Non-exponential hydrodynamical growth in density-stratified thin Keplerian discs

The short time evolution of three-dimensional small perturbations is studied. Exhibiting spectral asymptotic stability, thin discs are none the less shown to host intensive hydrodynamical activity in the shape of non-modal growth of initial small perturbations. Two mechanisms that lead to such behaviour are identified and studied, namely, non-resonant excitation of vertically confined sound waves by stable planar inertia-coriolis modes that results in linear growth with time, as well as resonant coupling of those two modes that leads to a quadratic growth of the initial perturbations. It is further speculated that the non-modal growth can give rise to secondary stratorotational instabilities and thus lead to a new route to turbulence generation in thin discs.