GAFD Seminar: Remi Tailleux (University of Reading)

The classical theory of available potential energy (APE), developed by Margules and Lorenz, establishes that only a fraction of the total potential energy of a stratified fluid can be converted reversibly into kinetic energy. Yet, the precise physical foundations of APE theory remain insufficiently understood, particularly in relation to other energy partitions such as mechanical energy and heat. The main aim of this talk is to clarify the physical origins of APE theory and show that it can be interpreted as a generalisation of Carnot’s theory of heat engines not restricted to closed cycles, leading to a more accurate and broadly applicable definition of thermodynamic efficiency. I will argue that APE also provides the natural conceptual framework underlying isentropic analysis in the atmosphere and isopycnal analysis in the oceans, with important implications for our understanding of mixing, energetics, and the definition of potential vorticity in ways directly connected to quasi-geostrophic theory. Finally, I will discuss the limitations of the widely used Lorenz approximation to APE, highlighting its unphysical features and the need for more consistent formulations.