Published online by Cambridge University Press: 11 October 2012
We investigate the spectral properties of the turbulence generated during the nonlinear evolution of a Lamb–Chaplygin dipole in a stratified fluid for a high Reynolds number $Re= 28\hspace{0.167em} 000$ and a wide range of horizontal Froude number
${F}_{h} \in [0. 0225~0. 135] $ and buoyancy Reynolds number
$\mathscr{R}= Re{{F}_{h} }^{2} \in [14~510] $. The numerical simulations use a weak hyperviscosity and are therefore almost direct numerical simulations (DNS). After the nonlinear development of the zigzag instability, both shear and gravitational instabilities develop and lead to a transition to small scales. A spectral analysis shows that this transition is dominated by two kinds of transfer: first, the shear instability induces a direct non-local transfer toward horizontal wavelengths of the order of the buoyancy scale
${L}_{b} = U/ N$, where
$U$ is the characteristic horizontal velocity of the dipole and
$N$ the Brunt–Väisälä frequency; second, the destabilization of the Kelvin–Helmholtz billows and the gravitational instability lead to small-scale weakly stratified turbulence. The horizontal spectrum of kinetic energy exhibits a
${{\varepsilon }_{K} }^{2/ 3} { k}_{h}^{\ensuremath{-} 5/ 3} $ power law (where
${k}_{h} $ is the horizontal wavenumber and
${\varepsilon }_{K} $ is the dissipation rate of kinetic energy) from
${k}_{b} = 2\lrm{\pi} / {L}_{b} $ to the dissipative scales, with an energy deficit between the integral scale and
${k}_{b} $ and an excess around
${k}_{b} $. The vertical spectrum of kinetic energy can be expressed as
$E({k}_{z} )= {C}_{N} {N}^{2} { k}_{z}^{\ensuremath{-} 3} + C{{\varepsilon }_{K} }^{2/ 3} { k}_{z}^{\ensuremath{-} 5/ 3} $ where
${C}_{N} $ and
$C$ are two constants of order unity and
${k}_{z} $ is the vertical wavenumber. It is therefore very steep near the buoyancy scale with an
${N}^{2} { k}_{z}^{\ensuremath{-} 3} $ shape and approaches the
${{\varepsilon }_{K} }^{2/ 3} { k}_{z}^{\ensuremath{-} 5/ 3} $ spectrum for
${k}_{z} \gt {k}_{o} $,
${k}_{o} $ being the Ozmidov wavenumber, which is the cross-over between the two scaling laws. A decomposition of the vertical spectra depending on the horizontal wavenumber value shows that the
${N}^{2} { k}_{z}^{\ensuremath{-} 3} $ spectrum is associated with large horizontal scales
$\vert {\mathbi{k}}_{h} \vert \lt {k}_{b} $ and the
${{\varepsilon }_{K} }^{2/ 3} { k}_{z}^{\ensuremath{-} 5/ 3} $ spectrum with the scales
$\vert {\mathbi{k}}_{h} \vert \gt {k}_{b} $.