Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, February 2001.Includes bibliographical references (p. 214-223).The chemistry of the stratosphere, in particular the balance between ozone production and loss, is very sensitive to transport into and out of the tropical stratosphere. There is a great deal of evidence that tropical air remains relatively isolated from extratropical air over timescales that are long compared to typical midlatitude mixing timescales. However, there are significant questions regarding the extent to which the tropics may be considered isolated, the mechanisms and variability of this isolation, and the implications of tropical isolation for global-scale transport. We address some of these issues using three very different tools: a simple model of stratospheric transport, which allows us to investigate the role of tropical transport in determining global transport timescales, satellite observations of long-lived tracers, which allow us to diagnose the seasonal variability of the tracer gradients that mark the transition between tropical and extratropical air, and a shallow water model, which allows us to investigate the mechanisms of tropical isolation in the simplest relevant dynamical framework. We first discuss the characteristics of analytical solutions for the mean age of air, a measure of the mean timescale for transport by large-scale processes in the stratosphere, in a simple, one-dimensional conceptual model of stratospheric transport. In this "leaky pipe" model, the stratosphere is divided into three regions: the tropics and the Northern and Southern extratropics. We examine the dependence of the mean age on advection, diffusive mixing, and quasi-horizontal transport between the tropics and the extratropics. This work provides insight into the role of the tropics in global chemical transport under the assumption of at least some degree of tropical isolation. We next examine the seasonal variability of the subtropical tracer gradients which mark the transition between tropical and extratropical air from both a diagnostic and a mechanistic standpoint. We use probability distribution functions of satellite measurements of long-lived tracers to define the transition regions, which are commonly called the subtropical "edges". We examine six and a half years of measurements and identify the central latitude, and in some cases the area, of these edges at eight pressure levels on quasi-monthly timescales. We compare the seasonal variability of the subtropical edges to the variability in several transport parameters and thus increase our understanding of the mechanisms of tropical isolation from a diagnostic standpoint. We then use a shallow water model, which represents many of the properties of the flow between two isentropic surfaces, to examine the mechanisms of the formation of the subtropical edges during each season. We include the effects of diabatic heating and cooling as well as planetary-scale wave propagation and examine the role of these processes in the formation of potential vorticity gradients that behave in much the same way as the observed subtropical tracer gradients. Our results indicate that the winter subtropical edge marks a mixing barrier. The rapid stirring in the winter hemisphere that results from planetary-scale wave breaking is generally confined to the midlatitudes, and the strong tracer and potential vorticity gradients in the winter subtropics likely result from "stripping" processes, as filaments of material are occasionally pulled out of the tropics by this mid-latitude stirring. The summer subtropical edge, however, does not mark a mixing barrier in the middle and upper stratosphere. Rather, it is likely that the strong subtropical tracer and potential vorticity gradients in the summer hemisphere result purely from the action of the residual circulation, which tends to increase potential vorticity and tracer values in the tropics and decrease them at high latitudes (for tracers with tropospheric sources and photochemical sinks) over the course of the summer. We show that the seasonal variability of the edges can, in some cases, contribute significantly to the mass budgets in simple "leaky pipe"-type models, but find that it is difficult to assess the role of this seasonal variability in tracer transport.by Jessica L. Neu.Ph.D
