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.

Ocean eddies play an important role in the distribution of heat, salt, and other tracers in the global ocean. But while surface eddies have been studied extensively, deeper eddies are less well understood. Here we study deep coherent vortices (DCVs) in the Northeast Atlantic Ocean using a high resolution numerical simulation. We perform a census of the DCVs on the $27.60$ kg/m$^3$ isopycnal, at the depth of $700-1500$ m, where DCVs of Mediterranean water (meddies) propagate. We detect a large number of DCVs, with maxima around continental shelves, and islands, dominated by small and short-lived cyclones. However, the large and long-lived DCVs are mostly anticyclonic. Among the long-lived DCVs, anticyclonic meddies, stand out. They grow in size by merging with other anticyclonic meddies. Cyclonic meddies are also regularly formed, but most of them are destroyed near their formation sites due to the presence of the energetic anticyclonic meddies, which destroy cyclones by straining and wrapping the positive vorticity around their core. During their life cycle, as they propagate to the southwest, anticyclonic meddies can interact with other DCVs, including anticyclones containing Antarctic Intermediate Water generated near the Moroccan coast, Canary anticyclonic DCVs and cyclonic DCVs generated south of $30^\circ$N along the African continental shelf. With these latter, they can form dipoles, and with the former, they co-rotate pro tempore. Thus, a more detailed view of the life cycle of anticyclonic meddies is proposed: they grow by merging, undergo multiple interactions along their path, and they decay at low latitudes.

Seasonal evolution of both surface signature and subsurface structure of a Mediterranean mesoscale anticyclones is assessed using the CROCO high-resolution numerical model with realistic background stratification and fluxes. In good agreement with remote-sensing and in-situ observations, our numerical simulations capture the seasonal cycle of the anomalies induced by the anticyclone, both in the sea surface temperature (SST) and in the mixed layer depth (MLD). The eddy signature on the SST shifts from warm-core in winter to cold-core in summer, while the MLD deepens significantly in the core of the anticyclone in late winter. Our sensitivity analysis shows that the eddy SST anomaly can be accurately reproduced only if the vertical resolution is high enough (~4m in near surface) and if the atmospheric forcing contains high-frequency. In summer with this configuration, the vertical mixing parameterized by the k-epsilon closure scheme is three times higher inside the eddy than outside the eddy, and leads to an anticyclonic cold core SST anomaly. This differential mixing is explained by near-inertial waves, triggered by the high-frequency atmospheric forcing. Near-inertial waves propagate more energy inside the eddy because of the lower effective Coriolis parameter in the anticyclone core. On the other hand, eddy MLD anomaly appears more sensitive to horizontal resolution, and requires SST retroaction on air-sea fluxes. These results detail the need of high frequency forcing, high vertical and horizontal resolutions to accurately reproduce the evolution of a mesoscale eddy.

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 analyse the material coherence of oceanic eddies sampled by ships during 9 oceanographic campaigns, 8 of which were conducted in the Atlantic Ocean (EUREC4A-OA, M124, MSM60, MSM74, M160, HM2016611, KB2017606, KB2017618) and one in the Indian Ocean (Physindien 2011). After reviewing previous definitions of coherence, we perform a relative error analysis of our data. To identify the eddy cores and assess the material coherence of the well-sampled eddies (19 out of 28 eddies in total), we use criteria based on active tracers (potential vorticity, temperature, salinity). The maximum tracer anomaly is often below the pycnocline (below the frequency stratification maximum). Therefore, some eddies are not considered to be materially coherent using only surface data, whereas they are when we study their three-dimensional structure. Two methods are then presented to extrapolate eddy volumes from a single ship section. The horizontal and vertical resolutions of the data are critical for this determination. Our results show that the outermost closed contour of the Brunt-Vaisala frequency is a good approximation for the materially coherent eddy core to determine the eddy volume.

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.

In this study, we revisit the role of curvature in modifying frontal stability. We first consider the statement “fq < 0 implies potential for instability”, where f is the Coriolis parameter and q is the Ertel potential vorticity (PV). This is true for any inviscid baroclinic flow. It is also evident in the transition of a governing equation for circulation within a front from elliptic to hyperbolic form as the discriminant changes sign. However, for curved fronts, an additional scale factor enters the discriminant owing to conservation of absolute angular momentum, L, leading to Solberg’s (1936) generalization of the Rayleigh criterion. In non-dimensional form, this expression also generalizes the classical instability criterion of Hoskins (1974) by accounting for centrifugal forces: modification of the front’s vertical shear and stratification owing to curvature tilts the absolute vorticity vector away from its thermal wind state and, in an effort to conserve the product of non-dimensional PV (q’) and absolute angular momentum (L’), this alters Rossby and Richardson numbers permitted for stable flow. The criterion, Φ’=L’q’ < 0, is then investigated in non-dimensional parameter space representative of low-Richardson-number vortices. An interesting outcome is that, for Richardson numbers near one, anticyclonic flows increase in q’, while cyclonic flows decrease in q’. Though stabilization is muted for anticyclones (owing to multiplication by L’), the de-stabilization of cyclones is robust, and may help to explain an observed asymmetry in the distribution of submesoscale coherent vortices in the global ocean.

Mesoscale eddies are ubiquitous in the ocean, and typically exhibit different characteristics to their surroundings, allowing them to transport properties such as heat, salt and carbon around the ocean. This takes place everywhere in the world’s ocean and at all latitude bands. Most of mesoscale eddies energy is generated by instabilities of the mean flow, and by air-sea interactions. Mesoscale dynamics can feed energy and momentum back into the mean flow and help drive the deep ocean circulation. Their suspected importance in transporting and mixing water properties as they propagate in the ocean, play a significant role in the global budgets of these tracers and climate. Increasing evidences point out at intense air-sea interaction at smaller scale than synoptical, especially in the extratropics that can strongly affect the Troposphere. However we do not have yet neither a global quantitative assessments nor a theoretical understanding of these processes. We will present new results from a recently developed eddy-atlas (ToEddies) that includes eddies merging and splitting. In particular, we will discuss properties of Agulhas Rings in the South Atlantic derived from satellite altimetry and the colocalization of these eddies with Argo floats. Our results show that these eddies are, in the South Atlantic, associated with strong thermal and haline anomalies. These are essentially due to Mode Waters (Agulhas Rings Mode Water: ARMW) formed in the core of the rings in the southeastern Cape Basin, just west of the Agulhas Retroflection, after intense air-sea interactions that can last for more than an entire season. These eddies are then advected in the South Atlantic and are responsible of an important flux of heat and salt into this basin (Laxenaire et al. 2018a,b). We corroborate such findings with full depth hydrography of selected eddies and very high-resolution modelling studies.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic — eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, \EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that \EURECA explored — from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation — are presented along with an overview \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.