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The authors investigate the influence of El Niño on midtropospheric CO2 from the Atmospheric Infrared Sounder (AIRS) and the Model for Ozone and Related Chemical Tracers, version 2 (MOZART-2). AIRS midtropospheric CO2 data are used to study the temporal and spatial variability of CO2 in response to El Niño. CO2 differences between the central and western Pacific Ocean correlate well with the Southern Oscillation index. To reveal the temporal and spatial variability of the El Niño signal in the AIRS midtropospheric CO2, a multiple regression method is applied to the CO2 data from September 2002 to February 2011. There is more (less) midtropospheric CO2 in the central Pacific and less (more) midtropospheric CO2 in the western Pacific during El Niño (La Niña) events. Similar results are seen in the MOZART-2 convolved midtropospheric CO2, although the El Niño signal in the MOZART-2 is weaker than that in the AIRS data. [ABSTRACT FROM AUTHOR]