Abstract: By the aid of NCEP/NCAR Reanalysis data sets,the low level wind fields are explored during the onset and development phase of ENSO and before.Evidences show that westerly anomalies in the western equatorial Pacific (140°E～180) experience two remarkable enhancements in the spring and summer of the year when El Niño emerges.The enhancements of westerlies are not only due to the eastward propagation of the westerlies in equatorial Indian Ocean,butalso caused by the strong convergence of the meridional wind from two hemispheres,which leads to the regionaly intensified zonal pressure gradient so as to result in the reinforcement and bursts of westerly wind.On the other hand,as the influence of the Earth-self-rotation,the north-easterly (south-easterly) wind from northern (southern) hemisphere changes into north-westerly (south-westerly) wind gradualy in the near-equatorial zone,strengthening the westerly wind directly.Comparing the contributions of the meridioanl wind from both hemispheres to the enhancements of westerly anomalies, we find that south-easterly wind of southern hemispere,appearing to the east of Australia, is stronger in vigor and more stable,which originates from the southern pole area and is related with Southern Oscilation.