1.  We also investigated the vertical cross section of the vertical pressure velocity (dP/dt) across 70°W to 10°E averaged over 20°S-5°S from December to March in 2003-2010. As shown in Figure 4a, it is clear that air sinks over 35°W-10°E and rises over 70°W-35°W. As a result of vertical motions, AIRS mid-tropospheric CO 2 concentrations are relatively low over 35°W- 10°E and relatively high over 70°W-35°W (Figure 4b). The difference of mid-tropospheric CO 2 between South Atlantic Ocean and South America areas is about 1 ppm from December to March. Variability of CO 2 From Satellite Retrievals and Model Simulations Xun Jiang 1, David Crisp 2, Edward T. Olsen 2, Susan S. Kulawik 2, Charles E. Miller 2, Thomas S. Pagano 2, and Yuk L. Yung 3 1 Department of Earth & Atmospheric Sciences, University of Houston, Houston, TX 77204; 2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109; 3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 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 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 mid-tropospheric CO 2 compared with other data sets in the northern hemisphere. Similar regression analysis is applied to the Model for OZone And Related chemicl Tracers-2 (MOZART-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 model tends to underestimate the CO 2 annual cycle amplitudes in the northern hemisphere mid-latitudes and underestimate the CO 2 semi-annual cycle amplitudes in the high latitudes. AIRS mid-tropospheric CO 2 data are also used to explore the variability of CO 2 over the South Atlantic Ocean. It was found that the CO 2 difference is ~1 ppm between the South Atlantic Ocean and South America during December to March. During December to March, there is sinking motion over the South Atlantic Ocean. The sinking air brings high altitude air with low concentrations of CO 2 to the mid-troposphere. Meanwhile, air rises over South America, which brings surface high concentrations of CO 2 to the mid-troposphere over South America. As a result, the mid-tropospheric CO 2 concentrations are lower over the South Atlantic Ocean than over South America during December to March. It is also found that the detrended AIRS mid-tropospheric CO 2 difference correlates well with the inverted and detrended 400 hPa vertical pressure velocity difference between South Atlantic and South America. Results obtained from this study demonstrate the strong modulation of large-scale circulation on the mid-tropospheric CO 2 and suggest that mid-tropospheric CO 2 measurements can be used as an innovative observational constraint on the simulation of large-scale circulations in climate models. CO 2 Annual Cycle & Semiannual Cycle Figure 1: (a) Latitudinal distributions of CO 2 annual cycle amplitudes. (b) Latitudinal distributions of CO 2 semiannual cycle amplitudes. Blue lines are results from AIRS mid-tropospheric CO 2. Green lines are results from GOSAT X CO2. Purple dots are results from NOAA-ESRL surface CO 2. Orange triangles are results from TCCON X CO2. Error bars are the uncertainties of CO 2 annual cycle and semiannual cycle amplitudes derived from the multiple regressions. Figure is from Jiang et al. [2014a]. We have applied a multiple regression method to all data sets (e.g., GOSAT XCO 2, AIRS mid- tropospheric CO 2, TES mid-tropospheric CO 2, TCCON XCO 2, and NOAA/ESRL surface CO 2 ). We regressed CO 2 data to the trend, annual, and semi-annual oscillation. The amplitudes for the CO 2 annual cycle and the CO 2 semiannual cycle are plotted in Figs. 1a and 1b, respectively. The CO 2 annual cycle amplitudes are ~5-10 ppm for the NOAA-ESRL surface CO 2 in the NH, which is almost a factor of two larger than those derived from the satellite CO 2 retrievals. The annual cycle amplitudes of the GOSAT X CO2 are consistent with those from TCCON X CO2. For these two X CO2 data sets, the NH (SH) annual cycle amplitudes are about 2-3 ppm (0.5-1 ppm). TES CO 2 annual cycle amplitudes are similar