Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism.

Through tropical basin interactions, the Indian Ocean Walker circulation (IWC) and basin mode (IOBM) covary with the Pacific Walker circulation (PWC) on interannual-decadal timescales. Coupling between IOBM and PWC has degraded considerably in recent decades due to greenhouse gas forcing; however, the instrumental record is too short to unequivocally determine whether such a decoupling could also have arisen naturally. Here we contextualise recent trends by reconstructing Indo-Pacific modes (IOBM, IWC, and PWC) over the past four centuries using a compilation of paleoclimate records. We demonstrate that both Indian Ocean modes were largely coupled to the Pacific throughout the pre-industrial era, but this coupling was disrupted by a series of volcanic eruptions during the early 19th century. Nonetheless, the PWC-IOBM relationship since the mid-20th century appears exceptional relative to equivalent-length intervals of the past millennium, including periods of strong volcanism. Given that stable basin interactions are a source of near-term predictability, our results have implications for climate risk management efforts.