Effects of high-frequency and balanced motions on Lagrangian pair dispersion at the ocean surface

We investigate the properties of relative dispersion of Lagrangian particles in a global-ocean simulation resolving both inertia-gravity waves (IGW) and meso and submesoscale (M/SM) turbulence. More specifically, we test if the dispersion laws depend on the shape of the Eulerian kinetic energy spectrum, as predicted from quasi-geostrophic turbulence theory. To this end, we focus on two areas, in the Kuroshio Extension and in the Gulf Stream, for which the relative importance of IGW compared to M/SM vary in summer and winter. In winter, Lagrangian statistical indicators return a picture in overall agreement with the shape of the kinetic energy spectrum. Conversely, in summer, when submesoscales are less energetic and higher-frequency internal waves gain importance, the expected relations between dispersion properties and spectra do not seem to hold. This apparent discrepancy is explained by decomposing the flow into nearly-balanced motions and internal gravity waves, and showing that the latter dominate the kinetic energy spectrum at small scales. Our results are consistent with the hypothesis that high-frequency IGWs do not impact relative dispersion, which is then controlled by the nearly-balanced, mainly rotational, flow component at larger scales. These results highlight that geostrophic velocities derived from wide-swath altimeters, such as SWOT, may present limits when estimating surface dispersion, and that current measuring satellite missions may provide the complementary information to do so.