Climate variability in the Indian Ocean region seems to be, in some aspects
, independent of forcing by external phenomena such as the El Nino/Southern
Oscillation(1-4). But the extent to which, and how, internal coupled ocean
-atmosphere dynamics determine the state of the Indian Ocean system have no
t been resolved. Here we present a detailed analysis of the strong seasonal
anomalies in sea surface temperatures, sea surface heights, precipitation
and winds that occurred in the Indian Ocean region in 1997-98, and compare
the results with the record of Indian Ocean climate variability over the pa
st 40 years. We conclude that the 1997-98 anomalies-in spite of the coincid
ence with the strong El Nino/Southern Oscillation event-may primarily be an
expression of internal dynamics, rather than a direct response to external
influences. We propose a mechanism of ocean-atmosphere interaction governi
ng the 1997-98 event that may represent a characteristic internal mode of t
he Indian Ocean climate system. In the Pacific Ocean, the identification of
such a mode has led to successful predictions of El Nino(5); if the propos
ed Indian Ocean internal mode proves to be robust, there may be a similar p
otential for predictability of climate in the Indian Ocean region.