Ill-Posedness of the Hydrostatic Euler–Boussinesq Equations and Failure of Hydrostatic Limit

We investigate the hydrostatic approximation for inviscid stratified fluids, described by the two-dimensional Euler–Boussinesq equations in a periodic channel. Through a perturbative analysis of the hydrostatic homogeneous setting, we exhibit a stratified steady state violating the Miles-Howard criterion and generating a growing mode, both for the linearized hydrostatic and non-hydrostatic equations. By leveraging long-wave nonlinear instability for the original Euler–Boussinesq system, we demonstrate the breakdown of the hydrostatic limit around such unstable profiles. Finally, we establish the generic nonlinear ill-posedness of the limiting hydrostatic system in Sobolev spaces.