METHODS OF CALCULATING TRANSPORT ACROSS THE POLAR VORTEX EDGE

Existing quantitative calculations of material transport across the st ratospheric polar vortex edge are difficult to interpret. This is beca use what is actually calculated has not been clearly shown to be irrev ersible transport, because of ambiguities inherent in defining the vor tex edge, and (relatedly) because the uncertainties in the various sor ts of calculations have not been quantified. The authors discuss some of the conceptual-and technical difficulties involved in such calculat ions. These typically use a tracer coordinate, so that an air parcel's ''position'' is defined as a function of some tracer that it carries. Also examined is the sensitivity to noise of a method that has been u sed in several prior studies, which the authors call the ''contour cro ssing'' method. When contour crossing is implemented with no explicit threshold to discriminate noise from signal, a realistic amount of noi se in the tracer data can cause apparent transports across the vortex edge in the range of ten percent to several tens of percent of the vor tex area per month, even if the true transport is zero. Moreover, cont our crossing does not discriminate between dynamically driven transpor t and that due to large-scale nonconservative effects acting upon the tracer used to define the coordinate. The authors introduce a new meth od, which is called the ''local gradient reversal'' method, for estima ting the dynamically driven component of the transport. This method is conceptually somewhat similar to contour surgery but applies to gridd ed fields rather than material contours. Like contour crossing, it can thus be used in conjunction with the reverse domain filling advection technique, while contour surgery is used with contour advection or co ntour dynamics. Local gradient reversal is shown to be less sensitive to noise than contour crossing.