Multi‐Year ENSO Events Over the Last Two Millennia

Abstract

Multi‐year El Niño‐Southern Oscillation (ENSO) events or consecutive ENSO events have received great research interest due to their far‐reaching societal, economic, climatic, and ecological influences. However, current knowledge and understanding of their characteristics and impacts are still limited by the availability of long‐term observations. In this work, the Paleo Hydrodynamics Data Assimilation product (PHYDA) is adopted to study the multi‐year ENSO events spanning the past 2,000 years. The analysis shows that the occurrence frequency of multi‐year El Niño and La Niña events is significantly higher than that could be expected from random probability, indicating that the 20th‐century multi‐year ENSO is not anomalous when considering the past 2,000 years. During periods with increased consecutive events, while the amplitude of the composited ENSO sea surface temperature (SST) anomalies shows no obvious changes, more extreme events are favored and correspond to the enhanced ENSO variability. Further investigation on the mechanism behind the variability of the multi‐year ENSO events over the past 2,000 years suggests that the multi‐year ENSO events are highly correlated with the low‐frequency variability in the background SSTs. The information flow analysis also confirms that there are two‐way interactions between the occurrence frequency of the multi‐year ENSO events and the low‐frequency variability of the background SST anomalies, which shows asymmetric characteristics between the El Niño and La Niña. Additionally, our findings indicate that multi‐year events can intensify climatic impacts on North American temperatures and droughts, and potentially contribute to compound extremes.