Nitrous oxide and methane during the 1994 SW monsoon in the Arabian Sea/northwestern Indian Ocean

Partial pressures of dissolved and atmospheric nitrous oxide, N₂O, and methane, CH₄, were measured during the latter stages of the southwest (SW) monsoon and subsequent intermonsoon transition of 1994 in the Gulf of Oman and the northern and central Arabian Sea, during Discovety cruises D210 and D212 of the United Kingdom Joint Global Ocean Flux Study of the northwestern Indian Ocean (NWIO). Mean observed atmospheric mixing ratios, 310 ± 3 ppbv N₂O, 1706±17 ppbv CH₄ (SW monsoon), and 311 ± 3 ppbv N₂O, 1784 ± 20 ppbv CH₄ (intermonsoon), were analytically indistinguishable from contemporary global baseline data. Mean surface mixed layer saturations were spatially and temporally heterogeneous. Largest variation was observed for N₂O in an upwelling region adjacent to the Oman coast; mean N₂O saturations were 140 ± 40% (SW monsoon) and 119 ± 17% (intermonsoon), with corresponding CH₄ saturations of 170 ± 55% and 179 ± 15%. These apparent differences were largely a consequence of less detailed station coverage during D212, reflecting large variability on a relatively small spatial-scale rather than true seasonal variation; for individual stations in the coastal upwelling, temporal changes in mean mixed layer saturations were not significant. This suggests that within this region the processes of gas exchange, net production, and supply/removal by advection and vertical mixing were more or less in balance during the period studied. Open ocean saturations were lower and less variable: 106 ± 7% N₂O, 130 ± 5% CH₄ (SW monsoon) and 104 ± 6% N₂O, 115 ± 2% CH₄ (intermonsoon). Large supersaturation maxima for N₂O (saturations ∼400-800%) and CH₄ (saturations ∼200-400%) just below the base of the mixed layer were ubiquitous and followed a trend of progressive deepening toward the south. All deep N₂O profiles were characterized by a second, more vertically extensive maximum (saturations ∼400-600%) between 500 and 1000 m. For these, plots of ΔN₂O versus AOU were consistent with their formation by coupled nitrification-denitrification, with denitrification becoming progressively more important with distance toward the core of the oxygen depleted zone. Sea-to-air fluxes for a 6-month period represented by the study were determined from measured air-sea partial pressure differences and gas transfer velocities derived from in situ wind speeds. Estimated semiannual emissions, ∼0.41-0.75 x 10¹² g N₂O, ∼0.1-0.18 x 10¹² g CH₄, were within most previously reported ranges for the NWIO. The data indicate that seasonal changes in wind speed rather than seasonal changes in air-sea partial pressure differences due to monsoon-driven mixing and upwelling are the dominant control on air-sea gas exchange in the NWIO.