The influence of tropospheric biennial oscillation on mid-tropospheric CO2

Mid‐tropospheric CO2 retrieved from the Atmospheric Infrared Sounder (AIRS) was used to investigate CO2 interannual variability over the Indo‐Pacific region. A signal with periodicity around two years was found for the AIRS mid‐tropospheric CO2 for the first time, which is related to the Tropospheric Biennial Oscillation (TBO) associated with the strength of the monsoon. During a strong (weak) monsoon year, the Western Walker Circulation is strong (weak), resulting in enhanced (diminished) CO2 transport from the surface to the mid‐troposphere. As a result, there are positive (negative) CO2 anomalies at mid‐troposphere over the Indo‐Pacific region. We simulated the influence of the TBO on the mid‐tropospheric CO2 over the Indo‐Pacific region using the MOZART‐2 model, and results were consistent with observations, although we found the TBO signal in the model CO2 is to be smaller than that in the AIRS observations. Key Points Tropospheric Biennial Oscillation was found in CO2 During strong monsoon years, there are more CO2 in the mid‐troposphere MOZART‐2 model results are consistent with observations