Impacts of Lower Thermospheric Atomic Oxygen and Dynamics on
Thermospheric Semiannual Oscillation using GITM and WACCM-X
Abstract
The latitudinal and temporal variation of atomic oxygen (O) is opposite
between the empirical model, MSIS and the whole atmosphere model,
WACCM-X at 97-100 km. The [O] from WACCM-X has maxima at solstices
and summer mid-high latitudes, similar to [O] from SABER. We use the
densities and dynamics from WACCM-X to drive the Global Ionosphere
Thermosphere Model (GITM) at its lower boundary, and compare it with the
MSIS driven GITM. We focus on the differences in the modeling of the
thermospheric and ionospheric semiannual oscillation (T-I SAO). Our
results reveal that driving GITM with WACCM-X shifts the phase of T-I
SAO to maximize around solstices. Nudging the dynamics in GITM towards
WACCM-X, reduces the amplitude of the oppositely-phased SAO but does not
completely correct its phase. We find that during solstices, WACCM-X
driven GITM has a smaller temperature gradient between the hemispheres
and weaker meridional and vertical winds in the summer hemisphere. This
leads to accumulation of [O] at lower latitudes due to weaker
meridional transport, resulting in solstitial maxima in global means.
WACCM-X itself has the right phase of SAO in the upper thermosphere but
wrong at lower altitudes. The exact mechanisms that can correct the
phase of SAO in IT models while using SABER-like [O] in the MLT are
currently unknown and warrant further investigation. We suggest
mechanisms that can reduce the solstitial maxima in the lower
thermosphere, for example, stronger interhemispheric meridional winds,
stronger residual circulation, seasonal variation in eddy diffusion, and
momentum from breaking gravity waves.