On the interpretation of the eliassen‐palm flux divergence

New expressions are presented for the wave‐activity density and the non‐conservative source of wave activity in the small amplitude Eliassen‐Palm theorem for the primitive equations in log(pressure) and isentropic coordinates. These expressions involve Ertel's potential vorticity, and include versions that are of Eulerian form. The theoretical results are used as a basis for investigating the interpretation of the Eliassen‐Palm flux divergence in terms of physical wave properties. Implications for modelling and observational studies of planetary waves are discussed and connections with stability theory and the definition of orthogonality of normal modes are mentioned.