Seasonal, inter-annual and long-term changes in the middle atmospheric Carbon Monoxide over the tropics

Seasonal, annual and long-term changes of carbon monoxide (CO) are investigated in the tropical (30° N ̶ 30° S) middle atmosphere (20 ̶ 80 km) for the period 2005 ̶ 2021. CO shows a semi-annual oscillation in the higher heights of both stratosphere and mesosphere. The CO response to the solar cycle increases with altitude (>2 ppmv/100 SFU) in the mesosphere. CO response to Quasi Biennial Oscillation (QBO) and El Niño and Southern Oscillation (ENSO) is negligible in stratosphere, however, that to ENSO is significantly negative in mesosphere. In the altitude range 60 ̶ 70 km, we find a positive trend (0.03 ppmv/dec) at 64 km. Furthermore, the CO trend is positive in mesosphere (70 ̶ 80 km), with a peak at 75 km (0.19 ppmv/dec). The observed changes in middle atmospheric CO in the tropics are well reproduced by the Whole Atmospheric Chemical Climate Model (WACCM). Therefore, this study provides new insights into the long-term changes in the tropical middle atmospheric CO and its connection to dynamics of the region.