The Roles of Greenhouse Gas and Solar Cycle Forcing on the Interannual Variability of Temperature and Ice Layers near the Mesopause Region

Polar Mesospheric Clouds (PMCs) are ice layers forming just below the mesopause in the summer polar region. Their frequency of occurrence and brightness have been shown to vary inversely with the solar cycle. This is not unexpected as increases in solar irradiance warm the atmosphere and increase the photolysis of water, both of which decrease the efficiency of ice layer formation. PMC frequency of occurrence and brightness have also been shown to have a long-term positive trend. It has been suggested that this trend is the result of increased greenhouse gas concentrations. The trends could be caused by increases in radiative cooling, but recent model results suggest that greenhouse gas increases in the stratosphere lead to a shrinking of the atmosphere which results in a small net cooling at PMC altitudes.

In May of 2007, the Aeronomy of Ice in the Mesosphere (AIM) satellite mission began observing PMCs for the purpose of understanding the causes of their formation and variability. The Solar Occultation for Ice Experiment (SOFIE) on AIM measures both PMC ice properties and the temperature of the air in which the ice forms. SOFIE has shown that over the course of the AIM mission, the temperature at PMC altitudes has generally decreased, while PMC ice mass has generally increased. This result is supported by observations from AIM’s Cloud Imaging and Particle Size (CIPS) experiment. While this inverse relationship is as expected, the trends are opposite to expectations as solar activity has generally increased during the period of AIM observations. This suggests that another forcing is overwhelming the effects of solar activity, at least for this particular solar cycle. The two key candidate forcings are hemispheric teleconnections, which have been shown to influence polar mesospheric temperatures and PMCs, and greenhouse gas increases.

In this talk, we examine observations from SOFIE, the Sounding of the Atmosphere by Broadband Emission Radiometry (SABER), and the Halogen Occultation Experiment (HALOE) to look at the decadal variation of polar mesosphere temperature at PMC altitudes and disentangle the relative effects of solar cycle and greenhouse gas forcings. We will also examine the possibility of trends in teleconnections.