Trends of Lower- to Mid-Stratospheric Water Vapor Simulated in Chemistry-Climate Models

Abstract

Using the outputs from 16 chemistry-climate models (CCMs), the trends of lower-to mid-stratospheric water vapor (WV) during the period 1980–2005 were studied. Comparisons were made between the CCM results and European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim). The results of most of the CCMs, and those based on ERA-Interim, showed the trends of lower-to mid-stratospheric WV during the period 1980–2005 to be positive, with the extent of the trend increasing with altitude. The trend of lower-to mid-stratospheric WV in the ensemble mean of the CCMs was 0.03 ppmv per decade, which was about twice as large as that based on ERA-Interim. The authors also used a state-of-the-art general circulation model to evaluate the impacts of greenhouse gas (GHG) concentration increases and ozone depletion on stratospheric WV. The simulation results showed that the increases of lower-to mid-stratospheric WV affected by the combined effects of GHG and ozone changes happened mainly via warming of the tropopause and enhancement of the Brewer-Dobson circulation (BDC), with the former being the greater contributor. GHG increase led to a higher and warmer tropopause with stronger BDC, which in turn led to more WV entering the stratosphere; while ozone depletion led to a higher and cooler tropopause, which caused the decreases of lowerto mid-stratospheric WV, despite also causing stronger BDC.