to GOSAT X CO2 in the NH. The NH CO 2 annual cycle amplitude is smallest in the AIRS mid-tropospheric CO 2. Since the CO 2 annual cycle amplitudes are small in the SH, the differences of CO 2 annual cycle amplitudes between different satellite CO 2 retrievals are correspondingly small. Amplitudes for CO 2 semiannual cycle are shown in Fig. 1b. The CO 2 semiannual signal is largest at the surface, since the source for the semiannual signal in CO 2 is mostly related to the CO 2 exchange between biosphere and atmosphere at the surface [Jiang et al., 2012]. The CO 2 semiannual signals are consistent between the GOSAT and TCCON CO 2, which are smaller than those from the surface NOAA-ESRL CO 2. The semiannual signal is smaller in the AIRS mid-tropospheric CO 2 than GOSAT CO 2 and TES CO 2. Figure 3: AIRS mid-tropospheric CO 2 averaged from December to March in 2003-2010. Units for CO 2 are ppm. Color represents AIRS mid-tropospheric CO 2. White contours are the NCEP2 400 hPa vertical pressure velocity (dP/dt). Solid white contours refer to the sinking air. Dashed white contours refer to the rising air. Figure is from Jiang et al. [2014b]. Conclusions  Results for the annual cycle and semiannual cycle amplitudes from the model convolved CO 2 are plotted against satellite and surface CO 2 in Fig. 2. The model convolved CO 2 annual cycle amplitudes are similar to those from satellite CO 2 and NOAA-ESRL surface CO 2. The values obtained by convolving the model CO 2 by the GOSAT averaging kernel are larger than the values obtained by convolving the model CO 2 by the AIRS CO 2 averaging kernel, because the GOSAT X CO2 averaging kernel’s maximum is closer to the surface than that for the AIRS CO 2 averaging kernel.  Both models show CO 2 semiannual cycles that are larger in the NH than SH. The CO 2 semiannual cycle amplitude obtained by convolving the models with the GOSAT averaging kernel is about 0.5-2 ppm in the NH, which is similar to the measured GOSAT CO 2 semiannual cycle shown in Fig. 2f. The amplitude of the CO 2 semiannual cycle obtained by convolving the models with the AIRS averaging kernel is about 0.5-1 ppm in the NH, which is weaker than that from AIRS CO 2 semiannual cycle in the high latitudes and requires further exploration with in-situ CO 2 profile data in the future. Figure 4: (a) Vertical pressure velocity (dP/dt) averaged over 20°S-5°S from December to March in 2003-2010. Units are 10 -2 Pa/s. Solid white contours refer to the sinking air. Dashed white contours refer to the rising air. (b) AIRS CO 2 averaged over 20°S-5°S from December to March in 2003-2010. Units are ppm. Figure is from Jiang et al. [2014b].  We investigated the temporal correlation between the South Atlantic Walker Circulation and the mid-tropospheric CO 2 difference between the South Atlantic Ocean (30°W-10°E; 20°S-5°S) and the South America (70°W-40°W, 20°S-5°S) in Figure 5. As shown in Figure 5a, the detrended AIRS CO 2 difference correlates well with the inverted and detrended vertical pressure velocity differences. The correlation coefficient between the detrended AIRS CO 2 difference (black solid line) and the inverted and detrended mean vertical pressure velocity difference (red dashed line) is 0.66. The corresponding significance level is 9%.  The correlation coefficients between the detrended AIRS CO 2 difference and the inverted and detrended vertical pressure velocity differences derived from reanalysis datasets and CMIP5 model simulations range from 0.55 to 0.72 (Figure 5b). The correlation coefficients are 0.67 for NCEP2, 0.64 for ERA-Interim, 0.55 for MERRA, between 0.57 and 0.72 for CMIP5 model simulations. Given the importance of large-scale circulation in driving global energy and water cycles, improving model simulations of large-scale circulation is critical to reducing the model spread in climate sensitivity estimates (Su et al. 2014). Since there are limited direct observations of vertical velocity, the mid-tropospheric