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
 $Re= 28\hspace{0.167em} 000$  and a wide range of horizontal Froude number   ${F}_{h} \in [0. 0225~0. 135] $  and buoyancy Reynolds number
 ${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
 $\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
 ${L}_{b} = U/ N$ , where   $U$  is the characteristic horizontal velocity of the dipole and
 $U$  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
 $N$  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
 ${{\varepsilon }_{K} }^{2/ 3} { k}_{h}^{\ensuremath{-} 5/ 3} $  power law (where   ${k}_{h} $  is the horizontal wavenumber and
 ${k}_{h} $  is the horizontal wavenumber and   ${\varepsilon }_{K} $  is the dissipation rate of kinetic energy) from
 ${\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} = 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} $  and an excess around   ${k}_{b} $ . The vertical spectrum of kinetic energy can be expressed as
 ${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
 $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}_{N} $  and   $C$  are two constants of order unity 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
 ${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
 ${N}^{2} { k}_{z}^{\ensuremath{-} 3} $  shape and approaches the   ${{\varepsilon }_{K} }^{2/ 3} { k}_{z}^{\ensuremath{-} 5/ 3} $  spectrum for
 ${{\varepsilon }_{K} }^{2/ 3} { k}_{z}^{\ensuremath{-} 5/ 3} $  spectrum for   ${k}_{z} \gt {k}_{o} $ ,
 ${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
 ${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
 ${N}^{2} { k}_{z}^{\ensuremath{-} 3} $  spectrum is associated with large horizontal scales   $\vert {\mathbi{k}}_{h} \vert \lt {k}_{b} $  and the
 $\vert {\mathbi{k}}_{h} \vert \lt {k}_{b} $  and the   ${{\varepsilon }_{K} }^{2/ 3} { k}_{z}^{\ensuremath{-} 5/ 3} $  spectrum with the scales
 ${{\varepsilon }_{K} }^{2/ 3} { k}_{z}^{\ensuremath{-} 5/ 3} $  spectrum with the scales   $\vert {\mathbi{k}}_{h} \vert \gt {k}_{b} $ .
 $\vert {\mathbi{k}}_{h} \vert \gt {k}_{b} $ .