Available Potential Energy

Available potential energy (APE) is the part of the total potential energy of a stratified fluid that can be converted to kinetic energy. Lorenz (1955) defined it as the difference between the actual total potential energy and the minimum total potential energy reachable by an adiabatic redistribution of mass to a reference state of flat isopycnals in hydrostatic balance. Only the APE, not the full potential energy, drives circulation. Baroclinic instability releases APE stored in tilted density surfaces, feeding mesoscale eddy kinetic energy; APE is the reservoir at the start of the ocean energy cascade.