What is El Niño? In a narrow sense, it's a giant puddle of heated water
that sloshes across the Pacific Ocean; a massive pool of warm seawater, builds in the western Pacific over a period of months. In one sense, it's like an iceberg; most of it is submerged, but part of it sticks out above the sea's surface, as the wedge floats in the surrounding ocean. Partly because warm water is less dense than cool water, and also
partly because El Niño waters are less salty than normal seawater. (It's always raining over an El Niño, and the rainwater dilutes the sea.) A sharp temperature and density change -- called the thermocline -- floats about 100 meters
below the surface, and marks the bottom of this warm "iceberg." The top layer of water may protrude 150 or more centimeters above sea level.
El Niño's energy reserve is vast, almost inconceivable. It contains more energy than has been procured from all the fossil fuels burned in the United States since the beginning of the century - that's all the gasoline in all the cars, the coal in all the power plants, the natural gas in all the furnaces.
Scientists are divided on what “makes” the buildup. Some say it begins when strong west-blowing winds near the equator-the trade winds-push against the sea, and drive sun-warmed surface waters against Indonesia and Australia. Others say that's backwards, that the wind is an effect, not a cause; warm masses of air in the western Pacific, heated by already-feverish tropical waters, create a vacuum beneath them as they rise like enormous hot air balloons. This vacuum sucks the trade winds toward it, making them stronger. Of course, these winds then pile more warm water up as they blow against the sea's surface, in turn causing more thermal updrafts, more vacuums, more winds. At some point, something breaks. The winds can no longer hold the wall of water up, and it begins flowing back "downhill." Like a teeter-totter, what was high becomes low, and what was low becomes high. Water levels actually dip below sea level near Australia, and as the pile traverses the Pacific, waters rise in the east near South America.
There are other, more elaborate theories about what causes El Niño. The truth is, no one knows what really causes El Niño. It might very well be the last El Niño that starts the next one. The official scientific name for El Niño, ENSO (El Niño-Southern Oscillation). An oscillation is a reverberation back and forth between two states, like between summer and winter. El Niño is one extreme of a years-long oscillation in the entire pacific basin and the atmosphere above it. As the cycle unfolds, an "anti-El Niño," dubbed La Niña, appears as El Niño disappears. Like a mirror image of El Niño, it produces extreme weather and abnormal conditions in the western Pacific similar to those El Niño produces in the east. Back and forth, east and west, this cycle has run continuously for many thousands of years.
Although the cycle is not as regular as the seasons, we can count on El Niño and its sister La Niña appearing about 30 times per century, with intervals as short as two years and as long as 10.
El Niño is a climate phenomenon, and despite all the predictions El Niño has generated, no one can say exactly what its effects will be. Scientists can, however, assign probabilities to its possible effects.
What you see when you look at a probability map of El Niño's effects is that, roughly, the closer you are to El Niño's Ground Zero, which is the tropical Pacific ocean, the higher the probabilities of predictable effects become. El Niño's day-to-day effects can never be fully predictable. That doesn't mean we can't make useful probability predictions, though. It affects the entire world's weather system, El Niño has lots of real effects on lots of lives, all expressed in this same probabilistic way.
In a sense, El Niño is like a slow-motion explosion of pent-up heat that makes a violent killer out of a small tropical storm...or that can bring life-giving rains to a
desert. It is because they hoard such stupendous quantities of energy that El Niño is second only to the change of seasons as shapers of global weather. Although El Niño's
water pooling phenomenon is regional, the effects this has on climate truly stretch around the world, as ocean currents and atmospheric winds ferry its monumental power out of the tropics and around the planet. El Niño is a heat engine, is a device for transforming heat energy into kinetic energy, the energy of motion; to produce the heat, El Niño uses sunlight. El Niño can wreak havoc halfway around the world because it is bound up in a Global Weather Machine.
Global Weather Machine
We live in an ocean of air, seething and flowing around us, changing-sometimes violently-every day. In the heart of this swirling machinery of rain clouds and jetstreams, hot desert winds and frozen arctic storms, there is one constant: change. A trillion and a half days have passed since the Earth was born in a spinning disk of stardust, and no two of those days have ever had the same weather.
Driven by the heat of the sun, weather is an interlocking system of cycles. Water evaporates, rises, cools, and falls as rain, only to evaporate once again. The sun rises and sets every day, with the air warming and cooling in response, and the cycle endlessly repeating.
Low pressure systems suck high pressure systems into their vacuum, creating spinning masses of wind and clouds bigger than Texas; these cyclones are swept across the skies by persistent high-speed winds miles up in the atmosphere, rivers of air in a relentless race around the globe.
Weather, in all its cycles and clashes, arises from a simple fact: the sun heats some parts of the Earth more than others. Because the Earth is a globe, and not a flat board, the sun shines almost straight down on the tropics, baking them every day of the year. But at the poles, the angle is small and the sun's rays are weak, and the poles are therefore cold.
Nature "abhors" this imbalance, and tries to fix it. As quickly as solar heat flows in to the tropics, it begins flowing out toward the poles, seeking to equalize the difference. The unrelenting march of this energy-on-the-move, from high concentration to low concentration, is the piston in the engine that propels weather.
When warm air leaves the tropics and heads toward the poles, cold air from near the poles is sucked back toward the tropics. This exchange sets up two-lane highways for air rushing to and from the tropics. These highways of air are called convection cells, and they are the reason wind blows.
Air flowing back and forth in these great cells is pushed sideways by the Earth's
rotation, dragged by friction with the land and the sea, and squeezed by gravity. All of these distortions cause turbulent mixing of the winds, and soon lead to the organization of storm centers due to unevenness between warm and cold. In particular, the sideways push given the winds by the spinning of the planet-called the Coriolis Effect-causes the constant convective flows to organize in bands, where the flow direction varies according to latitude. These bands are responsible for prevailing winds on the surface, and jetstreams high in the atmosphere.
We can see these bands of wind clearly in Jupiter's atmosphere, because Jupiter rotates at a furious pace, once every ten hours. We can also see them clearly on Earth when we take a picture from far out in space. El Niño exploits this organization of winds into bands when it causes major weather changes around the world. Specifically, El Niño can affect the path of flow in these bands, and the cyclones that are ushered across the surface by them are now delivered to different areas than normal. Think of the wind bands - both at the surface and high in the sky -- as a tram, a streetcar on which storm systems hitch a ride as they travel around the Earth. El Niño moves the tracks -- the stormtracks - of this tram. The answer to the puzzle of how this happens is literally blowing in the wind.