Mid-Holocene ENSO in PMIP2 and PMIP3

Using PMIP2 and PMIP3, we investigated the tropical Pacific climate state and El Niño-Southern Oscillation (ENSO) during the mid-Holocene period (6,000 years before present; 6ka run). The 6ka run in both the PMIP2 and PMIP3 recorded the reduced sea surface temperature (SST) and precipitation, and the intensified cross-equatorial surface winds, but the changes were more dominant in the PMIP3. The ENSO activity in the 6ka run was significantly suppressed in the PMIP2, but marginally reduced in the PMIP3. The weakened air-sea coupling associated with basin-wide cooling during 6ka, reduced precipitation, and a hemispheric contrast in the climate state led to the suppression of ENSO activity and the weakening of the annual cycle over the tropical eastern Pacific might lead to the intensification of ENSO through the frequency entrainment. On the whole, the variability of canonical (or conventional) El Niño tends to be reduced during 6ka, while that of CP/Modoki El Niño tends to be intensified. Central Pacific El Nino verse Eastern Pacific El Nino occurrence ratio obtained from (a) PMIP2 and (b) PMIP3. Blue and red bars indicate the ratio from 0ka and 6ka runs, respectively. Vertical error bar denotes the upper and lower limits associated with an increase and decrease of the occurrence ratio at the 95% confidence level, based on a bootstrap method