03 The mesospheric H2O and CO response to solar irradiance changes in the model and observed data
H2O and CO play an important role in the middle atmosphere. H2O, a strong greenhouse gas, is responsible for about two thirds of the natural radiative forcing whereas CO, being formed by CO2 photolysis, is suitable as a dynamical tracer and can increase the radiative forcing as reacts with OH radical. H2O and CO are strongly sensitive to solar irradiance variability that allows making a clear analysis of the solar signal in both model and observed data. However, current chemistry-climate models still have limitations in modelling the solar signal, mainly due to imperfect representation of chemistry, photolysis, and dynamics. Here, we estimate the mesospheric H2O and CO response to solar irradiance variability from the Chemistry-Climate Model Initiative (CCMI-1) model simulations and satellite data. We analyzed CCMI models in specified dynamics mode, REF-C1SD, covering the period from 1984 to 2011 and satellite observations performed with UARS/HALOE and Aura/MLS instruments. Using multiple linear regression, our results show a pronounced and statistically robust response of H2O and CO to changes in solar irradiance in the mesosphere and upper stratosphere. Additionally, we selected the period 2005-2017 to compare solar responses in CMAM, SOCOL, WACCM 3.5, EMAC-L90MA REF-C1SD simulations, and Aura/MLS observations. Signals from the CCMI-1 models generally agree with observations and reproduce an expected negative and positive correlation for H2O and CO, respectively, with solar irradiance. Such an analysis of H2O and CO from multiple models and observations has never been undertaken before. We stress the importance of this work for improving our understanding of the current ability and limitations of state-of-the-art models to simulate a solar forcing and its response in the chemistry and dynamic of the middle atmosphere.