Climatology of Mesosphere and Lower Thermosphere Residual Circulations and Mesopause Height Derived From NASA TIMED/SABER Observations

In the mesosphere and lower thermosphere (MLT) region, residual circulations driven by gravity wave and tidal breaking/dissipation significantly impact constituent distribution and the height and temperature of the mesopause.  Distributions of CO2 can be used as a proxy for the residual circulations. NASA TIMED Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) CO2 volume mixing ratio (VMR) and temperature measurements from 2003 to 2020 are used to study the monthly climatology of MLT residual circulations and mesopause heights. Our analyses show that (a) mesopause height strongly correlates with the CO2 VMR vertical gradient during solstices; (b) mesopause height has a discontinuity at midlatitude in the summer hemisphere, with a lower mesopause height at mid-to-high latitudes as a result of adiabatic cooling driven by strong adiabatic upwelling; (c) residual circulations have strong seasonal variations at mid- to high latitudes, but they are more uniform at low latitudes; and (d) the interannual variability of the residual circulations and mesopause heights is larger in the Southern Hemisphere (SH; 4–5 km) than in the Northern Hemisphere (NH; 0.5–1 km).