Formation mechanisms of latitudinal CO₂ gradients in the upper troposphere over the subtropics and tropics

Aircraft observations and numerical simulations using an atmospheric transport model exhibit large latitudinal gradients in the carbon dioxide (CO₂) mixing ratio around the subtropics and equator throughout the troposphere. The formation mechanisms of the latitudinal CO₂ gradients are investigated at aircraft flight altitudes (350-260 hPa) by considering the influences of atmospheric transport and carbon fluxes at the earth's surface. A meridional transport analysis demonstrates how various transport processes create the latitudinal CO₂ gradients in the upper troposphere. The analysis result shows that around the northern subtropics, the suppression of meridional mixing sharpens the CO₂ gradient during boreal winter and spring. In other words, extratropical cyclonic activity effectively flattens the mixing ratio along isentropic surfaces at midlatitudes and induces gradients around the subtropics. The southern subtropical CO ₂ gradient is also induced by the subtropical mixing barrier and convergence of the cross-equatorial eddy flux. Different from the subtropical gradients, a CO₂ gradient in the tropical upper troposphere is not created by the suppression of meridional transport but is created by the uplifting of low-level air during boreal winter and spring. The latitudinal CO₂ gradient in the tropical upper troposphere decreases due to interhemispheric transport. The seasonal migration of the mean Hadley circulation yields efficient interhemispheric transport and reduces the tropical CO₂ gradient.