How can Pacific winds and ocean temperatures reinforce one another during El Niño?

During El Niño, the tropical Pacific is not simply a patch of warmer water. Ocean temperature, winds and rainfall change together, with each influencing the others. The usual east-to-west trade winds push warm surface water toward the western Pacific. This helps make the warm upper layer deeper in the west and leaves colder water closer to the surface in the east.

The thermocline is the transition between the warm upper water and colder water beneath. Under typical conditions it slopes downward toward the west. In the east, winds and the ocean circulation can bring relatively cool water upward from near the shallower thermocline. This upwelling helps maintain the temperature contrast across the basin.

When the trade winds weaken, warm water can spread farther east and the thermocline becomes less strongly tilted. A deeper warm layer in the east means that water brought upward may not cool the surface as effectively. Wind changes therefore alter both the horizontal distribution of warmth and the source of water reaching the surface.

Warm ocean surfaces supply heat and moisture to the atmosphere. When the warm region shifts, the main region of rising air, cloud formation and heavy tropical rainfall can shift too. The changed pressure and circulation pattern can further weaken the usual trade winds. This closes a reinforcing loop: changed winds alter the ocean, and the altered ocean helps maintain changed winds.

The loop is not a guarantee of indefinite warming. Other ocean and atmospheric processes interrupt or reverse it, and events differ in timing and strength. ENSO can alter distant weather probabilities through atmospheric circulation, but it does not specify the weather in every place. The paired basin sections illustrate the coupled mechanism, not a forecast of a current event.