Significant response of methane in the upper troposphere to subseasonal variability in the Asian monsoon anticyclone

Substantial methane (CH4) emissions in Asia are efficiently transported to the upper troposphere through the monsoon dynamical system, which forms a remarkable seasonal CH4 enhancement in the upper troposphere. Using a chemical transport model, GEOS-Chem, driven by surface optimized CH4 flux, the CH4 enhancement over the Asian monsoon region is explored as a combined effect of the monsoon dynamical system and regionally increased emissions during the late monsoon season. The spatial distributions of CH4 at the upper troposphere show strong subseasonal variability, which is closely tied to the east–west oscillation of the Asian monsoon anticyclone (AMA). Besides, the AMA patterns influence the efficiency of methane-rich air transport to the upper troposphere and lower stratosphere. The AMA center around 80° E favors the upward transport from organized monsoon convective sources over the Indian subcontinent and Bay of Bengal, while the AMA center around 105° E favors the source from southwest China transported to the upper troposphere. When the AMA shifts over the Iranian Plateau, further vertical transport from the convective outflow level to the upper troposphere is weakened and the horizontal redistribution becomes dominant. According to our model sensitivity study, the differences in the upper-troposphere CH4 anomalies caused by large-scale circulation are 1–2 times the difference caused by regional surface emissions. Our research highlights the complex interaction between monsoon dynamics and surface emissions to determine the upper-troposphere methane.