Coupled model simulations of mid-Holocene ENSO and comparisons with coral oxygen isotope records

The sensitivity of El Niño-Southern Oscillation (ENSO) to changes in mean climate is investigated for simulations of pre-industrial and mid-Holocene (6000 years before present) climate using the Hadley Centre coupled atmosphere-ocean model, HadCM3. Orbitally-forced changes in insolation in the mid-Holocene produce changes in seasonality which may alter ENSO amplitude and frequency. The model simulations are compared with midHolocene fossil coral oxygen isotope records from the western Pacific Warm Pool. The coral records imply a reduction of around 60% in the amplitude of interannual variability associated with ENSO in the mid-Holocene, while the model simulates a smaller reduction in ENSO amplitude of around 10%. The model also simulates a slight shift to longer period variability and a weakening of ENSO phase-locking to the seasonal cycle in the mid-Holocene. There is little change in the pattern of ENSO tropical precipitation teleconnections in the simulated mid-Holocene climate.