Natural Variability of CO2 From Satellite Retrievals and Model Simulations Xun Jiang1, Edward T. Olsen2, Thomas Pagano2, Luke Chen2, Stephen J. Licata2, and Yuk L. Yung3 1 Department of Earth & Atmospheric Sciences, University of Houston, USA 2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA. 3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. The increasing level of the atmospheric CO2 has a significant influence on the global climate change. Superimposed on this trend is an annual cycle resulting from the uptake and release of CO2 by the vegetation. In addition to the trend and the annual cycle, atmospheric CO2 also exhibits variability at different time scales. Combining the satellite/in-situ observations and model simulations, we found that there is a semi-annual oscillation (SAO) signal in the middle tropospheric CO2 [Jiang et al., GBC 2012]. The SAO signal in the CO2, which can propagate from the surface to the midtroposphere, is related to the CO2 exchange between biosphere and atmosphere [Jiang et al., GBC 2012]. El Niño and Southern Oscillation (ENSO), an important large-scale climate inter-annual variability in the tropical region, can also influence CO2 concentrations. Using mid-tropospheric CO2 from satellite and model, we found that ENSO can influence the mid-tropospheric CO2 concentrations [Jiang et al., GRL, 2010; Jiang et al., JAS 2012]. Mid-tropospheric CO2 is enhanced in the central Pacific Ocean and diminished in the western Pacific Ocean during El Niño. Similar signal is also seen in the chemistry-transport model. However, the signal is weaker in the model than that in the observation [Jiang et al., JAS 2012]. In the high latitude, we have utilized the satellite CO2 data to explore the influence of the northern hemispheric annular mode (NAM) on the mid-tropospheric CO2 and found that there is less (more) CO2 at the polar region during the positive (negative) NAM years [Jiang et al., GRL 2010]. These results will help us better understand the natural variability of CO2, which is important to for investigating the carbon budget.