Growth of density inhomogeneities in a flow of wave turbulence
  We consider an advection of a passive scalar by a flow which is a superposition of random waves. We find that such a flow can lead to an exponential growth of the passive scalar fluctuations. We calculate the growth rate at the fourth order in wave amplitudes and find it non-zero when either both solenoidal and potential components are present in the flow or there are potential waves with the same frequencies but different wavenumbers. 
A. M. Balk. G. Falkovich and M.S. Stepanov