ATMOSPHERIC METHANE OVER THE NORTH PACIFIC FROM 1987 TO 1993

Atmospheric methane mixing ratios were measured over the North Pacific during the winter season from 1987 to 1993 to extend our methane reco rd since 1978. The latitudinal distribution of methane mixing ratio sh owed a north-to-south gradient from mid-latitudes to the equator every year. A sharp mixing ratio gradient often appeared at the boundary be tween the winter monsoon and the trade wind regions around 20 degrees N. No significant longitudinal gradient was found during the winter se ason, although methane levels along the equator showed a large differe nce between the western and eastern Pacific. The overall methane incre ase rate in the western Pacific was estimated to be 13 ppb/yr on the b asis of the long-term record for 15 years from 1978 to 1993. This reco rd indicates that the methane growth rate over this Pacific region was gradually slowing down until 1990, followed by no significant increas e in the 1990's. The overall deceleration of the growth rate was more rapid in the middle latitudinal zone (20 degrees N-30 degrees N) than in the lower latitudinal zone (3 degrees N-20 degrees N). This latitud inal difference suggests a rapid reduction of methane emissions from t he continental regions. The methane growth rate showed an interannual variation with an increasing trend around 1983 and 1987, which was rou ghly related to the El Nino events. It is suggested that the methane g rowth rate was affected by a change of interhemispheric transport due to the ENSO events.