[A-OS21] Dynamics of oceanic and atmospheric waves, vortices, and circulations

[AOS21-11] Floating tracer clustering in divergent random flows modulated by an unsteady mesoscale ocean velocity field

*Konstantin Koshel^1,2, Pavel Berloff³, Evgeny Ryzhov^1,3, Dmitry Stepanov¹ (1.V.I. Ilichev Pacific Oceanological Institute, FEB RAS, 2.Institute of Applied Mathematics, FEB RAS, 7, Radio Street, Vladivostok 690022, Russia, 3.Department of Mathematics, Imperial College London, London SW7 2AZ, United Kingdom)

Keywords:Mesoscale, Submesoscale, Tracer, clustering, mixing, advection

Surface evolution and clustering of floating tracer induced by a combination of a regular mesoscale field and kinematic random field mimicking the influence of the submesoscale is considered. The theory of exponential clustering in random velocity fields is applied to characterize possible clustering scenarios in steady and unsteady mesoscale flows, which are an hourly output of an eddy-resolving circulation model for the Japan/East Sea. The mesoscale field abounds in transient eddy-like structures that dominate the dynamical pattern by modulating various currents and their branching. The steady mesoscale component, obtained by time-averaging the hourly output, features coherent closed recirculation zones, where tracer can be entrapped. The submesoscale component is kinematically modeled using a divergent random velocity field with a prescribed correlation radius and variation. The linear superposition of the mesoscale and submesoscale fields sustains tracer clustering, which is the exponential growth of tracer density in patches of vanishing areas. Statistical topography integral characteristics are used to quantify the emerging clustering indicating drastic variations in clustering rates when subject to a steady or unsteady mesoscale component. A steady mesoscale component favors potent exponential clustering, similar to the theoretically tractable case of purely divergent random velocity fields without any constraints imposed by mean or regular flows. An unsteady component, on the other hand, hinders exponential clustering due to intense advection processes, which spread tracer out of enclosed eddying regions at faster rates compared to characteristic clustering ones. The aptitude of the proposed approach to studying tracer evolution at the ocean's surface is assessed arguing that a fine adjustment of the random velocity field parameters may prove useful in obtaining reliable estimates of the submesoscale influence at the ocean's surface.


Results are in Koshel et al. (2019); Stepanov et al. (2020).

Koshel, K.V., Stepanov, D.V., Ryzhov, E.A., Berloff, P. and Klyatskin, V.I., Clustering of floating tracers in weakly divergent velocity fields. Phys. Rev. E, 2019, 100, 063108.

Stepanov, D. V., Ryzhov, E. A., Zagumennov, A. A., Berloff, P., & Koshel, K. V. (2020). Clustering of floating tracer due to mesoscale vortex and submesoscale fields. Geophysical Research Letters, 48, e2019GL086504. https://doi.org/10.1029/2019GL086504

Figure. Example of tracer density evolution (density is color-coded) for the cyclonic eddy region for the unsteady regular velocity. The submesoscale influence changes according to EXP1-4.

EXP1 - no small scale velocity; EXP2 – nondivergent random velocity; EXP3 – divergent random velocity; EXP4 – combined divergent and solenoidal random velocity components. Upper panel: t = 2x10⁴; bottom panel: t = 4x10⁴.