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

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.

The Northwest Tropical Atlantic (NWTA) is a region with complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8ºN that leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km2. These two processes affect the spatial variability of the region’s surface latent heat flux (LHF). First, the presence of surface freshwater modifies the vertical stratification of the ocean limiting the amount of heat that can be released to the atmosphere. Second, they create a highly heterogeneous mesoscale sea-surface temperature (SST) field that directly influences near-surface atmospheric circulation. These effects are illustrated byd from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC4A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m2, of which 100 W m2 are associated with wind speed changes and 40 W m2 with SST variations. Surface currents or stratification-change associated heat release remain as second-order contributions with LHF variations of less than 10 W m2 each. Although this study is limited by the paucity of collocated observations, it highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales.

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 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.

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.