Extreme cooling in Northeast China in January and concurrent maximum in the lower stratospheric temperature and ozone: Case studies

This paper analyzes the four severe cooling events in Changchun, Northeast China, in January for selected years. The events in the absence of sudden stratospheric warmings were chosen. This excludes the possible effects of warm stratospheric anomalies in the polar vortex region, which may affect midlatitudes. The meridional profiles and maps for the coldest and warmest days of January are compared using the temperature and ozone data from observations and reanalysis. Based on the vertical temperature structure compared to the coldest–warmest days, the lower stratosphere–surface thermal coupling was analyzed, which has not been studied previously. It has been shown that (i) temperature increase by 5–10 °C in the lower stratosphere, (ii) lowering the tropopause by about 2–3 km, (iii) pushing the cold midtropospheric layer from climatological about 3 km to the surface with formation of the midlatitude temperature minimum, and (iv) a decrease in surface temperature in Changchun by 15–24 °C, concurrently occurred on the coldest days compared to the warmest days. Cold air in the region of the midlatitude minimum temperature, merging with the cold air of higher latitudes, contributes to the formation of the cold air outbreak pattern. The main elements of stratospheric and tropospheric dynamics (quasi-stationary and traveling planetary waves, zonally asymmetric Brewer–Dobson circulation, deformation of the tropopause and tropospheric isotherms) involved in extreme cooling events are analyzed. Because of the positive correlation between total ozone and temperature in the lower stratosphere, extremely high total ozone over the midlatitude region may serve as an indicator of warm stratospheric anomaly and possible downward thermal forcing.