Floating tracer clustering in divergent random flows modulated by an unsteady mesoscale ocean field

Abstract

Clustering of tracers floating on the ocean surface and evolving due to combined velocity fields consisting of a deterministic mesoscale component and a kinematic random component is analysed. The random component represents the influence of submesoscale motions. A theory of exponential clustering in random velocity fields is applied to characterise the obtained clustering scenarios in both steady and unsteady timedependent mesoscale flows, as simulated by a comprehensive realistic, eddy-resolving, general circulation model for the Japan/East Sea. The mesoscale flow field abounds in transient eddy-like patterns modulating and branching the main currents, and the underlying time-mean flow component features closed recirculation zones that can entrap the tracer. The submesoscale flow component is modelled kinematically, as a divergent random velocity field with a prescribed correlation radius and variance. The combined flow induces tracer clustering, that is, the exponential growth of tracer density in patches with vanishing areas. The statistical topography methodology, which provides integral characteristics to quantify the emerging clusters, uncovers drastic dependence of the clustering rates on whether the mesoscale flow component is taken to be steady or time-dependent. The former situation favours robust exponential clustering, similar to the theoretically understood case of purely divergent and zero-mean random velocity. The latter situation, on the contrary, hinders exponential clustering due to significant advection of the tracer out of the nearly enclosed eddies, at the rate faster than the clustering rate.

Mesoscale Dynamics and Eddy Heat Transport in the Japan/East Sea from 1990 to 2010: A Model-Based Analysis

“…Since the JES circulation is derived by the water exchange through the JES Straits [22], open boundary conditions must be formulated in these simulations. In [41,42], the modified INMOM configuration was used, where this water exchange was naturally formed.…”

On Clustering of Floating Tracers in Random Velocity Fields

“…In the real ocean, potential sources of the weakly divergent flow include the ageostrophic or submesoscale velocities (Haza et al., 2016; Stepanov, Ryzhov, Berloff, & Koshel, 2020; Stepanov, Ryzhov, Zagumennov, et al., 2020). Observations of strong subduction at submesoscale fronts show the potential for strong surface convergence (Archer et al., 2020).…”

“…This spectrum can be chosen to mimic the narrow band, isotropic structure from the classical picture of turbulence first proposed by Kolmogorov (1941). Kinematic fields such as these have often been used as a model for oceanic turbulence (Klyatskin, 2003; Koshel et al., 2019; Stepanov, Ryzhov, Berloff, & Koshel, 2020; Stepanov, Ryzhov, Zagumennov, et al., 2020; Wilson & Sawford, 1996). Traditionally, an assumption of delta‐correlations in time is used to generate a time series of the velocity.…”

“…Theoretical investigations of the clustering process have previously utilized kinematic velocity fields (Klyatskin, 2003; Koshel et al., 2019; Stepanov, Ryzhov, Berloff, & Koshel, 2020; Stepanov, Ryzhov, Zagumennov, et al., 2020; Wilson & Sawford, 1996). Often, Gaussian Random fields are generated with imposed spatial correlations, but delta‐correlated in time.…”

Long-term mean circulation in the Japan Sea as reproduced by multiple eddy-resolving ocean circulation models

“…Lagrangian passive tracer is a useful tool to trace the origin and paths of large ocean current systems, which has been involved into the 3D regional models by Stepanov et al (2020) in the middle Japan Sea and by Prants et al (2022) and Fayman et al (2019) in the Peter the Great Bay. Stepanov et al (2020) explained the clustering phenomena of floating tracer in the middle Japan Sea by identifying a mesoscale ocean current field with various eddy structures. Prants et al (2022) tracked the formation and deep convection of dense shelf water in winter by using Lagrangian maps in the Regional Ocean Modeling System (ROMS).…”