Pathways of influence between northern hemisphere blocking and stratospheric polar vortex variability

Yessimbet, K., Shepherd, T. G. ORCID: https://orcid.org/0000-0002-6631-9968, Ossó, A. C. and Steiner, A. K. (2022) Pathways of influence between northern hemisphere blocking and stratospheric polar vortex variability. Geophysical Research Letters, 49 (23). e2022GL100895. ISSN 0094-8276 doi: 10.1029/2022GL100895

Abstract/Summary

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 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 stratospheric polar vortex variability.