The Indian Ocean Dipole (IOD) affects climate and rainfall across the world, and most severely in nations surrounding the Indian Ocean1–4. The frequency and intensity of positive IOD events increased during the twentieth century5 and may continue to intensify in a warming world6. However, confidence in predictions of future IOD change is limited by known biases in IOD models7 and the lack of information on natural IOD variability before anthropogenic climate change. Here we use precisely dated and highly resolved coral records from the eastern equatorial Indian Ocean, where the signature of IOD variability is strong and unambiguous, to produce a semi-continuous reconstruction of IOD variability that covers five centuries of the last millennium. Our reconstruction demonstrates that extreme positive IOD events were rare before 1960. However, the most extreme event on record (1997) is not unprecedented, because at least one event that was approximately 27 to 42 per cent larger occurred naturally during the seventeenth century. We further show that a persistent, tight coupling existed between the variability of the IOD and the El Niño/Southern Oscillation during the last millennium. Indo-Pacific coupling was characterized by weak interannual variability before approximately 1590, which probably altered teleconnection patterns, and by anomalously strong variability during the seventeenth century, which was associated with societal upheaval in tropical Asia. A tendency towards clustering of positive IOD events is evident in our reconstruction, which—together with the identification of extreme IOD variability and persistent tropical Indo-Pacific climate coupling—may have implications for improving seasonal and decadal predictions and managing the climate risks of future IOD variability.
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Acknowledgements This research was supported by an Australian Research Council QEII Fellowship to N.J.A. (DP110101161; including C.-C.S., H.C. and R.L.E.) and the ARC Centre of Excellence for Climate Extremes (CLEX; CE170100023; to N.J.A., N.M.W., M.H.E. and C.C.U.). Further support to N.J.A. was provided by ARC Discovery Project DP140102059 and Future Fellowship FT160100029. B.E. was supported by an Australian Research Training Program scholarship and B.C.D. received scholarship support from the ARC Centre of Excellence for Climate System Science (CE110001028). M.H.E. is also supported by the Earth Science and Climate Change Hub of the Australian Government’s National Environmental Science Programme (NESP). C.C.U. acknowledges support by the US National Science Foundation (AGS-1602455). C.-C.S. thanks the Science Vanguard Research Program of the Ministry of Science and Technology (108-2119-M-002-012) and the Higher Education Sprout Project of the Ministry of Education, Taiwan, China (108L901001) for support. H.C. acknowledges support by the National Natural Science Foundation of China (NSFC 41888101). We gratefully acknowledge the Ministry of Research, Technology and Higher Education, and the Director of Intellectual Property Management as Secretary of the Coordinating Team for Foreign Research Permit (TKPIPA) for the research permit in Indonesia. Fieldwork was carried out in 2001 under research permit 2889/II/KS/2001, supported by W. Hantoro and the Indonesian Institute of Sciences. We thank W. Hantoro, B. Suwargadi, D. Prayudi, I. Suprianto, M. Gagan, K. Glenn, T. Watanabe, H. Scott-Gagan, and K. Sieh for assistance with fieldwork, J. Cowley, J. Cali, D. Becker, A. Kimbrough, S. Wong, B. Plunkett, S. Sosdian, H. Scott-Gagan and C.-H. Hsu for laboratory support, the NCAR CESM1 modelling group for making their last millennium ensemble simulations available, and Australia's National Computational Infrastructure and CLEX Computational Modeling Systems team for data hosting and support. We acknowledge Python Software Foundation (Python version 3.7.2), MathWorks Inc. (MATLAB Release 2014a) and R. Pawlowicz (M_Map mapping package for MATLAB, version 1.4g) for software used in analysis and figure generation. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government. We thank J. Addison and R. Halley (US Geological Survey) for internal reviews of this manuscript before submission, and S. Eggins for leadership and guidance.
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