Impact of the 1994–1997 temporary decrease in Northern Hemisphere stratospheric methane on the 1990s methane trend

Methane (CH4) ranks as the second most significant anthropogenic greenhouse gas following carbon dioxide (CO2). It originates from a wide range of surface sources and subsequently enters the stratosphere through the tropical tropopause. In line with the observed positive trend in tropospheric CH4, stratospheric CH4 has shown an overall increase in the long-term trend. However, contrary to the continuous increase in tropospheric CH4, stratospheric CH4 exhibits a temporal decrease in the Northern Hemisphere middle and upper stratosphere during short-time periods. This study investigates the causes behind the decreasing trend of stratospheric CH4 in the Northern Hemisphere from 1991 to 2000. We find that the extreme decrease of stratospheric CH4 from July 1994 to May 1997 contributes to the overall decreasing trend of CH4 from 1991 to 2000. This extreme decrease is attributed to the weakened meridional component of the residual circulation. The weakened meridional component attenuates the transport of CH4-rich air from the low-latitude lower stratosphere to the mid-latitude middle and upper stratosphere, leading to the observed decrease in CH4. It is further found that the smallest SST gradient in the North Pacific and adjacent regions from 1980 to 2020 is identified as a significant factor contributing to the weakened residual circulation and the decrease in CH4. Simulations by a chemistry-climate model support the results.