The synoptic-scale potential vorticity intrusion over the Northeastern China during winter and its influence on surface air temperature

The regional potential vorticity (PV) intrusion (PVI) (RPVI) index, which is defined as the sum of the numbers of grids occurring PVI within a certain area for each day, is used to reflect the day-to-day PVI variability over the Northeastern China during winters from 1979 to 2016. The synoptic-scale PVI variations and resultant surface air temperature (SAT) anomalies are identified by comparing the high with low RPVI index cases. In high (low) RPVI cases, significantly strong positive (negative) PV anomalies are found in the stratospheric mid-latitudes, which intrude downward mostly within 90°E-110°E into the upper troposphere to reach around 300 hPa and extend eastward to the east of 120°E, forcing an anomalous cyclonic (anticyclonic) circulation in middle and lower troposphere over East Asia, with the anomalous northerlies (southerlies) of the forced lower-tropospheric cyclone (anticyclone) leading to significant negative (positive) SAT anomalies less (greater) than -0.9°C (0.9°C) especially over the northeastern China. In the stratosphere, the positive (negative) mid-latitude PV anomalies are actually associated with the weakening (strengthening) of the polar vortex over the Eurasian continent for the high (low) RPVI cases, facilitating (impeding) high stratospheric PV anomalies over the polar region to transport southwestward toward northeastern China along the southeastern side of the anomalous anticyclone (cyclone).