Vertical fluxes conditioned on vorticity and strain reveal submesoscale ventilation

Abstract

It has been hypothesized that submesoscale flows play an important role in the vertical transport of climatically important tracers, due to their strong associated vertical velocities. However, the multi-scale, non-linear, and Lagrangian nature of transport makes it challenging to attribute proportions of the tracer fluxes to certain processes, scales, regions, or features. Here we show that criteria based on the surface vorticity and strain joint probability distribution function (JPDF) effectively decomposes the surface velocity field into distinguishable flow regions, and different flow features, like fronts or eddies, are contained in different flow regions. The JPDF has a distinct shape and approximately parses the flow into different scales, as stronger velocity gradients are usually associated with smaller scales. Conditioning the vertical tracer transport on the vorticity-strain JPDF can therefore help to attribute the transport to different types of flows and scales. Applied to a set of idealized Antarctic Circumpolar Current simulations that vary only in horizontal resolution, this diagnostic approach demonstrates that small-scale strain dominated regions that are generally associated with submesoscale fronts, despite their minuscule spatial footprint, play an outsized role in exchanging tracers across the mixed layer base and are an important contributor to the large-scale tracer budgets. Resolving these flows not only adds extra flux at the small scales, but also enhances the flux due to the larger-scale flows.

Effects of Mesoscale Dynamics on the Path of Fast‐Sinking Particles to the Deep Ocean: A Modeling Study

“…Below 500 m, vertical velocities are negative close to the line σ = ζ, corresponding to the cyclonic side of fronts, and positive on the anticyclonic side (Figure 8h). Such a pattern is expected from frontal dynamics, and again in accord with results shown in Balwada et al (2021) below the mixed-layer. However, between 200 and 500 m, only upwelling is visible in the SD region on both sides, apparently contradicting classical expectations.…”

“…Vertical velocity patterns are qualitatively similar between the Eulerian and Lagrangian diagnostics. At all depths, the AVD region is featured with downwelling, whereas the CVD region is dominated by upwelling, in accord with the picture of the vertical velocity below the mixed layer in Balwada et al (2021). In the SD region, vertical velocity patterns change with depth.…”

Oceanic Mesoscale Cyclones Cluster Surface Lagrangian Material

“…We carried out several diagnostics to confirm those visual impressions. Inspired by Balwada et al (2021), we examine the mean particle concentration as a function of vertical vorticity (ζ = ∂ x v − ∂ y u, with (u, v) the horizontal velocity in the (x, y), that is, zonal and meridional, coordinate system) and strain ( 4b strongly supports the first observation. Indeed, particle concentration is rather homogeneous (≈1-2 × 10 −8 m −2 ) in most of the vorticity-strain domain but a clear two-to-three times increase in concentration (up to 6 × 10 −8 m −2 ) is found approaching and past the σ = ζ line on the cyclonic side (ζ > 0).…”

“…At first sight, the clustering asymmetry is unanticipated. Indeed, while in submesoscale-dominated regimes, clustering of material in cyclonic regions is documented and rationalized (Balwada et al, 2021;Berta et al, 2020;D'Asaro et al, 2018;Esposito et al, 2021), such an asymmetry is unexpected at the mesoscale − see the introductory review of literature in Section 1. Our model resolution formally allows an accurate representation of mesoscales but hardly permits submesoscales.…”

Observed Equatorward Propagation and Chimney Effect of Near‐Inertial Waves in the Midlatitude Ocean

“…Apart from the background vorticity field, mesoscale straining processes may also affect the vertical propagation of NIWs Bühler & McIntyre, 2005;Noh & Nam, 2021). To further link the chimney effect to ambient flow properties, it can be instructive to apply the vorticity-strain parameter space (Balwada et al, 2021), which allows us to distinguish between strain-dominated regions (SD; i.e., fronts), anticyclonic vorticity-dominated regions (AVD; i.e., anticyclonic eddies) and cyclonic vorticity-dominated regions (CVD; 4a and 4b). The strain rate is defined as…”