New paper: why is the North Atlantic Oscillation (NAO) associated with wet/dry conditions over northern & southern Europe?

Why is the NAO associated with wet/dry conditions over northern & southern Europe? The common answer that the storm track shifts and supplies more or less moisture to each region is wrong – instead it is the change in the mean atmospheric circulation that matters most.

Our new paper (https://journals.ametsoc.org/jcli/article/doi/10.1175/JCLI-D-20-0011.1/348497/Mechanisms-of-winter-precipitation-variability-in) finds out why the NAO affects precipitation over Europe by breaking the moisture budget into components and finding the relationship between each component and the NAO.

What changes during positive NAO winters and why?

North Atlantic storms do transport moisture northward and also eastward into Europe, so it is often assumed that the storm track changes associated with NAO fluctuations would be the main reason for changes in precipitation over Europe.

However, in positive NAO winters, northern Europe is wetter because changes in mean circulation and humidity increase moisture transport into the north of Europe and transport less moisture to southern Europe (which is then drier). Turns out that storms oppose this, moving anomalous moisture away from N Europe during positive NAO winters. The opposite happens during negative NAO winters.

Find out more:

Seager R, Liu H, Kushir Y, Osborn TJ, Simpson IR, Kelley CR and Nakamura J (2020) Mechanisms of winter precipitation variability in the European-Mediterranean region associated with the North Atlantic Oscillation. Journal of Climate https://doi.org/10.1175/JCLI-D-20-0011.1