The available potential energy of overturns as an indicator of mixing in the seasonal thermocline

Using empirical observations, it is shown that the available potential energy of fluctuations (the APEF) is proportional to the product of the Cox number C x , the local buoyancy frequency N , and the molecular diffusivity of temperature D . The nondimensional number C pe = 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.