Recent Enhanced Deep Troposphere-to-Stratosphere Air Mass Transport Accompanying the Weakening Asian Monsoon

The Asian monsoon (AM) region is a well-known region with prevailing stratosphere–troposphere exchange (STE). However, how the STE across this region changes with the weakening AM remains unclear. Here, we particularly diagnose the air mass transport between the planetary boundary layer (PBL) and the stratosphere over the AM region during 1992–2017 using the Lagrangian particle dispersion model FLEXPART based on the ERA-Interim reanalysis data. The results show that both the downward and upward deep STEs exhibit a detectable increasing trend, while the latter, namely, the deep troposphere-to-stratosphere transport (DTST), is relatively more significant. Further analysis reveals that the long-term trend of DTST over the AM region could be partly attributed to changes in the Pacific Walker circulation and the air temperature (especially at upper levels). Additionally, it is found that DTST increases markedly over the tropical oceanic regions, while the increasing DTST into the stratosphere can be attributed to the enhanced air masses originated from the PBL over the terrestrial regions, where large amounts of pollutant emissions occur. The results imply that the influence of the DTST on the chemical composition and the climate of the stratosphere over the AM region is expected to become increasingly important, and is thereby of relevance to climate projection in an evolving climate.