Ekman pumping is the vertical velocity forced at the base of the surface Ekman layer by the divergence or convergence of wind-driven Ekman transport; convergent Ekman transport (as occurs in the centers of subtropical gyres) produces downward Ekman pumping (downwelling), which stretches the underlying water column vertically (increasing its length) by conservation of mass; by conservation of potential vorticity, this vertical stretching requires an increase in the column's absolute vorticity, and since relative vorticity remains small, this is achieved by a decrease in the planetary vorticity (Coriolis parameter f), meaning the water column must move toward the equator (where f is smaller) -- consistent with the classical Sverdrup relation linking downwelling (negative Ekman pumping) to equatorward interior ocean transport.