Are noctilucent clouds harbingers of global change in the middle atmosphere?

Abstract

Noctilucent clouds (NLC) and their space counterpart, Polar Mesospheric Clouds (PMC) are water-ice clouds that form in the cold high-latitude mesopause region during summertime. These clouds have been suggested to be sensitive markers of long-term mesospheric change. It was previously reported that NLC numbers over northwestern Europe increased in the late 20 th century. However, when the observations are sorted properly by location, the time series of NLC observations no longer shows an upward trend. A 40-year data set from Moscow also shows no significant long-term trend in NLC numbers, but an increase in their average brightness. There is also new satellite evidence that PMCs have brightened over the past 23 years. Recent mesospheric cloud occurrences have been reported at unusually-low latitudes (40°–50°N). Together with the satellite evidence, these developments have promoted speculation that permanent atmospheric changes near the high-latitude mesopause region have occurred. Before such conclusions can be drawn, natural variations in NLC activity need to be understood. The solar cycle appears to be the dominant natural influence. The published data (both from the ground and space-based) are consistent in showing a pronounced 10–11 year period oscillation in cloud occurrence, which is approximately anti-correlated with the solar cycle. The brighter the clouds, the more their numbers change over the solar cycle. The cyclic change is likely due to small changes in mesospheric temperature and water vapor, induced by the solar cycle variation of UV irradiance. There is an unexplained several-year time lag between solar cycle minimum and the peak of cloud activity. Longer time series and more comprehensive models are needed before the link with global change can be established.