The Path Density of Interhemispheric Surface-to-Surface Transport through the Troposphere and Stratosphere

An understanding of interhemispheric transport is essential for the interpretation of the observed global-scale structure of long-lived anthropogenic trace gases, which have their dominant sources in the Northern Hemisphere. Here we quantify interhemispheric surface-to-surface transport using a new, general path-density diagnostic. The path density, η, is constructed from Green-function tracers and provides the joint probability that air whose last surface contact occurred on region Ωi ("Ωi air") will make its next surface contact with region Ωf, that it will have surface-to-surface transit time τ∈(τ, τ+dτ), and that it can be found in volume element d3r during its surface-to-surface journey. The probability ηdτd3r also has the interpretation of being the number of mass-years spent in d3r by Ωi air that makes its next surface contact with Ωf after a transit time τ∈(τ, τ+dτ). The dependence on τ allows the average flow rate along the paths to be determined. The diagnostic is formulated to be straightforwardly computed using only a forward transport model without the need of its adjoint. The zonally averaged path density for interhemispheric transport from the earth's surface north of 32.4○N (ΩN) to the surface south of 32.4○S (ΩS) is computed with the MATCH transport model driven by NCEP reanalyses. The ΩN→ΩS and ΩN→ΩN path densities are examined for air that had last ΩN contact ("ΩN air") during January and July. The distribution of surface-to-surface transit times, the probability of ΩN air making next contact with ΩS, and the probability of finding ΩN air destined for ΩS in the stratosphere are computed from suitable integrations of the path density. Approximately one third of the ΩN air undergoes interhemispheric transport, with a ~20% probability of being found in the stratosphere during its surface-to-surface journey. The stratospheric fraction is about equally divided between the stratospheric Middle and Overworlds, with the flow rate through the stratospheric Middleworld being about twice as large as the flow rate through the Overworld.