Text below from: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/hgt.shtml
The daily geopotential height anomalies at 17 pressure levels are shown for the previous 120 days as indicated, and they are normalized by standard deviation using 1979-2000 base period. The anomalies are calculated by subtracting 1979-2000 daily climatology, and then averaged over the polar cap poleward of 65° N.
The blue (red) colors represent a strong (weak) polar vortex. The black solid lines show the zero anomalies.