The Western Pacific Warm Pool (WPWP) plays a key role in modulating tropical climate and in the initiation of El Niño - Southern Oscillation (ENSO) events, however it is not known how the warm pool and ENSO will respond to future greenhouse warming. This study aims to use records of Sea Surface Temperature (SST) and rainfall extracted from mid-Holocene fossil corals to examine the nature of ENSO when SSTs appear to have been slightly warmer than present.

Modern and fossil Porites sp corals were drilled from Koil and Muschu Islands, Papua New Guinea, located in the central WPWP and within the path of the Sepik River flood plume. Here reduced SST and rainfall manifest ENSO events and changes in these parameters are reflected in coral skeletal oxygen isotope (d ¹⁸O) and Sr/Ca ratios.

Fossil coral d ¹⁸O results show an average decrease in d ¹⁸O values from 7055 to 4675 yrs BP followed by a small increase to present d ¹⁸O values. Reduced interannual range in d ¹⁸O values (~0.4‰ range) compared to the modern corals (~0.6‰ range) persists until 1865 yrs BP. The fossil coral D d ¹⁸O results for 7055 yrs BP indicate cooler/drier than present conditions, assuming that the d ¹⁸O of monsoonal rainfall for the Warm Pool area was similar to modern values in the mid-Holocene. The dry condition persisted to 4985 yrs BP when coral Sr/Ca ratios indicate a warm SST peak ~1° C higher than present. By 4675 yrs BP conditions are perhaps slightly cooler and wetter than present.

The d ¹⁸O results suggest that the ENSO may have been weaker from 7055 to 1865 yrs BP, however some years with higher d ¹⁸O values may reflect weak-moderate ENSO events. Although the initial Sr/Ca results need to be verified, the 1° C warmer than present temperature at 4985 yrs BP, coupled with drier conditions, suggests there was no thermostat effect acting to regulate SSTs in the mid-Holocene Warm Pool. This result indicates that it may be possible to have warmer SSTs coupled with ENSO events.