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 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 local-time perturbations peaks above 85ĝ€¯km and has a latitude structure resembling the (1,1) mode in temperature. On the other hand, SD-WACCM-X DW1 also peaks above 85ĝ€¯km and has a latitude structure resembling the (1,1) mode, but it simulates two local maximum of the (1,1) mode between 85 and 92ĝ€¯km. Despite the differences in altitude structure, a tendency analysis and the adiabatic displacement method revealed that, on seasonal and interannual timescales, observed and modeled CO's (1,1) component can be reproduced solely using vertical advection. It was also found that both observed and modeled CO's (1,1) component contains interannual oscillations with periodicities close to that of the quasi-biennial oscillation and the El Niño-Southern Oscillation. From these results, this work concludes that on seasonal and interannual timescales, the observed and modeled (1,1) mode affects the global structure of upper mesospheric CO primarily through vertical advection.