Characterizing the Climatological Composition and Intraseasonal and Interannual Variability of the Asian Summer Monsoon Anticyclone Based on 12 Years of Aura Microwave Limb Sounder Measurements

Satellite measurements are invaluable for investigating the composition of the upper troposphere / lower stratosphere (UTLS) in the region of the Asian summer monsoon (ASM) anticyclone, which has been sparsely sampled by other means. The Aura Microwave Limb Sounder (MLS), launched in July 2004, makes simultaneous colocated measurements of trace gases and cloud ice water content (a proxy for deep convection) in the UTLS on a daily basis. Its dense spatial and temporal sampling, long-term data record, extensive measurement suite, and insensitivity to aerosol and all but the thickest clouds make Aura MLS uniquely suited to characterizing the climatological composition of the ASM region and quantifying the considerable spatial, seasonal, and interannual variability therein. Here we examine 12 years of version 4 MLS measurements of both tropospheric (H₂O, CO, CH₃Cl, CH₃CN, CH₃OH, HCN) and stratospheric (O₃, HNO₃, HCl) tracers, along with cloud ice, on four potential temperature surfaces in the UTLS (350–410 K) over the course of the complete ASM lifecycle, from April through October. In addition to describing the average evolution of the anticyclone during the monsoon season, we investigate intraseasonal and interannual variability in the UTLS response to the ASM. We explore the relationships between the observed trace gas behavior and variations in the strength or location of surface emissions as well as meteorological factors.