Dynamical processes related to the atomic oxygen equinox transition

Both ground based and satellite observations have shown a "springtime transition" in the night-time OI 557.7 nm airglow emission and O₂ atmospheric band emission characterized by a relatively persistent period of very low emission rates around spring equinox at mid and high latitudes, indicating depletion of the peak atomic oxygen between 95 and 100 km. The salient feature of this depletion is reproduced by the NCAR thermosphere- ionosphere-mesosphere-electrodynamics general circulation model simulation. Analysis of the simulation results shows that the mean circulation in the upper mesosphere/mesopause and lower thermosphere is altered significantly by changes in gravity wave filtering and gravity wave forcing around equinox. The change first starts in form of a distinctive circulation cell in the summer lower thermosphere when the westward jet in the summer stratosphere and mesosphere begins to reverse. The circulation cell then becomes deeper and expands to lower altitudes and to the other hemisphere. This circulation can span a large vertical depth and drives an exchange of mass and chemical species between the upper mesosphere and lower thermosphere. The variation of the peak atomic oxygen concentration around equinox is caused by this exchange, and the vertical transport of atomic oxygen dominates the diffusive transport in this process. Additional variability of the atomic oxygen and corresponding airglow emissions may result from the variability of gravity wave sources, planetary waves and transience of their amplitudes and phases, and circulation change due to late and final stratospheric sudden warming.