Stratospheric water vapor trends in a coupled chemistry-climate model

Stratospheric H2O trends are examined in a detailed, coupled chemistry-climate model (CCM). The modeled H2O trend in the upper stratosphere (US) is mainly caused by CH4 oxidation while the trend in the lower stratosphere (LS) is largely related to changes in temperature and transport near the tropopause. Incomplete CH4 oxidation leads to a maximum upward H2O trend in the US of about 1.4x the imposed tropospheric CH4 trend. Cross-tropopause exchange of water vapor gives rise to much larger trends (similar to+50 ppbv/yr) in the LS. A trend of +0.44 K/decade in the 100 hPa temperature (T) contributes up to 70% (+35 ppbv/yr) of the model LS H2O trend while the remainder (around 30%) can be caused by changes in transport processes near the tropopause. In the LS the maximum modeled trend is close to observations although the globally averaged value is smaller than observed. Given an observed decrease in tropopause temperatures, our results suggest that the observed LS water vapor increases would require a significant change in transport.