Integration of proxy data and model scenarios for the mid-Holocene onset of modern ENSO variability

Climate model studies have shown a gradual insolation-forced intensification of the El Niño–Southern Oscillation (ENSO) during the Holocene. Proxy records of past climate variability provide important test cases for such model simulations, and are needed to determine the exact mechanisms and dynamics of the ENSO system. We provide an integrated overview of marine and terrestrial paleoclimatic proxy data relevant for detecting ENSO variability. We reconstruct spatial climate patterns during two time-slices, 6–5 and 4.5–3.5 ka cal BP, to examine the mid-Holocene intensification of ENSO. The proxy data consistently indicate that a state change occurred at ∼5 ka cal BP towards active ENSO cyclicity in the equatorial Pacific. Furthermore, from around 3 ka cal BP the ENSO-teleconnected regions are characterized by an increased impact of ENSO, comparable to the present-day high-amplitude fluctuations of ENSO. Model studies have thus far explained the late-Holocene intensification of ENSO by insolation-forced Pacific trade wind reduction during summer. Our review shows that this single mechanism cannot completely explain the observed Holocene changes. An additional mechanism is proposed, involving increased Indo-Pacific Warm Pool (IPWP) heat charging, which is a possible explanation for the late-Holocene increase in ENSO amplitude.