In 2017, the phase of the SNAO defined over July and August as in Folland et al.
(a),(b) SOI (provided by the Australian Bureau of Meteorology); (c),(d) AO (NCEP Climate Prediction Center); (e),(f) A AO (NCEP Climate Prediction Center); (g),(h) winter (Dec-Feb) NAO average (NCAR; presented for winter at the beginning of each year so winter 2017/18 is not shown); (i),(j) summer (Jul-Aug) SNAO average (Folland et al.
2012) and influenced the related anomalies in the SNAO and western European climate (Folland et al.
The circulation anomalies correspond to a positive anomaly (mean = 1.2 hPa, which is only 0.5 standard deviations) in the SNAO index relative to the CONTROL, which is smaller than the observed anomaly (2.7 hPa; Supplementary Fig.
S18.2c), may have favored a positive phase of the SNAO and a northward shift of North Atlantic summer storm track (e.g., Folland et al.
S18.1 that the signal-to-noise ratio for the changes in SAT is large, which permits more confident conclusions, whereas that for changes in circulation and precipitation is much lower (though it is interesting to note that the model suggests a stronger forced signal in western European summer precipitation than in the SNAO).