Defining Lagrangian coherent vortices from their trajectories

We study the transport properties of mesoscale eddies (i.e., vortices of 100–200 km in diameter) over a finite time duration. While these oceanic structures are well-known to stir and mix surrounding water, they can also carry and transport water properties in a coherent manner. In this paper, we are interested in dynamic transport properties of these coherent structures, despite their chaotic environment. Here, we reveal that such vortices can be identified based a simple decomposition of their Lagrangian trajectories. We identify and extract coherent vortices as material lines along which particles’ trajectories share similar polar rotations. The proposed method identifies coherent vortices and their centers in an automatic manner. We illustrate our new method by identifying and extracting Lagrangian coherent vortices in different two-dimensional flows.