Climate Indices

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

  • NAO Index (Station-Based): Index of the NAO based on the difference of normalized sea level pressures (SLP) between either Lisbon, Portugal (or Ponta Delgada, Azores) and Stykkisholmur, Iceland. Lisbon is used for the winter-time (DJFM) index, while Ponta Delgada is used for the monthly, seasonal, and annual indices.
  • NAO Index (PC-Based): Index of the NAO based on the time series of the leading Emperical Orthogonal Function (EOF) of SLP anomalies over the Atlantic sector, 20-80N, 90W-40E. These indices are used to measure the NAO throughout the year, tracking the seasonal movements of the Icelandic low and Azores high.

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 [pdf]

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 [pdf]
  • 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, 5, 1117-1130. [pdf]

Miscellaneous Figures of Selected Climate Indices:


Other Climate Index Sites: