Climate Data

Hurrell wintertime SLP-based Northern Annular Mode (NAM) Index

The NAM (or Arctic Oscillation) is defined as the first EOF of NH (20°-90°N) winter SLP data. It explains 23% of the extended winter mean (December-March) variance, and it is clearly dominated by the NAO structure in the Atlantic sector. Although there are some subtle differences from the NAO regional pattern over the Atlantic and Arctic (see Figure 1 below), the main difference is larger amplitude anomalies over the North Pacific of the same sign as those over the Atlantic. This feature gives the NAM a more annular (or zonally-symmetric) structure. Positive values of the NAM are associated with lower-than-normal sea level pressures over the Arctic and westerly wind anomalies along ~55º-60ºN.

Key Strengths:

  • Long NAM Index extending back to 1899
  • Characterizes changes in the position and strength of the NH mid-latitude jet

Key Limitations:

  • Physical distinction from NAO and North Pacific indices is not completely agreed upon by researchers
  • Dependent on any inherent weaknesses in the source data set and its gridding scheme

Technical Notes

Observational data is subject to corrections after it has been published and used in our indices. Once noted, these corrections are applied to our input data, and the corresponding climate indices may change. As is the nature of PC-based indices, every time additional data is used to compute the EOF the individual PC values will likely change. It is thus recommended that one downloads an entire climate index each time they wish to update their holdings.

The NCAR Sea Level Pressure dataset is used for the calculation of the NAM index.

Questions about these datasets? Contact Adam Phillips (asphilli (AT) and/or Jim Hurrell (jhurrell (AT)

Years of Record

1899/01 to 2016/03
temporal metadataID:




Data Time Period Extended?

yes, data set is extended

Ocean or Land


Missing Data Flag

spatially complete

Input Data

SLP from NCAR (Trenberth/Palino) SLP

Earth system components and main variables

Download Data

Plain text icon DJFM Northern Annular Mode Index (SLP-Based) [1.9 KB file]


1.9 KB updated through: 2016/03
missing value: -999.0 Units: Next update 2017/04 or as requested.


NAM Index Data provided by the Climate Analysis Section, NCAR, Boulder, USA. Updated regularly. Accessed DD Month YYYY [list date you accessed the data].


  • The DJFM PC index value for year N refers to an average of December year N-1 and January, February, and March year N SLP values prior to the EOF calculation. (Example: The 1999 PC value was based on the average of December 1998 and January, February, and March 1999 SLP values.)

Data Access: Please Cite data sources, following the data providers' instructions.

  1. For the 1899-present NAM index calculated by the NCAR Climate Analysis Section, please see the "Hosted Data" tab.
  2. Arctic Oscillation page at NOAA; daily data available 1950-present
  3. Annular Modes Website from Colorado State

Key Figures

Click the thumbnails to view larger sizes



Cite this page

National Center for Atmospheric Research Staff (Eds). Last modified 18 Aug 2016. "The Climate Data Guide: Hurrell wintertime SLP-based Northern Annular Mode (NAM) Index." Retrieved from

Acknowledgement of any material taken from this page is appreciated. On behalf of experts who have contributed data, advice, and/or figures, please cite their work as well.