Simply put by Billy Kessler, an Oceanographer for the Pacific Marine Environmental Laboratory at NOAA-Seatle in his FAQ about El Nińo: Suppose a relatively brief opposing wind occurs over the west Pacific warm pool. It may last for as little as one month. This starts and estward current that pushes the warm pool a little bit east of it's usual position. If the ocean and atmosphere were not coupled, then this motion would soon stop when it ran into the trade winds. But we have shown that the trade winds exist because of the temperature contrast between east and west. If the central Pacific is warmed by flow from the west, even a small amount, then the region of rising air will ten to move east with the warm water.



That means the trade winds will also shrink back east a little, since those winds are caused by the rising air. But then the pressure of the trades holding up the sea surface slope to the west is weakened, and even more west Pacific water tries to slosh eastward. That wardms the central Pacific a little bit more, and the rising air moves further east, and the trades shrink more. This collapse continues until the water is warm across the Pacific, the trades are weak, and the thermocline zone between the warm upper-sunlit waters and the colder deep-sundeprived waters] and sea surface slope flatten out. When a cataylist weakens the easterly trade winds around the equator, warm water that is usually stockpiled around Indonesia slowly moves toward the South American coastline.