Upper ocean turbulence from high 3-D resolution simulations

We reexamine the turbulent properties of a baroclinically unstable oceanic flow, in a large domain (1000km*2000km*4000m), using Primitive-Equation simulations with high resolution (2km and 100 vertical levels). This resolution allows to fully represent the small-scale (or sub-mesoscale) dynamics and its coupling with the mesoscales. We present evidence that turbulence dynamics in the surface layers strongly differs from that in the interior. Surface properties include a noticeable k^{-5/3} velocity spectrum as well as a conspicuous local spectral relationship between surface kinetic energy, Sea Surface Height and density variance over a large range of scales (from 400km to 4km). Dynamics in the surface layers is mostly driven by energetic small-scale frontogenesis. On the other hand the interior dynamics properties are close to the 3-D quasi-geostrophic turbulence involving a k^{-3} velocity spectrum and a steeper spectrum slope for the density. At intermediate depths a smooth transition is observed between surface and interior dynamics. The strong restratification in the surface layers compensated by a destratification at depth suggests a connection, between the surface and interior, induced by the small-scale divergent motions.