Surfaces of neutral static stability (neutral surfaces) along which lateral ocean mixing occurs, can be quite different to potential density surfaces. For example, the lateral gradients of potential temperature may differ in the two surfaces by up to a factor of four in some extreme but important regions. Vertical mixing processes cause vertical upwelling velocities that can be modelled as vortex-stretching terms in layered models of ocean circulation. However, since potential density surfaces are commonly inclined with respect to neutral surfaces, lateral motion along neutral surfaces also gives rise to unwanted vertical motion through isopycnals that is not related to vertical mixing processes.

When considering the conservation of potential vorticity in the real ocean, the differences between the depths of neutral surfaces and potential density surfaces becomes quite important. Neutral surface potential vorticity (NSPV), is introduced, defined to be proportional to f/h, where f is the Coriolis parameter and h is the vertical distance between adjacent neutral surfaces. NSPV is often significantly different (by factors of 2 or 3) to the two other commonly used potential vorticity variables, isopycnal potential vorticity, IPV, and fN2.