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

Abstract 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.