Review of three studies devoted to the available potential energy (APE) leads to the proposal of a diagnosis for APE, well-suited for rotating stratified flows within the primitive equations (PE) framework in which anharmonic effects (due to large vertical displacements of isopycnals) are permitted. The chosen diagnosis is based on the APE definition of Holliday & McIntyre (J. Fluid Mech., vol. 107, 1981, pp. 221–225) and uses the background stratification of Winters et al. (J. Fluid Mech., vol. 289, 1995, pp. 115–128). Subsequent evaluation of the APE in a PE direct simulation (1/100°, 200 levels) of oceanic mesoscale turbulence indicates that anharmonic effects are significant. These effects are due to large vertical displacements of the isopycnals and the curvature of the background density profile.