Scientists can predict an episode of El Niño by calculating the difference in pressure between Darwin; Australia and Tahiti. This calculation gives us an index which is known as the SOI, or the southern oscillation index. When the SOI begins to fall, it indicates that our climate is slipping into what we know as El Niño. Since the pressure changes along with the direction of the trade winds (which transport the warm water) during an onset of El Niño, the SST will get warmer as the SOI gets smaller, as seen below for the year 1983.

(Taken from www.fb4.noaa.gov:80)