Impacts of Rossby wave and local transient wave forcing on a heavy precipitation event in early April 2023 in eastern China

To explore the evolution mechanism of atmospheric circulation anomalies during the extreme heavy precipitation event in eastern China in early April 2023, Rossby wave energy dispersion and transient forcing are diagnosed based on the NCEP daily reanalysis dataset. Impacts of Rossby wave propagation and local transient wave forcing on the establishment and maintenance of the double-blocking circulation distribution in the middle to high latitudes over Eurasian region and possible precursor are investigated, and possible mechanisms behind the double-blocking circulation distribution in the middle to high latitudes that affected the extreme heavy precipitation in eastern China in early April 2023 are explored. Results are as follows: Under the strong polar vortex in earlier period, a teleconnection similar to the polar Eurasian distant correlation (POL) caused abnormal positive anomalies of geopotential height from the Ural mountains to Northeast Asia. Meanwhile, Rossby waves triggered by heavy precipitation in the North Atlantic propagated downstream. The combined effects of upstream Rossby waves and transient forcing of cold air in the west over lake Baikal caused a relatively stable "+−+" pattern of double-blocking circulation from the Ural mountains to Northeast Asia, namely the pattern of the "Ural mountains blocking high, Baikal lake trough, and Northeast Asia blocking high". The Ural mountain blocking high and the Northeast Asia blocking high are abnormal heat sources, both of which showed a barotropic atmospheric structure; the lake Baikal low trough is an abnormal cold source with baroclinic structure. The meridional wind anomalies under the double-blocking circulation strengthened southward (northward) transport of cold (warm) air. When the weak fluctuations of the subtropical westerly wind jet over the eastern China were in phase with Rossby waves in the middle to high latitudes, the "+−+" pattern of Rossby waves extended to the south and strengthened meridional winds. Under the combined effects of the strong southerly winds on the southwest side of the low-level anticyclone and the warming of the sinking airflow associated with the Northeast Asia blocking high, temperature increased in eastern China. When the cold air over western lake Baikal descended and moved to the south, strong cold front, cyclone and shear line formed one after another from north to south in eastern China, leading to snowstorms, cold waves, strong convections and heavy precipitation. When Rossby waves were triggered again in the North Atlantic and propagated downstream, the eastern North Atlantic blocking high strengthened and moved eastward, the Ural mountain blocking high was weakened and collapsed, and the precipitation in the eastern China ended.