Atmospheric seasonal forecasts of the twentieth century: multi-decadal variability in predictive skill of the winter North Atlantic Oscillation (NAO) and their potential value for extreme event attribution.

The combination of a marginally significant time series of skill correlated with various diagnostics of the circulation will lead us to hypothesise the existence of genuine multi‐decadal fluctuations in the level of seasonal forecast skill of the NAO. 3.1 Linkages to the NAO phase and amplitude Comparing the 30‐year correlation skill from Figure 2 (a) with the 30‐year running mean NAO index in Figure 2 (b) reveals that, in general, periods of significant forecast skill coincide with periods when the 30‐year averaged NAO index is positive.

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Whilst these inter‐decadal differences in skill are, by themselves, only marginally statistically significant, the variations in skill strongly co‐vary with statistics of the general circulation itself suggesting that such differences are indeed physically based.

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