Cool and Wet Mesosphere : Precursor or Spinoff of Climate change?

The interannual variations of mesospheric temperature and water vapor (H2O) in the vicinity of the polar mesospheric clouds (PMCs) are investigated. Satellite measurements of the middle atmosphere from Microwave Limb Sounder (MLS) and solar irradiance from SOlar Radiation and Climate Experiment (SORCE) fully cover solar cycle 24 including two solar minima, SC 24 minimum during 2007-2009 and SC 25 minimum during 2018-2020. The H2O amount within the H2O hole, a region of exceptionally depleted H2O in the mesosphere over the Arctic and Antarctic region during summer solstice is characterized. To eliminate solar cycle signals, we estimate the decadal trend in the temperature and water vapor (H2O), by taking a difference of the variables between the SC 24 and SC 25 minima. During the SC25 minimum, the near-global mean annual mesopause temperature is generally more than 1K cooler than the temperature during the SC24 minimum. Similarly, the increase of H2O between two minima is found to be 0.1 - 0.3 ppmv/decade, which corresponds to a nearly 8%/decade of H2O increase in northern high latitudes. Combination of the mesospheric cooling and increment of the mesospheric H2O provides a favorable condition for the formation of the PMCs. The global increase in mesospheric H2O during the last decade may be responsible for the diminished solar cycle signals in PMCs occurrence in recent years.