Insights into mesoscale eddy dynamics: a three-dimensional perspective on potential density anomalies

Abstract. Mesoscale eddies are fundamental components of the global ocean circulation. In situ observations and Lagrangian analyses have shown that some eddies are materially coherent, transporting within their cores a water mass distinct from the surrounding environment. Additionally, laboratory experiments indicate that eddies locally deflect isopycnal surfaces in accordance with thermal wind balance, regardless of whether they trap a water mass. These two mechanisms drive spiciness mode anomalies and heaving mode anomalies, respectively, associated with mesoscale eddies. In this study, we quantitatively assess the physical processes governing mesoscale eddy dynamics by introducing a novel theoretical decomposition of the potential density field within eddy cores that accounts for both effects. We apply this framework to six anticyclonic eddies sampled during the EUREC4A-OA, METEOR 124, and Physindien 2011 oceanographic campaigns. Unlike previous studies, we evaluate not only the amplitude of these anomalies but also their vertical structure. Our results confirm that heaving mode anomalies dominate the total density anomaly. However, contrary to previous assumptions, we demonstrate that their vertical structure is dictated by the local background stratification and often exhibits a nearly Gaussian profile. In contrast, spiciness anomalies provide only a second-order contribution to the total potential density anomaly, making them negligible for most dynamical processes governing mesoscale eddies. By bridging experimental results with observational eddy datasets, our study refines the understanding of the mesoscale eddy vertical structure, providing a more accurate predictive framework for their shape and role in the transport of oceanic properties.