= Available potential energy
{title2=$A$}
The portion of a fluid system's <potential energy> that can be released by admissible rearrangement relative to a stable reference state. In a uniformly stratified incompressible fluid, the quadratic density is $A=\rho_0N^2\xi_z^2/2=\rho_0b^2/(2N^2)$, where $\xi_z$ is vertical <fluid displacement> and $b$ the buoyancy perturbation. It is positive for stable stratification and measures the restoring energy of <internal gravity waves>.
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