Anomalous warming in the Indian Ocean coincident with El Ni\~no

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\~no in the Pacific and has yielded important clues to the dynamics driving the warming. The signal is especially strong during the 1997 El Ni\~no. 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\~no 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\~no 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\~no 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\~no in the Pacific.