Testing chemistry-climate models' regulation of tropical lower-stratospheric water vapor

Abstract. Climate models predict that tropical lower stratospheric humidity will increase as the climate warms, with important implications for the chemistry and climate of the atmosphere. We analyze the trend 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. Within CCMs, the long-term trend in humidity is primarily driven by warming of the troposphere. 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 regression model to individual decades from the 21st century CCM runs and compare them to observations. Many of the CCMs, but not all, compare well with observations, lending credibility to their predictions. One notable deficiency in most CCMs is that they underestimate the impact of the quasi-biennial oscillation on lower stratospheric humidity. Our analysis provides a new and potentially superior way to evaluate model trends in lower stratospheric humidity.

Smalley, K. M., Dessler, A. E., Bekki, S., Deushi, M., Marchand, M., Morgenstern, O., ... Zeng, G.

Smalley, Kevin M||Dessler, Andrew E||Bekki, Slimane||Deushi, Makoto||Marchand, Marion||Morgenstern, Olaf||Plummer, David A||Shibata, Kiyotaka||Yamashita, Yousuke||Zeng, Guang

Testing chemistry-climate models' regulation of tropical lower-stratospheric water vapor Institutional Repository Document