I downloaded daily and monthly values of NAO index from a reliable URL on the web. I calculated the monthly values of NAO from daily values and compared the calculated monthly values with the monthly ones obtained from URL. Surprisingly, there were no similarity between the two series. Does anyone know how to be calculated the monthly values of NAO index existing on the web?
Hello Dr. Bazrafshan,
You can easily check the following links, read the information given in the captions and notes in the expert guidance sections.
https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-pc-based
https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-station-based
Kind Regards
Dear Dr. Bazrafshan,
The NAO values can be calculated based on different calculation procedures, datasets or periods for the series normalization, making a difference between the series from different sources. The daily NAO series you got from the NOAA Climate Prediction Center were calculated based on the Rotated Principal Component Analysis (RPCA). You can find the full description of the procedure in the following link:
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/history/method.shtml
Dears Dr. Vazifehkhah and Dr. Tabari,
Many thanks for your time and and consideration to the question.
Your answers are very helpful for me.
Best wishes
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