Super El Niño can raise coastal extreme sea-level risk tenfold

Thomas Carey Monahan , Christopher G. Piecuch, Ivan D. Haigh

El Niño-Southern Oscillation is a dominant mode of global climate variability with a range of societally relevant impacts. However, its effect on global coastal sea-level extremes remains poorly quantified, especially for very strong ("super") El Niños, which are rare. Here we analyse global reanalyses, hindcasts, and reconstructions to quantify how El Niño alters extreme sea level frequencies through mean sea level, tide, surge, and wave processes with distinct geographic fingerprints. We find low-frequency mean sea-level anomalies are the primary source of increased risk in the tropical Pacific during El Niño events, but that wave, tide, and surge play important secondary roles. Cross-basin thermocline tilt produces sea-surface height anomalies that correspond to an order-of-magnitude increase in exceedance risk along the tropical east Pacific, with the response steepening for strong events. The coincidence of the projected 2026–2027 Super El Niño with local maxima of long-period astronomical tidal cycles may produce 99th-percentile extremes in up to half of all high waters at some sites with potential major flooding and ecological impacts---the background shift alone is equivalent to 40–50 years of projected sea-level rise under SSP1-2.6.