Abstract
Atmospheric CH4 observations from two ground-based stations within Indian subcontinent, namely, Sinhagad (SNG) and Cape Rama station (CRI) showed a strong seasonality with a minima (∼1800 ± 20 ppb) during southwest monsoon (SWM; i.e. June–September, JJAS) and a maxima (2000 ± 30 ppb) during northeast monsoon (NEM i.e. December–February, DJF) with a peak-to-peak seasonality close to 200 ppb. The Indian summer (winter) monsoon is characterized with strong southwesterly (northeasterly) winds of oceanic (land) origin at the surface level and strong easterly (westerly) jet streams aloft. The monsoon dynamics has pronounced impact on CH4 variability over India and is analyzed with winds, Lagrangian trajectories, and 3-dimentional distributions of CH4 simulated by a general circulation model. The model simulations suggest a consistent annual vertical structure (mean and sub-seasonal uncertainty) of CH4 over India with a stark contrast in concentration from summer to winter at surface levels (below 750 mb) in confirmation with what is identified by the ground-based observations. During SWM (NEM) the air with comparatively lower (higher) CH4 concentrations from southern (northern) hemisphere reduces the CH4 over India by 1814 ± 26 ppb (enhances by 1950 ± 51 ppb). The contribution of local fluxes to this seasonality appears to be albeit weak as the synthesized CH4 fluxes (from EDGAR dataset) of the Indian peninsula itself show a peak in summer and a dip in winter. Similar property of CH4 is also common to nearby oceanic region (i.e. over Arabian Sea, 1765 ± 10 ppb during summer) suggesting the role of monsoon dynamics as the controlling factor. Further the mixing and convection carries the CH4 to the upper atmosphere and advect inward or outward aloft according the seasonal monsoon dynamics.
The weaker CH4 concentrations above Sinhagad (SNG) and Cape Rama (CRI) India during summer monsoon months JJAS may characterize a southern hemisphere origin. In order to verify this we compared the SNG and CRI CH4 with that of the southern hemispheric values taken from Seychelles at Mahe Island (SEY). Noticeably the SEY CH4 has similar seasonal variability as that of SNG and CRI, but with weaker amplitude probably because of its location over the open ocean. The in-phase seasonality between SNG, CRI and SEY thus suggest a possible southern hemisphere origin of CH4 that is observed over the Indian peninsula during JJAS. Model (LMDz) simulated CH4 concentration compared with observations a) SNG (Sinhagad) and SEY (Seychelles), and b) CRI (Cape Rama) and SEY (Seychelles). [Display omitted]
•A Summer minima and winter maxima of CH4 at surface is observed over India.•A seasonality of ∼200 ppb is linked with the atmospheric circulation and convection.•Seasonality of CH4 is confined to below 750 mb with uniform profile of CH4 aloft.•The low-level convergence, uplift & upper level divergence shapes CH4 over India.