The mutual adaptation of mass and momentum toward a steady geostrophic state in rotating fluids. The adjustment problem was first considered by Rossby (1938), who derived the geostrophically balanced steady end state for an ocean to which momentum is impulsively imparted. The end state always possesses less energy than the initial state, a fact due to the end state being achieved through decaying inertial oscillations which disperse energy away in pulses of Poincare waves.