A Large Stratospheric Chemical Climate Feedback in GEOS-5

Concentrations of stratospheric ozone and water vapor are strongly affected by stratospheric temperature and circulation changes due to increasing greenhouse gases (GHGs). Specifically, global warming accelerates the Brewer-Dobson circulation, which causes decreases in tropical lower stratospheric ozone, leading to radiative cooling and decreases in stratospheric water vapor. The decreases in stratospheric water vapor and ozone in turn modify climate response to increasing GHGs. This stratospheric chemical feedback can be quantified as the difference of equilibrium climate sensitivity between simulations with and without interactive chemistry. Previous studies have reported a wide spread of stratospheric chemical feedback - from negligibly small to 20% decrease of surface warming. The purpose of this study is to understand the mechanisms controlling the magnitude of the stratospheric chemical feedback in the Goddard Earth Observing System Model, Version 5 (GEOS-5). Two doubling CO2 experiments are conducted using GEOS-5: one with interactive chemistry and the other with prescribed ozone climatology. A large stratospheric chemical feedback is found in GEOS-5 - interactive chemistry reduces the equilibrium climate sensitivity by 13%, or 0.6 K. We quantitatively identify the relationships between Brewer-Dobson circulation acceleration, tropical lower stratospheric ozone decrease, tropical lower stratospheric cooling, and stratospheric water vapor decrease. The lower stratospheric processes that control the magnitude of stratospheric ozone and water vapor decreases are clarified. We also perform a series of sensitivity experiments to isolate the effects of changes in lower stratospheric ozone and water vapor on climate sensitivity