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The Indian Ocean dipole is a prominent mode of coupled ocean-atmosphere variability1-4, affecting the lives of millions of people in Indian Ocean rim countries5-15. In its positive phase, sea surface temperatures are lower than normal off the Sumatra-Java coast, but higher in the western tropical Indian Ocean. During the extreme positive-IOD (pIOD) events of 1961, 1994 and 1997, the eastern cooling strengthened and extended westward along the equatorial Indian Ocean through strong reversal of both the mean westerly winds and the associated eastward-flowing upper ocean currents1,2. This created anomalously dry conditions from the eastern to the central Indian Ocean along the Equator and atmospheric convergence farther west, leading to catastrophic floods in eastern tropical African countries13,14 but devastating droughts in eastern Indian Ocean rim countries8-10,16,17. Despite these serious consequences, the response of pIOD events to greenhouse warming is unknown. Here, using an ensemble of climate models forced by a scenario of high greenhouse gas emissions (Representative Concentration Pathway 8.5), we project that the frequency of extreme pIOD events will increase by almost a factor of three, from one event every 17.3 years over the twentieth century to one event every 6.3 years over the twenty-first century. We find that a mean state change-with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean-facilitates more frequent occurrences of wind and oceanic current reversal. This leads to more frequent extreme pIOD events, suggesting an increasing frequency of extreme climate and weather events in regions affected by the pIOD.
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In austral winter and spring, southeasterly trade winds that feed the tropical convergence zone near the maritime continent are a feature of the southerntropicalIndianOcean.DuringapIODevent,aninitialcoolingoff Sumatra-Java,theeasternpoleoftheIndianOceandipole,suppresseslocal convection, inducing easterly wind anomalies and a shallowing thermo- cline.Thispromotesupwellingthatinturnreinforcestheinitialcooling1,2,18, a process referred to as Bjerknes feedback. The growth of cool anomalies causesanorthwestwardextensionofthesoutheasterlytradewinds1,2,16,with anomalous easterlies along the equatorial Indian Ocean (Fig. 1a), where weak westerlies normally prevail. Thechangeinwindpromotesconver- gence,rainfallandwarmanomaliesintheequatorialwesternIndianOcean. The altered circulations induce droughts and bushfires in eastern Asia and Australia5-8, floods in parts of the Indian subcontinent11 and eastern Africa13,14,coralreefdeathacrosswesternSumatra12,andmalariaoutbreaks ineasternAfrica15.DuringextremepIODevents,asoccurredin1961,1994 and 1997, the anomalies, particularly the anomalous equatorial easterlies, arefarstronger(Fig.1b),withcommensuratelygreaterimpacts.Duringthe 1997 event, devastating floods in Somalia, Ethiopia, Kenya, Sudan and Uganda...