Model study of the impacts of emissions, chemical and dynamical processes on the CO variability in the tropical upper troposphere and lower stratosphere

The Whole Atmosphere Community Climate Model (WACCM) is used to investigate the relative importance of CO emissions, chemical and dynamical processes on temporal variations of CO in the tropical upper troposphere (UT) and the lower stratosphere (LS). The semi-annual oscillation (SAO) in the tropical UT and the annual oscillation (AO) in the tropical LS detected in the MLS CO observations can be well captured by the model. The model simulations reveal that the CO surface emissions explain most of the SAO signals in the tropical UT, with the remainder being attributed to dynamical and chemical processes. The CO AO in the LS primarily results from combined effects of dynamical and chemical processes while the dynamical and chemical processes make opposite contributions to the CO AO signals, consistent with the previous findings. Our analysis further reveals that CO surface emissions tend to weaken the amplitude of the CO annual cycle in the tropical LS, while the annual variations in the meridional component of the Brewer–Dobson (BD) circulation can amplify the annual variations of CO above 30 hPa. The model simulations also indicate that the CO annual cycle in the LS has a mixed behaviour with the annual variations of tropical upwelling reflected in CO between 70 and 50 hPa and a standard tape-recorder signal above 50 hPa. Moreover, the AO signals of CO exist up to 10 hPa when the chemical processes are switched off. The temporal and spatial variations of CO in the UT and near the tropopause are mainly driven by the upward transport of CO by tropical deep convection and the Asian summer monsoon circulation. In the early stage of the South Asian summer monsoon over the Bay of Bengal and the South China in the late spring and early summer, the transport of the CO surface emissions over Southeast Asia by the South Asian summer monsoon leads to an increase in the tropical CO, but the horizontal transport from the extratropics into the tropics (termed in-mixing) driven by the Asian summer monsoon anticyclone in the boreal summer decreases the tropical CO.