The upper stratosphere: Is it time to pay attention?

Oral Presentation 

In the pursuit of improving sub-seasonal to seasonal North Atlantic winter forecasting, our focus centres on better prediction of the North Atlantic Oscillation (NAO), the dominant mode of variability in the Northern Hemisphere. The NAO is understood to have several sources of predictability; the stratosphere is one of them. Despite recent research indicating the importance of stratospheric initial conditions and stratosphere-troposphere coupling in this context, the processes governing these remain elusive. This study reveals that the October upper stratosphere is highly relevant to the predictability of winter NAO. We derive a simple index based on the zonal-mean meridional wind in the mid-latitude October upper stratosphere. This index serves as a precursor in the study of the boreal polar vortex seasonal development and the subsequent changes to tropospheric circulation through the lens of the NAO and North Atlantic weather regimes. It is found that a strong index is associated with a significantly stronger vortex, with westerly anomalies that penetrate the full height of the stratosphere and persist throughout the extended winter period. These stratospheric conditions significantly project onto the NAO, with the index explaining 16.6\% of middle winter NAO variance. The mechanisms responsible are investigated by examining the behaviour of the stratospheric waveguide, upward-propagating planetary waves, and wave mean-flow interactions.