Forecasting Meridional Swings of the Mei-Yu Rain Belt and Western Pacific Subtropical High During 2020 Summer Extreme Flood

Abstract The persistent extreme rainfall along the mei-yu front, as a pivotal weather phenomenon in East Asia in the boreal summer, often triggers devastating flood disasters. Notably, during June–July 2020 (JJ2020), the superheavy mei-yu rain belt exhibited pronounced meridional swings, rather than extreme precipitation intensity or duration, but this has received less attention in the literature. This study first identifies the latitudinal location of the rain belt (LLR) during the 2020 mei-yu season and then examines its relationship with the ridge line of the western Pacific subtropical high (WPSH-RL) and further evaluates its forecasting, utilizing real-time forecasts from the Global Ensemble Prediction System of the European Centre for Medium-Range Weather Forecasts (ECMWF-GEPS). We demonstrate that the LLR has a strong correlation with the WPSH-RL 2 days earlier during JJ2020 and the ECMWF-GEPS exhibits considerable performance in predicting the meridional swings of both the LLR and WPSH-RL though the prediction skills tend to decrease slightly when removing their seasonal northward migrations. These clearly indicate the ECMWF-GEPS’s good capability of capturing their synoptic and intraseasonal meridional swings and emphasize the importance of the WPSH-RL in forecasting the LLR. Moreover, for the short- and medium-term forecasts, the range between the 25% and 75% percentiles of ensemble members can well predict the recurring meridional swings in both the LLR and WPSH-RL. Therefore, a probabilistic forecast approach incorporating the ensemble range could provide a more skillful prediction than only using the ensemble mean. Significance Statement It is well known that the persistent extreme rainfall along the mei-yu front often triggers devastating flood disasters, especially in June–July 2020. However, the recurring meridional swings, as a vital characteristic, have received less attention in the literature. In this paper, to investigate the recurring meridional swings, we first profile the lead–lag relationship between the ridge line of the western Pacific subtropical high (WPSH-RL) and the mei-yu rain belt. Further, we use real-time forecasts from the Global Ensemble Prediction System of the European Centre for Medium-Range Weather Forecasts (ECMWF-GEPS) to reveal the importance of the seasonal northward migrations on the forecasts of WPSH-RL and the rain belt. Finally, our results suggest that the probabilistic forecasts (interval between the 25th and 75th percentiles) can be a key signal to judge whether the WPSH-RL and the latitudinal location of the rain belt (LLR) will have recurring latitudinal swings. The findings in this study can assist us in profiling the recurring meridional swings, as well as in predicting them in advance.