1. Institute for Global Change Research, SEAVANS N 7F, 1-2-1 Shibaura, Minato-ku, Tokyo 105 6791, Japan
2. Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560 012, India
3. Department of Earth and Planetary Physics, Graduate School of Science, The University of Tokyo, Tokyo 113 0033, Japan

Correspondence to: T. Yamagata^1,3 Correspondence and requests for materials should be addressed to T.Y. (Email: e-mail: yamagata@geoph.s.u-tokyo.ac.jp).

For the tropical Pacific and Atlantic oceans, internal modes of variability that lead to climatic oscillations have been recognized^{1, 2}, but in the Indian Ocean region a similar ocean–atmosphere interaction causing interannual climate variability has not yet been found³. Here we report an analysis of observational data over the past 40 years, showing a dipole mode in the Indian Ocean: a pattern of internal variability with anomalously low sea surface temperatures off Sumatra and high sea surface temperatures in the western Indian Ocean, with accompanying wind and precipitation anomalies. The spatio-temporal links between sea surface temperatures and winds reveal a strong coupling through the precipitation field and ocean dynamics. This air–sea interaction process is unique and inherent in the Indian Ocean, and is shown to be independent of the El Niño/Southern Oscillation. The discovery of this dipole mode that accounts for about 12% of the sea surface temperature variability in the Indian Ocean—and, in its active years, also causes severe rainfall in eastern Africa and droughts in Indonesia—brightens the prospects for a long-term forecast of rainfall anomalies in the affected countries.