Comments on a paper by Sakuma and Ghil

New stability criteria, for steady nonparallel solutions of the shallow‐water equations, are shown to be incorrect through the use of a counterexample: the sine profile in an f‐plane channel. If the channel is wide enough and the Coriolis parameter does not vanish, the conditions in question predict stability but the flow is found to be unstable to quasigeostrophic normal modes.