Mesoscale to Submesoscale Transition in the California Current System. Part II: Frontal Processes

Abstract

This is the second of three papers investigating the regime transition that occurs in numerical simulations for an idealized, equilibrium, subtropical, eastern boundary, upwelling current system similar to the California Current. The emergent upper-ocean submesoscale fronts are analyzed from phenomenological and dynamical perspectives, using a combination of composite averaging and separation of distinctive subregions of the flow. The initiating dynamical process for the transition is near-surface frontogenesis. The frontal behavior is similar to both observed meteorological surface fronts and solutions of the approximate dynamical model called surface dynamics (i.e., uniform interior potential vorticity q and diagnostic force balance) in the intensification of surface density gradients and secondary circulations in response to a mesoscale strain field. However, there are significant behavioral differences compared to the surfacedynamics model. Wind stress acts on fronts through nonlinear Ekman transport and creation and destruction of potential vorticity. The strain-induced frontogenesis is disrupted by vigorous submesoscale frontal instabilities that in turn lead to secondary frontogenesis events, submesoscale vortices, and excitation of even smaller-scale flows. Intermittent, submesoscale breakdown of geostrophic and gradient-wind force balance occurs during the intense frontogenesis and frontal-instability events.

Formation of intrathermocline eddies at ocean fronts by wind-driven destruction of potential vorticity

“…Calculations of the energetics of the flow (not shown) reveal that the dominant source of energy for the instabilities is the release of available potential energy from the mean flow. The meandering motions can hence be classified as a form of baroclinic instability, similar to the submesoscale mixed layer instabilities described in Boccaletti et al (2007) and Capet et al (2008). In contrast to the front with the eastward frontal jet, the front that was initially at y = 0, 200 km, remains two-dimensional throughout the experiment.…”

Submesoscale turbulence in the upper ocean

“…Primitive equation and Boussinesq simulations can also address how non-QG effects modify mesoscale-driven surface frontogenesis and allow an estimate of the importance of corrections to surface QG turbulence (cf. Hakim et al, 2002;Capet et al, 2008b;Klein et al, 2008;Roullet et al, 2012;Badin, 2012), providing another stepping stone for understanding observations.…”

Energy pathways and structures of oceanic eddies from the ECCO2 state estimate and simplified models

“…Fu et al, 2010), and investigating mechanisms for the damping of mesoscale eddies, such as radiation of Rossby waves, frontogenesis and lee internal waves (e.g. Flierl, 1984;Capet et al, 2008;Nikurashin and Ferrari, 2010).…”