CO 2 can be utilized as an indirect constraint on model representation of large-scale circulation, for example, the vertical velocity of the South Atlantic Walker Cell. Influence of South Atlantic Walker Circulation on CO 2 Abstract Figure 2: (a) Comparison of annual cycle amplitudes between AIRS mid-tropospheric CO 2 and model-convolved CO 2 from MOZART (dotted line) and CarbonTracker (dash-dot line) (b) Comparison of semiannual cycle amplitudes between AIRS mid-tropospheric CO 2 and model-convolved CO 2, (c) and (d) are the comparisons of annual and semiannual cycle amplitudes between TES mid- tropospheric CO 2 and model convolved CO 2. (e) and (f) are the comparisons of annual and semiannual cycle amplitudes between GOSAT X CO2 and model-convolved CO 2. (g) and (h) are the comparisons of annual and semiannual cycle amplitudes between NOAA- ESRL surface CO 2 and model surface CO 2. Units are ppm. Figure is from Jiang et al. [2014a].  To understand the influence of the large-scale circulation on the mid-tropospheric CO 2, we examined the AIRS mid-tropospheric CO 2 distributions from December to March averaged over 2003-2010. NCEP2 400 hPa vertical pressure velocity (dP/dt) calculated in the same time period was overlain on the AIRS mid-tropospheric CO 2 in Figure 3. Solid (dashed) white contours refer to the sinking (rising) air. Mid-tropospheric CO 2 concentrations are the lowest over the South Atlantic Ocean, coincident with sinking air as shown by the white solid contours in Figure 3. Sinking air can bring low concentrations of CO 2 from high altitude to the mid-troposphere, leading to low concentrations of mid-tropospheric CO 2 over the Southern Atlantic Ocean. The rising air over South America brings high concentrations of CO 2 from the surface to the mid-troposphere, leading to relatively high concentrations of mid-tropospheric CO 2 over South America. As shown in Figure 3, mid-tropospheric CO 2 concentrations over the South Atlantic Ocean are about 1 ppm lower than that over South America from December to March. References: Jiang, X., D. Crisp, E. T. Olsen, S. S. Kulawik, C. E. Miller, T. S. Pagano, M. Liang, and Y. L. Yung, (2014a), CO 2 annual and semiannual cycles from multiple satellite retrievals and models, Submitted to ESS. Jiang, X., E. T. Olsen, T. S. Pagano, H. Su, and Y. L. Yung, (2014b), Modulation of mid-tropospheric CO 2 by the South Atlantic Walker Circulation, Submitted to JAS. A41H-3164 Figure 5: (a) Difference of the detrended AIRS mid-tropospheric CO 2 between the South Atlantic Ocean (30°W-10°E; 20°S-5°S) and South America (70°W-40°W, 20°S-5°S) (black solid line) and difference of the inverted and detrended 400 hPa vertical pressure velocity (dP/dt) between the South Atlantic Ocean and the South America from reanalysis data and CMIP5 models. Different colored dashed lines are from different reanalysis data and CMIP5 models. Bold red dashed line is the averaged vertical pressure velocity difference from all reanalysis data and model simulations. (b) Correlation coefficients between detrended CO 2 difference and detrended and inverted vertical pressure velocity differences from reanalysis data and CMIP5 models. A 3-month running mean has been applied to all time series to remove the high frequency signals. Figure is from Jiang et al. [2014b].  CO 2 annual cycle and semiannual cycle amplitudes decrease with altitudes. Model-convolved CO 2 annual cycle and semiannual cycle amplitudes are similar to those from the satellite CO 2 retrievals.  Low concentrations of CO 2 are seen over the Southern Atlantic Ocean, which is related to the sinking branch in the Atlantic Walker Circulation.  AIRS mid-tropospheric CO 2 difference correlates well with the inverted and detrended 400 hPa vertical pressure velocity difference between South Atlantic and South America. Satellite CO 2 retrievals can be used as an innovative observational constraint on the simulation of large-scale circulation in climate models.