Seasonal and regional characterization of horizontal mixing in the global ocean

Abstract

Recent work on Lagrangian descriptors has shown that Finite Time Lyapunov Exponents (FTLE) or Finite Size Lyapunov Exponents (FSLE) can be applied to real or simulated data to characterize the mixing properties and transport barriers of the oceanic flow. However, a more detailed analysis on regional and seasonal variability, and its interpretation, was still lacking. In this paper, we have analyzed the near-surface velocity field derived from the Ocean general circulation model For the Earth Simulator (OFES) using Finite-Size Lyapunov Exponents (FSLE). We have characterized regional and seasonal variability by means of some statistical descriptors. Our results show that horizontal mixing, as measured by FSLEs, is seasonally-varying, attaining its maximum in Summer. FSLEs also strongly vary depending on the region: with our descriptors we have first characterized the mixing properties of Northern and Southern Hemispheres, then the main oceanic basins and ocean currents. We have finally studied the relation between Lagrangian FSLEs and some Eulerian descriptors such as Eddy Kinetic Energy (EKE) and vorticity (w) over the different regions. We have found that Lagrangian and Eulerian descriptors are linked by means of dispersion relations. These relations are useful to characterize the dynamics of different ocean areas, and undoubtedly have an influence on the evolution of biological markers, among other variables.