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North Pacific (NP) Index by Trenberth and Hurrell; monthly and winter

The North Pacific Index (NP index or NPI) is the area-weighted sea level pressure over the region 30°N-65°N, 160°E-140°W. The NP index is defined to measure interannual to decadal variations in the atmospheric circulation. The dominant atmosphere-ocean relation in the North Pacific is one where atmospheric changes lead changes in sea surface temperatures by one to two months. However, strong ties exist with events in the tropical Pacific, with changes in tropical Pacific SSTs leading SSTs in the north Pacific by three months.

Key Strengths:

  • Long timeseries running from 1899 to present.
  • Depicts changes in the intensity of the Aleutian low, and is highly correlated with the 500hPa PNA index from Wallace and Gutzler (1981).
  • Official North Pacific Index cited in Trenberth & Hurrell (1994).

Technical Notes

The North Pacific Indices produced by NCAR's Climate Analysis Section are based on Trenberth and Hurrell (1994). They are currently offerred as ascii text files in a 2-column YYYYMM and VALUE format, for monthly index, November thru March (NDJFM) anomaly, and NDJFM index values. 

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. It is thus recommended that one downloads an entire climate index each time they wish to update their holdings. Use this page to find the latest file rather than bookmarking the file that you downloaded last time.

Questions about this dataset? Contact Adam Phillips (asphilli (AT) ucar.edu).

Years of Record

1898/12 to 2020/05
temporal metadataID:
CDG-hosted-4848

Formats

Timestep

Monthly | Seasonal

Data Time Period Extended?

yes, data set is extended

Domain

NH - Northern Hemisphere | Pacific Ocean

Input Data

SLP from NCAR (Trenberth/Palino) SLP

Earth system components and main variables

Atmosphere, Sea Level Pressure

Download Data

Plain text icon Monthly North Pacific Index [22.93 KB file]

Details:

22.93 KB updated through: 2020/06
missing value: -999.0 Units: hPa Next update 2020/11 or as requested.

Citation:

NP Index Data provided by the Climate Analysis Section, NCAR, Boulder, USA, Trenberth and Hurrell (1994). Updated Regularly. Accessed DD Month YYYY [list date you accessed the data].

Download Data

Plain text icon NDJFM North Pacific Index [1.96 KB file]

Details:

1.96 KB updated through: 2020/03
missing value: -999.0 Units: hPa Next update 2021/05 or as requested.

Citation:

NP Index Data provided by the Climate Analysis Section, NCAR, Boulder, USA, Trenberth and Hurrell (1994). Updated Regularly. Accessed DD Month YYYY [list date you accessed the data].

Notes:

  • NDJFM North Pacific Indices: The station index value for year N refers to an average of November and December year N-1 and January, February, and March year N. (Example: The 1999 value contains the average of November and December 1998 and January, February, and March 1999.)

Download Data

Plain text icon NDJFM North Pacific Index anomalies [1.97 KB file]

Details:

1.97 KB updated through: 2020/03
missing value: -999.0 Units: hPa Next update 2021/05 or as requested.

Citation:

NP Index Data provided by the Climate Analysis Section, NCAR, Boulder, USA, Trenberth and Hurrell (1994). Updated Regularly. Accessed DD Month YYYY [list date you accessed the data].

Notes:

  • NDJFM North Pacific Index anomalies based on 1925-1989 mean of 1008.9 hPa
  • NDJFM North Pacific Indices: The station index value for year N refers to an average of November and December year N-1 and January, February, and March year N. (Example: The 1999 value contains the average of November and December 1998 and January, February, and March 1999.)

Key Figures

Click the thumbnails to view larger sizes

Thumbnails

Captions
NDJFM North Pacific Index NDJFM North Pacific Index (anomalies) standardized in bar chart form. The smooth black line is a weighted (1-3-5-6-5-3-1) running average. (Climate Data Guide; A. Phillips)

Cite this page

Hurrell, James & National Center for Atmospheric Research Staff (Eds). Last modified 14 Aug 2020. "The Climate Data Guide: North Pacific (NP) Index by Trenberth and Hurrell; monthly and winter." Retrieved from https://climatedataguide.ucar.edu/climate-data/north-pacific-np-index-trenberth-and-hurrell-monthly-and-winter.

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.

North Pacific (NP) Index by Trenberth and Hurrell; monthly and winter

The North Pacific Index (NP index or NPI) is the area-weighted sea level pressure over the region 30°N-65°N, 160°E-140°W. The NP index is defined to measure interannual to decadal variations in the atmospheric circulation. The dominant atmosphere-ocean relation in the North Pacific is one where atmospheric changes lead changes in sea surface temperatures by one to two months. However, strong ties exist with events in the tropical Pacific, with changes in tropical Pacific SSTs leading SSTs in the north Pacific by three months.

1898-12/2020-05

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