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The general nature of two-dimensional mixing on isentropic surfaces in the troposphere has been investigated. The daily time series of isentropic winds is obtained from a global general circulation model and is used to drive a high-resolution fully Lagrangian passive tracer model. Results are compared with the extremes of chaotic mixing by organized waves on the one hand and classical diffusion on the other and are found to lie in the middle ground. Advection by planetary-and synoptic-scale eddies generates small scales from an initially smooth tracer field exponentially fast, but given a modest degree of smoothing the tracer field evolving from a localized release rapidly attains the form of an algebraically spreading cloud. The zonal size of the cloud increases linearly with time (superdiffusively), owing to the systematic shear in the extratropical zonal jets, while the meridional spread has the square-root-of-time increase characteristic of classical diffusion. It is argued, however, that the small-scale tracer structure missing from current general circulation models and from diffusive mixing models severely compromises the fidelity with which chemical reactions and the hydrological cycle can be modeled. Several different analyses, including a study of the spatial distribution of finite-time Lyapunov exponents, indicate the presence of a partial barrier to mixing between the tropics and extratropics. In contrast with previous studies using simpler advecting flow fields, the extratropical mixing regions appear to be zonally homogeneous, and there are no impediments to zonal homogeneization. Our calculations indicate that a zonal wave 3 tracer pattern would be mixed away in 10–15 days. If the results can be taken as indicative of potential vorticity mixing, the implication is that under normal circumstances mixing due to synoptic eddies exerts a damping effect on planetary-scale waves. Implications of the results for the general circulation of the troposphere are discussed.
Pierrehumbert et al. (Sun,) studied this question.