Nonlinear surface Ekman effects on cyclonic and anticyclonic vortices

Upper-ocean turbulent flows at horizontal length scales smaller than the deformation radius depart from geostrophic equilibrium and develop important vertical velocities, which are key to marine ecology and climatic processes. Due to their small size and fast temporal evolution, these fine scales are difficult to measure during oceanographic campaigns. Instruments such as Lagrangian drifters have provided another way to characterize these scales through the analysis of pair-dispersion evolution and have pointed out striking particle convergence events. By means of numerical simulations, we investigate such processes in a model of surface-ocean turbulence that includes ageostrophic motions. This model originates from a Rossby-number expansion of the primitive equations and reduces to the surface quasi-geostrophic model, a paradigm of submesoscale dynamics, in the limit of vanishing Rossby number. We focus on the effect of the ageostrophic dynamics on the pair-dispersion and clustering properties of Lagrangian tracer particles at the ocean surface. Our results indicate that while over long times the pair separation process is barely affected by the ageostrophic component of the velocity field, the latter is responsible for the formation of temporary particle aggregates, and the intensity of this phenomenon increases with the Rossby number. We further show that Lagrangian tracers preferentially accumulate in cyclonic frontal regions, which is in agreement with observations and other more realistic modeling studies. These findings appear interesting to improve the understanding of the turbulent transport by ocean fine scales and in light of upcoming, new high-resolution satellite data of surface velocity fields.

Particle dispersion and clustering in surface ocean turbulence with ageostrophic dynamics

Maalouly,

Lapeyre,

Cozian

et al. 2023

Horizontal and vertical Lagrangian dispersion in decaying cyclonic and anticyclonic vortices with lateral and bottom friction

Ocampo Jaimes,

Zavala Sansón,

Graef

2024

The horizontal and vertical dispersion of passive particles in a decaying circular vortex in a rotating system is investigated analytically and numerically. The vortex decay is due to lateral viscosity and bottom friction effects (associated with the Ekman boundary layer). The vortex model comprises the three-dimensional velocity field, where the azimuthal component is much larger than the radial and vertical components, so the structure remains circular. The particles are dispersed by the deterministic velocity field plus stochastic perturbations. The analytical model allows the examination of frictional effects separately. The experiments show that an initial point charge of particles is dispersed around the vortex. The role of lateral viscosity is to delay the angular distribution of the particles. Bottom friction, on the other hand, generates radial motions, thus inducing outward advection in cyclones and inward advection in anticyclones. The intensity of lateral and bottom friction slows down the expulsion or retention of particles. Regarding vertical dispersion, cyclonic vortices can lift particles by a substantial fraction of the fluid column, while anticyclones sink particles as in a bathtub vortex. It is shown that the vertical distributions of the particles are significantly affected by the strength of the decaying mechanisms. Some consequences for the dispersion of tracers in oceanic vortices are discussed.

Horizontal and vertical dispersion in a wind-driven oceanic gyre model

Barrientos Valencia,

Zavala Sansón,

Beron-Vera

2024

This study addresses the horizontal and vertical dispersion of passive tracers in idealized wind-driven subtropical gyres. Synthetic particles within a closed basin are numerically advected to analyze their dispersion under different theoretical velocity fields. Horizontal dispersion simulations incorporate the classic wind-driven Stommel circulation along with (i) surface Ekman drift associated with the Stommel wind field and (ii) inertial effects due to particle size and buoyancy. Results reveal that the Ekman drift inhibits particle dispersion across the entire domain leading to tracer concentration in a quasi-stable distribution skewed toward the western side of the basin. Similar behavior is observed with inertial particles. The equilibrium state is quantified for different diffusivity values, particle sizes, and buoyancies. For vertical dispersion, simulations incorporate the three-dimensional Ekman velocity, which includes a negative vertical component, while ignoring inertial effects. Initially, surface particles accumulate around the gyre center while slowly sinking, but they disperse across the basin once they surpass the Ekman layer and are free from surface effects. Tracers sink more on the western side of the basin, regardless of horizontal diffusivity. On average, ignoring inertial effects, particles sink less with higher diffusivity and more with lower diffusivity, suggesting a potential for high horizontal distribution of sunken tracers in the ocean.