Greenhouse gas effects on the solar cycle response of water vapour and noctilucent clouds

The responses of water vapour (Hinline-formula₂O) and noctilucent clouds (NLCs) to the solar cycle are studied using the Leibniz Institute for Middle Atmosphere (LIMA) model and the Mesospheric Ice Microphysics And tranSport (MIMAS) model. NLCs are sensitive to the solar cycle because their formation depends on background temperature and the Hinline-formula₂O concentration. The solar cycle affects the Hinline-formula₂O concentration in the upper mesosphere mainly in two ways: directly through the photolysis and, at the time and place of NLC formation, indirectly through temperature changes. We found that Hinline-formula₂O concentration correlates positively with the temperature changes due to the solar cycle at altitudes above about 82 km, where NLCs form. The photolysis effect leads to an anti-correlation of Hinline-formula₂O concentration and solar Lyman-inline-formulaα radiation, which gets even more pronounced at altitudes below inline-formula∼ 83 km when NLCs are present. We studied the Hinline-formula₂O response to Lyman-inline-formulaα variability for the period 1992 to 2018, including the two most recent solar cycles. The amplitude of Lyman-inline-formulaα variation decreased by about 40 % in the period 2005 to 2018 compared to the preceding solar cycle, resulting in a lower Hinline-formula₂O response in the late period. We investigated the effect of increasing greenhouse gases (GHGs) on the Hinline-formula₂O response throughout the solar cycle by performing model runs with and without increases in carbon dioxide (COinline-formula₂) and methane (CHinline-formula₄). The increase of methane and carbon dioxide amplifies the response of water vapour to the solar variability. Applying the geometry of satellite observations, we find a missing response when averaging over altitudes of 80 to 85 km, where Hinline-formula₂O has a positive response and a negative response (depending on altitude), which largely cancel each other out. One main finding is that, during NLCs, the solar cycle response of Hinline-formula₂O strongly depends on altitude.