Modulation of midtropospheric methane by El Niño
Atmospheric Infrared Sounder (AIRS) midtropospheric methane (CH4) data are utilized to study the variation of methane concentrations over the Pacific Ocean with an emphasis on the correlation to El Niño–Southern Oscillation (ENSO). When El Niño events happen, the rising air over the central Pacific can bring low surface concentrations of CH4 over the ocean into midtroposphere, resulting in a reduction of midtropospheric CH4 over the region. On the contrary, the rising air over the western Pacific brings low surface CH4 to the midtroposphere during the La Nina events, which leads to negative midtropospheric CH4 anomalies over the western Pacific. In the horizontal direction, there are stronger southward winds during El Niño than La Niña months in the region of the western Pacific Ocean. The stronger southward winds during El Niño can enhance the transport of high-concentration CH4 from the Northern Hemisphere to the tropical western Pacific region and contribute to the positive CH4 anomalies over the region. The difference of midtropospheric CH4 can reach +15 ppb (−15 ppb) over the western (central) Pacific between El Niño and La Niña events. The noteworthy difference of CH4 has a significant correlation to the Southern Oscillation Index with a correlation coefficient of 0.74. The change in the transports associated with the ENSO event is an important factor for CH4 anomalies in the middle troposphere. Results found in this study can help us better understand the spatiotemporal variability of methane.