Dispersion characteristics in the East Australian Current system: Insights from 20 years of Lagrangian drifter data

Abstract

Dispersion is a fundamental physical mechanism that shapes marine ecosystems through transporting and mixing heat, salt, and nutrients, as well as affecting the distribution and connectivity of organisms. Western Boundary Currents (WBC) are key to driving regional heat circulation, have immense economic and social value, and also have a significant impact on dispersion. Despite its importance, the understanding of dispersion in the East Australian Current (EAC) system, a dynamic WBC that is experiencing rapid changes, remains limited. Using 20 years of data collected as part of the Global Drifter Program, we conduct an in-depth analysis of dispersion characteristics in the EAC system. We show that both single (absolute) and pairwise (relative) dispersion metrics closely align with theoretical quasi- or surface quasi-geostrophic regimes based on both surface (undrogued drifter) and near surface ($\sim 15$ m; drogued drifter) observations, but absolute dispersion can be super-diffusive after the integral time scale, especially at the surface where wind and wave energy may play a role. Diffusivity is spatially heterogeneous, generally being higher where the current has a strong influence, but particularly so where the current separates into a vast eddy field in the Tasman Sea. Strengthening of the EAC jet and the eddy kinetic energy at its separation during austral summer can lead to an approximate doubling of the eddy diffusivity at 33° S and prolonged Richardson pairwise dispersion. This important baseline study demonstrates the influence that EAC dynamics can have on dispersion characteristics, and emphasizes the possible need for better model parameterizations that can account for anisotropic or spatially inhomogeneous dispersion prominent in dynamic WBC regions.