What controls the seasonal cycle of columnar methane observed by GOSAT over different regions in India?

Methane (CH4) is one of the most important shortlived climate forcers for its critical roles in greenhouse warming and air pollution chemistry in the troposphere, and the water vapor budget in the stratosphere. It is estimated that up to about 8% of global CH4 emissions occur from South Asia, covering less than 1% of the global land. With the availability of satellite observations from space, variability in CH4 has been captured for most parts of the global land with major emissions, which were otherwise not covered by the surface observation network. The satellite observation of the columnar dry-air mole fractions of methane (X CH4) is an integrated measure of CH4 densities at all altitudes from the surface to the top of the atmosphere. Here, we present an analysis of X CH4 variability over different parts of India and the surrounding cleaner oceanic regions as measured by the Greenhouse gases Observation SATellite (GOSAT) and simulated by an atmospheric chemistrytransport model (ACTM). Distinct seasonal variations of X CH4 have been observed over the northern (north of 15 degrees N) and southern (south of 15 degrees N) parts of India, corresponding to the peak during the southwestern monsoon (July-September) and early autumn (October-December) seasons, respectively. Analysis of the transport, emission, and chemistry contributions to X CH4 using ACTM suggests that a distinct X CH4 seasonal cycle over northern and southern regions of India is governed by both the heterogeneous distributions of surface emissions and a contribution of the partial CH4 column in the upper troposphere. Over most of the northern Indian Gangetic Plain regions, up to 40% of the peak-to-trough amplitude during the southwestern (SW) monsoon season is attributed to the lower troposphere (similar to 1000-600 hPa), and similar to 40% to uplifted high-CH4 air masses in the upper troposphere (similar to 600-200 hPa). In contrast, the X CH4 seasonal enhancement over semi-arid western India is attributed mainly (similar to 70 %) to the upper troposphere. The lower tropospheric region contributes up to 60% in the X CH4 seasonal enhancement over the Southern Peninsula and oceanic region. These differences arise due to the complex atmospheric transport mechanisms caused by the seasonally varying monsoon. The CH4 enriched air mass is uplifted from a high-emission region of the Gangetic Plain by the SW monsoon circulation and deep cumulus convection and then confined by anticyclonic wind in the upper tropospheric heights (similar to 200 hPa). The anticyclonic confinement of surface emission over a wider South Asia region leads to a strong contribution of the upper troposphere in the formation of the X CH4 peak over northern India, including the semi-arid regions with extremely low CH4 emissions. Based on this analysis, we suggest that a link between surface emissions and higher levels of X CH4 is not always valid over Asian monsoon regions, although there is often a fair correlation between surface emissions and X CH4. The overall validity of ACTM simulation for capturing GOSAT observed seasonal and spatial X CH4 variability will allow us to perform inverse modeling of X CH4 emissions in the future using X CH4 data.