Role of Madden-Julian Oscillation in predicting the 2020 East Asian summer precipitation in subseasonal-to-seasonal models.

The 2020 summer monsoon season in East Asia was unusually long and intense, and the Madden-Julian Oscillation (MJO) has been proposed as an underlying reason. This study analyzes the role of the MJO in the 2020 East Asian precipitation forecasts of the subseasonal-to-seasonal (S2S) model. The S2S models underestimated the cumulative precipitation over East Asia, and the models with good forecast performance yielded a distinct precipitation band over East Asia and a western pacific subtropical high (WPSH) during the analysis period. East Asian precipitation forecast performance was more closely related to the location of the center than the strength of the WPSH, with precipitation increasing with a decrease in the latitude at the center. MJO Phases 1-3 activation intensified the WPSH and shifted the center of WPSH to lower latitudes. Our results confirm that the strong East Asian precipitation in summer 2020 was partly due to changes in the characteristics of the MJO and indicate the importance of accurately estimating the MJO-WPSH coupling for reliable East Asian precipitation forecasts.