Climate Indices

In this section a short description of the most commonly used climate indices will be provided. Internal dynamics is an important factor that determines the climate of the Earth. Interactions between various elements of the system, in the absence of an external forcing mechanism, can affect the systems variability. The El Nino Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the Southern Annular Mode (SAM) indices are characteristic examples.

North Atlantic Oscillation Index (NAO)

Definition: NAO is defined as the difference of normalised sea level pressure anomalies between meteorological stations in Azores and in Iceland. A high NAO index is associated with stronger than average westerlies in the North Atlantic mid latitudes, while the opposite is true for negative values of the index. The state of the NAO index also affects the winter storm path, rainfall patterns, heat and moisture transport from the North Atlantic to the surrounding land masses.

The NAO index is calculated by applying rotated principal component analysis. This method isolates the primary teleconnection patterns for all months, from which a time series of the patterns is constructed. Further details can be found here

Data source: The data can be found on daily and monthly resolution covering the period from 1950 to 2012, provided by NOAA following the link


El Nino Southern Oscillation Index (NINO3)

Definition: NINO3 is one of several climate indicators of El Nino Southern Oscillation derived from sea surface temperature anomaly estimates in the Eastern Tropical Pacific (5N-5S,150W-90W).  The NINO3 Index is calculated by subtracting the long-term mean. Following this method, an El Niño or La Niña event can then be identified if the 5-month running-average of the index exceeds 0.4°C for at least 6 consecutive months (Trenberth, 1997). However, there is no single definition of an El Niño or La Niña event.

Data source: The data can be found on monthly resolution covering the period from 1950 to 2012 provided by NOAA following the link


Pacific Decadal Oscillation Index (PDO)

Definition: The PDO is most frequently referred to as a long-lived El-Niño like pattern of the Pacific climate variability. The extreme phases of the PDO are classified as warm or cool, derived as the leading principal component of monthly sea surface temperature anomalies in the North Pacific Ocean poleward of 20°N (Zhang et. al. 1997). The main difference of PDO to ENSO is that is acting on longer time scales (20-30 years) affecting mainly the North Pacific sector. Causes for the PDO are not currently known.

Data Source: The data can be found on monthly resolution covering the period from 1948 to 2012, provided by NOAA following the link


Indian Ocean Dipole Mode Index (IOD)

Definition: The IOD is defined as the SST anomaly difference between the western equatorial Indian Ocean (50E-70E, 10S-10N) and the south eastern equatorial Indian Ocean (90E-110E, 10S-0N), referred to as Dipole Mode Index (DMI). IOD is a coupled ocean-atmosphere phenomenon. Positive IOD values are characterised by cooler than normal water in the tropical eastern Indian Ocean and warmer than normal water in the tropical western Indian Ocean, while positive IOD is also associated with  a decrease in rainfall over parts of central and southern Australia. The opposite is true for negative IOD periods.

Data Source: The data can be found on monthly resolution covering the period from 1958 to 2010, provided by JAMSTEC following the link


Southern Annular Mode Index (SAM)

Definition: SAM is defined as the normalised difference mean sea-level pressure between 40°S and 65°S.  A high SAM index is associated with stronger westerlies in a broad band around 55°S and anomalously dry conditions over southern South America, New Zealand and Tasmania and wet conditions over much of Australia and South Africa. Over the ocean, the stronger westerly winds tend to generate stronger eastward currents which diverge at the ocean surface due to enhanced wind-driven Ekman transport leading to stronger upwelling in ˜60°S. The departures of SAM from its annular pattern enhance meridional exchanges and thus large heat transport.

Data Source: The data can be found on monthly resolution covering the period from 1957 to 2012, provided by LASG following the link


Southern Oscillation Index (SOI)

Definition: The SOI is calculated using the pressure differences between Tahiti and Darwin. A negative phase of the SOI is characterised by below-normal atmospheric pressure at Tahiti and above-normal atmospheric pressure at Darwin. Long periods of negative SOI are associated with warmer than normal ocean waters across the eastern tropical Pacific (El Niño). The opposite is true for long periods of positive SOI (La Niña). Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean. This results in a decrease in the strength of the Pacific trade wind, and a reduction in rainfall over eastern and northern Australia. Two of the strongest El Niño episodes of the century occurred on 1982/83 and 1997/98.

Data Source: The data can be found on monthly resolution covering the period from 1951 to 2012, provided by NOAA following the link


HadEX2 - global gridded climate extremes indices

Definition:HadEX2 is a global land-based climate extremes dataset derived from RClimDex using high quality daily data from about 7400 temperature stations and 11600 precipitation stations. The dataset consists of 27 indices of temperature and precipitation on a 2.5° x 3.75° grid from 1901 to 2010, representing annual and monthly values derived from daily station data. 

Available indices are: Max Tmax (TXx), Max Tmin (TNx), Min Tmax (TXn), Min Tmin (TNn), Cool nights (TN10p), Cool days (TX10p), Warm nights (TN90p), Warm days (TX90p), Diurnal temperature range (DTR), Max 1-day percipitation amount (Rx1day), Max 5-day percipitation amount (Rx5day).

When using the dataset in a paper, please use the following citation: Donat, M.G., et al. (2012), Updated analyses of temperature and precipitation extreme indices since the beginning of the twentieth century: The HadEX2 dataset, JGR Atmospheres.

Data source: The data can be found in monthly and annual resolution through the external link or through ICDC at /data/icdc/climate_indices/hadex2/



Bond, N.A. and D.E. Harrison (2000): The Pacific Decadal Oscillation, air-sea interaction and central north Pacific winter atmospheric regimes. Geophys. Res. Lett., 27(5), 731-734.

Gershunov, A. and T. P. Barnett.  Interdecadal modulation of ENSO teleconnections.  Bull. Amer. Meteor. Soc., 79: 2715-2725.

Goosse H., P.Y. Barriat, W. Lefebvre, M.F. Loutre and V. Zunz, (date of view). Introduction to climate dynamics and climate modeling. Online textbook available at www.climate.be/textbook.

Saji N.H., Goswami B.N., Vinayachandran P.N., Yamagata T., 1999: A dipole mode in the tropical Indian Ocean, Nature,401, 360-363

Trenberth, K. E., 1997. The Definition of El Niño. Bull. Amer. Met. Soc., 78, 2771-2777.

Zhang, Y., J.M. Wallace and D.S. Battisti 1997: ENSO-like Interdecadal Variability: 1900-93. Journal of Climate, Vol. 10, 1004-1020.

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