Identifying vortex air using Carbon Monoxide Observations

This study uses observations of carbon monoxide (CO) from the MLS instrument to quantify transport in the polar region in winter. 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 without additional information. This is achieved by fitting two Gaussian distributions to the PDF for a specific period and altitude, pressure or isentropic 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 shows that the statistical methodology detailed can be used to characterize measurements made in the interior of the vortex during periods when the regions are chemically distinct. Statistical analysis for a number of years and a range of isentropic levels also unambiguously shows that the fitting scheme can be used to characterize measurements made inside the Southern hemisphere vortex during periods when the regions are chemically distinct. Given the weaknesses of reanalyses at high altitudes, this technique potentially provides a simple alternative to the dynamically derived calculation of values characteristic of the vortex interior.