A 4‐year zonal climatology of lower tropospheric CO 2 derived from ocean‐only Atmospheric Infrared Sounder observations

A 4‐year zonally averaged climatology of atmospheric CO 2 , ocean only, between ±60° latitude has been derived from the Atmospheric Infrared Sounder (AIRS) radiances. Using only very clear fields of view, the CO 2 profile in the computed radiances is scaled until agreement is found with observations. ECMWF forecast and analysis fields are used for the temperature profile in the computed radiances. The AIRS channels used to derive CO 2 amounts are nominally sensitive to CO 2 variability in the ∼300–800 mbar region (2–9 km), significantly lower in the atmosphere than that in previous studies using AIRS. Validation using aircraft measurements of CO 2 at 650 mbar indicates that the AIRS CO 2 results presented here are accurate to the 0.5–1.0 ppm level. The AIRS‐derived climatology clearly exhibits the CO 2 rectifier effect, with mean CO 2 values several parts per million lower than in those in the boundary layer. The AIRS CO 2 seasonal cycle has a relatively constant amplitude of ∼3 ppm from +10° to +60° latitude, which matches the boundary layer seasonal cycle amplitude near +10° latitude but is about three times smaller than that in the boundary layer amplitude at +60° latitude. Phase comparisons between the AIRS and boundary layer CO 2 seasonal cycles show the boundary layer phase leading AIRS in the Northern Hemisphere until ∼+10° latitude, where the phases cross and the AIRS higher‐altitude CO 2 begins to lead the boundary layer phase down to ∼−10° latitude. These results may offer new insight into CO 2 interhemispherical transport. Growth rates derived from the AIRS CO 2 climatology are 2.21 ± 0.24 ppm/year, in good agreement with in situ measurements.