Extreme precipitation over northern China in autumn 2021 and joint contributions of tropical and mid-latitude factors

Frequent heavy precipitation and severe flood disasters struck northern China in early autumn 2021,with seven provinces/municipalities surpassing their precipitation records since 1961.Causes for such extreme events from the climate perspective are unclear because the variability of autumn precipitation in northern China is poorly understood.This study revealed that the event was directly caused by the extremely north-westward location of the western Pacific subtropical high(WPSH),which was jointly induced by several tropical and mid-latitude factors.The developing La Niña and warmer than normal tropical Atlantic sea surface temperature altered the Pacific Walker Circulation and enhanced the convection over the tropical western Pacific.Meanwhile,the Madden-Julian Oscillation stagnated for a long time in its phases 3-5 in early autumn and contributed constructively to the active convection over the tropical eastern Indian Ocean and western Pacific.These effects altogether excited a lower-tropospheric cyclone anomaly to the west of the Philippines and shifted the WPSH northwestwards in early autumn.A mid-latitude cyclonic centre was also observed upstream of East Asia.It dispersed Rossby wave energy southeastwards in the form of cold air invasions.The resultant convergence of cold and dry air from the north with warm and moist air from the south together caused extreme precipitation over northern China in autumn 2021.Relevant conclusions can enhance our current understanding of the causes of autumn extreme precipitation events in northern China and improve our ability to predict such events and prevent resultant disasters.