El Niño rules the climate

Temperature plot of the Pacific Ocean during a non-El Niño period.
  No running, no diving, no horseplay...
Normally, when El Niño is not operating, a giant convection cell forms in the western Pacific between the South American coast and Australia. The tradewinds push surface water westward and the ocean piles up near Indonesia (sea level around Indonesia is actually 2 feet higher than at Peru!).

As surface water travels west, the sun heats this water which forms a warm pool in the western Pacific Ocean. Creatively named the "West Pacific Warm Pool", this solar-heated hot-tub usually lies just northeast of Australia.





 

The Warm Pool contains an enormous amount of energy. Because water has a high specific heat, it takes a huge amount of energy to raise the ocean temperature by just a few degrees C. Therefore, as the sun heats the ocean, solar energy is stored as a colossal quantity of heat energy in the Warm Pool. (The Warm Pool has more energy than a half million 20 megaton hydrogen bombs!)

While the warm pool piles up in the west Pacific, cooler deep-sea ocean water is drawn up and surfaces in the eastern Pacific. This deep, cool water is rich in nutrients which feeds a vibrant fishing industry along Peru and Ecuador.

The power of the West Pacific Warm Pool is nothing to be taken lightly. Its heat pumps moisture into the atmosphere where it is carried around the globe. This moisture is then released as thunderstorms all over the world. The Pool also helps control the direction of jet streams and reinforces prevailing winds. In other words, the West Pacific Warm Pool creates storms and distributes them to certain areas all around the globe.

So, under "normal" conditions, prevailing winds at the equator push a warm body of water westward where it releases moisture, deflects jet streams, and helps control the weather patterns of the world.

During an El Niño event, the tradewinds weaken, and the ocean sloshes back toward the east, seeking to level itself out. When this happens, the Warm Pool moves eastward too and the upward movement of cold water along South America stops.


Image adapted from UCAR.


When the Warm Pool moves east during El Niño, so does the giant weather machine. This means that now storms are being created in a different part of the ocean and prevailing winds and jet streams are being distributed to different parts of the Earth.

When those thunderstorms are shifted from their normal position, the global atmospheric circulation is also changed. And by changing atmospheric circulation, including the jet streams that circulate planet-wide, the effects spread far beyond the Pacific basin.

At the end of El Niño, the tradewinds pick up their pace and start pushing the Western Pacific Warm Pool back to the western Pacific. Thus, we return to what's considered "normal" conditions in the Pacific Ocean.

But, just when the tradewinds have gotten out of their lazy gait and have started to jog along again, they tend to get carried away. Following an El Niño, the winds often strengthen beyond their normal pace. When this happens, more and more surface water gets pushed west. This also means that even larger amounts of cold water are drawn up off the coast of S. America.

This strong upwelling of water and cooling of the east Pacific, the opposite of El Niño, is dubbed La Niña. Eventually, the tradewinds will weaken again and the Pacific ocean will begin another cycle of warming and cooling.

This system of oscillation between El Niño (warm) and La Niña (cool) episodes in the eastern Pacific occurs about every 3-4 years. Climatologists call the swing of changing patterns of winds and hot water the Southern Oscillation, which is why you'll often see El Niño called "El Niño-Southern Oscillation" or just ENSO.

The key to remember is that conditions oscillate between El Niño and its opposite, La Niña. In other words, what's really normal is change.

I heard the 1997-98 El Niño was a monster.


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