Interhemispheric Stratospheric Symmetry: Seasonal Precursors to Surface Extratropical Variability

Seasonal prediction of surface climate relies on influences external to the troposphere, of which the stratosphere is a key source. The October stratosphere is particularly important for predictability of the coming winter and summer seasons in the Northern and Southern Hemisphere respectively, as the seasonal transition in zonal winds creates an interhemispherically symmetric window of stratosphere-troposphere coupling. Building on a recently identified teleconnection between the October upper stratospheric meridional wind and the December-January-February (DJF) North Atlantic Oscillation, we use reanalysis to demonstrate an analogous Southern Hemisphere teleconnection predicting the DJF Southern Annular Mode with skill (�=0.53)
comparable to current operational seasonal forecasts. We show that CMIP6 models fail to capture either teleconnection and that model skill is contingent on representation of the stratospheric pathway connecting the October meridional wind indices to the respective DJF polar vortex, a relationship that is poorly captured across models in both hemispheres.