I have investigated features of decadal climate variability in the North Pacific and clarified the role of the ocean in controlling them, by analyzing historical data sets and executing experiments with a low-order model. Detailed investigation of the variability of the wintertime Aleutian low and water temperature in the subarctic frontal region implies a new scenario for the mechanism of the decadal variability. The decadal variability can be interpreted as the transition process between two stable states (cold and warm period) in midlatitude atmosphere. When the transition to a cold period occurs, sea surface temperature in the subarctic frontal region begins to be lowered not only through the surface processes but by westward propagation of cold anomaly from eastern subarctic region as a baroclinic Rossby wave. As a result, the meridional gradient of sea surface heat flux (∂yQ0) decreases its magnitude and then the forcing of the Aleutian low is weakened. When this weakening exceeds a threshold, the transition from a cold to a warm period occurs abruptly. After the transition to a warm period, ∂yQ0, in turn, increases its magnitude through the opposite processes to cause the reverse transition. The timescale of this cycle is mainly determined by the propagation speed of a baroclinic Rossby wave. The El Ni¿o and Southern Oscillation variability both in the atmosphere and ocean can also affect the timescale by causing the thresholds for transitions to be temporarily exceeded. Experiments with a four-variable low-order model support the proposed mechanism of the decadal variability.