An Analysis of Interhemispheric Transport Pathways Based on Three-Dimensional Methane Data by GOSAT Observations and Model Simulations

Interhemispheric transport (IHT) of airmass is an important parameter for understanding the transport of air pollutants in the troposphere. Although the IHT time of surface emissions is about 1.39 years, the mechanisms and location for transport through the tropics and the role of monsoonal circulations are poorly understood due to lacking observational evidence. This study clarifies the transport pathways that connect the Northern and Southern Hemispheres by analyzing global three-dimensional distributions of methane (CH4), observed by the thermal infrared (TIR) sensor onboard the Greenhouse gases Observation SATellite (GOSAT), and simulations of an atmospheric chemistry-transport model (ACTM). CH4 has a chemical lifetime of about 1 year in the tropical troposphere and increased quickly to about 100 years in heights of the tropopause and above, which makes CH4 an ideal species for studying IHT. We found that the most active IHT occurs in the altitude region of the upper troposphere, in a layer of 200 hPa below the tropopause. The IHT over tropical South America and Africa is active almost all year long, while in the Asia region there is significant seasonality caused by the South Asian summer monsoon (ASM). We confirm and clarify the influence of zonally asymmetric heating on IHT in the ASM. We highlight an important role of the monsoon-induced eddy circulation in the cross-Equator transport of long-lived trace gases in the extended zone between tropical Africa and Southeast Asia. The zones of the most active IHT are clearly marked and quantified over the Asian, African, and American continents.