Relative dispersion at the surface of the ocean: role of balanced motions and internal waves

Ocean flows at scales smaller than few hundreds of kilometers display rich dynamics, mainly associated with quasi- geostrophic motions and internal gravity waves. Although both of these processes act on comparable lengthscales, the former, which include meso and submesoscale turbulent flows, are considerably slower than the latter, which take part in the ocean fast variability. Understanding how their effects overlap is crucial for several fundamental and applied questions, including the interpretation and exploitation of new, high-resolution satellite altimetry data, and the characterization of material transport at fine scales.

In this study we investigate these points by examining Lagrangian pair-dispersion statistics in a high-resolution global-ocean numerical simulation including high-frequency motions, such as internal gravity waves. In particular, we aim at assessing the sensitivity of the particle relative-dispersion process on ageostrophic, fast fluid motions. For this purpose we select a study area close to Kuroshio Extension, characterized by energetic submesoscales, and focus on the seasonal variability of the Lagrangian dynamics.

We find that in winter pair dispersion is predominantly influenced by meso and submesoscale motions, meaning nearly balanced dynamics. The behavior of the different Lagrangian indicators considered agrees in this case with the theoretical predictions, based on the shape of the kinetic energy spectrum, in quasi-geostrophic turbulent flows. Conversely, in summer, when high-frequency motions gain importance and submesoscales are less energetic, the situation is found to be more subtle, and the usual relations between dispersion properties and spectra do not seem to hold. We explain this apparent inconsistency relying on a decomposition of the flow into nearly-balanced motions and internal gravity waves. Through this approach, we show that while the latter contribute to the kinetic energy spectrum at small scales, they do not impact relative dispersion, which is essentially controlled by the nearly-balanced, mainly rotational, flow component at larger scales.