[1] Mid-tropospheric CO 2 retrieved from the Atmospheric Infrared Sounder (AIRS) was used to investigate CO 2 interannual variability over the Indo-Pacific region. A signal with periodicity around two years was found for the AIRS mid-tropospheric CO 2 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) CO 2 transport from the surface to the mid-troposphere. As a result, there are positive (negative) CO 2 anomalies at mid-troposphere over the Indo-Pacific region. We simulated the influence of the TBO on the mid-tropospheric CO 2 over the IndoPacific region using the MOZART-2 model, and results were consistent with observations, although we found the TBO signal in the model CO 2 is to be smaller than that in the AIRS observations. Citation: Wang, J., X. Jiang, M. T.