The Climate Prediction Center defines. . .

"El Niño conditions" as existing when:

A one-month positive sea surface temperature anomaly of 0.5C or greater is observed in the Niño-3.4 region of the equatorial Pacific Ocean (5�N-5�S, 120�W-170�W) and an expectation that the 3-month Oceanic Ni�o Index (ONI) threshold will be met AND

An atmospheric response typically associated with El Niño is observed over the equatorial Pacific Ocean (see The ENSO Cycle).

"La Niña conditions" as existing when:

A one-month negative sea surface temperature anomaly of -0.5C or less is observed in the Niño-3.4 region of the equatorial Pacific Ocean (5�N-5�S, 120�W-170�W) and an expectation that the 3-month Oceanic Ni�o Index (ONI) threshold will be met AND

An atmospheric response typically associated with La Niña is observed over the equatorial Pacific Ocean (see The ENSO Cycle).