Spontaneous inertia-gravity-wave generation by surface-intensified turbulence

Abstract

The spontaneous generation of inertia-gravity waves (IGWs) by surface-intensified, nearly balanced motion is examined using a high-resolution simulation of the primitive equations in an idealised oceanic configuration. At large-and meso-scales, the dynamics, which is driven by baroclinic instability near the surface, is balanced and qualitatively well described by the surface quasi-geostrophic model. This however predicts an increase of the Rossby number with decreasing spatial scales, and hence a breakdown of balance at small scale; the generation of IGWs is a consequence of this breakdown.The wave field is analysed away from the surface, at depths where the associated vertical velocities are of the same order as those associated with the balanced motion. Quasi-geostrophic relations, the omega equation in particular, prove sufficient to separate the IGWs from the balanced contribution to the motion. A spectral analysis indicates that the wave energy is localised around dispersion relation for free IGWs, and decays only slowly as the frequency and horizontal wavenumber increase. The IGW generation is highly intermittent in time and space: localised wavepackets are emitted when thin filaments in the surface density are formed by straining, leading to large vertical vorticity and correspondingly large Rossby numbers. At depth, the IGW field is the result of a number of generation events; away from the generation sites it takes the form of a relatively homogeneous, apparently random wave field. The energy of the IGW field generated spontaneously is estimated and found to be several orders of magnitude smaller than the typical IGW energy in the ocean.

Drifter observations of submesoscale flow kinematics in the coastal ocean

“…The distribution of ζ is expected to be asymmetric and skewed toward positive values when Ro = O(1) (or larger [e.g., Hoskins and Bretherton, 1972;Rudnick, 2001]). The positively skewed asymmetry is found in highresolution numerical simulations whenever submesoscale flows are not overshadowed by mesoscale eddies [Capet et al, 2008a;Gula et al, 2015]. Numerical studies suggest that submesoscale dynamics will ultimately dominate when scales become sufficiently small.…”

“…Ocean submesoscale turbulence is ubiquitous at length scales ranging from 10 m to 10 km and plays a critical role in surface layer mixing [Capet et al, 2008a;McWilliams, 2016]. Submesoscale flows are characterized by large Rossby number, Ro = ζ /f, where ζ is the relative vertical vorticity and f is the local planetary vorticity.…”

Gravity wave emission in an atmosphere-like configuration of the differentially heated rotating annulus experiment

“…In the case of a quasi-geostrophic flow, the balanced part of δ can be obtained from the omega equation for the quasi-geostrophic vertical velocity and equation (3.6d) (e.g. Zhang et al 2000;Viúdez & Dritschel 2006;Plougonven, Snyder & Zhang 2009;Danioux et al 2012). Subtracting the balanced horizontal divergence from the total would highlight the gravity-wave signal especially clearly.…”

Generation and backreaction of spontaneously emitted inertia‐gravity waves

“…Applied to the interior, geostrophic eddy field, this yields a flux of energy of about 0.3 TW. Three limitations need to be emphasized: first, more complex flows may generate stronger gravity waves, although there is fragmentary evidence that such increase would not be very large [ Afanasyev , ; Danioux et al , ]. Second, the estimate above is based on the emission found for strong dipoles (decaying jets), with Ro = 0.3, for fixed diffusion and stratification.…”