Abstract

Coastal El Ni & ntilde;o events-marine heatwaves instances in the far eastern Tropical Pacific during otherwise basin-scale neutral or cold conditions-can have severe societal impacts for countries along the west coast of South America, as exemplified by the 2017 and 2023 Peru-Ecuador floods. Due to the brevity of the observational record, it is not well understood whether these events are driven by local or large-scale processes. Here, to overcome this limitation we use a data-driven modeling approach to address their return period and forcing mechanisms. It is shown that extreme coastal El Ni & ntilde;o events are a local manifestation in the eastern tropical Pacific of the constructive interactions of the Pacific Meridional Modes (PMM). Specifically, the North PMM yields a dipole-like anomaly SST pattern along the equator that favors its development, while the positive phase of the South PMM reinforces it. A smaller group of more moderate coastal events are remotely driven by zonal wind anomalies in the western tropical Pacific without the PMMs' influence. The role of PMMs in the development of extreme coastal El Ni & ntilde;o suggests that they may be more predictable than previously thought.