Methane in the Asian Monsoon anticyclone and global UTLS behavior observed in ACE-FTS satellite data

Methane in the upper troposphere and lower stratosphere (UTLS) plays a critical role in atmospheric composition and radiative forcing, yet the processes governing its vertical structure and variability remain poorly understood. Here, we use high-quality satellite observations from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) to characterize the vertical structure and global seasonal behavior of methane in the UTLS, with emphasis on the role of Asian Summer Monsoon (ASM). In the ASM, a maximum in methane concentration occurs near the tropopause, at a higher altitude than the corresponding carbon monoxide maximum. The largest relative enhancement for methane – the difference between air inside and outside the anticyclone – occurs above the tropopause and extends poleward to the north. This behavior contrasts with carbon monoxide, which is confined within the anticyclone. The seasonal cycle of methane near the tropical tropopause peaks in boreal autumn, following the summer peak in surface fluxes and deep convection. This phase lag reflects the combined effects of ASM transport and subsequent large-scale redistribution following the breakdown of the anticyclone. Positive anomalies occur in the tropical lower stratosphere in boreal winter, consistent with the upwelling branch of the Brewer–Dobson circulation (BDC). Beyond the seasonal cycle, ACE-FTS data show a pronounced hemispheric asymmetry in lower stratospheric methane trends, consistent with reported changes in other long-lived trace constituents. Together, these results provide new observational evidence that ASM transport imprints a distinct methane structure above the tropopause and plays a key role in shaping UTLS variability on a global scale.