The uploaded file (NAO.csv) contains the North Atlantic Oscillation (NAO) index presented in Cropper, Hanna, Valente and Jónsson (2015): 
A DAILY AZORES-ICELAND NORTH ATLANTIC OSCILLATION INDEX BACK TO 1850, accepted in Geoscience Data Journal (http://onlinelibrary.wiley.com/journal/10.1002/%28ISSN%292049-6060).

The file contains 5 columns of data:

1. Year (YYYY)
2. Month (MM)
3. Day (DD)
4. Daily.NAO (The value of the daily NAO)
5. Natural.NAO (The value of the daily 'Natural NAO')

The accepted article describes the sources of early data/ homogenisation/ normalisation procedures applied.

For a copy of the article, check the Geoscience Data Journal website or please email T Cropper (ggp10tec@shef.ac.uk)