Spatial Distribution of Mixing and Transport in the Northern Middle Atmosphere

We apply a novel approach to studying eddy mixing and transport in the northern middle atmosphere during winter (December-January-February), based on the concept of Lagrangian diffusivity – a measure of how quickly air parcels mix together. Unlike traditional diagnostics that rely on longitudinal or contour-based averages, Lagrangian diffusivity provides a detailed three-dimensional view of the mixing process. Our formulation of Lagrangian diffusivity requires the calculation of parcel trajectories, performed on isentropic surfaces using ERA5 reanalysis data. Additionally, we have applied several diagnostic techniques to contextualize our results on Lagrangian diffusivity within the broader framework of the stratospheric polar vortex and quasi-geostrophic wave properties. Specifically, we investigate the influence of quasi-geostrophic motions on the stratospheric polar vortex using wave activity flux and local wave activity. Furthermore, we employ a Lagrangian descriptor, a tool based on parcel trajectory length, to locate the boundary of the stratospheric polar vortex (SPV).

The results reveal pronounced zonal asymmetries in Lagrangian diffusivity and wave activity flux. Mixing is highest at mid-latitudes around the prime meridian and at locations within the SPV. Local wave activity is elevated at high latitudes and upstream of the climatological vortex boundary opening, highlighting the role of quasi-geostrophic waves in the southward displacement of mid-latitude westerlies.