Testing chemistry-climate models' regulation of tropical
lower-stratospheric water vapor
Kevin M. Smalley1, Andrew E. Dessler1, Slimane Bekki2, Makoto Deushi3, Marion Marchand2, Olaf Morgenstern4, David A. Plummer5, Kiyotaka Shibata6, Yousuke Yamashita7,a, and Guang Zeng41Department of Atmospheric Science, Texas A & M, College Station, Texas, USA 2LATMOS, Institut Pierre Simon Laplace (IPSL), Paris, France 3Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan 4National Institute of Water and Atmospheric Research (NIWA), Lauder, New Zealand 5Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada 6School of Environmental Science and Engineering, Kochi University of Technology 7National institute for Environmental Studies (NIES) aNow at: Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan
Received: 28 Oct 2016 – Accepted for review: 07 Nov 2016 – Discussion started: 08 Nov 2016
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., Plummer, D. A., Shibata, K., Yamashita, Y., and Zeng, G.: Testing chemistry-climate models' regulation of tropical
lower-stratospheric water vapor, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-964, in review, 2016.