Calculation of stratosphere–troposphere exchange in East Asia cut-off lows: cases from the Lagrangian perspective

In this study, the authors focus on the cut-off low pressure systems (COLs) lingering over East Asia in late spring and early summer and quantify the two-way stratosphere–troposphere exchange (STE) by 3D trajectory integrations, achieved using a revised version of the UK Universities Global Atmospheric Modelling Programme Offline Trajectory Code (Version 3). By selecting 10 typical COLs and calculating the cross-tropopause air mass fluxes, it is found that stratosphere-to-troposphere transport (STT) fluxes exist in the center of COLs; and in the periphery of the COL center, troposphere-to-stratosphere transport (TST) fluxes and STT fluxes are distributed alternately. Net transport fluxes in COLs are from stratosphere to troposphere, and the magnitude is about 10−4 kg m−2 s−1. The ratio between the area-averaged STT and TST fluxes increases with increasing strength of the COLs. By adopting appropriate residence time, the spurious transports are effectively excluded. Finally, the authors compare the results with previous studies, and find that the cross-tropopause fluxes (CTFs) induced by COLs are about one to two orders of magnitude larger than global CTFs. COLs play a significant role in local, rapid air mass exchanges, although they may only be responsible for a fraction of the total STE.