Influences of atmospheric ventilation on the composition of the upper troposphere and lower stratosphere during the two primary modes of the South Asia high

Abstract

There are two key modes of the South Asia high (SAH) during the boreal summer: the Iranian Plateau (IP) and Tibetan Plateau (TP) modes. The anomalies of chemical constituents in the upper troposphere and lower stratosphere (UTLS) region within the SAH area largely depend on this bimodality. To better understand the underlying mechanisms of this dependence, the ensemble of 30-day backward trajectories, initialized in the UTLS region within the SAH, was simulated by a Lagrangian model FLEXPART. The comparative diagnostic was performed from the perspective of atmospheric ventilation. The results show that vertical transport from the lower troposphere to the UTLS during the TP mode was very efficient, resulting in tropospheric air mass transported into the UTLS within a shorter timescale than during the IP mode. Furthermore, the effect of SAH isolation during the TP mode was stronger than during the IP mode. This stronger trapping is likely to force the tropospheric air mass to reside in the SAH area for a longer period of time. In addition, compared to the IP mode, near-surface air mass sources during the TP mode overlapped more with areas of severe air pollution (CO emissions). The above three factors associated with the processes of atmospheric ventilation, i.e., the efficiency of vertical transport, the strength of the SAH isolation, and the boundary layer sources, provide potential explanations as to why the anomalies of atmospheric constituents in the UTLS are different between the TP and the IP mode.