Strengthening and Westward Shift of the Tropical Pacific Walker Circulation during the Mid-Holocene: PMIP Simulation Results

Based on the zonal mass streamfunction, the mid-Holocene annual and seasonal changes in the tropical Pacific Walker circulation (PWC) are examined using numerical simulations from the Paleoclimate Modelling Intercomparison Project Phases 2 and 3. Compared to the preindustrial period, the annual mean of the PWC intensity strengthened (with an average increase of 0.26 X 10(14) kg(-2) m(-22) s(-1) or 5%), and both the western edge and center of the PWC cell shifted westward (by an average of 4 degrees and 3 degrees, respectively) in the majority of the 29 models used for analysis during the mid-Holocene. Those changes were closely related to an overall increase in the equatorial Indo-Pacific east-west sea level pressure difference and low-level trade winds over the equatorial Pacific. Annual mean PWC changes come mainly from boreal warm seasons. In response to the mid-Holocene orbital forcing, Asian and North African monsoon rainfall was strengthened due to large-scale surface warming in the Northern Hemisphere in boreal warm seasons, which led to an intensified large-scale thermally direct east-west circulation, resulting in the enhancement and westward shift of the tropical PWC. The opposite occurred during the mid-Holocene boreal cold seasons. Taken together, the change in the monsoon rainfall over the key tropical regions of Asia and North Africa and associated large-scale east-west circulation, rather than the equatorial Pacific SST change pattern, played a key role in affecting the mid-Holocene PWC strength.