The Effect of Moderate Rotation on Stratified Turbulence

Abstract:

We report results on rotating strafied turbulence in the absence of forcing, with large-scale
isotropic initial conditions, using direct numerical simulations computed on grids of up to 4096^3
points. The Reynolds and Froude numbers are respectively equal to Re = 5:510^4 and Fr = 0:02.
The ratio of the Brunt-Vaisala to the inertial wave frequency, N=f, is taken to be equal to 5, a
choice appropriate to apply to the dynamics of the southern abyssal ocean at mid latitudes. This
gives a global buoyancy Reynolds number RB = ReF r2 = 32, a value sufficient for some isotropy
to be recovered in the small scales beyond the Ozmidov scale, but still moderate enough that the
intermediate range where waves are prevalent is well resolved. We concentrate on the large-scale
dynamics and confirm that the Froude number based on a typical vertical length scale is of order
unity, with strong gradients in the vertical. Two characteristic scales emerge from this computation,
and are identied from sharp variations in the spectral distribution of total energy and helicity. A
spectral break is also observed at a scale at which the partition of energy between the kinetic and
potential modes changes abruptly and beyond which a Kolmogorov-like spectrum recovers. Large
slanted layers are ubiquitous in the ow in the velocity and temperatureelds, and a large-scale
enhancement of energy is also observed, directly attributable to the effect of rotation.