Influence of compound zonal displacements of the South Asia high and the western Pacific subtropical high on Meiyu intraseasonal variation

Meiyu shows substantial intraseasonal variation at periods of 10–30 days and 30–60 days, which often leads to extreme precipitation and disastrous flooding over the Yangtze River basin. Monitoring and prediction of the intraseasonal variation of Meiyu is crucial for disaster prevention and mitigation. Here, we proposed two sets of novel indices for Meiyu real-time monitoring and prediction based on the compound zonal displacements of the South Asia high (SAH) and the western Pacific subtropical high (WPH) at 10–30-day and 30–60-day period, respectively. For the 10–30-day period of Meiyu, the zonal displacement of the SAH is associated with a mid-latitude Eurasian Rossby wave train, whereas the WPH is related to the second mode of the boreal summer intraseasonal oscillation. On the 30–60-day timescale, the zonal displacement of the SAH and the WPH are both associated with the first mode of the boreal summer intraseasonal oscillation. The subtle differences in zonal displacement of the SAH and the WPH determine eight type configurations, corresponding to distinct influences on Meiyu. Meiyu intraseasonal variation can be well reconstructed by using the relationship between these two indices and rainfall anomalies pattern over China. Given that the ECMWF S2S model is more skillful in forecasting upper- and lower-level circulation than in directly forecasting precipitation, a hybrid dynamical–statistical model is conducted to subseasonal prediction of Meiyu using the ECMWF model forecast indices. The hybrid model outperforms the ECMWF model in subseasonal prediction of the Meiyu variation at 17–40-day lead times.