This summer of 2019/20 in Australia, a positive Indian Ocean Dipole (IOD) event set up extremely hot and dry conditions for the historically worst fires that ravaged Australia. An unprecedented 20% of the forest has been burned, resulting in ecological havoc, environmental pollution, and enormous property damage. Climate simulations also pointed out that this extreme climate will severely change the climate and environment of the countries around the Indian Ocean under the current global warming trend. But will this implication be a future tragedy? Can the positive IOD result in any severe impact on Taiwan?
Prof. Chuan-Chou Shen, a Distinguished Professor at the Department of Geosciences and the Chairman of the Global Change Research Center, National Taiwan University (NTU), supported by the Ministry of Science and Technology (MOST) and NTU, was invited by Dr. Nerilie Abram of the Australian National University (ANU) to this project in 2001. Dr. Shen and his team at the NTU used radiometric U-Th dating to determine ages of the fossil corals collected from Sumatran Islands to establish a millennial-long record of Indian Ocean climate oscillation. The newly developed record shows that strong events like the one in 2019/20 have been very rare, only 10 of these events over the reconstruction, but four of those have occurred in just the last 60 years. Records show that after the twentieth century, the frequency of strong dipole events has become increasingly dense and the situation has worsened with the increase of atmospheric greenhouse gas contents. The latest results were first published online as an 8-page ARTICLE in the renowned journal “Nature” on March 9, 2020. Combined with previous studies, this international collaborative team found that positive dipole events often occur in conjunction with the "central Pacific El Niño" (CP El Niño) episodes, which indirectly affects Taiwan's seasonal rainfall patterns.
This team used drilled cores of an alive and eight fossil corals to produce a semi-continuous reconstruction of IOD variability that covers five centuries of the last millennium since 1240. This record shows a tropical climate transition in the frequency of positive IOD events, which have a mean periodicity of 25 years prior to 1590, and 8 years after 1590. The highest frequency of positive events is in the 17th century and after 1960.
Results show that a positive IOD event that occurred in 1675 was up to 42% stronger than the strongest event observed so far in the instrumental record, which was in 1997. The 1675 event points to the fact that such extremes are possible even without human-caused climate change. Under the current global warming trend, extreme events could happen again in the near future.
This new coral record also features a persistent and tight coupling between the positive IOP and the central Pacific El Niño (CP El Niño). Our previous study using tree ring records also showed that the variability of CP El Niño after 1990 has been the most obvious period in the past eight centuries, attributable to anthropogenic greenhouse forcing. Subsequent model simulations suggested that the positive IOD and central Pacific El Niño will become intensive and increasingly fierce in the coming decades. It could induce the declining spring rainfall in Taiwan, especially in the southern regions. If the climate pattern remains unchanged, the formation of summer typhoons and invasion of autumn typhoons in Taiwan are expected to increase and the rainfall could be more violent in the two seasons. Our government should actively respond to these hazards and deal efficiently by adopting necessary precautions for future disaster prevention as soon as possible.
This study was supported by the Science Vanguard Research Program of the MOST, the Higher Education Sprout Project of the Ministry of Education, the Core Consortiums Research Project and the Research Center for Future Earth of NTU. Dr. Tsai-Luen Yu, ex-graduate student, Chia-Hao Hsu, and Prof. Shen of the Department of Geosciences, NTU, were invited to join in this international team. The 14 collaborative affiliations are universities and institutes from Australia, Asia, and America.
Abram, N. J.*, Wright N. M., Ellis B., Dixon B. C., Wurtzel J. B., England M. H., Ummenhofer C. C., Philibosian B., Cahyarini S. Y., Yu T.-L., Shen C.-C., Cheng H., Edwards R. L., and Heslop D. (2020) Coupling of Indo-Pacific climate variability over the last millennium. Nature 579, 385-392. March 19
Prof. Chuan-Chou Shen, Distinguished Professor, Department of Geosciences, National Taiwan University
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