• ENSO;
  • atmospheric transport;
  • biomass burning;
  • hydroxyl;
  • methane

[1] We present an analysis of interannual variability (IAV) and trends in atmospheric methane (CH4) mixing ratios over the western Pacific between 55°N and 35°S from 1994 to 2010. Observations were made by the Center for Global Environmental Research (CGER) of the National Institute for Environmental Studies (NIES), using voluntary observation ships sailing between Japan and Australia/New Zealand and between Japan and North America, sampling background maritime air quasi-monthly (∼10 times per year) with high latitudinal resolution. In addition, simulations of CH4 were performed using NIES atmospheric transport model. A large CH4 increase was observed in the tropics (10°N–5°S) during 1997 (between 15 ± 3 and 19 ± 3 ppb yr−1) and during 1998 for other regions (40°N–50°N: 10 ± 2–16 ± 1 ppb yr−1; 10°S–25°S: 12 ± 2–22 ± 4 ppb yr−1). The CH4 increase leveled off from 1999 to 2006 at all latitudes. The CH4 growth rate was enhanced in 2007 (25°N–50°N: 10 ± 1–12 ± 3 ppb yr−1; 15°S–35°S: 7 ± 1–8 ± 1 ppb yr−1) but diminished thereafter; however, a large CH4 growth (10 ± 1–17 ± 1 ppb yr−1) was observed in 2009 over the northern tropics (0°–15°N). These observations, combined with the simulation results, suggest that to explain the CH4 increase in 2007 would require an increase in surface emissions of ∼20 ± 3 Tg-CH4 yr−1 globally and an increase in the Northern Hemisphere (NH) of 4–7 ± 3 Tg-CH4 yr−1 more than that in the Southern Hemisphere (SH), assuming no change in OH concentrations; alternatively, a decrease in OH concentrations of 4.5 ± 0.6%–5.5 ± 0.5% yr−1 globally would be required if we assume no change in surface emissions. Over the western Pacific, the IAV in CH4 within the northern tropics was characterized by a high growth rate in mid-1997 and a reduced growth in 2007. The present data indicate that these events were strongly influenced by the IAV in atmospheric circulation associated with El Niño and La Niña events. Our observations captured the CH4 anomaly in 1997 associated with forest fires in Indonesia. The IAV and trends in CH4 as seen by our data sets capture the global features of background CH4 levels in the northern midlatitudes and the SH, and regional features of CH4 variations in the western tropical Pacific.