Impacts of Increasing CO₂ on Diurnal Migrating Tide in the Equatorial Lower Thermosphere

Abstract. We investigate impacts of increased CO₂ concentration on migrate diurnal tide (DW1). A future climate simulation is conducted using a WACCM-X model, with surface CO₂ levels increasing according to the RCP 8.5 scenario. The DW1 (1,1) mode, a propagating tide peaking near the equator, exhibits a positive trend of ~+1 % per decade in a range of 20–70 km, and a negative trend of ~-2 % per decade in a range of 90–110 km. The positive trend is likely driven by depression in atmospheric density in the mesosphere and enhanced equatorial convective activity, while the negative trend appears to result from increased eddy diffusion in the mesosphere, which overwhelms the positive trend. Two potential mechanisms may explain the negative trend. First, increasing CO₂ enhances mesospheric stability, reducing tidal vertical wavelengths. In our simulation, equatorial temperatures around ~50–70 km become cooler than those in ~70–90 km. This strong cooling could be linked to CO₂ mixing and transport, as well as the contraction of the mesospheric ozone layer due to CO₂-induced cooling. Second, stronger convective activity intensifies gravity wave generation, increasing gravity wave diffusion in the mesosphere. This strong convective activity also likely intensifies the tide below ~70 km. While our positive DW1 trend is consistent with McLandress and Fomichev (2006), the negative trend in the lower thermosphere contrasts with their results. This discrepancy might arise because their model used a time-independent diffusion coefficient, whereas WACCM-X accounts for CO₂-driven changes in gravity wave diffusion.