Red line is a 9-point binomial filter. The filter removes noise from the signal to highlight trends.
Yellow line is an 11-year moving average.
The 15-JAN-13 AO (-0.646) ranks 26th (41th percentile).
Same idea for AO/s sister...the North Atlantic Oscillation (NAO).
The 15-JAN-13 NAO (0.011) ranks 26th (41th percentile).
Three things to note...
1) The highly variable AO index on D+45 compared to its sister index...then NAO.
2) The 11-year moving average between 1960 and1980 is negative...followed by a 30-year period were the average is above zero.
3) This season/s mid-winter AO and NAO values are slightly below normal for all mid-winter values during the period-of-record (Z = -0.207 and Z = -0.045, respectively).