The relative contributions of internal variability and external forcing to Pacific Walker Circulation over the last millennium

Pacific Walker Circulation (PWC) is one of the most active components of tropical atmospheric circulation. Current studies have mainly focused on PWC changes over recent decades and the near future, while less effort has been devoted to long-term PWC variability. In this study, we examine PWC variability over the last millennium (LM) based on the Community Earth System Model Last Millennium Ensemble (CESM-LME). The simulated PWC variations show no significant trend but do reveal decadal fluctuations during the LM, which underestimate the strengthened LIA-MCA PWC differences indicated by proxy records. A quantitative estimation of the contributions made to PWC variability from internal variability and external forcing is conducted by using multiple linear regression (MLR) analysis. The Interdecadal Pacific Oscillation (IPO) accounts for approximately 40% of the PWC variability at the decadal time scale. Volcanic forcing is also an important contributor to PWC variability during the LM. The positive IPO-like SST anomalies are associated with a weakened Indo-Pacific SLP gradient. The low-level wind change is characterized by enhanced westerlies in the tropical western Pacific, which is consistent with a weakened PWC. A significant PWC weakening appears during the first post-eruption year and returns to normal conditions thereafter. This east‒west contrast in equatorial Pacific SSTs, occurring in conjunction with strengthened surface westerly winds, has been suggested to be responsible for the change in PWC strength following large volcanic eruptions.