Abstract
Abstract. Variations in tropical lower-stratospheric humidity influence both the chemistry and climate of the atmosphere. We analyze tropical lower-stratospheric water vapor 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 variability. 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 regression model to individual decades from the 21st century CCM runs and compare them to a regression of a decade of observations. Many of the CCMs, but not all, compare well with these observations, lending credibility to their predictions. One notable deficiency is that most CCMs underestimate 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.
Highlights
Stratospheric water vapor is well known to be a greenhouse gas (e.g., Manabe and Wetherald, 1967; Forster and Shine, 1999; Solomon et al, 2010; Maycock et al, 2014)
Climate models predict that tropical lower-stratospheric humidity ([H2O]entry) will increase as the climate warms, with important implications for the chemistry and climate of the atmosphere
We demonstrate in this paper that the regression used by Dessler et al (2013, 2014) can be used to quantify the physical processes underlying these model trends and variability in an ensemble of chemistry–climate models (CCMs)
Summary
Stratospheric water vapor is well known to be a greenhouse gas (e.g., Manabe and Wetherald, 1967; Forster and Shine, 1999; Solomon et al, 2010; Maycock et al, 2014). This is mainly caused by radiative heating of the TTL from increased upwelling radiation from a warming troposphere (Lin et al, 2017) In addition to this mechanism, Dessler et al (2016) demonstrated in two CCMs that a warming climate increases the amount of water directly injected into the stratosphere via deep convection, providing another mechanism for tropospheric temperature to affect [H2O]entry. Dessler et al (2013) analyzed the 21st century trend in one chemistry– climate model (hereafter, CCM; similar to general circulation models, but with a more realistic stratosphere and higher vertical resolution in the TTL) and found that the regression model worked well in reproducing the CCM’s [H2O]entry trend over the 21st century They concluded that the increase in [H2O]entry was driven by the increase in tropospheric temperatures, which was partially offset by a strengthening BDC. The purpose of this paper is to see whether this linear decomposition of [H2O]entry variability holds in most CCMs and whether the same factors dominate
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