We explored mechanisms for the thermospheric zonal mean wind responses to doubled CO2 concentration through investigating the zonal mean momentum balance in the thermosphere using GAIA model simulations in June. The analysis shows that ion drag, molecular viscosity, and meridional pressure gradient force vary 3–20 times more than the other forces due to the doubled CO2 concentration. The three forces strongly attenuate each other; consequently, the increase in zonal mean zonal ion drag dominantly strengthens the southward wind (∼15 ms−1 at maximum) in the northern (summer) hemisphere and latitudes north of 35 (Formula presented.) S. This strengthened ion drag was attributed to increased ion density/relative velocity between ions and neutral particles in the northern/southern hemisphere. Southward of 35 (Formula presented.), the zonal pressure gradient force and the meridional advection of zonal wind strengthen the southward wind by 4–12 ms−1 in total. On the other hand, the zonal wind is mainly altered by increased meridional pressure gradient force (∼15 ms−1 at maximum) caused by a latitudinally asymmetric response of the thermosphere density to increasing CO2 concentration. Our results suggest that the increased ion density in the northern hemisphere is the underlying trigger for the wind responses to increasing CO2 concentration. Furthermore, we show that the meridional advection was mainly (∼70%) due to that of the DW1 tidal component.
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