El Niño is believed to be brought on by the weakening and reversal of the trade winds in the western equatorial Pacific, which changes the SLP that defines the Southern Oscillation. These wind and pressure changes may trigger the equatorial Kelvin waves that propagate warm sea surface temperatures to the cooler eastern Pacific sending the warm water rushing back eastward, thus changing our climate into a phase of El Niño.

(Taken from www.carleton.ca)