Seasonal variability in distribution and fluxes of methane in the Arabian Sea

Methane, a biogeochemically important gas in Earth's atmosphere was measured in the water column and air in the Arabian Sea in different seasons, viz., northeast monsoon, intermonsoon, and southwest monsoon, as part of the Joint Global Ocean Flux Study (India). These observations record its distributions in the water column as well as its fluxes a,nd their seasonal variations. Methane is mainly produced in subsurface water, and its supersaturation occurs in the upper 400 m. The CH4 peak concentration and its location vary with latitude and season. Below about 400 m, seawater CH4 concentrations, in general, are observed to be undersaturated, suggesting its consumption. Production of CH4 in oxygenated water appears to be under biological control; however, the peak in deep anoxic water does not show any particular relation with any single chemical, biological, or physical variable and rather suggests it to be maintained by the quasi-horizontal transport. The average surface supersaturations are found to be 140 +/- 37, 173 +/- 54, and 200 +/- 74 in the northeast monsoon, intermonsoon, and southwest monsoon, respectively. Wind speed dependent flux estimation reveals the coastal region of the Arabian Sea to be a stronger source of methane compared to the open oceanic region, although a zonal transect along 10(0)N also shows higher flux of methane. The effects of the Findlater Jet induced downwelling on the distributions of CH4 in the near surface water as well as on its emissions have been discussed. Its annual emission rate of 0.03-0.05 Tg CH4 yr(-1) from the Arabian Sea is nearly the same as that observed from the global oceans.