Changes in ENSO Activity During the Last 6,000 Years Modulated by Background Climate State

Various proxy records show that El Nino-Southern Oscillation (ENSO) activity has changed from calm to active during the last 6,000 years. However, it is so far unclear whether orbital forcing has solely induced such a dramatic change. In this study, we performed a transient run for the last 6,000 years using an Earth system model of intermediate complexity affected by orbital forcing only without changes due to other climate forcing, and then its time-varying background states were implemented into an intermediate atmosphere-ocean coupled model for ENSO. ENSO activity simulated by the intermediate atmosphere-ocean coupled model during the last 6,000 years resembled the observed proxy data, inferring that orbital forcing mainly leads to changes in ENSO activity during the last 6,000 years. From additional sensitivity experiments, we found that a change in sea surface temperature background conditions is primarily responsible for the observed ENSO activity over the last 6,000 years through modifying the anomalous horizontal thermal advection of the mean SST gradient.