Dominant mode of winter atmospheric circulation affected by autumn SST anomalies over North Pacific：Thermal and dynamic perspectives

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.