Blue line is the observed AO on 31-DEC.
Red line is a 9-point binomial filter. The filter removes noise from the signal to highlight trends.
The 31-DEC-12 AO ranks 31th (52th percentile).
Same idea for AO/s sister...the North Atlantic Oscillation (NAO).
The 31-DEC-12 NAO ranks 53th (84th percentile).