Maintenance of PDO variability during the mid-holocene in PMIP2

Using the Paleoclimate Modeling Intercomparison Project Phase 2 (PMIP2), we investigate Pacific Decadal Oscillation (PDO) variability during the Mid-Holocene (6,000 years ago), especially maintenance of the PDO variability during the mid-Holocene despite reduced El Nino-Southern Oscillation (ENSO) activity. Most of the models participating PMIP2 identified the reduced ENSO activity during the mid-Holocene, but essentially little difference in PDO variability between the pre-industrial and mid-Holocene periods. Through multiple regression and partial correlation analysis, we found that the influence of the ENSO on the PDO was reduced due to reduced ENSO activity, but that the influence of local atmospheric circulation (Aleutian Low; AL) was enhanced during the mid-Holocene even though the variability of AL itself did not change much between the mid-Holocene and the present. That is, the reduction in PDO activity due to the weaken ENSO is offset by the enhanced PDO attributable to the intensified influence of the AL. As a result, the PDO variability during the mid-Holocene was as active as that during the pre-industrial era.