oceanic models?

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How well mesoscale eddies are represented in oceanic models?

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