Fig. 4: AAM fluctuations precede changes in extratropical climate.

a,b, Correlation between the ensemble mean predicted AAM (from forecasts started in November) and the following observed winter NAO (a) and Pacific jet-stream winds (b). Forecasts were started in November, and the NAO and jet-stream winds are predicted at a lead time of 13 months for all years between 1960 and 2017, inclusive. The NAO is the two-point difference in sea-level pressure between the Azores and Iceland, and the jet-stream wind is the zonal mean wind at 300 hPa and 60° N averaged over the Pacific (150° E to 150° W). The correlation with the following winter NAO and winds is plotted at each latitude and for each month as the forecasts progress. Positive correlations indicate that AAM anomalies precede the same-sign NAO and winds in the following winter as expected. Note the poleward migration with lead time (months), consistent with predictability arising from the poleward-migrating AAM anomalies. Hatching shows regions where the correlation between AAM and NAO is significant at the 90% level according to a one-sided t test.