Decadal and inter-hemispheric variability in polar mesospheric clouds, water vapor, and temperature

Variability in polar mesospheric clouds (PMCs), temperature, and water vapor over decadal time scales and also between hemispheres was examined using measurements from the Halogen Occultation Experiment (HALOE) covering 1991 to present. HALOE measurements were compared to results from a zonally averaged chemical/dynamics model (CHEM2D). HALOE indicates decadal cycles in temperature, water vapor, and PMCs that are correlated with the 11-year solar cycle. During solar cycle 23, variations in temperature and water vapor were nearly identical in the north and south. Temperatures varied by roughly 5 K at 85 km to 1 K at 30 km, with colder temperatures during solar minimum. Water vapor varied by roughly 30% at 85 km to less than 1% at 30 km, with more water vapor during solar minimum. Solar cycle variations in PMC extinction were roughly 23% in both the south and north, with brighter PMCs occurring during solar minimum. The overall picture given by HALOE is consistent with expectations, where a cooler and wetter mesosphere during solar minimum corresponds to brighter PMCs. CHEM2D confirms the solar cycle variations in temperature indicated by HALOE, but underestimates the observed solar cycle changes in H2O. Comparing southern and northern HALOE measurements reveals warmer temperatures in the south throughout the mesosphere. CHEM2D results show the same pattern, although the model appears to overestimate the magnitude of these north-south differences. HALOE indicates that water vapor is nearly identical in the north and south, while CHEM2D predicts a wetter southern mesosphere. HALOE measurements show that northern PMCs are 30% brighter than southern clouds on average, a difference that must be related to the cooler northern summer mesosphere.