The tropical Pacific has taken a decidedly circuitous path over the past several years toward its present extraordinary warmth. Until 2014, persistent and increasingly anomalous warmth in the tropical West Pacific Ocean (near Indonesia) had been in place for the better part of two decades—not coincidentally since the last big El Niño event in 1997-1998.
The tropical East Pacific, on the other hand, experienced no such net warming between 1998 and 2014. Since the tropical West Pacific is already warmer on average than the East, this trend led to a substantial increase in the west-to-east temperature differential across the Pacific Ocean basin over a 15 year period. This mean state change generally resembled the ocean temperature pattern associated with La Niña, and as a result the Pacific entered a persistent “La Niña-like” state after the 1997-1998 El Niño.
Source: Daniel Swain, The California Weather Blog, Weather West, September 2, 2015