Meridional wind in the upper stratosphere: A source of winter NAO predictability

Improvement of subseasonal to seasonal North Atlantic winter forecasting requires better prediction of the North Atlantic Oscillation (NAO), the dominant mode of variability in the Northern Hemisphere. Despite recent research demonstrating the importance of stratosphere-troposphere coupling for NAO predictability, the driving mechanisms and implications are not fully understood. This study reveals that the October upper stratosphere is highly relevant to polar vortex development and predictability of winter NAO. We derive a simple index based on the strength of meridional wind in the upper stratospheric surf zone and find that anomalously poleward motion is associated with a significantly stronger polar vortex, which predicts the subsequent winter surface NAO with a correlation coefficient of r = 0.40.