--updated Feb. 11, 2016

The Oceanic Niño Index (ONI) is NOAA's primary indicator for monitoring El Niño and La Niña, which are opposite phases of the climate pattern called the El Niño-Southern Oscillation, or “ENSO” for short. NOAA considers El Niño conditions to be present when the Oceanic Niño Index is +0.5 or higher, indicating the east-central tropical Pacific is significanty warmer than usual. La Niña conditions exist when the Oceanic Niño Index is -0.5 or lower, indicating the region is cooler than usual.

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.

The ONI tracks the running 3-month average sea surface temperatures in the east-central tropical Pacific between 120°-170°W. Scientists call the area the Niño 3.4 region.

Location of the Niño regions for measuring sea surface temperature in the eastern and central tropical Pacific Ocean. The sea surface temperature in the Niño3.4 region, spanning from 120˚W to 170˚W longitude, when averaged over a 3-month period, forms NOAA’s official Oceanic Niño Index (the ONI). NOAA Climate.gov image by Fiona Martin.

To calculate the ONI, scientists from NOAA's Climate Prediction Center calculate the average sea surface temperature in the Niño 3.4 region for each month, and then they average it with values from the previous and following months. This running three-month average is compared to a 30-year average. The observed difference from the average temperature in that region—whether warmer or cooler—is the ONI value for that 3-month "season."

Maps of sea surface temperature anomaly in the Pacific Ocean during a strong La Niña (top, December 1988) and El Niño (bottom, December 1997). Maps by NOAA Climate.gov, based on data provided by NOAA View. large versions La Niña | El Niño

About El Niño and La Niña

ENSO shifts irregularly back and forth between El Niño and La Niña every two to seven years. Each phase triggers predictable disruptions of temperature, precipitation, and winds. These changes disrupt the large-scale air movements in the tropics, triggering a cascade of global side effects.

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.

Further Reading

Climate.gov ENSO Blog

Frequently asked questions about El Niño and La Niña

NOAA's ENSO alert system

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

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