Between September, 1997, and the spring of 1998, the greatest warming of the century has occurred in the western Indian Ocean. This warming is superimposed on a general warming trend that has placed the equatorial Indian Ocean in its warmest state in the last 100 years. Occurring at the time of the Pacific ocean El Nino and driven by strong, anomalous surface easterly winds, the near-equatorial sea surface temperature gradient and the slope of the ocean surface reversed itself for a prolonged period with warmer temperatures (+ 3°C) and an elevated surface (+30 cm) in the west and cooler temperatures (-3°C) and depressed sea levels (-30 cm) in the east. Rainfall caused by converging surface winds into East Africa was the largest on record and was attended by epizootic and epidemic outbreaks of Rift Valley fever and cholera. A conceptual model of the coupled ocean-atmosphere system identifies dynamic oceanic modes in the Indian Ocean and suggests that the changes in the regional climate produce a growing instability in a manner similar to the El Nin˜o of the Pacific Ocean. The identification of such modes in the Indian Ocean suggests the feasilibility of the quantitative prediction of the intensity of the South Asian monsoon and the strength of the East African equinoctial rainfall some months ahead using coupled ocean-atmosphere-land models. Furthrmore, the ability to understand elementary behavior in the Indian Ocean will allow connections between the Asian monsoon and ENSO phenomena to be understood better and hopefully exploited for the prediction of joint ENSO-monsoon modes.