Novel mechanism for troposphere-to-stratosphere transport due to the interaction between typhoons and orography

Troposphere-to-Stratosphere Transport (TST) can inject anthropogenic pollutants from the Earth’s surface into the Upper Troposphere – Lower Stratosphere (UTLS), changing its chemical composition and influencing the radiative processes. Furthermore, TST may play a key role in sustaining the long-range transport of pollutants across the globe, particularly during extreme weather events. Indeed, during such events, a large quantity of pollutants can be transported from the Boundary Layer (BL) to the free atmosphere, enhancing the probability of long-range transport as the contaminants reach higher altitudes. The various mechanisms that contribute to TST have not yet been fully resolved due to the multi-scale nature of this transport process. In our study, we investigated the TST processes triggered by the transition of the typhoon Molave over the Philippines in the autumn 2020, combining a Lagrangian modeling tool with the Weather Research and Forecasting model. Our findings supported the proposal of a novel TST mechanism based on the interaction between typhoon updrafts, convection, orographic lifting, and gravity waves. Firstly, our results demonstrate that this interaction can rapidly transport air from the BL to the UTLS region, carrying a significant amount of pollutants despite deposition processes. Secondly, our work highlights the importance of gravity waves in the mixing processes close to the tropopause region. Overall, our study suggests that the interplay between typhoon episodes and mountainous regions can play an important, yet previously insufficiently considered, role in TST. This interaction influences key topics that are relevant to our society, such as the long-range dispersion of pollutants.