The coherence of mesoscale eddies and its implication to characterize eddies structures

– Yan Barabinot, Doctorant, LMD –

Résumé :

Mesoscale eddies are ubiquitous features of the global ocean. They are generally referred to as “coherent” structures because they are organized, 
rotating fluid elements that propagate within the ocean and have long lifetimes (months or even years). To assess this definition, 
that we refer to as the kinematic coherence, studies often rely on Eulerian criteria to characterize mesoscale eddies. 
On the other hand, according to the Lagrangian theory, these eddies often transport a distinct water mass within their cores, 
making them materially coherent as long as trajectories remain closed. Despite the rigorous mathematical formalism of the material coherence, 
this definition cannot be assessed by in situ data as it required temporal series. In the literature, both definitions co-exist and are sometimes confused, 
which is not without consequence, as it has a direct impact on transport estimates by these structures. In this context, 
I rely on 25 mesoscale eddies sampled during oceanographic cruises to determine if a distinct water mass exists in eddy cores, 
thereby verifying their material coherence using in situ data, despite the lack of temporal continuity. 
We introduce the term thermohaline coherence to describe this approach. 
Identifying such a water mass would signal Lagrangian transport from the eddy formation region. 
This new definition aims to reconcile the kinematic coherence and the material coherence. 
Once this definition has been proposed, it helps to estimate eddies volume using in situ data or analyze the position of the coherent cores with respect to the sea surface. 
The first main result is that surface data alone are insufficient to characterize the eddy material coherence as the coherent core often lies below the mixed layer depth even for surface eddies. 
The second main result is that the Ertel Potential Vorticity (EPV) appears as a suitable parameter to isolate and characterize the eddy cores and their boundaries. 
The latter appear as regions of finite horizontal extent, characterized by a local extremum of the vertical and horizontal components of the EPV. 
These are found to be closely related to the presence of a different water mass in the core (relative to the background) and the steepening of the isopycnals
due to eddy occurrence and dynamics.


Lien Zoom :

https://cnrs.zoom.us/j/95829059783?pwd=U1R0aVAxY2psRXV0MjBLbGRaZGxRdz09