Climate Variability: Oceanic Niño Index


The El Niño - Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. On periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal.

This oscillating warming and cooling pattern, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world. El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.

El Niño conditions occur when abnormally warm waters accumulate in tropical latitudes of the central and eastern Pacific Ocean. Consequently, tropical rains that usually fall over Indonesia shift eastward. During El Niño winters, northwestern North America is more likely to experience warmer-than average temperatures and the southeastern U.S. is more likely to receive more rain than average.

La Niña conditions occur when cooler-than-average waters accumulate in the central and eastern tropical Pacific and tropical rains shift to the west. In the United States, seasonal precipitation impacts are generally opposite those of El Niño. Compared with El Niño conditions, La Niña conditions are generally more favorable for the formation of Atlantic hurricanes.

How do we tell what phase ENSO is in? 
NOAA's Climate Prediction Center has determined the average monthly sea surface temperature for a particular swath of the tropical Pacific Ocean by averaging measurements collected there over the past 30 years. Scientists refer to that swath as the Niño 3.4 region. The observed difference from the average temperature in that region-whether warmer or cooler-is used to indicate the current phase of ENSO.

Nino 3.4 Region

The shaded rectangle shows the Niño 3.4 region, the area of the Pacific Ocean where observed sea surface temperature is compared to average sea surface temperature to calculate the Oceanic Niño Index. The region spans a swath from 5°N to 5°S latitude and 120°W to 170°W longitude.

To filter out month-to-month variability, average sea surface temperature in the Niño 3.4 region is calculated for each month, and then averaged with values from the previous month and following month. This running three-month average value is compared with average sea surface temperature for the same three months during 1971 - 2000. The departure from the 30-year average of the three-month average is known as the Oceanic Niño Index or ONI.

Explore this interactive graph: Click and drag to display different parts of the graph. To squeeze or stretch the graph in either direction, hold your Shift key down, then click and drag. This graph shows monthly values of the Oceanic Niño Index from 1950 through present.

ENSO Conditions
For real-time monitoring and prediction, NOAA considers El Niño conditions to be present when the Oceanic Niño Index is at least +0.5. In other words, El Niño conditions exist when the three-month average sea surface temperature in the Niño 3.4 region is at least 0.5°C warmer than average.

Conversely, NOAA declares that La Niña conditions exist when the Oceanic Niño Index is less than -0.5. This means that the three-month sea surface temperature in the Niño 3.4 region is at least 0.5°C cooler than average.

Whenever the Oceanic Niño Index is between +0.5 and -0.5, conditions are ENSO-neutral. A table of ONI values for each three-month period from 1950 to present is available from NOAA's Climate Prediction Center.

Further Reading
Climate Prediction Center's ENSO Cycle Page, Accessed December 10, 2009.

El Niño Theme Page, NOAA's Pacific Marine Environmental Laboratory. Accessed October 2, 2009.

ENSO-tagged posts at International Research Institute for Cimate and Society. Accessed October 2, 2009.

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