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, an expectation that the 3-month Oceanic Niño Index (ONI) threshold will be met (see graphs below), and an atmospheric response typically associated with El Niño is observed over the equatorial Pacific Ocean.
El Niño/Southern Oscillation (ENSO) DIAGNOSTIC Discussion
El Niño/ENSO: Recent Evolution, Current Status and Predictions
Southern California Coastal Ocean Observing System El Niño page
The NOAA 1/4° daily Optimum Interpolation Sea Surface Temperature data (http://www.ncdc.noaa.gov/oisst).