ABSTRACT

Atmospheric methane mixing ratios were measured over the North Pacific during the winter season from 1987 to 1993 to extend our methane record since 1978. The latitudinal distribution of methane mixing ratio showed a north-to-south gradient from mid-latitudes to the equator every year. A sharp mixing ratio gradient often appeared at the boundary between the winter monsoon and the trade wind regions around 20°N. No significant longitudinal gradient was found during the winter season, although methane levels along the equator showed a large difference between the western and eastern Pacific. The overall methane increase rate in the western Pacific was estimated to be 13 ppb/yr on the basis of the long-term record for 15 years from 1978 to 1993. This record indicates that the methane growth rate over this Pacific region was gradually slowing down until 1990, followed by no significant increase in the 1990's. The overall deceleration of the growth rate was more rapid in the middle latitudinal zone (20°N–30°N) than in the lower latitudinal zone (3°N–20°N). This latitudinal difference suggests a rapid reduction of methane emissions from the continental regions. The methane growth rate showed an interannual variation with an increasing trend around 1983 and 1987, which was roughly related to the El Niño events. It is suggested that the methane growth rate was affected by a change of interhemispheric transport due to the ENSO events.