Quantitative measures of polar vortex dynamics using the function M

Changes in the dynamics of the stratospheric polar vortices can have a significant effect on the composition of air in the polar stratosphere, including mixing ratios of ozone, as well as chemical species that descend to the stratosphere from the mesosphere via the energetic particle precipitation indirect effect (EPP IE). The dynamics of the vortex barrier are particularly important since the barrier isolates polar stratosphere from air at lower latitudes.

The “function M” is a recently proposed measure for quantifying transport in dynamical systems and a potential measure of the strength of the vortex as a barrier to horizontal mixing. A number of quantitative measures of vortex dynamics based on the function M have been developed and calculated daily for December 2009-February 2010 in the Northern Hemisphere and August-October in 2009 and 2010 in the Southern Hemisphere, for potential temperatures of 600, 700 and 900K. These include measures of vortex barrier strength and permeability based on the values of the function M at the equivalent latitude related to the vortex edge. The area associated with gridded values of M above a threshold value is also used to calculate a measure associated with the area of the vortex. The reliability and accuracy of these measures is tested by comparing them to known vortex dynamics. These quantitative measures of vortex dynamics can potentially be used to help find relationships between the behaviour of the vortex and mixing ratios of tracer chemicals, or phases of climatic cycles and modes. Qualitative plots of the function M showing the shape and extent of the polar vortex are also compared with CO tracer plots in order to assess the metrics.