Three-Dimensional Structure of Oceanic Mesoscale Eddies

Abstract

Mesoscale eddies contribute to the global oceanic energy budget and cascade; they induce substantial mass and heat transport, thus playing a key role in the global climate system. The three-dimensional (3D) structures of mesoscale eddies should be studied in order to quantify their impact. In this paper, mesoscale eddies are reviewed from the perspective of their horizontal and vertical structures, temporal evolution, and fine structures. The universal 3D structure of mesoscale eddies is revealed via observations, and findings show that their tendency to achieve lowest-energy or minimum-dissipation states shapes their lowest-order coherent structure. Recent efforts also push forward to the higher-order structure of eddies: The understanding of horizontal structures is gradually evolving from symmetric to asymmetric, and the vertical structure based on separable variable assumption and vertical mode decomposition should further take the eddy vertical alignment and tilt into account. The temporal evolution of mesoscale eddies’ 3D structure is in a dynamical balance influenced by multiple competitive factors, such as eddies’ self-sustaining ability, background gradient and deformation, and generation of submesoscale fine structures. The submesoscale processes associated with mesoscale eddies have been intensively studied in recent years, and they should be considered as a fine-scale part of the 3D structure of eddies rather than separate instability processes. This review hopes to provide the readers with an enlightening, yet not thorough, review of the development of mesoscale eddies’ structures and brings up potential topics and unresolved difficulties for further research and development.