“…Continuous measurements from space (Buchwitz et al, 2005;Chahine et al, 2008;Crevoisier et al, 2009;Eldering et al, 2017;Kuze et al, 2009;Nassar et al, 2011) and ground (Nevison et al, 2008;Newman et al, 2016;Wong et al, 2015;Wunch et al, 2011) have been devoted to monitoring atmospheric CO 2 variability on daily to monthly basis. With midtropospheric CO 2 observations from NASA's spaceborne Atmospheric Infrared Sounder (AIRS) becoming available in the last decade (Chahine et al, 2008), several short-term (intraseasonal to interannual) natural variability have been unveiled, including the tropical intraseasonal variability (with peak-to-peak amplitude~1.5 ppm) related to the Madden-Julian oscillation (MJO) (Li et al, 2010), the semiannual variability (~3 ppm) (Jiang et al, 2012), the biennial variability (~1.0 ppm) related to Asian monsoons (Wang et al, 2011), interannual variability (2-3 ppm) related to the El Niño-Southern Oscillation (Jiang et al, 2010), and polar variability related to the Arctic oscillation and stratospheric sudden warming (Jiang et al, 2010(Jiang et al, , 2013.…”