Notes about these visualisations of the NAO index:
They all use the Jones/CRU NAO index (Gibraltar minus Iceland). This is most useful during the winter half of the year when Gibraltar and Iceland sea level pressures (SLP) are strongly anti-correlated and the NAO index measures a coherent pattern of stronger/weaker atmospheric flow into NW Euope. Results are also shown for the summer, but these should be interpreted carefully due to more complex circulation regimes in summer that may not be well-described by this NAO index, not least because Gibraltar SLP is influenced by the Iberian thermal low during summer months.
The index was generated using a 1951-1980 reference period and therefore has a mean of zero during this period. It is inconvenient that this is a period of anomalous NAO index for most seasons, so the long-term mean of the index is not very close to zero when this reference period is used. Therefore, timeseries are also shown after expressing as anomalies from their 1901-2000 means, a longer and more representative baseline. Note that the index itself was still derived using the means and standard deviations of Gibraltar and Icelan SLP from the 1951-1980 reference period, but the index timeseries is subsequently re-anomalised relative to 1901-2000.
