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 seven mesoscale eddies sampled with relative accuracy during four 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 at the mesoscale. We test our approach using a theoretical framework and explore the possible magnitude of this new criterion, including its upper bound.

Key Points

A new criterion that compare the horizontal and vertical components of Ertel PV is introduced to characterize eddies boundaries

Eddy boundaries behave like a front

The Rossby number and the vertical stratification anomaly drive eddies boundaries intensity

Plain Language Summary

Mesoscale eddies are ubiquitous rotating currents in the ocean. They are considered as one of the most important sources of ocean variability because they can live for months and transport and mix heat, salt, and other properties within and between ocean basins. They have been studied extensively from satellite observations because they are often at or near the ocean surface. However, observations of their 3D structure are rare, and calculations of eddy transport are often approximated without precise knowledge of their true vertical extent. In addition, recent studies suggest the existence of subsurface eddies that are not detectable from satellite observations. Here, we characterize and attempt to generalize 3D eddy properties by analyzing observations collected during specific high-resolution field experiments in the Atlantic Ocean. We also propose a criterion, based on geophysical fluid dynamics theory, that defines the lateral and vertical eddy boundaries. This criterion can be applied broadly to assess eddy structure, volume, transport, and evolution more quantitatively than in previous studies. We also provide insight into why these boundaries are substantial, which may explain why oceanic eddies are coherent structures that can span long distances and have long lifetimes.