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

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

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

    Missing Value
    -999

    Units
    std.dev.

    Updated Through
    2022-03-31

    Next Update
    2023-05-01

    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

Key Figures

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)

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

Other Information

Earth system components and main variables
Atmosphere, Sea Level Pressure
Type of data product
Climate Indices, Circulation, NAM
Dataset collections
None

Years of record
to
Metadata ID
CDG-hosted-4839
Data time period extended
Yes, data set is extended
Timestep
Seasonal
Domain
Atlantic Ocean, NH - Northern Hemisphere, Pacific Ocean
Formats:
ascii
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