Two types of anomaly pathways associated with decadal thermocline variability in the North Pacific Ocean are identified on constant density surfaces (isopycnals): one westward and the other southwestward around the subtropical gyre. A theory, based on two baroclinic planetary wave modes in the thermocline, is proposed to account for the observed variability pattern. The variability along the westward wave path is ascribed to the non-Doppler-shift 1st mode and seems to be forced mainly by wind stress forcing; the anomaly along the southwestward subduction path is associated with the advective nature of the 2nd mode and could be excited mainly by surface buoyancy anomalies. This study may provide a dynamically unified, three-dimensional picture of decadal thermocline variability in the North Pacific Ocean. ¿ 1999 American Geophysical Union