Causes and predictability of the inter-month evolution of extreme precipi-tation over eastern China in midsummer 2021
In July-August 2021,the evolution of the rain belt in eastern China deviated significantly from the typ-ical northward progression of the climatological monsoon.The center of positive precipitation anomalies was loca-ted in Jianghuai-North China region in July and shifted southward to the Central China region in August.These re-gions experienced the highest and second-highest average precipitation anomalies on record since 1979,respec-tively.The inter-month differences in precipitation anomalies were primarily associated with the northeast(south-west)displacement of the western Pacific subtropical high(WPSH),the northward(southward)shift of the East Asian subtropical westerly jet,and the continuous eastward extension of the South Asia high(SAH)in July(Au-gust)2021.The active tropical convection and enhanced warming of the North Atlantic Ocean were key factors influencing the southward shift of the precipitation center.In July 2021,the Madden-Julian Oscillation(MJO)was active over the Maritime Continent,intensifying tropical convection and triggering the northward propagation of a Pacific-Japan-like wave train.This caused the WPSH to shift northeastward,promoting water vapor convergence in the Jianghuai-North China region and resulting in increased precipitation.In August,the MJO reactivation over the tropical Indian Ocean strengthened local meridional circulation,leading to anomalous downward motion from southern China to the northwest Pacific Ocean.This favored the southward and westward extensions of the WPSH.Additionally,anomalously warm North Atlantic Sea surface temperature(SST)in August 2021 simulated the southeastward propagation of Rossby waves in the upper troposphere,intensifying the SAH and causing the East Asian subtropical westerly jet to strengthen and shift southward.Consequently,warm and moist air from the tropi-cal western Pacific converged in Central China,shifting the precipitation center southward in August.The Climate Forecast System,version 2(CFSv2,reported in June),accurately predicted positive precipitation anomalies for July 2021 in most parts of the Jianghuai-North China.However,it incorrectly predicted negative precipitation a-nomalies for August in southern China.The model successfully reproduced the northward movement of the WPSH and the influence of tropical convective activity on the Maritime Continent during July precipitation in the Jiang-huai-North China.However,it failed to predict the effects of tropical Indian Ocean convection and anomalously warm North Atlantic SSTs in August 2021.Consequently,the model could not replicate the southward and strengthened deviation of the East Asian subtropical westerly jet or the intensified and eastward deviation of the SAH,leading to inaccurate precipitation predictions for Central China in August.The extreme precipitation anoma-lies in eastern China during midsummer 2021 were significantly influenced by the frequency and intensity of ty-phoons.Additionally,the predictive efficiency of dynamic models for the inter-monthly evolution of midsummer precipitation was limited.Developing an effective prediction model that integrates dynamic and statistical approa-ches is necessary to improve monthly-to-seasonal climate predictions in the future.
extreme precipitation in eastern China in July-Augustinter-month evolution and variabilityCFSv2 predictabilitycirculation anomaliestropical convection and MJONorth Atlantic SST
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