Abstract The Indian Ocean Dipole (IOD) has major climate impacts worldwide, and most profoundly for nations around the Indian Ocean basin. It has been 20 years since the IOD was first formally defined and research since that time has focused primarily on examining IOD dynamics, trends and impacts in observational records and in model simulations. However, considerable uncertainty exists due to the brevity of reliable instrumental data for the Indian Ocean basin and also due to known biases in model representations of tropical Indian Ocean climate. Consequently, the recent Intergovernmental Panel on Climate Change (IPCC) report on the Ocean and Cryosphere in a Changing Climate (SROCC) concluded that there was only low confidence in projections of a future increase in the strength and frequency of positive IOD events. This review examines the additional perspectives that palaeoclimate evidence provides on IOD trends, variability, interactions with the El Nino–Southern Oscillation (ENSO), and impacts. Palaeoclimate data show that recent trends towards more frequent and intense positive IOD events have been accompanied by a mean shift toward a more positive IOD-like mean zonal SST gradient across the equatorial Indian Ocean (due to enhanced warming in the west relative to the east). The increasing frequency of positive IOD events will imminently move outside of the range of natural variability in the last millennium if projected trends continue. Across a range of past climate states, periods of a mean positive IOD-like state in the Indian Ocean have been accompanied by elevated IOD variability. This includes events that exceed the magnitude of even the strongest measured historical events, demonstrating that the Indian Ocean can harbour even stronger variability than what has been observed to date. During the last millennium there has been a tight coupling between the magnitude of interannual IOD and ENSO variability, although positive IOD events have frequently occurred without any indication of a co-occurring El Nino event. This IOD–ENSO coupling may not have persisted across past climates that were very different from the present, raising questions of whether their interaction will change in a rapidly warming future. Palaeoclimate evidence for hydroclimate changes during the last millennium further points to the importance of interannual IOD and ENSO variability in providing the rainfall that breaks droughts in regions that are impacted by these modes of variability. Overall, this review highlights the rich insights into the IOD that can be gained from palaeoclimate evidence. Palaeoclimate data helps to overcome known limitations in observations and model simulations of the IOD, and demonstrates that strong conclusions about the IOD and its impacts can be reached when the evidence for past, present and future behaviour of the IOD are assessed together.

中文翻译：

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印度洋偶极子的古气候观点

摘要 印度洋偶极子 (IOD) 对全球气候产生重大影响，对印度洋盆地周围的国家影响最为深远。自 IOD 首次正式定义以来已经过去 20 年了，从那时起，研究主要集中在检查观测记录和模型模拟中的 IOD 动态、趋势和影响。然而，由于印度洋盆地可靠仪器数据的简短以及热带印度洋气候模式表示中的已知偏差，存在相当大的不确定性。因此，最近政府间气候变化专门委员会 (IPCC) 关于气候变化中的海洋和冰冻圈 (SROCC) 的报告得出的结论是，对未来 IOD 积极事件强度和频率增加的预测只有低信度。本综述考察了古气候证据提供的关于 IOD 趋势、变异性、与厄尔尼诺-南方涛动 (ENSO) 的相互作用和影响的其他观点。古气候数据显示，最近趋向于更频繁和更强烈的正 IOD 事件的趋势伴随着跨越赤道印度洋的更正的 IOD 样平均纬向 SST 梯度的平均转移（由于西部相对于东部的变暖增强） . 如果预测的趋势继续下去，正 IOD 事件的频率将很快超出上个千年的自然变化范围。在过去的一系列气候状态中，印度洋平均正 IOD 样状态的时期伴随着 IOD 变异性的升高。这包括甚至超过最强烈的历史事件的幅度的事件，这表明印度洋可以拥有比迄今为止观察到的更大的变化。在过去的千年中，年际 IOD 和 ENSO 变异性之间存在紧密耦合，尽管正 IOD 事件经常发生，但没有任何迹象表明厄尔尼诺事件同时发生。这种 IOD-ENSO 耦合可能不会在过去与现在非常不同的气候中持续存在，这引发了在快速变暖的未来它们的相互作用是否会发生变化的问题。过去千年中水文气候变化的古气候证据进一步指出了年际 IOD 和 ENSO 变异在提供降雨以打破受这些变异模式影响的地区的干旱方面的重要性。总的来说，这篇综述强调了可以从古气候证据中获得的对 IOD 的丰富见解。古气候数据有助于克服 IOD 观测和模型模拟中的已知局限性，并表明当对 IOD 过去、现在和未来行为的证据进行评估时，可以得出关于 IOD 及其影响的有力结论。