The Tropical Impedance of Interhemispheric Transport

Sulfur hexafluoride (SF6) is an anthropogenic greenhouse gas emitted primarily in the mid-latitude regions of the Northern Hemisphere (NH) that can be used to quantify the “mean age theorem the NH mid-latitude surface layer. The mean age calculated from SF6 observations show large meridional gradients (suggesting slow meridional transport) within the tropics. Here we examine the processes limiting interhemispheric transport using age simulations from the Community Atmospheric Model with atmospheric chemistry (CAM-chem) model driven by Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis meteorology. The model captures the observed large tropical gradients, including longitudinal and seasonal variations. The regions of highest age gradient areas are shown to be in the vicinity of regions with maximum surface convergence and precipitation, implying the upward motion in the tropics creates the “barrier effect” for the interhemispheric transport. Results indicate both seasonal and interannual variations in the tropical age are linked to changes in location of ITCZ and monsoon circulation.