A Transport Pathway for Polluted Air from the Asian Summer Monsoon Anticyclone to the Global UTLS

Recent model studies have shown that pollutants such as carbon monoxide (CO) get lofted to the upper troposphere by convection within the Asian Summer Monsoon Anticyclone (ASMA) during the northern hemisphere (NH) summer. It was also shown that multiple times each summer, a portion of this polluted air gets transported out of the confines of the eastern flank of the ASMA due to eddy shedding. Further study has shown that these eddy shedding events are usually coincident with the deepening of a quasi-stationary anticyclonic circulation near Japan that is also present during the NH summer, called the Bonin High. In this study, we investigate these eddy shedding events and the role that the Bonin High plays in the process of transporting polluted air away from the confines of the ASMA. Analysis of MLS CO and meteorological fields in the upper troposphere during NH summer shows that about 80% of the time, the deepened Bonin High contains elevated CO concentrations of similar intensity as the ASMA. Using Lagrangian trajectories initialized from 100-250 hPa within the center of each ‘polluted’ Bonin High observed between 2005-2016, we show that the majority of the backward trajectories trace back to the ASMA region. Forward trajectories reveal that these air masses tend to follow two primary pathways after exiting the circulation: eastward over the Pacific Ocean or along the Pacific subtropical jet towards the United States, and westward along the tropical jet towards Africa. These findings suggest that the Bonin High may play a key role in the transport of pollution tracers away from the confines of the ASMA where they can get redistributed throughout the global UTLS.