Contribution of different processes to changes in tropical lower-stratospheric water vapor in chemistry–climate models
Résumé
Variations in tropical lower-stratospheric humid- ity influence both the chemistry and climate of the atmo- sphere. We analyze tropical lower-stratospheric water va- por in 21st century simulations from 12 state-of-the-art chemistry–climate models (CCMs), using a linear regression model to determine the factors driving the trends and vari- ability. Within CCMs, warming of the troposphere primarily drives the long-term trend in stratospheric humidity. This is partially offset in most CCMs by an increase in the strength of the Brewer–Dobson circulation, which tends to cool the tropical tropopause layer (TTL). We also apply the regres- sion model to individual decades from the 21st century CCM runs and compare them to a regression of a decade of ob- servations. Many of the CCMs, but not all, compare well with these observations, lending credibility to their predic- tions. One notable deficiency is that most CCMs underesti- mate the impact of the quasi-biennial oscillation on lower- stratospheric water vapor. Our analysis provides a new and potentially superior way to evaluate model trends in lower- stratospheric humidity.
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