  We investigate experimentally stratified turbulence forced by waves.
Stratified turbulence is present in oceans and it is expected to be dominated
by nonlinear interaction of internal gravity waves as described by the Garrett
& Munk spectrum. In order to reach turbulent regimes dominated by
stratification we use the Coriolis facility in Grenoble (France) which large
size enables us to reach regimes with both low Froude number and large Reynolds
number. Stratification is obtained by using vertically linearly varying salt
concentration and we force large scale waves in a $6\times6\times 1$ m$^3$
domain. We perform time-resolved PIV to probe the space-time structure of the
velocity field. We observe a wide band spectrum which is made of waves.
Discrete modes are observed due to the square shape of the flow container as
well as a continuum part which appears consistent with an axisymmetric
superposition of random weakly nonlinear waves. Our observations support the
interpretation of turbulence of a strongly stratified fluid as wave turbulence
of internal waves although our spectrum is quite different from the Garrett &
Munk spectrum. Weak turbulence proceeds down to a small cutoff length scale
(the buoyancy wavelength) at which a transition to more strongly nonlinear
turbulence is expected.
