La Niña is characterized by unusually cold ocean temperatures in the equatorial Pacific.
El Niño is characterized by unusually warm ocean temperatues in the Equatorial Pacific Ocean.

These are graphics of the Reynolds Sea Surface Temperature (SST) analysis from the National Center for Environmental Prediction (NCEP) for the entire Pacific Ocean (70N to 70S). 

Also see plots of the Sea Surface Temperature Anomalies (the amount that the temperature differs from the normal value for the month).

Graphics are provided by the TAO project and created by Dai McClurg.

You can also see animations of the Reynolds SST and TAO buoy SST .