Why this matters

Aerosols from surroundings of the Tibetan Plateau (TP) play a substantial role in the rapid shrinking of glaciers in the TP. Particularly, with nearly half of the world’s population and heavy industry, South and East Asia adjacent to the TP are the world’s hotspots for aerosol pollution and large amounts of aerosols over the TP come from South and East Asia. Previous studies mainly investigated the transboundary transport of aerosols over the TP from the tropospheric perspective, with little attention paid to the stratospheric dimension. The downward propagation of anomalous signal pulses of the Arctic stratospheric polar vortex has significant impact on weather, climate, and atmospheric environment in the troposphere. Currently, there is still a lack of systematic and comprehensive understanding of the impact of the Arctic stratospheric polar vortex anomaly on the transboundary transport of aerosols over the TP, which deserves urgent investigation.

What I found

Therefore, this study, for the first time, examined the impact of the sudden stratospheric warming (SSW) in January 2021 on the transboundary transport of aerosols from South Asia to the TP. The results indicated that the SSW event in January 2021 contributed 30%–40% to the reduction in the aerosol transport from South Asia to the TP. This was because the Rossby wave train, disturbed by the downward propagation of abnormal signal pulses of the SSW, propagated eastward and formed an anticyclone from the Persian Gulf to northern China. Located to the south of this anomalous anticyclone, easterly anomalies dominated over northwest South Asia and the southern TP, hindering the transport of aerosols from South Asia to the TP. This study provides a reference for the prediction of air quality over the TP. 

What I have done

WRF-Chem version 3.9 was used in this study. To validate the model performance on meteorology and chemistry over South Asia and the TP, and to quantitatively calculate the cross-border transport flux of aerosols from South Asia towards the TP, one free-running simulation was conducted, specifically namely CONT. The model domain of this CONT experiment was centered at 33 °N, 88.20 °E, encompassing the entire TP and its neighboring regions. The model domain had 190 grid points in the west-east direction and 130 grid points in the south-north direction, with a horizontal resolution of 30 km. Vertically, it spanned 54 sigma levels from the ground to 20 hPa.

To demonstrate the impact of the SSW event in January 2021 on the transboundary transport flux of aerosols from South Asia towards the TP, we conducted two nudging simulations, namely NSSW and NCLM. In the nudging run of NSSW, the stratospheric circulation was nudged towards the observed evolution represented by the ERA5 reanalysis dataset from November 2020 to February 2021. In the nudging run of NCLM, the stratospheric circulation was nudged towards the climatological state of ERA5 dataset from November to February during 1991 to 2020. The tropospheric circulation remained unchanged in the two sets of nudging runs. The physical and chemical configurations in these two nudging runs were identical to those in the CONT run, with the exception that the spectral nudging technique was applied to temperature, horizontal winds, and geopotential height. Then, the difference in the transport flux of aerosols between these two nudging runs can be considered as the effect of the SSW event in January 2021 on the aerosol transport flux from South Asia towards the TP. The results were also validated by the MERRA-2 reanalysis.

To illustrate the impact of the SSW event in January 2021 on the tropospheric circulation over Eurasian continent, we designed two additional sets of nudging simulations solely for meteorology, with no chemistry included, namely NC_SSW and NC_CLM. In the nudging run of NC_SSW, the stratospheric circulation was nudged towards the observed evolution represented by the ERA5 reanalysis dataset from November 2020 to February 2021. In the nudging run of NC_CLM, the stratospheric circulation was nudged towards the climatological state of ERA5 dataset from November to February during 1991 to 2020. The tropospheric circulation remained unchanged in the two sets of nudging runs. Thus, the difference in tropospheric circulation between the simulations of NC_SSW and NC_CLM can be considered as the tropospheric circulation anomalies induced by the SSW event in January 2021. The physical parameterization options in these two nudging runs were identical to those in the CONT run. The spectral nudging technique was applied to temperature, horizontal winds, and geopotential height. It should be noted that the model domain of these two nudging runs was centered at 40° N, 50° E, encompassing the Eurasian continent and its neighboring regions. The model domain had 380 grid points in the west-east direction and 260 grid points in the south-north direction, with a horizontal resolution of 30 km. Vertically, it spanned 54 sigma levels from the ground to 20 hPa.