A diagnostic analysis has been conducted based on the January 1975-December 1987 NMC 850hPa wind field data. The results show that the northern Pacific high latitude anomalous westerly on the quasi-4-year time scale, which occurs ahead of the beginning of the positive tropical central/eastern Pacific SSTA, propagates southward along a zigzag path,i.e., first southwest then southeast and so does the Aleutian SSTA.