Where Does El Niño Come From? Here's How It Forms

El Niño is expected to form in the coming months and that means we should see warming waters across part of the equatorial Pacific Ocean that seemingly appear out of nowhere. But there is a reason it happens, and the answer partially lies below the ocean's surface.

El Niño's latest status: NOAA announced last week that El Niño was 70 to 80% likely to form by this summer or early fall.

Why it matters: It could have an effect on summer and winter temperatures and hurricane season.

(​MORE: Summer Outlook | Hurricane Season Outlook)

How we'll know El Niño conditions have arrived: This would be marked by the warming (to a certain threshold) of a strip of water straddling the equator in the Pacific Ocean. That warm water must also persist. Its counterpart La Niña, which is the cooling of this same patch of the ocean, finally faded earlier this year.

Where the warm water comes from: You might think this water periodically warms because of heating from the sun or some source in the atmosphere, but it actually comes from beneath the ocean's surface.

The graphic below shows warmer than average water pooling below the Pacific Ocean's surface at depths of roughly 200 to 800 feet as of early April. It stretches from southeast Asia eastward across central and eastern parts of the ocean basin.

T​his water is the source for El Niño's likely development since it's expected to rise up and warm the ocean's surface waters to some extent in the coming months. The supply of warm water also gives increased confidence in the forecast for El Niño's development by supporting what computer model projections are showing, NOAA said.

These aren't the waves you're used to, you know the ones that break into whitecaps in the ocean. They are more massive, as in big enough to encompass the entirety of the Pacific Ocean, and the water slowly sloshes around versus breaking above the surface.

A change in trade wind speeds is what can set this oceanic downwelling Kelvin wave into motion.

Trade winds typically blow from east to west across the Pacific when El Niño or La Niña conditions are not present and allow warm water to collect in the western part of that ocean basin closer to southeast Asia.

If those typical surface winds slacken up, then that pool of warm water can slosh back eastward but remain below the ocean's surface (the downwelling part) as it moves toward the central and eastern part of the ocean basin. That's what has occurred this spring.

The next step in El Niño's formation: As alluded to earlier, in order for El Niño to officially form the warmer water that's waiting in the depths of the central and eastern Pacific must rise to the surface.

A slackening of the typical east-to-west trade winds near the equator is what can allow that to occur, and NOAA says conditions are favorable for that to be the case over the next couple of weeks.

NOAA declares an El Niño when sea-surface temperatures in a certain region of the equatorial Pacific Ocean have reached at least 0.5 degrees Celsius above average for at least a month and are expected to persist for several more months. Changing weather patterns in response to that warmer water is another factor taken into consideration.

See this link for further details on what a developing El Niño could mean from meteorologist Jon Erdman.

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