“…The terms on the right-hand side represent the forces acting (per unit mass): Coriolis (−2 × u), centrifugal (− × ( × r)), Newtonian gravity (−∇ N ), pressure gradient (−α∇p), and all other forces (G). For discussion of (2.1) as an expression of Newton's second law of motion see, for example, Lorenz (1967), Phillips (1973, Gill (1982), Pedlosky (1987), Holton (1992 and Dutton (1995). As described in these references, it is usual to combine the centrifugal force (− × ( × r)) with Newtonian gravity (−∇ N ) to give apparent gravity, −∇ A , where A = N − 2 s 2 /2 is a new scalar potential, s being perpendicular distance from the axis of rotation of J.…”