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Abstract Proxy-based reconstructions of the Asian Summer Monsoon exhibit variability over multiple scales, from inter-annual to centennial. The relationship between East Asia Summer Monsoon (EASM) variability and the El Niño-Southern Oscillation (ENSO) over the last millennium remains unclear and a focus of current research. This study investigates this relationship using observations, reanalysis data, proxy-based reconstructions, and a control simulation using the Community Earth System Model (CESM). The results derived from both the observation/reanalysis/proxy-based reconstruction and the simulation show consistently that the relationship between EASM strength and Niño3.4 SST is divergent. The observation/reanalysis data illustrate strong negative correlations between Niño3.4 SST and EASM strength at an annual scale in the 1920s-1930s and 1980s-1990s, with weak correlations during other periods. The proxy-based reconstruction demonstrates strong negative correlations at inter-decadal scales from the 1360s to the 1530s, with either weak correlations or positive correlations during other periods. Moreover, the simulations reveal positive correlations in some periods and negative or weak correlations in other periods. The simulations demonstrate that the divergent relationship may be derived from multi-decadal variations in Pacific SST caused by internal variability in the climate system, likely due to the Inter-decadal Pacific Oscillation (IPO). The EASM variation is usually in (out of) phase with Niño3.4 SST during the positive (negative) phase of the IPO. In the positive phase of the IPO, there is a tripolar pattern expressed in a cyclone-anticyclone-cyclone arrangement at pressure level of 850 hPa from the tropics to mid-latitudes over the northwest Pacific. Warming of the Niño3.4 strengthens the tropical cyclone by weakening the trade winds and subsequently strengthening the subtropical anticyclone, leading to a strong Meiyu front, which suggests a strong EASM. However, in the negative phase of the IPO, a huge cyclone occupies the northwest Pacific. Warming of the Niño3.4 strengthens the cyclone by weakening the trade winds, which leads to a weak Meiyu front, which suggests a weak EASM. These results may be helpful in further understanding the mechanisms through which the ENSO and IPO impact the atmospheric circulations in the Asian Pacific, especially regarding the long-term variations in EASM at inter-annual to inter-decadal scales. [ABSTRACT FROM AUTHOR]