A document by Yan Barabinot. Click on the document to view its contents.

Mesoscale eddies are found throughout the global ocean. Generally, they are referred to as “coherent” structures because they are organized rotating fluid elements that propagate within the ocean and have a long lifetime. Since in situ observations of the ocean are very rare, eddies have been characterized primarily from satellite observations or by relatively idealized approaches of geophysical fluid dynamics. Satellite observations provide access to only a limited number of surface features and exclusively for structures with a fingerprint on surface properties. Observations of the vertical sections of ocean eddies are rare. Therefore, important eddy properties, such as eddy transports or the characterization of eddy “coherence”, have typically been approximated by simple assumptions or by applying various criteria based on their velocity field or thermohaline properties. In this study, which is based on high-resolution in-situ data collection from the EUREC4A-OA field experiment, we show that Ertel potential vorticity is very appropriate to accurately identify the eddy core and its boundaries. This study provides evidence that the eddy boundaries are relatively intense and intimately related to both the presence of a different water mass in the eddy core from the background and to the isopycnal steepening caused by the volume of the eddy. We also provide a theoretical framework to examine their orders of magnitude and define an upper bound for the proposed definition of the eddy boundary. The results suggest that the eddy boundary is not a well-defined material boundary but rather a frontal region subject to instabilities.

In this paper we analyze the effect of material coherence on the transport properties of 9 eddies sampled during research cruises. We check the accuracy of our data and, after reviewing different definitions of coherence, we assess whether these eddies have retained a heterogeneous water mass in their cores. T, S anomalies on isopycnal surfaces are computed to highlight the different thermohaline properties between the eddy core and its surroundings. The maximum of the tracer anomaly is often located below the pycnocline. We find that while some of these eddies are not coherent according to surface data only, they are when their entire 3D structure is considered. We then present two methods for extrapolating eddy volumes from a single hydrographic section. The volume obtained from T,S anomalies on isopycnical surfaces is compared with that obtained by other criteria. Our results show that the outermost closed contour of the Brunt-Väisäla frequency is a good approximation for the eddy boundary when calculating its volume.

Mesoscale eddies are ubiquitous in the global ocean. Most of them are materially coherent: they advect a different water mass in their core than in the surrounding water, according to studies based on in situ observations and Lagrangian techniques. In parallel, laboratory experiments have shown that eddies have the ability to locally modify the stratification according to the thermal wind balance, without necessarily contain heterogeneous water. These two types of density anomalies associated with mesoscale eddies are often erroneously confused in the literature. Here we propose a new theoretical decomposition of the potential density field in the core of eddies to assess their respective amplitude and dynamical effect. This allows the modelling of their 3D shape and the estimation of the importance of each term. This decomposition is applied to 6 anticyclonic eddies sampled during the EUREC4A-OA, METEOR 124 and PHYSINDIEN 2011 in situ experiments. We show that the anomaly corresponding to the slope of the isopycnals is the largest contributor to the total density anomaly. Its vertical shape is nearly Gaussian, but also depends on the local background stratification. The horizontal density gradient associated with the trapped water mass adds a second order term to the total anomaly and can be neglected for the study of eddy dynamics. The horizontal structures of the eddies studied are consistent with previous studies and show an exponential-alpha shape.

Mesoscale eddies play an important role in transporting water properties, enhancing air-sea interactions, and promoting large-scale mixing of the 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). Eddies have been sampled by sparse in-situ vertical profiles, but because in-situ ocean observations are limited, they have been characterized primarily from satellite observations, numerical simulations, or relatively idealized geophysical fluid dynamics methods. However, each of these approaches has its limitations. Many questions about the general structure and “coherence” of ocean eddies remain unanswered. In this study, we investigate the properties of 7 mesoscale eddies sampled with relative accuracy during 4 different field experiments in the Atlantic. Our results suggest that the Ertel Potential Vorticity (EPV) is 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. Based on these results, we propose a new criterion for defining eddies. We test our approach using a theoretical framework and explore the possible magnitude of this new criterion, including its upper bound.