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

Pacific Walker Circulation (PWC) is an important component 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 variations. In this study, we investigate PWC changes 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-based reconstructions. 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 internal variabilities contribute approximately 80% to the changes of PWC during the LM, among which the Interdecadal Pacific Oscillation (IPO) has the largest contribution. In the positive phase of the IPO, the Indo-Pacific sea-level pressure (SLP) gradient decreases, and anomalous westerlies occur in the tropical western Pacific, corresponding to a weakened PWC. The relationships between the IPO and PWC show multidecadal to centennial fluctuations, suggesting that other internal modes or external forcings may have modulated the IPO-PWC relationship. Volcanic forcing is also an important contributor to PWC variability during the LM. The simulated PWC significantly weakens and lasts for 1-2 years after large volcanic eruptions. The El Niño-like SST pattern and the corresponding zonal SLP gradient lead to the PWC weakening following large volcanic eruptions.