Coupling of Equatorial Pacific Mean Climate State and ENSO Variability during the Holocene: Evidence from a High-Resolution Borneo Speleothem Record

The El Niño-Southern Oscillation (ENSO) is the primary driver of interannual climate variability in the tropics. Despite substantial progress in understanding ocean-atmosphere feedbacks that drive ENSO today, debates remain about its relation to the mean climate state of the equatorial Pacific, as well as its response to external climate forcing on centennial and longer timescales. Here we present a δ18O record from a U-Th dated speleothem in Malaysian Borneo sampled at sub-annual resolution in six time windows of the Holocene. Our estimates of stalagmite δ18O variance at ENSO periods (2-7 years) show a significant reduction in interannual variability during the mid-Holocene relative to both the late Holocene and early Holocene. Our results support coral, mollusc and foraminiferal records from the central and eastern equatorial Pacific that show a mid-Holocene minimum in ENSO variance. Reduced mid-Holocene interannual δ18O variability in Borneo coincides with an overall minimum in mean δ18O in Borneo stalagmites from 3.5 to 5.5 kyr BP. Persistent warm pool convection would tend to enhance the Walker circulation during the mid-Holocene, potentially driving reduced ENSO variance. Thus both convective intensity and interannual variability in Borneo may be linked to precessional insolation forcing. A minimum in ENSO variance occurred around 8.2 kyr BP, with an increase in mean δ18O that is comparable in magnitude to Heinrich event-driven δ18O changes in Borneo, suggesting that the tropical climate system responds to meltwater discharge events in the North Atlantic on relatively short timescales. Isolating the exact mechanisms that drive long-term ENSO evolution will require additional high-resolution paleoclimatic reconstructions and further investigation of the tropical climate system using coupled climate models.