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Holocene Records of the Indian Ocean Dipole N.J. Abram and M.K. Gagan

Holocene Records of the Indian Ocean Dipole

N.J. Abram and M.K. Gagan

The Indian Ocean Dipole is a recently discovered mode of inter-annual climate variability, which results in anomalous winds, sea surface temperatures and rainfall throughout the Indian Ocean region, bringing drought to Indonesia and Australia and floods to eastern Africa. Developing a better understanding of the natural dynamics and effects of the Indian Ocean Dipole is essential for improved long-range forecasts of droughts and floods in the Indian Ocean region.

Corals from the Mentawai Islands (south-west Sumatra, Indonesia) provide an excellent opportunity to reconstruct the dynamics of the Indian Ocean Dipole. This area lies in the eastern Indian Ocean and experiences upwelling, strong sea surface cooling and drought during Indian Ocean Dipole events. The delta18O and Sr/Ca composition of modern corals from the Mentawai Island clearly preserve the 1994 and 1961 Indian Ocean Dipole events (Figure 1a). A previously unrecognised dipole event is also identified during 1876. Dipole events in the modern coral records are characterised by distinct cooling signals in coral Sr/Ca and delta18O as well as low rainfall in residual delta18O records. These modern coral records confirm that corals from the Mentawai Islands can be used to reconstruct Indian Ocean Dipole events.

 

The Holocene coral records preserve similar signals (Figure 1b) and provide the first evidence that the Indian Ocean Dipole has operated since at least the mid-Holocene. These dipole events also occurred during times when the Mentawai Island corals indicate that mean sea surface temperature and/or rainfall conditions were different to present day, showing that the Indian Ocean Dipole is a robust climatic feature that persists in a range of mean climatic states. The fossil coral records also provide evidence that the Indian Ocean Dipole system can operate independently of the El Niño-Southern Oscillation (ENSO), with dipole events continuing in the mid-Holocene when ENSO is thought to have been substantially weaker than present day.