Aura/MLS observes, and SD-WACCM-X simulates the seasonality, quasi-biennial oscillation and El Nino Southern Oscillation of the migrating diurnal tide driving upper mesospheric CO primarily through vertical advection

. This work uses 17 years of upper mesospheric carbon monoxide (CO) and temperature observations by the Microwave Limb Sounder (MLS) on-board the Aura satellite to present and explain the seasonal and interannual variability of the migrating diurnal tide (DW1) component of upper mesospheric CO. This work then compares these observations to 15 simulations by the Specified Dynamics – Whole Atmosphere Community Climate Model with Ionosphere/Thermosphere eXtension (SD-WACCM-X). Results show that, for all seasons, MLS CO DW1 peaks above 85 km and has a latitude structure resembling the (1,1) mode in temperature. On the other hand, SD-WACCM-X DW1 also peaks above 85 km and has a latitude structure resembling the (1,1) mode but it simulates 2 local maximum of the (1,1) mode between 85 km and 92 km. Despite the differences in altitude structure, a tendency analysis and the adiabatic displacement method revealed that, on seasonal and 20 interannual timescales, observed and modelled CO’s (1,1) component can be reproduced solely using vertical advection. It was also found that both observed and modelled CO’s (1,1) component contains interannual oscillations with periodicities close to that of the Quasi-biennial Oscillation and the El Nino Southern Oscillation. From these results, this work concludes that on seasonal and interannual timescales, the observed and modelled (1,1) mode affects the global structure of upper mesospheric CO primarily through vertical advection. MLS CO μ′ and MLS CO μ w′ in June solstice for regions less than 90 km between latitudes 30° S and 60° N. On the other hand, between latitudes 30° S and 60° S, largest differences are found between 80 km and 92 km. This also indicates that unlike in March equinox, MLS CO μ′ and MLS CO μ w′ are not similar throughout all latitudes in June solstice. al, 2022). For example, Smith et al (2010) proved that upper mesospheric SABER atomic oxygen’s local time variation is primarily driven by vertical advection, but their analysis only involved the use of 7-year climatological average of vernal equinox atomic oxygen. Salinas et al (2022) recently did something very similar but for mesospheric SABER water vapor. Our work adds to these studies by presenting an approach involving the adiabatic displacement method that involves proving the importance of vertical advection at both seasonal and interannual 450 timescales. From these results, we can conclude that the global structure of upper mesospheric MLS CO’s DW1 component is primarily driven by DW1-induced vertical advection over all latitudes during equinox seasons and over all latitudes except the winter middle to high latitudes during solstice seasons. This could suggest that MLS CO’s DW1 component over the winter middle to high latitudes may be driven by other mechanisms such as meridional advection, eddy diffusion and/or chemistry. It could also suggest that the data over the winter middle to high latitudes may be affected by inadequate sampling. On the other hand, the global structure of upper mesospheric SD-WACCM-X CO’s DW1 component is primarily driven by DW1- 650 induced vertical advection over all latitudes for both equinox and solstice seasons. We also conclude that the dominant DW1 tidal mode in upper mesospheric MLS CO DW1 and SD-WACCM-X CO DW1 is the (1,1) mode and that for seasonal and interannual timescales, the (1,1) mode affects MLS CO DW1 and SD-WACCM-X CO DW1 primarily through vertical advection. However, MLS only observes one pulse of the (1,1) mode between 80 km and 95 km while SD-WACCM-X simulates two pulses. This could be due to MLS’ limited vertical resolution, or it could be due to inaccuracies in SD-WACCM- 655 X simulation of the background atmosphere and/or tidal vertical propagation. In addition, we find that the interannual variability of MLS CO (1,1) and SD-WACCM-X CO (1,1) is primarily driven by the QBO and ENSO’s effects on DW1-induced vertical advection. These conclusions suggest that we can use CO as a tracer for vertical advection due to the DW1 tide and the (1,1) mode on seasonal and interannual timescales. h T′ and the MEI index. This spectrum reveals that both have a statistically significant oscillation with period of around 24 to 36 months. This period of statistical significance