Changes in the dynamics of the stratospheric polar vortices can significantly affect the composition of air in the polar stratosphere, with the dynamics of the vortex barrier being particularly important. The "Function M" is a recently proposed measure for quantifying transport in dynamical systems. We show that it can be used not only to visualize the structure of the stratospheric polar region in detail but also to provide a basis for quantitative measures capturing important aspects of vortex dynamics. Two such measures have been calculated daily for August-October 2009 and 2010 in the Southern Hemisphere for potential temperatures of 600, 700, and 900 K, as well as for three different Northern Hemisphere winter periods for 900 K. We discuss a measure of vortex barrier strength and permeability based on the average value of the function M near the vortex edge. The second measure, associated with vortex barrier area, is obtained by calculating the area associated with values of M above a threshold. Both measures are found to be potentially useful, with the area-based measure providing the most convincing results. The measures are based on a Lagrangian framework and follow the vortex edge, allowing periods when the vortex retains its dynamical integrity to be identified even when the vortex is greatly distorted. We also discuss a strong linear correlation near the vortex edge between values of the function M calculated over different time periods, suggesting that the structure of the polar vortex is coherent over periods of at least 30 days.
[1] This study uses observations of carbon monoxide (CO) from the Microwave Limb Sounder instrument to identify vortex air in the winter polar regions. In particular, we use the probability distribution function (PDF) of the CO data to delineate CO concentrations characteristic of the interior of the vortex core as a function of space and time. This is achieved by fitting two Gaussian distributions to the PDF for a specific period and vertical level. These Gaussian fits are then examined to determine whether two chemically distinct regions exist by inspecting the intersection area between the Gaussians relative to their individual areas. When chemically distinct regions exist, the values of the fitted mean CO concentrations are representative of the interior and exterior of the polar vortex. To prove this point, a domain-filling analysis is performed to produce high-resolution maps.Comparison of these maps with the vortex edge derived using the equivalent latitude technique, and a statistical analysis over a range of years and isentropic levels, shows that the fitting scheme can be used to characterize measurements made inside the vortex during periods when chemically distinct populations exist. The statistical analysis also suggests that a threshold CO concentration can be derived based on the Gaussian fits which defines a good mixture between reducing the quantity of CO observations incorrectly identified as vortex interior air, whilst also minimizing the number of observations misclassified as vortex exterior air. This technique therefore provides an alternative to the dynamically derived calculation of values characteristic of the vortex interior.Citation: McDonald, A. J., and M. Smith (2013), A technique to identify vortex air using carbon monoxide observations,
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