To explore the impact of Sea Surface Temperature Anomaly (SSTA) in the North Pacific on extreme weathers in China, the thermal and dynamic diagnostic method that combines ocean-atmosphere coupling and weather transient eddies is adopted to study the impact and possible mechanism of SSTA in the North Pacific on atmospheric circulation. The Maximum Covariance Analysis (MCA), regression, and correlation analysis are applied to daily and monthly NCEP/NCAR reanalysis data from 1977 to 2023 with ENSO influences being removed. The results indicate that early autumn (August to October, ASO) North Pacific SSTA significantly affects early winter (Octber to Decenber, OND) atmospheric circulation and the corresponding spatial distribution pattern is the main mode. The leading mode of early autumn North Pacific SSTA has a clear "negative in the west-positive in the east" dipole distribution characteristic, and the corresponding early winter atmospheric circulation anomaly shows a "PNA (Pacific-North American teleconnection pattern) like" pattern. The positive and negative abnormal circulation centers shift northward and eastward compared to the SSTA centers, and their vertical distributions show an equivalent barotropic structure. The ocean and atmosphere in the North Pacific during autumn and winter is coupled through diabatic heating and transient forcing. In late summer (July to September, JAS), the anomalous wave train of the "east-west dipole" type atmospheric circulation deepens the low-level Aleutian low pressure circulation, strengthens the cold (warm) atmospheric advection, while turbulent heat flux is released (obtained) and positive (negative) diabatic heating anomalies occur. Correspondingly, the "east-west dipole" SSTA in early autumn strengthens, the sea front in the southern part of the negative SSTA area becomes stronger, baroclinic atmospheric activities in the lower atmosphere and transient activities in the upper levels both increase. Positive anomalies of diabatic heating are transported upwards, generating baroclinicity in the atmosphere. The transient activities lead to the formation of negative (positive) potential height anomalies and (anti) cyclonic circulation to the north (south) of the upper-level jet stream, which are stronger in the upper levels. The forcing of upper transient activities increases with time and moves southward, which, combined with diabatic heating anomalies, lead to the change of the anomalous wave train of atmospheric circulation from an "east-west dipole" to a "PNA like" mode distribution. The upper and lower structures and intensities undergo a transition process of "equivalent barotropic—weak baroclinic—equivalent barotropic" and "strong—slightly weakened—significantly strengthened", respectively. On the contrary, the anomalous atmospheric circulation forces the SSTA to transform from an "east-west" dipole to a "horseshoe shaped" pattern, and the SSTA undergoes a "strong—weak—weak" transition in intensity from early autumn to early winter. The North Pacific SSTA in early autumn causes significant increases in atmospheric circulation anomalies in early winter through sustained effects and transient activity forcing, and these anomalies are statistically significant. The atmospheric circulation anomaly corresponding to the "PNA like mode" in early winter is a significant wave train propagating downstream to North America, which contributes to the formation of the intrinsic variability PNA mode in the winter atmosphere. When the atmospheric circulation anomaly in early winter is in the same (opposite) phase with the winter PNA mode, the PNA mode strengthens (weakens), and the changes in the strength of the PNA mode have certain indicative significance for predicting extreme weathers in 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.