The Path Density of Interhemispheric Surface-to-Surface Transport. Part II: Transport through the Troposphere and Stratosphere Diagnosed from NCEP Data

Interhemispheric transport from the earth’s surface north of 32.48N (region VN) to the surface south of 32.48S (region VS) is quantified using the path-density diagnostic developed in Part I of this study. The path density is computed using the Model of Atmospheric Transport and Chemistry (MATCH) driven by NCEP reanalyses. The structure of both the VN / VS and VN / VN zonally averaged path densities is examined in detail for air that had last VN contact (‘‘VN air’’) during January and July. The path density provides the joint probability that VN air will make its next surface contact with either VN or VS, that it will have surface-tosurface transit time t 2 (t, t 1 dt), and that it can be found in volume element dr during its surface-tosurface journey. The distribution of surface-to-surface transit times, the probability of VN air making next contact with VS, and the probability of finding VN air destined for VS in the stratosphere are computed from suitable integrations of the path density. Approximately one-third of the VN air undergoes interhemispheric transport to VS, with a ;20% probability of being found in the stratosphere during its surface-to-surface journey. The stratospheric fraction has about equal contributions from the part of the stratosphere that is isentropically isolated from the troposphere and from the part that is isentropically connected to the troposphere (i.e., from the overworld and the stratospheric middleworld in the terminology of Hoskins). The flow rate through the stratospheric middleworld is about twice as large as the flow rate through the overworld.