Based on various observations and model simulations, the long-term changes in the present and future tropical Pacific Walker circulation (PWC) have remained inconclusive. In order to understand the PWC changes at varying time scales, the annual and seasonal changes in the 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 15%, and both the western edge and center 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 large ice sheets and lower atmospheric greenhouse gas concentrations, the 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.
图: 相对于工业化革命前期，16个模式集合平均模拟的赤道太平洋（5°S-5°N）末次冰盛期（a）年平均纬向质量流函数变化（填色和等值线；10^10 kg/s）和（b）垂直积分的纬向质量流函数的逐月变化（10^14 kg2/m2/s），打点区表示70%以上的模式与多模式集合平均变化符号一致。（a）中箭头为16个模式集合平均模拟的垂直速度（ω × -50；Pa/s）和纬向辐散风（m/s）的合成。
Figure. Last glacial maximum minus preindustrial changes in (a) annual mean of zonal mass streamfunction (shading and contours; units: 10^10 kg/s) and (b) monthly means of vertically integrated zonal mass streamfunction (units: 10^14 kg2/m2/s) along the equatorial Pacific (5°S-5°N) from the ensemble mean of 16 PMIP2/3 models. The dotted areas represent regions where at least 70% of the models agree on the sign of the ensemble mean change. Vectors in (a) are the composite of pressure velocity (ω × -50; units: Pa/s) and zonal divergent wind (units: m/s) from the ensemble mean of 16 PMIP2/3 models.
Zhiping Tian and Dabang Jiang, 2020: Weakening and eastward shift of the tropical Pacific Walker circulation during the last glacial maximum. Boreas, 49, 200–210, https://doi.org/10.1111/bor.12417.