where u is the fluid velocity, ρ ρ is the fluid density, and p the pressure. If we assume that F = ∇ ∇ Φ Φ = − − 2 ρ ρ Ω Ω × × u is a scalar potential and the advective term on the left may be neglected (reasonable if the Rossby number is much less than unity) and that the flow is incompressible (density is constant), the equations become: