Pathways of Influence Between Northern Hemisphere Blocking and Stratospheric Polar Vortex Variability

We apply a causal inference-based framework to test and quantify previously suggested causal relationships between Northern Hemisphere blocking, upward wave-activity fluxes and stratospheric polar vortex (SPV) variability using reanalysis data. We show that the influence of blocking on the polar vortex is entirely mediated by upward wave-activity fluxes, as the classical view would suggest. However, the causal pathway is not completely straightforward. In contrast to the vortex-weakening effect of European blocking, the vortex-strengthening effect of west Pacific blocking on lower stratospheric wave-activity fluxes is only partially mediated by upper tropospheric wave-activity fluxes. In addition, only two-thirds of the effect of upper tropospheric wave-activity fluxes on polar vortex variability is mediated by lower stratospheric wave-activity fluxes. We also show that sudden stratospheric warmings are not entirely explainable in terms of upward wave-activity fluxes. These findings help clarify the pathways of influence between blocking and SPV variability.