Oceanic Restratification Forced by Surface Frontogenesis

Abstract

Potential vorticity (PV) conservation implies a strong constraint on the time evolution of the mean density at a given depth. The authors show that, on an f plane and in the absence of sources and sinks of PV, it only depends on two terms, namely, the time evolution of the product between density anomaly and relative vorticity and the vertical PV flux. This primitive equation result, which applies at any depth, suggests that the ageostrophic dynamics induced by baroclinic eddies strongly affect the mean oceanic stratification profile. This result is illustrated for two simple initial-value simulations of a baroclinic, balanced jet. For initial situations propitious to surface frontogenesis, the simulations show a restratification over the whole water column characterized by the amplification in time of the Brunt-Väisälä frequency in the upper oceanic layers. In the absence of surface frontogenesis, such as when the jet is initialized at the middepth of the water column, the restratification is much weaker and slower. Because both simulations have similar kinetic energy and growth rate of baroclinic instability, the results clearly reveal that the restratification is driven by surface frontogenesis in the first case and by vertical PV flux in the interior in the second case. The authors also point out that the dynamics of the interior PV is tightly related to the surface dynamics because of total mass conservation.

Surface kinetic energy transfer in surface quasi-geostrophic flows

“…This term should be positive, in particular in the highwavenumber region, where density gradients are the strongest (since the density gradient spectrum has a k 1/3 slope). Equation (4.6) bears some similarity with a relation found by Lapeyre, Klein & Hua (2006) that couples frontogenesis and restratification (through w z ρ) and redistributes density vertically. Equation (4.7) further indicates that Π a is related to the release of available PE by the ageostrophic circulation associated with frontogenesis.…”

Friction, Frontogenesis, and the Stratification of the Surface Mixed Layer

“…Hence, potential vorticity is a more natural variable than stratification to describe the dynamics of the mixed layer. Lapeyre et al (2006) show that restratification at an outcropping ocean front can also be conveniently described in terms of potential vorticity. They consider an idealized front with no-flux and free-slip surface boundary conditions to minimize nonconservative processes.…”

“…Traditional models assume that the vertical structure of the mixed layer is set by the vertical mixing of buoyancy and momentum driven by atmospheric surface fluxes and stresses. Lapeyre et al (2006) and Fox-Kemper et al (2008) have recently pointed out that during times of weak air-sea fluxes, lateral dynamics become leading order and tend to restratify the mixed layer through ageostrophic slumping of lateral buoyancy gradients. The goal of this paper is to extend recent work of Thomas (2005) and show that frictional forces acting on buoyancy fronts can also modify the stratification of the mixed layer.…”

Satellite Altimetry and Key Observations: What We've Learned, and What's Possible with New Technologies

“…Traditional altimeters revealed the fundamental role of mesoscale eddies in the horizontal transport of tracers. Recent theoretical work suggests that submesoscale motions play a leading role in the vertical transport [74], [75], [70] and [76]. Vertical velocities in the ocean require higher spatial resolution because they result from convergences and divergences in the horizontal velocity field.…”

“…Vertical velocities associated with divergences and convergences of geostrophically balanced velocities on 10 km scale penetrate down to a few hundred meters below the ocean surface [74]. Hence, the resolution of delay-Doppler altimeters and SWOT will allow one to compute the exchange of properties between the ocean surface boundary layers and the interior.…”