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

Abstract. 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 insufficiently understood. Here, we use high-quality satellite observations from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), together with in situ measurements, to characterize the vertical structure and global seasonal behavior of methane in the UTLS, with particular emphasis on the role of Asian Summer Monsoon (ASM). Methane exhibits a relative maximum that peaks near the tropopause inside the ASM anticyclone. The largest relative enhancement – defined as the difference between air inside and outside the anticyclone–occurs above the tropopause and extends poleward to the north. This behavior contrasts sharply with carbon monoxide, which is confined within the anticyclone. The seasonal cycle of methane near the tropical tropopause reaches maximum 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. The enhanced methane signal propagates upward into the lower stratosphere, 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 previously 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.