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

Teaser image
NAM Spatial Pattern (DJFM)
Main content

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

Key Strengths

Long NAM Index extending back to 1899

Characterizes changes in the position and strength of the NH mid-latitude jet

Key Limitations

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

Please cite data sources, following the data providers' instructions
Dataset DOIs
None
Hosted Climate Index Files
  1. DJFM Northern Annular Mode Index (SLP-Based)

    Missing Value
    -999

    Units
    std.dev.

    Updated Through
    2024-03-31

    Next Update
    2024-05-01

    Start Date
    1899-01-01

    File Notes
    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.)

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

Data Access
None
Usage Restrictions
None

Cite this page

Acknowledgement of any material taken from or knowledge gained from this page is appreciated:

Phillips, Adam & National Center for Atmospheric Research Staff (Eds). Last modified "The Climate Data Guide: Hurrell wintertime SLP-based Northern Annular Mode (NAM) Index.” Retrieved from https://climatedataguide.ucar.edu/climate-data/hurrell-wintertime-slp-based-northern-annular-mode-nam-index on 2024-05-29.


Citation of datasets is separate and should be done according to the data providers' instructions. If known to us, data citation instructions are given in the Data Access section, above.


Acknowledgement of the Climate Data Guide project is also appreciated:

Schneider, D. P., C. Deser, J. Fasullo, and K. E. Trenberth, 2013: Climate Data Guide Spurs Discovery and Understanding. Eos Trans. AGU, 94, 121–122, https://doi.org/10.1002/2013eo130001

Key Figures

Time series of the various NAO and NAM indices.

Time series of the various NAO and NAM indices. They are highly correlated. (Climate Data Guide; A. Phillips)

NAM/NAO comparison

The NAM pattern (top) explains ~24% of the extended winter-mean (DJFM) variance, and is clearly dominated by the NAO structure in the Atlantic sector. Although there are some subtle differences from the regional pattern of the NAO (EOF1 of DJFM North Atlantic SLP; bottom) over the Atlantic and Arctic, 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. (Climate Data Guide; A. Phillips)

Other Information

Earth system components and main variables
Type of data product
Dataset collections
None

Years of record
to
Data time period extended
Yes, data set is extended
Timestep
Seasonal
Formats:
Input Data

SLP from NCAR (Trenberth/Palino) SLP

Vertical Levels:
None
Missing Data Flag
Spatially complete
Ocean or Land
Ocean & Land
Spatial Resolution
None
Model Resolution (reanalysis)
None
Data Assimilation Method
None
Model Vintage (reanalysis)
None