Mixing by low- and high-resolution surface geostrophic currents

The nature of mixing near the ocean's surface is investigated by considering passive tracer advection by currents derived geostrophically using sea surface height fields that differ in their spatiotemporal resolution. One is a low-resolution field that can be thought of as being derived using current-generation groundtrack altimetric measurements. The other is a high-resolution field that may be envisioned as derived using hypothetical next-generation wideswath altimetric measurements. The investigation is carried out in two world ocean regions with quite distinct characteristics and which have been the focus of recent studies on mixing by altimetry-derived currents. One region corresponds to an ample domain in the South Atlantic, which is traversed by vortices shed from the Agulhas current. The other region corresponds to a portion of the Pacific sector of the Southern Ocean, which is dominated by multiple zonal jets associated with the Antarctic Circumpolar Current. It is argued that while altimetry-derived currents capture the most salient characteristics of surface ocean mixing, deterministic Lagrangian calculations may be improved by considering surface geostrophic currents with greater spatiotemporal resolution. This is supported on analysis of probability distribution functions of finite-time Lyapunov exponents (FTLEs); inspection of Lagrangian coherent structures (or LCSs), which constitute distinguished material fluid curves that organize mixing; computation of kinetic energy and FTLE variance spectra; and realization of explicit passive tracer advection experiments.