Prolonged El Niño conditions modulate heatwaves across the Americas

The El Niño-Southern Oscillation (ENSO) is the main source of climate variability in the tropical Pacific, affecting global patterns and extreme events like heatwaves, with significant consequences. ENSO is marked by warmer (El Niño) and cooler (La Niña) sea surface temperature (SST) anomalies over the eastern Pacific, with a typical seasonal cycle: development in fall, a winter peak, and rapid decay in spring. However, some ENSO events last beyond this canonical cycle, extending into northern summer. A notable recent example occurred in 2018-2019, when El Niño conditions persisted until July 2019 and were implicated in driving a record-breaking marine heatwave in the North Pacific. Although prolonged El Niño events have occurred in the recent past and are projected to become more frequent under future climate conditions, their summer impacts, particularly their role in modulating heatwave characteristics, remain poorly understood.

This study compares the frequency, duration, and intensity of summer heatwaves in the Americas following summer-persistent El Niño events with those following normal El Niño events. We define a summer-persistent El Niño as an event in which SST anomalies in the Niño3.4 region remain above 0.5 °C through June of the decaying year. Heatwaves are defined as periods of at least three consecutive days with daily maximum temperatures above the 90^th percentile of the 1961-1990 reference period. To assess these relationships, we analyse reanalysis datasets (ERSSTv5, NCEP20CR, ERA5) and perform AMIP-type simulations using the atmospheric component of the Earth System Model EC-Earth3. For ENSO-neutral, conventional El Niño, and summer-persistent El Niño conditions, monthly SST composites are generated that capture the annual cycle by averaging all historical events. Based on these composites, we construct a 10-member ensemble, each spanning a six-year simulation. To further isolate ENSO-related forcing, we perform sensitivity experiments by uniformly increasing SSTs across the ENSO-active region by 1 °C.

We identify six summer-persistent El Niño events since 1895. These events are associated with reduced heatwave activity over the western United States, characterised by less frequent, shorter, and cooler events, linked to a sustained but distorted Pacific-North American (PNA)-like pattern. In contrast, northeastern South America experiences pronounced positive heatwave anomalies, with more frequent, longer-lasting, and more intense heatwaves than those observed following conventional El Niño events. These regional differences are driven by a prolonged weakening and eastward shift of the Walker Circulation, accompanied by intensified descending motion over South America. Collectively, these findings underscore the extended influence of ENSO beyond its typical spring termination and highlight the importance of accounting for ENSO persistence in seasonal heatwave forecasts and climate adaptation strategies in ENSO-sensitive regions.