Impacts of increasing CO ₂ on diurnal migrating tide in the equatorial lower thermosphere

Abstract. We investigate the impacts of increased CO2 concentration on migrating diurnal tide (DW1). A future climate simulation is conducted using a WACCM-X model, with surface CO2 levels increasing according to the RCP8.5 scenario. The DW1 (1,1) mode, a propagating tide peaking near the equator, exhibits a statistically significant positive trend in a range of 20–70 km, and a significant negative trend in a range of 90–110 km. The positive trend is likely driven by a reduction in atmospheric density in the mesosphere and enhanced equatorial convective activity, while the negative trend appears 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 atmospheric descent induced by CO2-driven 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. The negative trend is confirmed in SABER observation for the last two decades, while the positive trend is not verified.