The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020

摘要

During June and July of 2020,the Yangtze River basin suffered from extreme mei-yu rainfall and catastrophic flooding.This study explores the seasonal predictability and associated dynamical causes for this extreme Yangtze River rainfall event,based on forecasts from the Met Office GloSea5 operational forecast system.The forecasts successfully predicted above-average rainfall over the Yangtze River basin,which arose from the successful reproduction of the anomalous western North Pacific subtropical high(WNPSH).Our results indicate that both the Indian Ocean warm sea surface temperature(SST)and local WNP SST gradient were responsible for the westward extension of the WNPSH,and the forecasts captured these tropical signals well.We explore extratropical drivers but find a large model spread among the forecast members regarding the meridional displacements of the East Asian mid-latitude westerly jet(EAJ).The forecast members with an evident southward displacement of the EAJ favored more extreme Yangtze River rainfall.However,the forecast Yangtze River rainfall anomaly was weaker compared to that was observed and no member showed such strong rainfall.In observations,the EAJ displayed an evident acceleration in summer 2020,which could lead to a significant wind convergence in the lower troposphere around the Yangtze River basin,and favor more mei-yu rainfall.The model forecast failed to satisfactorily reproduce these processes.This difference implies that the observed enhancement of the EAJ intensity gave a large boost to the Yangtze River rainfall,hindering a better forecast of the intensity of the event and disaster mitigation.