Bi-decadal climate variation is dominant over the North Pacific on inter-decadal timescale; however the mechanism has not been fully understood. We here find that the bi-decadal variations in the North Pacific climate and intermediate waters possibly relate to the 18.6-year period modulation of diurnal tide. In the period of strong diurnal tide, tide-induced diapycnal mixing makes surface salinity and density higher and the upper-layer shallower along the Kuril Islands and the east coast of Japan. Simple model results suggest that the coastal depth adjustment by baroclinic Kelvin waves enhances the thermohaline circulation, the upper-layer poleward western boundary current and associated heat transport by about 0.05PW. This could also explain the warmer SST in the Kuroshio-Oyashio Extension regions, where positive feedback with Aleutian Low might amplify the bidecadal variations. The 18.6-year tidal cycle hence could play a role as a basic forcing for the bi-decadal ocean and climate variations.