What controls the seasonal cycle of columnar methane observed by GOSAT over different regions in India?
Authors: Chandra Naveen , Hayashida Sachiko , Saeki Tazu , Patra Prabir K. 
Published: April 2017

Abstract. Methane (CH4) is one of the most important short-lived climate forcer (SLCFs) according to the United Nations Environment Programme as well as it plays also a critical role in air pollution chemistry in the troposphere. With the availability of satellite observations from space, variabilities in CH4 have been captured for most parts of the global land with major emissions. The satellite observations however do not allow us to derive emission information straightforwardly, unlike in-situ measurements near the source region, without separating the role of transport and chemistry in the columnar dry-air mole fractions of methane (XCH4), which involves the CH4 densities at all altitudes along the solar light path. Observations of enhanced XCH4 are often linked to the local/regional emissions by simple correlation, without separating transport and chemistry contributions to XCH4 variability. Here, we present the analysis of XCH4 variability over different inland and surrounding cleaner oceanic regions of India using GHGs Observation SATellite (GOSAT). We also use the JAMSTEC's state-of-the-art atmospheric chemistry-transport model (ACTM) for simulating the observed XCH4 concentrations by varying surface emissions. The model-observation comparisons help us to elucidate the synoptic and seasonal variabilities in XCH4 in relation with coupled monsoon meteorology and surface fluxes. Distinct seasonal variations of XCH4 have been observed over the regions lying in the northern (north of 15° N) and southern part (south of 15° N) of India, corresponding to the peak during southwest (SW) monsoon (July–September) and early autumn season (October–December), respectively. The detailed study of all possible governing factors (transport, emission and chemistry) responsible for XCH4 seasonal cycle suggests that distinct XCH4 seasonal cycle over northern and southern regions of India is not only governed by the heterogeneous distributions of surface emissions, but also distribution of CH4 in the upper tropospheric layer. We have observed different contributions from lower troposphere (~1000–600 hPa), affected mainly by surface emissions, and transport dominated upper atmosphere (~600–0 hPa) in the XCH4 seasonal cycle. Over most of northern part of the Indian regions, up to 40 % of the peak during the SW monsoon season is attributed to the lower troposphere, while ~ 60 % to uplifted high-CH4 air masses in the upper atmosphere. In contrast, XCH4 enhancement over the semi-arid western India is mainly (~ 88 %) attributed to the upper atmosphere. The ratios of contribution changed over the southern peninsula and cleaner oceanic region; up to 60 % of seasonal cycle of XCH4 is contributed by lower tropospheric region. These differences arise due to the complex atmospheric transport mechanisms, caused by the seasonally varying monsoon. The CH4 enriched air masses uplifted from high emission region by the SW monsoon circulation and deep cumulus convection, and then confined by anticyclonic wind in the upper troposphere (~ 200 hPa), cause the strong contribution of the upper troposphere in the peak of XCH4 over most of the regions lying in the northern part of India. Based on this analysis, we suggest that a link between surface emissions and higher levels of XCH4 is not always valid over Asian monsoon regions, although there is often a fair correlation between surface emissions and XCH4.