The TOPEX/POSEIDON altimeter has provided further evidence that interannual warming occurs in the Indian Ocean with a frequency similar to that of El Ni¿o in the Pacific and has yielded important clues to the dynamics driving the warming. The signal is especially strong during the 1997 El Ni¿o. The altimeter observes long waves which move westward from the southeastern Indian Ocean at about the same time as westwardly wind anomalies appear in the east-central portion of the basin. The sea level peaks in the southwestern Indian Ocean and causes a sea level variation signal that is a near mirror image of El Ni¿o in the eastern Pacific. Sea surface temperature data also show a similar correlation. An analysis of the altimeter data indicates significant variability in the Indian Ocean during the 1994 and 1997 El Ni¿o events at the first and second baroclinic Rossby wave modes. Sea surface temperature and wind data suggest that the Indian Ocean warming has occurred during several previous El Ni¿o events, particularly during the large events of 1982 and 1987. Based on these observations, it is suggested that the warming begins with wind-forced Rossby waves in the southeastern Indian Ocean associated with the Southern Oscillation, similar to the forcing of Kelvin waves which precede El Ni¿o in the Pacific. ¿ 1999 American Geophysical Union