Reduced ENSO Variability During the Onset of the 4.2 ka Event

Abstract Atmosphere‐ocean dynamics in the tropics play a key role in global climate change. An abrupt cooling event that occurred between 4500 and 3900a BP, known as the 4.2 ka event, has long been believed to be linked to changes in sea surface temperatures (SSTs) associated with intensified El Niño and Southern Oscillation (ENSO) variability. However, the precise timing and amplitude of ENSO variability during the 4.2 ka event remain uncertain, largely due to the lack of resolution in records from the ENSO‐sensitive regions. Here we present a 104‐year‐long monthly resolved coral record from the South China Sea that spans from 4400 to 4300 years BP, corresponding to the onset of the 4.2 ka event. Using the Sr/Ca paleo‐thermometry, we show a significant decrease in ENSO variability during this time interval compared to the modern period. The ENSO events decrease in the frequency from one every 3.5 years during the modern era to one every 5.6 years during the onset of the 4.2 ka event, with weaker magnitude in the latter period. This reduction in ENSO variability is largely associated with an intensification of the Pacific Walker circulation. We therefore suggest that ENSO variability was a response to, not a driver of, the development of the 4.2 ka event.