Increased frequency of multi-year El Niño–Southern Oscillation events across the Holocene

Abstract El Niño–Southern Oscillation (ENSO) events, whether in warm or cold phases, that persist for two or more consecutive years (multi-year), are relatively rare. Compared with single-year events, they create cumulative impacts and are linked to extended periods of extreme weather worldwide. Here we combine central Pacific fossil coral oxygen isotope reconstructions with a multimodel ensemble of transient Holocene global climate simulations to investigate the multi-year ENSO evolution during the Holocene (beginning ~11,700 years ago), when the global climate was relatively stable and driven mainly by seasonal insolation. We find that, over the past ~7,000 years, in proxies the ratio of multi-year to single-year ENSO events increased by a factor of 5, associated with a longer ENSO period (from 3.5 to 4.1 years). This change is verified qualitatively by a subset of model simulations with a more realistic representation of ENSO periodicity. More frequent multi-year ENSO events and prolonged ENSO periods are being caused by a shallower thermocline and stronger upper-ocean stratification in the Tropical Eastern Pacific in the present day. The sensitivity of the ENSO duration to orbital forcing signals the urgency of minimizing other anthropogenic influence that may accelerate this long-term trend towards more persistent ENSO damages.