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North Atlantic Oscillation (NAO) Index from Hurrell (1995): Science 269:676-679

Winter (December through March) index of the NAO based on the difference of normalized sea level pressures (SLP) between Lisbon, Portugal and Stykkisholmur, Iceland. Other seasons, annual averages, daily values, and PC-based NAO indices are also available from Jim Hurrell's Climate Indices page.

North Pacific (NP) Index from Trenberth and Hurrell (1994): Climate Dynamics 9:303-319

Area-weighted sea level pressure over the region 30N-65N, 160E-140W.

Atlantic Multi-decadal Oscillation (AMO) from Trenberth & Shea (2006): Geophysical Research Letters 33, L12704, doi:10.1029/2006GL026894 (updated)

Southern Oscillation Index (SOI) from Trenberth (1984): Monthly Weather Review 112:326-332

The Southern Oscillation Index (SOI) presented here is computed using monthly mean sea level pressure anomalies at Tahiti (T) and Darwin (D). The SOI [T-D] is an optimal index that combines the Southern Oscillation into one series. These SOI values are slightly different than those calculated by the Climate Prediction Center due to the normalization used. The [T+D] series is a measure small scale and/or transient phenomena that are not part of the large scale Southern Oscillation.

Niño Regions 3 and 3.4 SST Indices from Trenberth, K. E. (1997) The Definition of El Niño. Bulletin of the American Meteorological Society, 78, 2771-2777. Figures and indices (SST anomalies) are current through December 1999.

Niño 3 (90°W-150°W, 5°S-5°N) and Niño 3.4 (120°W-170°W, 5°S-5°N) SST anomalies relative to a base period climatology of 1950-1979 are shown along with a listing of the monthly anomalies (indices) for the period January 1950 to December 1999. The Niño 3.4 anomalies may be thought of as representing the average equatorial SSTs across the Pacific from about the dateline to the South American coast.

NOAA uses a slight variant of the above called the Ocean Nino Index [ONI]. NOAA uses a 3-month rather than a 5-month running average of the Niño 3.4 SST anomalies. Further, to be classified as a full-fledged El Niño or La Niña the anomalies must exceed +0.5C or -0.5C for at least five consecutive months.

TNI (Trans-Niño Index) and N3.4 (Niño 3.4 Index) from Trenberth, K. E., and David P. Stepaniak (2001) Indices of El Niño evolution. Journal of Climate., 14, 1697-1701.

TNI (Trans-Niño Index), defined to be the difference in normalized SST anomalies between Niño 1+2 and Niño 4 regions. N3.4 represents normalized SST anomalies from the Niño 3.4 region. Based on HADISST data from the U.K. Meteorological Office, and the NCEP OI SST analysis. The index begins in January 1871 and ends in September 2000, with updates as current data is made available.

Palmer Drought Severity Indices from Dai, A., K.E. Trenberth, and T. Qian (2004) A global data set of Palmer Drought Severity Index for 1870-2002: Relationship with soil moisture and effects of surface warming. J. Hydrometeorology, in press.

Miscellaneous Figures of Selected Climate Indices:

Darwin SOI and GISST Anomalies spanning the period 1870 to 1998. Presented by Kevin E. Trenberth at January 1999 AMS meeting in Dallas, Texas.

Other Climate Index Sites:

• NCEP Climate Prediction Center (CPC)
• Joint Institute for the Study of the Atmosphere and Ocean (JISAO), U. of Washington

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