Using Aura Microwave Limb Sounder Measurements to Place the 2017 Asian Summer Monsoon Observed by the StratoClim Campaign into Context

The Asian summer monsoon (ASM) is characterized by persistent deep convection and a strong upper-level anticyclone that spans the region from East Asia to the Middle East. The rapid vertical transport strongly influences constituent distributions in the upper troposphere / lower stratosphere (UTLS), bringing up relatively moist, ozone-poor air from near the surface. The intense convection also rapidly lofts boundary layer pollutants into the UTLS, where air masses can remain confined within the closed circulation of the anticyclone for several weeks. Satellite measurements are invaluable for investigating the impact of pollution transport on the composition of the ASM anticyclone, which until very recently had been sparsely sampled by other means. The Microwave Limb Sounder (MLS), launched as part of NASA’s Aura mission in July 2004, makes simultaneous co-located measurements of trace gases and cloud ice water content (a proxy for deep convection) in the UTLS on a daily basis. With its long data record, relatively dense spatial and temporal sampling, extensive measurement suite, and insensitivity to aerosol and all but the thickest clouds, Aura MLS is well suited to characterizing UTLS composition in the ASM region and quantifying its considerable spatial, seasonal, and interannual variations. Here we examine MLS measurements of cloud ice and both tropospheric (CO, CH3Cl, CH3CN) and stratospheric (O3, HNO3, HCl) tracers, along with meteorological reanalyses, to place the 2017 ASM observed in detail by the EU StratoClim campaign into the context of other recent monsoon seasons. The signature of pollution trapped inside the anticyclone was considerably weaker than average in 2017. Although the anticyclone was particularly weakly polluted in 2017, CO enhancements have generally been smaller than the climatological norm since ∼2014, not only inside the anticyclone, but also throughout the hemisphere. Analysis of the suite of MLS measurements suggests that factors other than changes in the severity of biomass burning or the strength of deep convection, such as the declining trend in anthropogenic emissions in the region reported in recent studies, may be playing a primary role in controlling year-to-year variations in the signature of pollution trapped in the anticyclone.