Downward propagation of sudden stratospheric warming signals and the local environment in the Beijing-Tianjin-Hebei region: A comparative study of the 2018 and 2019 winter cases

Abstract

Based on the reanalysis data and PM2.5 concentration in the BTH (Beijing-Tianjin-Hebei) region, the two most recent sudden stratospheric warmings (SSWs, February 11, 2018 and January 2, 2019) in the Northern Hemisphere and their potential impact on the environmental condition are investigated. The stratospheric polar vortex changes more radically from a strong to a weak state during the 2018 SSW than the 2019 SSW, and the stratospheric signal during the 2018 SSW propagates downward deeper than that during the 2019 SSW. In the pre-SSW period, enhanced climatological wave activities lead to more exchange between midlatitudes and high latitudes, and cold air outbreak in this period corresponds to a low BTH PM2.5 concentration. In the tropospheric wave weakening period due to its upward propagation to disturb the stratosphere (days 5–20), the BTH PM2.5 concentration increase rapidly, reaching the light pollution level. The February 2018 SSW shows a stronger downward propagating signals in the post-SSW period, which favors weakening of the pressure contrast between Arctic and midlatitudes and strengthening of the East Asian winter monsoon systems in the middle troposphere, resulting in diffusion and dilution of the pollutants. In contrast, the PM2.5 concentration is still high in the post-SSW period during the 2019 event, consistent with the non-downward propagation of the stratospheric signal. The lower tropospheric systems do not necessarily cooperate with the stratospheric variability to impact the regional pollutions.