Sometimes the warm El Niño current arrives in the Pacific almost on schedule, and sometimes it disappears for decades. Scientists have found an explanation: the ocean has two different "moods," and it can jump between them.
Think of a pendulum that swings now on one set of swings, now on another. On one set, it moves smoothly and predictably, and strong El Niños occur regularly. On the other, it jerks about chaotically, and no powerful currents appear. Both sets of swings exist at the same time, and the ocean can leap from one to the other.
In a computer simulation lasting one hundred thousand years, such jumps happened irregularly. There is no way to guess when the pendulum will leap next. So instead of a precise forecast of "will El Niño occur this year," scientists can only estimate the odds of which set of swings the ocean is on right now.
In the JT model, a strong El Niño occurs when the east–west temperature difference across the tropical Pacific approaches zero: the ocean briefly becomes almost 'flat' in temperature.