Perhaps the best time series index available to measure the strength and persistence of the NAO is available through the Climate Prediction Center. This index accounts for the inherent spatial variability and seasonality of the pattern, as opposed to the many other indices that are  based on point location observations. The advantage of this index is that is stems from a RPCA procedure of the standardized 500mb height anomalies in the region of 20-90ºN.

One problem with currently available NAO indices is that a long-term trend exists within the time series. This trend biases the index prior to ~1979 toward negative values and, following 1979, toward positive values. It is difficult at this stage to definitively say what is causing the long term trend. Perhaps it is a global change signal. However, it is crucial to remove the noise from the index before using it in any long term investigation. In addition, winter values are especially biased so the index must be adjusted before any NAO winter examinations. I'm in the process of developing a new NAO index that has the trend removed. For more info. on this, see my 'research projects' tab.