Michael Maalouly (Unité de Mécanique de Lille J. Boussinesq, Université de Lille , France); Apolline Dekens (Unité de Mécanique de Lille J. Boussinesq, Université de Lille , France); Guillaume Lapeyre (LMD/IPSL, CNRS, Ecole Normale Supérieure, PSL Research University, Paris, France); Aurélien L. S. Ponte (Ifremer, Université de Brest, CNRS, IRD, Laboratoire D’Océanographie Physique et Spatiale, IUEM, Brest, France)

Ocean flows at scales smaller than few hundreds of kilometers display rich dynamics, mainly associated with nearly-balanced, meso and submesoscale turbulence, and internal gravity waves. Although these processes may act on comparable lengthscales, the former 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, such as those from SWOT, 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 resolving both submesoscale and high-frequency motions, such as internal gravity waves. In particular, we aim at assessing the sensitivity of the particle relative-dispersion process on the latter, fast and strongly ageostrophic fluid motions. For this purpose we select a study area close to Kuroshio Extension, in which the relative importance of internal waves and balanced motions varies in summer and winter, 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. In this case, the behavior of the different Lagrangian indicators considered is in overall agreement with their predictions based on the shape of the kinetic energy spectrum, as in quasi-geostrophic turbulence. 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. Our results then also suggest that high-resolution data from SWOT should allow predicting Lagrangian transport and dispersion properties via the advection of synthetic drifters, provided the satellite-derived, nearly- balanced flow component is sufficiently accurate.