Characterizing the Spatial Distribution of Mixing and Transport in the Northern Middle Atmosphere During Winter.

A three‐dimensional winter (DJF) climatology of Lagrangian diffusivity characterizing eddy mixing and transport in the northern middle atmosphere is presented. To emphasize aspects other than zonal averages, we use the theory of Lagrangian diffusivity (κyy) hitherto not applied in stratospheric contexts to our knowledge. Our formulation of Lagrangian diffusivity requires the calculation of parcel trajectories, which is made on isentropic surfaces. A Lagrangian descriptor is used to estimate the boundary of the stratospheric polar vortex (SPV). To characterize quasi‐geostrophic motions and their influence on the SPV we apply the wave activity flux (W) and Local Wave Activity (A) $(\mathcal{A})$. Our data set is the ERA5 reanalysis for the period 1979–2013. Results for κyy show important zonal asymmetries. In the lower and middle stratosphere, κyy is highest at midlatitudes, particularly around the prime meridian. This location is surrounded by manifolds associated with hyperbolic trajectories emanating from the outer SPV boundary. κyy is also high within the SPV, and near the locations where the SPV boundary is open. Zonal asymmetries are also clear in W at midlatitudes. The larger values of A $\mathcal{A}$ are at high latitudes and upstream of the opening of the vortex boundary. The role of quasi‐geostrophic waves on the south‐north shift of the midlatitude westerlies is highlighted. In particular, the waves contribute to open the SPV boundary at around 90W. The interannual variability of κyy is explored by contrasting winters with positive‐negative Northern Annular Mode index, and Sudden Stratospheric Warmings of displacement‐split type. Plain Language Summary: This paper applies a novel approach to studying eddy mixing and transport in the northern middle atmosphere during winter (December–January–February). The approach is 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. This approach, hitherto restricted to oceanographic applications, is a significant contribution of the present work. We employ a Lagrangian descriptor, a tool based on parcel trajectory length, to locate the stratospheric polar vortex SPV boundary. Also, we investigate the influence of quasi‐geostrophic motions on the SPV using wave activity flux and local wave activity (LWA). The data set is the ERA5 reanalysis from 1979 to 2013. The results reveal pronounced zonal asymmetries in Lagrangian diffusivity and wave activity flux. Mixing is highest at midlatitudes around the prime meridian and at locations within the SPV. LWA 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 midlatitude westerlies. Comparisons are made of diffusivity during weak and strong vortex winters, as well as of during winters with and without Sudden Stratospheric Warmings. Key Points: Locally, the largest values of Lagrangian diffusivity around the vortex are along the prime meridian at all levels in the stratosphereThe location of maximum diffusivity is bounded by the edge of the climatological displaced vortex and a manifold emanating from the vortex edgeWhen the Northern Annular Mode is positive, the vortex boundary closes and diffusivity is much lower inside [ABSTRACT FROM AUTHOR]