Abstract
Middle atmospheric carbon monoxide (CO) has been considered as a dynamical tracer to study the transport in the stratosphere and mesosphere because of its long lifetime. However, most trend studies of CO are limited to troposphere and the Upper Troposphere Lower Stratosphere (UTLS). Here, we investigate the seasonal, annual and long-term changes of CO in the tropical (30° N–30° S) middle atmosphere (20–80 km) for the period of 2005–2021. CO shows a semi-annual oscillation in the higher altitudes of both stratosphere and mesosphere. The response of solar cycle to CO increases with altitude and is largest in the mesosphere. On the other hand, Quasi Biennial Oscillation (QBO) response to CO change is highest in the lower stratosphere. The CO trends are statistically insignificant at all altitudes, although they are positive in the height range 20–25 km and 32–40 km, at about 0.01–0.035/yr, depending on altitude. The observed changes in the middle atmospheric CO in the tropics are well reproduced by the Whole Atmospheric Chemical Climate Model (WACCM). Therefore, this study provides new insights on the long-term changes in the tropical middle atmospheric CO and its connection to dynamics of the region.
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