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1. Introduction

Transport of air across the tropopause plays an important role in determining the chemical composition, and hence radiative properties, of both the troposphere and stratosphere. Quantifying this transport presents a significant challenge on account of the many multiscale processes involved, from the global scale mean meridional circulation, through intermediate advective and convective processes, to molecular diffusion. Further, crucial to any such quantification is the ability to distinguish between reversible and irreversible motion (see Holton et al. 1995 for a general overview).

An important mechanism for cross-tropopause transport is through the quasi-horizontal motion on those isentropic surfaces that intersect the tropopause, both in middle and low latitudes. Transport can occur by strong deformations of the tropopause and the subsequent generation of fine scale structures on which molecular diffusion may operate. When viewed on an isentropic surface the tropopause is typically associated with the region of strong gradients of potential vorticity (PV), or often by a single PV value typically in the range of 1-3pvu. Transport is then associated with the irreversible generation of small-scale PV filaments, and may be estimated by the area of stratospheric PV air entrained into the tropospheric PV air or vice versa, although such estimates are notoriously sensitive on the choice of the PV value used to define the tropopause. On the other hand, more general insight may be obtained by considering the mixing properties of the flow in the upper troposphere/lower stratosphere region; these are directly linked to the generation of small scales and in some cases may also be used to define a transport.

In this work we consider some aspects of transport and mixing near the tropopause using a variety of techniques. The main tool will be the advection of material contours or particles on isentropic surfaces by observed wind fields, described in Section 2. In Section 3, we show an example of Rossby wave breaking event and the subsequent filamentation of stratospheric air into the upper tropical troposphere. In Section 4 and Section 5 we consider the seasonal and interannual variation of the mixing properties near the tropopause by two separate diagnosics, contour lengthening rates and effective diffusivity. In Section 6 we use some ideas from the theory of dynamical systems to describe the geometry of the flow field near the tropopause. A short summary is given in Section 7.

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