Using empirical observations, it is shown that the available potential energy of fluctuations (the APEF) is proportional to the product of the Cox number Cx, the local buoyancy frequency N, and the molecular diffusivity of tenn Perature D. The nondimensional number Cpe=APEF/(ND) is shown to be a good predictor of the Cox number and can be used as an indicator of mixing in stratified fluids. Analysis of the APEF at ocean station P shows that turbulence in the seasonal thermocline responds to strong surface forcing. ¿American Geophysical Union 1987