Impact of tropical waves on extreme rainfall events during coastal El Niño

Abstract We examine the relationship between convectively coupled waves, the Madden–Julian Oscillation (MJO), and extreme precipitation over the western coast of South America during Coastal El Niño (COEN) events for the period spanning 1980–2023. Two types of COEN can be distinguished: (i) that occur in association with large-scale El Niño Southern Oscillation (ENSO) (e.g. 1982/83, 1997/98), and (ii) more ‘local’ COEN–when anomalous sea surface temperature take place over the far-eastern equatorial Pacific only (e.g. 2017, 2023). During both types of COEN events, increased rainfall along the western coast of South America is associated with intense Kelvin wave activity. In addition, westward inertio-gravity (WIG) waves, Rossby waves, and the MJO exhibit increased activity during local COEN events. During the recent extreme COEN 2017 and 2023, heavy rainfall occurred alongside significant WIGs, Kelvin, Rossby, and MJO events with unprecedented amplitudes propagating along western South America. Our results suggest that the probability of extreme precipitation under Coastal ENSO in western South America is strongly modulated by wave activity.