Figure 12.5 Conservation of potential vorticity can clarify why western boundary currents are necessary. Left: Vorticity input by the wind ζt balances the change in potential vorticity ζ in the east as the flow moves southward and f decreases; but the two do not balance in the west where ζ must decrease as the flow moves northward and f increases. Right: Vorticity in the west is balanced by relative vorticity ζb generated by shear in the western boundary current.