Weakening and eastward shift of the tropical Pacific Walker circulation during the Last Glacial Maximum

Abstract

The annual and seasonal changes in the tropical Pacific Walker circulation (PWC) during the Last Glacial Maximum (LGM) are investigated using all available numerical experiments from the Paleoclimate Modelling Intercomparison Project Phases 2 and 3. Compared to the preindustrial period, the annual mean of the PWC intensity weakened by an average of 0.96×1014 kg2 m−2 s−1 or 15%, and both the western edge and centre of the PWC cell shifted eastward by an average of 9° and 8°, respectively, as obtained from the ensemble mean of the 16 models used for analysis during the LGM. Those changes were closely linked with an overall weakening of the equatorial Indo‐Pacific east–west sea‐level pressure difference and low‐level trade winds over the equatorial west/central Pacific. On the seasonal scale, the LGM PWC generally weakened and shifted eastward throughout the seasons of year. In response to the LGM's large ice sheets and lower atmospheric greenhouse gas concentrations, large‐scale uneven surface cooling in the Northern Hemisphere led to an increased (a decreased) land–sea thermal contrast in boreal cold (warm) seasons. These induced decreases in the North Asian and African monsoon rainfall and hence suppressed a large‐scale thermally direct east–west circulation in the two seasons. As a result, the LGM PWC weakened and shifted eastward in both boreal cold and warm seasons, which jointly contributed to the weakening and eastward shift of the annual mean PWC.