In s coordinates, a variable f e(x, y, s, t) whose horizontal gradient 2=f e is equal to the irrotational part of the horizontal pressure gradient force is referred to as an equivalent isobaric geopotential height. Its inner part can be derived from the solution of a Poisson equation with zero Dirichlet boundary value. Because 2=f (x, y, p, t) is also the irrotational part of the horizontal pre...