Interdecadal Shift in the Influence of East Asian Winter Monsoon on El Niño: The Role of the Arctic Oscillation

Abstract El Niño events can be triggered by the westerly wind anomalies typically occurring over the western tropical Pacific (WTP). Previous studies have demonstrated that a strengthened East Asian winter monsoon (EAWM) can induce westerly wind anomalies over the WTP, which in turn can lead to the occurrence of El Niño events. This study encompasses data spanning from 1948 to 2020, revealing an interdecadal shift in the relationship between EAWM and El Niño events around the mid-1980s. Specifically, between 1948 and 1986, a strong EAWM was associated with westerly anomalies over the WTP, favoring the initiation of subsequent El Niño events. However, from 1987 to 2020, the connection between the EAWM and anomalous westerly winds over the WTP became insignificant, with the EAWM no longer triggering El Niño events. The mid-1980s coincided with a significant interdecadal shift in the winter Arctic Oscillation (AO) transitioning from an interdecadal negative phase to a positive phase. It is demonstrated that during the interdecadal negative phase of the winter AO from 1948 to 1986, the westerly anomalies over the WTP were predominantly driven by strong EAWM, with little influence from the winter AO. In contrast, during the interdecadal positive phase from 1987 to 2020, the WTP westerly anomalies were primarily related to AO, with the EAWM playing a negligible role.