Quantifiying seasonal-scale changes in El Niño southern oscillation for the past millennia

Abstract

The El Niño-Southern Oscillation (ENSO) is the greatest source of interannual climate variability, yet model forecasts of the response of this system to global warming are inconsistent. The brevity of the instrumental record and lack of detailed knowledge of ENSO under different background states contribute to the uncertainty. Here we present a sequence of Porites coral microatoll !18O records from Kiritimati (Christmas) Island in the central equatorial Pacific showing ENSO variability during discrete “windows” spaced between 1500 and 6000 years ago (mid- to late Holocene), when background climate conditions were different due to changes in the Earth’s orbit around the sun. Our sequence includes a 175-year monthly-resolved microatoll !18O record showing ENSO variability 4,300 thousand years ago. The record shows a 60% reduction in the ENSO variance, a stronger annual cycle that persisted for the full 175 years of the record, and limited low frequency (multi-decadal) modulation of the ENSO signal. El Niño events were ‘damped’ during their June-December growth phase, but still phaselocked to the seasonal cycle. La Niña events were reduced and together ENSO seasonal phasing was likely similar to that observed during the weak ENSO period of the 1920-1950s.Further, results from corals aged between 1,500 and 2,000 years ago also show reduced ENSO, as well as changes in the contribution of the annual cycle, El Niño and La Niña events to the overall coral !18O signal. Our results show fundamental metrics on the seasonal characteristics of ENSO during the altered background conditions of the mid- to late Holocene. The results suggest that Holocene ENSO responded to changes in orbital forcing and that there was limited, unforced variability. This may have implications for ENSO under future global warming conditions.