1. Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory 0200, Australia
2. Center for Climatic Research, University of Wisconsin-Madison, 1225 W. Dayton Street, Madison, Wisconsin 53706, USA
3. Earth Environment Institute, Chinese Academy of Science, Xi'an 710075, China
4. The Ocean University of China, Qingdao 266003, China
5. Research and Development Center for Geotechnology, Indonesian Institute of Sciences (LIPI), Bandung 40135, Indonesia
6. Present address: British Antarctic Survey, Natural Environment Research Council, Cambridge CB3 0ET, UK.

Correspondence to: Nerilie J. Abram^1,6Michael K. Gagan¹ Correspondence and requests for materials should be addressed to N.J.A. (Email: nabr@bas.ac.uk) and M.K.G. (Email: michael.gagan@anu.edu.au).

Abstract

The Indian Ocean Dipole^{1, 2} (IOD)—an oscillatory mode of coupled ocean–atmosphere variability—causes climatic extremes and socio-economic hardship throughout the tropical Indian Ocean region^{1, 2, 3, 4, 5}. There is much debate about how the IOD interacts with the El Niño/Southern Oscillation (ENSO) and the Asian monsoon, and recent changes in the historic ENSO–monsoon relationship⁶ raise the possibility that the properties of the IOD may also be evolving. Improving our understanding of IOD events and their climatic impacts thus requires the development of records defining IOD activity in different climatic settings, including prehistoric times when ENSO and the Asian monsoon behaved differently from the present day. Here we use coral geochemical records from the equatorial eastern Indian Ocean to reconstruct surface-ocean cooling and drought during individual IOD events over the past 6,500 years. We find that IOD events during the middle Holocene were characterized by a longer duration of strong surface ocean cooling, together with droughts that peaked later than those expected by El Niño forcing alone. Climate model simulations suggest that this enhanced cooling and drying was the result of strong cross-equatorial winds driven by the strengthened Asian monsoon of the middle Holocene. These IOD–monsoon connections imply that the socioeconomic impacts of projected future changes in Asian monsoon strength may extend throughout Australasia.

1. Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory 0200, Australia
2. Center for Climatic Research, University of Wisconsin-Madison, 1225 W. Dayton Street, Madison, Wisconsin 53706, USA
3. Earth Environment Institute, Chinese Academy of Science, Xi'an 710075, China
4. The Ocean University of China, Qingdao 266003, China
5. Research and Development Center for Geotechnology, Indonesian Institute of Sciences (LIPI), Bandung 40135, Indonesia
6. Present address: British Antarctic Survey, Natural Environment Research Council, Cambridge CB3 0ET, UK.

Correspondence to: Nerilie J. Abram^1,6Michael K. Gagan¹ Correspondence and requests for materials should be addressed to N.J.A. (Email: nabr@bas.ac.uk) and M.K.G. (Email: michael.gagan@anu.edu.au).

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