Out-of-phase tropical Atlantic-Pacific response after volcanic eruptions over the Last Millennium in PMIP4/past1000 simulations.

The Atlantic Niño, a key mode of tropical Atlantic variability, strongly influences precipitation over South America and Africa and interacts with Pacific El Niño through intricate ocean-atmosphere dynamics. Despite its significance, the Atlantic-Pacific response to natural external forcings remains poorly understood due to the short observational record. Using multi-model simulations of the past millennium, we show that January volcanic eruptions induce a tropical out-of-phase lagged response, with long-lasting El Niño-like warming in the Pacific beginning in austral winter, followed by austral summer Niña-like cooling in the Atlantic. These responses are triggered independently, driven by persistent equatorial atmospheric wave activity resulting from the interplay between relative cooling over the Indian Ocean and surrounding landmasses as opposed to the warmer Pacific. Our findings underscore the role of volcanic forcing in modulating Atlantic Niño intrinsic dynamics, significantly modifying the associated precipitation patterns, and contributing to a more comprehensive view of tropical interbasin ocean-atmosphere interactions.