The Henan extreme rainfall in July 2021: Modulation of the northward-shift monsoon trough on the synoptic-scale wave train

Abstract

The synoptic-scale wave train is a dominant pattern of the synoptic variability over the tropical western Pacific and usually affects the extreme weather over South China and Southeast Asia. Whether it could extend its influence and contribute to the Henan extreme rainfall in July 2021 still needs to be unraveled. We found that during the Henan extreme rainfall days a positively synoptic-scale vorticity disturbance dominated Henan province, China, which was embedded in the synoptic-scale wave train that originated from the western North Pacific. Moreover, the propagating pathway of this synoptic-scale wave train located northward and was likely modulated by the latitudinal location change of the monsoon trough over the western North Pacific. A northernmost displacement of the monsoon trough in July 2021 (∼23.2°N) would facilitate the synoptic-scale wave train to propagate farther northwestward via shifting the related barotropic conversion northward. Therefore, the synoptic-scale wave train from the tropics could reach Henan, provide the necessary lifting forcing, and supply abundant water vapor associated with the anomalous southerly for the occurrence of Henan extreme rainfall event. The results implicate that the pre-existing synoptic-scale wave train regulated by the location of the monsoon trough may be a potential precursor for heavy rainfalls in northern Central China.