# Influence of Droughs on Mid-Tropospheric CO\({}_{2}\)
## 3 Results
There is stronger rising motion in dry years than in wet years over northern California, Nevada, Utah, and Arizona. The stronger rising motion in the dry years can bring more CO\({}_{2}\) from surface to the mid-troposphere, which contributes to the high CO\({}_{2}\) concentrations in the mid-troposphere during dry years. The surface temperature is higher over inland regions during dry years than wet years, resulting in an unstable environment, which will move air from the surface to the mid-troposphere. Rising air can further bring high concentration CO\({}_{2}\) from the surface to the mid-troposphere, which leads to positive CO\({}_{2}\) anomalies during dry conditions. Figure 2: **(a)** The mean value of AIRS CO\({}_{2}\) concentration in dry years (JJAS of 2003, 2007, and 2010); **(b)** the mean value of AIRS CO\({}_{2}\) concentration in wet years (JJAS of 2006, 2008, 2011, and 2012); **(c)** CO\({}_{2}\) differences between dry and wet years; and **(d)** CO\({}_{2}\) differences within 5% significance level and 1% significance level are highlighted in light green and dark green. Units for CO\({}_{2}\) are ppm in **(a–c)**. In addition to the vertical pressure velocity, we also explore the CO\({}_{2}\) surface emissions to see if CO\({}_{2}\) surface emissions can influence mid-tropospheric CO\({}_{2}\) concentrations during drought conditions. The mean values of Net Ecosystem Exchange (NEE) during dry years and wet years are shown in Figure 4a,b. NEE represents the net exchange between the biosphere and the atmosphere. Positive NEE means that CO\({}_{2}\) is released from biosphere to atmosphere, while negative NEE suggests that CO\({}_{2}\) is removed from the atmosphere by vegetative uptake. As shown in Figure 4a,b, NEE is positive over southern California and Arizona and negative over northern California and part of Utah. NEE differences between dry years and wet years are shown in Figure 4c. As shown in Figure 4c, there are positive NEE anomalies over most regions of Nevada, Utah, and part of California and Arizona during dry years, which mean more CO\({}_{2}\) is released to the atmosphere from biosphere during dry years in these regions. The increased CO\({}_{2}\) released from the biosphere can contribute to high CO\({}_{2}\) concentrations during dry years. There are also some negative NEE anomalies over California and Arizona during dry years. Since NEE is related to photosynthesis and respiration from the biosphere, we explore Gross Primary Production (GPP) and ecosystem respiration in Figures S1 and S2 in the Supplementary Materials. GPP represents photosynthesis by the biosphere. The difference in GPP between dry years and wet years is shown in Figure S1c. Negative GPP anomalies are seen over Nevada and California in Figure S1c, which suggests that there is less photosynthesis by the biosphere during dry years. Less photosynthesis means less CO\({}_{2}\) uptake from biosphere. Thus, there will be more CO\({}_{2}\) remaining in the atmosphere during dry years over Nevada and California. There are also positive anomalies of GPP over Arizona and Utah, which means more CO\({}_{2}\) is removed from the atmosphere over these regions during the dry years, which will be partly canceled by the positive anomalies of ecosystem respiration in the same regions (Figure S2c). As shown in Figure S2c, the difference in respiration  
Figure 3: **(a)** The mean value of 500 hPa vertical pressure velocity, from NCEP2, in dry years (JJAS of 2003, 2007, and 2010); **(b)** the mean value of 500 hPa vertical pressure velocity in wet years (JJAS of 2006, 2008, 2011, and 2012); **(c) 500 hPa vertical pressure velocity differences between the dry and wet years; and **(d)** vertical pressure velocity differences within 5% significance level and 1% significance level are highlighted in light green and dark green. Units for Vertical pressure velocity are 10\({}^{-2}\) Pa/s in **(a–c)**.