Estimation of isentropic stirring and mixing and their diagnosis for the stratospheric polar vortex

Abstract. Isentropic stirring and mixing are important processes that determine the distribution of long-lived trace gases in the stratosphere. Stirring stretches tracer contours into filaments and mixing dissipates tracer variance. The combined effects on tracer transport by stirring and mixing are quantified by the effective diffusivity in the modified Lagrangian-mean (MLM) theory that diagnoses tracer transport in an areal coordinate. Here a method is developed to diagnose transport processes based on tracer contours in geographic coordinates. Compared to the MLM theory this method has resolving ability along tracer contours and quantifies stirring and mixing separately. Also, the influence of diabatic motion on the diagnosed stirring is reduced, which is useful for stratospheric analysis where diabatic motion is uncertain. The developed method is validated in a methane simulation experiment. The diagnosed stirring effects are consistent with established knowledge about stratospheric dynamics. Finally, stirring and mixing effects on trace gases in the polar vortex are diagnosed during a northern polar vortex period. According to the diagnosis stirring and mixing always increase the methane concentration in the polar vortex. However, their effects are reversed by vortex movement and deformation in most cases. Only in a few cases, planetary waves can penetrate into the vortex and stirring increases the methane concentration in the vortex. The developed method is readily applicable to diagnose stratospheric transport processes from satellite observed trace gas distributions.