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

Abstract

Interhemispheric transport from the earth?s surface north of 32.4°N (region ΩN) to the surface south of 32.4°S (region ΩS) 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 ΩN ? ΩS and ΩN ? ΩN zonally averaged path densities is examined in detail for air that had last ΩN contact (?ΩN air?) during January and July. The path density provides the joint probability that ΩN air will make its next surface contact with either ΩN or ΩS, that it will have surface-to-surface transit time τ ? (τ, τ + dτ), and that it can be found in volume element d 3r during its surface-to-surface journey. 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 to ΩS, 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.