Noctilucent clouds as possible indicators of global change in the mesosphere Original Research Article

Noctilucent clouds (NLC) are ice clouds near the high-latitude mesopause region, occurring in summer. These clouds have been implicated as possible harbingers of permanent changes in the upper atmosphere. Their year-to-year variations have been characterized by a semi-quantitative index, the number of nights per season in which they were reported. In this paper we compare early studies of NLC year-to-year variability with data from the modern era, including observations from Europe, the USSR and North America. This review focuses on the possible influence on NLC of water vapor variability. We compare the noctilucent cloud time series, which indicate a strong upward trend in the 1964–1986 period, with changes expected of atmospheric water vapor at the mean height of the clouds (83 km). At this height, both methane-induced changes and 11-year solar-UV induced changes are expected to be the main forcings. Using the data available for surface methane and for solar Lyman-alpha fluxes, we estimate the water vapor changes due to methane oxidation, and to Lyman-α induced photodissociation of water vapor. For the periodic (10-year) component, the NLC time series was found to significantly correlate with Lyman-α flux data, for nearly all available multi-decadal NLC data sets. As first shown by Gadsden for the European data, the correlations are highest when the time lag of NLC following solar cycle minimum is two to three years. This result places into considerable doubt the hypothesis of direct solar Lyman-α control of NLC. Volcanism appears to have had a negligible influence, with the possible exception of the 1883 Krakatoa eruption.