Prediction skill and predictability of precipitation during Meiyu and rainy season in North China using ECMWF subseasonal forecasts

The rainy seasons in the Yangtze River valley (called Meiyu) and North China represent two main stages with the northward advance of the East Asian summer monsoon. This study investigates the precipitation prediction skills between these two rainy seasons using the S2S hindcast data from the European Centre for Medium-Range Weather Forecast (ECMWF) during 2002-2021. Meiyu exhibits higher precipitation forecast skill compared to the rainy season in North China. In particular, the forecast skill is more pronounced in the southeast region during the Meiyu, while it has negative performance in central regions of the rainy season in North China. Precipitation patterns during the Meiyu are strongly influenced by the single-blocking system in the high latitude extending from the east of Lake Baikal to the Sea of Okhotsk, as well as the subtropical high at 500 hPa geopotential height in the low latitude. For the rainy season in North China, anomalies in the 500 hPa geopotential height over northeast China has a significant impact on the precipitation according to reanalysis data. The forecast skill of precipitation differs between the two rainy seasons, as well as their associared biases. The model biases are attribute to both circulation simulation and its influence on precipitation. Specifically, the prediction skills of circulation in the low latitudes are high during the Meiyu, and the relationship between circulation and precipitation is well captured. However, the prediction of circulations in the high latitude shows lower skill. During the rainy season in North China, the prediction skills for circulation in key areas are relatively high, but the relationship between circulation in key areas and precipitation is not captured, or may even exhibite an opposite relationship. Meanwhile, the model circulations at different latitudes have different responses to the El Nino state, with more biases observed in the high latitudes. In summary, the decline in prediction skills with the northward advance of the East Asian summer monsoon emphasizes the crucial role of predictability in the high latitudes.