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).

Please cite data sources, following the data providers' instructions

Suggested Data Citation

Dataset DOIs

None

Hosted Climate Index Files

North Pacific Index (monthly)
Missing Value
-999

Units
hPa

Updated Through
2023-12-31

Next Update
2024-05-01

Start Date
1899-01-01

File Notes

CAS 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].
North Pacific Index (NDJFM)
Missing Value
-999

Units
hPa

Updated Through
2023-03-31

Next Update
2024-05-01

Start Date
1899-01-01

File 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.)

CAS 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].
North Pacific Index Anomalies (NDJFM)
Missing Value
-999

Units
hPa

Updated Through
2023-03-31

Next Update
2024-05-01

Start Date
1899-01-01

File 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.)

CAS 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].

Data Access

None

Usage Restrictions

None

Key Figures

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)