Subseasonal forecast barrier of the North Atlantic oscillation in S2S models during the extreme mei-yu rainfall event in 2020

Enhanced predictability of high-impact weather events is a Subseasonal to Seasonal Prediction Project (S2S) priority. In early summer 2020, a record-breaking heavy rainfall event occurred over the Yangtze River valley during the mei-yu season (June and July). Here we evaluate the S2S model forecast performance concerning the summer 2020 extreme mei-yu event over the Yangtze River valley. Our results show all operational S2S models exhibit fluctuating high-low–high forecast skill patterns during this three-stage mei-yu event, determined by different dominant circulations. In particular, the poor mid-latitude circulation forecast (low trough over northern China) in the North Atlantic Oscillation (NAO) negative phase, with relative more contribution than that of the low-latitude circulation (western North Pacific subtropical high and South Asian high), decreases the model forecast skill for mei-yu rainfall in early-mid July. The prediction barrier of the NAO pattern and its downstream response, via the eastward-propagating wave train, jointly decrease the rainfall forecast skill during the NAO active negative period. Therefore, the skillful model prediction of the active summer NAO could help predict mei-yu events in the East Asian summer monsoon, suggesting further model improvements to the S2S forecasting of summertime mid-latitude variations are urgently needed.