Coastal El Niño and La Niña Events in a Changing Climate: Insights from the CESM2 Large Ensemble

Coastal El Niño events in the Eastern Boundary Upwelling system off Peru have garnered significant attention due to their substantial societal impacts. The recent events, 2017 and 2023, rank amongst the strongest on record, raising concerns about their future behavior. This study relies on the CESM2 Large Ensemble (CESM2-LE) to explore how the frequency, intensity and spatial patterns of coastal events may evolve throughout the 21st century. Initially, an evaluation of the model revealed a pattern bias associated with a too energetic South Pacific Meridional Mode (SPMM) and a weaker North Pacific Meridional Mode (NPMM), both patterns known to affect coastal warming. Nevertheless, the model realistically simulates precipitation during coastal events in both their cold and warm phases and captures a strong link to Pacific Meridional Modes (PMMs). At the end of the 21st century, warm coastal events are expected to become 40% less frequent but are associated with a precipitation increase of approximately 2 mm/day due to increased sea surface temperatures in the mean state. Future climatological precipitation levels during February-March-April (FMA) from the third decade of the 21st century onward are projected to match those currently seen during extreme events, such as the 2017 Coastal El Niño episode. Coastal La Niña, conversely, exhibits no meaningful change in frequency or intensity, but may serve as intervals of moderate rather than extreme precipitation in the future.