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Satellite CO_2 retrievals from the Greenhouse gases Observing SATellite (GOSAT), Atmospheric Infrared Sounder (AIRS), and Tropospheric Emission Spectrometer (TES) and in situ measurements from the National Oceanic and Atmospheric Administration - Earth System Research Laboratory (NOAA-ESRL) Surface CO_2 and Total Carbon Column Observing Network (TCCON) are utilized to explore the CO_2 variability at different altitudes. A multiple regression method is used to calculate the CO_2 annual cycle and semiannual cycle amplitudes from different data sets. The CO_2 annual cycle and semiannual cycle amplitudes for GOSAT X_(CO2) and TCCON X_(CO2) are consistent but smaller than those seen in the NOAA-ESRL surface data. The CO_2 annual and semiannual cycles are smallest in the AIRS midtropospheric CO_2 compared with other data sets in the Northern Hemisphere. The amplitudes for the CO_2 annual cycle and semiannual cycle from GOSAT, TES, and AIRS CO_2 are small and comparable to each other in the Southern Hemisphere. Similar regression analysis is applied to the Model for OZone And Related chemical Tracers-2 and CarbonTracker model CO_2. The convolved model CO_2 annual cycle and semiannual cycle amplitudes are similar to those from the satellite CO_2 retrievals, although the models tend to underestimate the CO_2 seasonal cycle amplitudes in the Northern Hemisphere midlatitudes and underestimate the CO_2 semiannual cycle amplitudes in the high latitudes. These results can be used to better understand the vertical structures for the CO_2 annual cycle and semiannual cycle and help identify deficiencies in the models, which are very important for the carbon budget study.

Midtropospheric CO2 data from the Atmospheric Infrared Sounder (AIRS) are used in this study to explore the variability of CO2 over the South Atlantic Ocean. It was found that the area-averaged CO2 over the South Atlantic Ocean is less than that over South America by about 1 ppm during December–March. This CO2 contrast is due to the large-scale vertical circulation over this region. During December–March, there is sinking motion over the South Atlantic Ocean. The sinking motion brings high-altitude air with a slightly lower concentration of CO2 to the midtroposphere. Meanwhile, air rising over South America brings near-surface air with a higher concentration of CO2 to the midtroposphere. As a result, the AIRS midtropospheric CO2 concentration is lower over the South Atlantic Ocean than over South America during December–March. The detrended AIRS midtropospheric CO2 difference correlates well with the inverted and detrended 400-hPa vertical pressure velocity difference between the South Atlantic and South America. Results obtained from this study demonstrate the strong impact of large-scale circulation on the vertical distribution of CO2 in the free troposphere and suggest that midtropospheric CO2 measurements can be used as an innovative observational constraint on the simulation of large-scale circulations in climate models.

Using CO_2 data from the Atmospheric Infrared Sounder (AIRS), it is found for the first time that the mid-tropospheric CO_2 concentration is ~1 part per million by volume higher during dry years than wet years over the southwestern USA from June to September. The mid-tropospheric CO_2 differences between dry and wet years are related to circulation and CO_2 surface fluxes. During drought conditions, vertical pressure velocity from NCEP2 suggests that there is more rising air over most regions, which can help bring high surface concentrations of CO_2 to the mid-troposphere. In addition to the circulation, there is more CO_2 emitted from the biosphere to the atmosphere during droughts in some regions, which can contribute to higher concentrations of CO_2 in the atmosphere. Results obtained from this study demonstrate the significant impact of droughts on atmospheric CO_2 and therefore on a feedback cycle contributing to greenhouse gas warming. It can also help us better understand atmospheric CO_2, which plays a critical role in our climate system.