Abstract: By using long rangesea surface temperature,850hPa wind and out going long-wave radiation data,the relat ionship of the interannual variation of sea surface temperature anomalies (SSTA) between the tropical Indian ocean (TIO) and equatorial mideastern Pacific ocean has been investigated.The empirical orthogonal function (EOF) analyses of SSTA show that the dominant modes in the TIO are the monopole mode with same SST Asign in basin scale and the dipole mode with a reversal in sign across the basin,respectively.The dominant pattern in the tropical Pacific ocean is the El Niño mode.All monopole,dipole and El Niño modes display noticeable interannual variations.The wavelet squared coherency analyses show that in the interannual time scale,the SSTA in NinO3 region (5°S-5°N,150-90°W) are closely related to the monopole and dipole modes in the TIO,i.e.,the El Niño from its developing phase to the decaying phase corresponds to SSTA in the TIO varying from the dipole mode to the monopole mode.In average,the El Niño lags behind the dipole mode 4 months and precedes the monopole 6 months.It is revealed by the evolution of the air-sea coupled charact eristics in the TIO and Pacific ocean that the interactions in the western Pacific ocean associated with the El Niño shifting from the developing episode to the decaying play an import ant role on the transformation of SSTA in the TIO from the dipole to the monopole.The variations of the zonal wind anomalies over both TIO and Pacific ocean are affected greatly by the convections over the area in the eastern TIO and western tropical Pacific.The variations of the convection accom panied with the evolution of El Niño alter the zonal winds and hence associate oceanic processes,which are responsible for SSTA in the TIO changing from the dipole to monopole mode.