Evidence of the Lower Thermospheric Winter-to-Summer Circulation From SABER CO₂ Observations

Numerical studies have shown that there is a lower thermospheric winter-to-summer circulation that is driven by wave dissipation and that it plays a significant role in trace gas distributions in the mesosphere and lower thermosphere, and in the composition of the thermosphere. However, the characteristics of this circulation are poorly known. Direct observations of it are difficult, but it leaves clear signatures in tracer distributions. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite has obtained CO₂ concentration from 2002 to present. This data set, combined with simulations by the Whole Atmosphere Community Climate Model, provides an unprecedented opportunity to infer the morphology of this circulation in both the summer and winter hemispheres. Our study show that there exists a maximum vertical gradient of CO₂ at summer high latitudes, driven by the convergence of the upwelling of the mesospheric circulation and the downwelling of the lower thermospheric circulation; in the winter hemisphere, the maximum vertical gradient of CO₂ is located at a higher altitude, driven by the convergence of the upwelling of the lower thermospheric circulation and the downwelling of the solar-driven thermospheric circulation; the bottom of the lower thermospheric circulation is located between ~ 95 km and 100 km, and it has a vertical extent of ~10 km. Analysis of the SABER CO₂ and temperature at summer high latitudes showed that the bottom of this circulation is consistently higher than the mesopause height by ~10 km.