On the Effects of Mesospheric and Lower Thermospheric Oxygen Chemistry on the Thermosphere and Ionosphere Semiannual Oscillation

This study quantifies mesosphere/lower thermosphere (MLT) oxygen chemical contributions to the global thermosphere‐ionosphere (T‐I) semiannual oscillation (SAO) using a series of numerical experiments from the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) that isolate essential chemical processes affecting O and O2 in the MLT region. We track the vertical dynamical, diffusive, and chemical fluxes of O and O2 in and out of two control volumes between ∼80 and 130 km using a finite volume approach to the individual species continuity equation to investigate their relative importance on the global T‐I SAO. TIME‐GCM results indicate that the global T‐I SAO amplitude and phase is fairly insensitive to significant changes in odd oxygen chemical reaction rates in the MLT. While chemistry has an appreciable effect on O in the MLT region, sensitivity to changes in odd oxygen and odd hydrogen chemical rates appear to be offset by a consequent adjustment in the vertical bulk wind and eddy diffusive transport of O locally, rendering their effects inconsequential to the global T‐I SAO aloft. The implications of our findings for reproducing a self‐consistent global T‐I SAO in the NCAR thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) with a lower boundary near ∼100 km are discussed. Specifically, including latitude‐season variations in O, O2, and N2 from NRLMSIS® 2.0 at the lower boundary of the TIE‐GCM near ∼100 km improves its representation of the climatological T‐I SAO. However, reformulating the TIE‐GCM temperature lower boundary condition could further improve its ability to simulate the T‐I SAO from first‐principles. Key Points The thermosphere‐ionosphere (T‐I) semiannual oscillation (SAO) is insensitive to mesosphere/lower thermosphere (MLT) oxygen chemical rates Vertical transport and photochemistry act to set up a steady‐state balance of MLT oxygen (O) important for the global T‐I SAO Including a seasonally varying compositional lower boundary condition improves the T‐I SAO in TIE‐GCM, but it is still smaller than observed