Stratospheric Water Vapor Feedback and Its Modification by Interactive Ozone Chemistry

Stratospheric water vapor has a positive climate feedback: a warming climate increases stratospheric water vapor, and the increased stratospheric water vapor enhances surface warming. However, there has been a strong debate on the importance of this feedback. Furthermore, stratospheric water vapor feedback is modified by interactive ozone chemistry through changes in the Brewer-Dobson circulation and tropical tropopause temperature. But the magnitude of this chemical modification is highly uncertain. The purpose of this study is to quantify stratospheric water vapor feedback and its modification by interactive ozone chemistry in the NASA Goddard Earth Observing System Model Chemistry-Climate Model (GEOS CCM).

Two sets of doubling CO₂ simulations with prescribed ozone fields were conducted: one that includes the stratospheric water vapor feedback, and the other that eliminates the feedback by fixing the stratospheric water vapor concentrations. The differences between these two simulations quantify stratospheric water vapor feedback. We find a large stratospheric water vapor feedback in GEOSCCM that reduces the climate feedback parameter by 0.12 Wm^-2K^-1 and increases global surface warming by nearly 20%. Additional doubling CO₂ simulations with interactive ozone chemistry were conducted in order to quantify the chemical modification of stratospheric water vapor feedback. The results show that interactive ozone leads to tropical tropopause cooling and reduces stratospheric water vapor feedback by nearly 50%. Our results suggest that stratospheric water vapor and ozone feedbacks significantly contribute to climate sensitivity.