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.