by Mark Hoover
What is El Niño? In a narrow sense, it's a giant puddle of heated water
that sloshes across the Pacific Ocean. From NASA satellites, we can see
the big picture; a massive pool of warm seawater, half again as large as
the United States, builds in the western Pacific over a period of months.
This gargantuan wedge of warm sea contains 20 or 30 times as much water as
all the Great Lakes put together. 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.) Both
of these conditions contribute to buoyancy. 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. This isn't so hard to picture if you think about tides,
which also pile water up 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. It would take more than a million large power plants, at
1,000 megawatts each, running full tilt for a year, to heat the ocean that
much. Or if you want to think in more violent terms, that's all the energy
produced by about half a million 20 megaton hydrogen bombs, gone into
Scientists are divided on what triggers 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. There the waters pile up like driftwood blown into a harbor.
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 snaps. 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.
The birth of El Nino. This animation shows anomalies, or departures from normal, in Sea Surface Temperature (SST) over the past year. As spring
became summer in 1997, a Kelvin Wave of warm water crossed the Pacific and
accumulated off the coast of South America, shown here in red. See the 775K animation of the wave.
There are other, more elaborate theories about what causes El Niño, but one
element seems common to all of them: when you look closely, you see a dog
chasing its own tail. 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) reflects an understanding of this teeter-totter nature. 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.
Scientists looking back through time find that El Niño has left tantalizing
records of its past appearances in unexpected places. Signs can be read in
Arizona tree rings and Himalayan ice-core layers. Corals in New Guinea and
lake bottoms in the Sierra Nevada attest to previous comings (come back on
February 10 for "El Niño's Reach Into Living Things" to
find out more). 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. Only a
few of those appearances will be whoppers like 1982 or 1997; it's the big
ones that tend to grab our attention.
Asking why El Niños occasionally reach giant proportions- as this year's
has-is like asking why one thunderstorm is larger than all the others in a
summer, or why two dice come up snake eyes only once in three dozen tosses.
The answer is hidden in the laws of probability. Climate science is all
about understanding probabilities. El Niño is a climate phenomenon, and
despite all the hype and dire predictions this El Niño has generated, no
one can say exactly what its effects will be. Scientists can, however,
assign probabilities to its possible effects.
Will California be deluged? There's a 60 percent chance of double the
normal rainfall, and a 10 percent chance of triple. (There's also a 10
percent chance of drier than normal.) Will Florida be cloudier and cooler
than normal? There's a 70 percent chance the answer is yes. Will the
Galapagos Islands be drenched? There's over a 95 percent chance that the
answer is yes-a virtual certainty. 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.
In North America, which is halfway between the equator and the pole, not
many of El Niño's predicted effects have higher than about a 60 percent
probability. That's not a flaw of science as much as a recognition that in
the temperate zones far from the center of the action, El Niño's day-to-day
effects can never be fully predictable. If you want to be trendy, you can
say this is because Earth's weather machinery is chaotic, a property of
many natural systems which unfold in mysterious and complex ways.
That doesn't mean we can't make useful probability predictions, though.
Here's a question for you: if the weather forecast calls for a 70 percent
chance of rain tomorrow, do you carry an umbrella? How about 30 percent?
We all have some sense of probability when it has real effects on our
lives. Because it affects the entire world's weather system, El Niño has
lots of real effects on lots of lives, all expressed in this same
We may have to use the language of probability to gauge El Niño's future
effects, but we can use our own five senses to grasp its physical reality.
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 parched Chilean desert. It is because they hoard
such stupendous quantities of energy that El Niños are 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, in some ways like the heat engine that propels you when you're
cruising down the freeway. A heat engine, whatever its size, is a device
for transforming heat energy into kinetic energy, the energy of motion.
Your car uses burning gasoline to produce its heat; El Niño uses sunlight.
Typhoon Winnie, a monster fueled by El Niño energy, is a study in what
kinetic energy of the atmosphere looks like.
But how exactly is it that El Niño—after all no more than a large pool
of water in the Pacific—can wreak havoc halfway around the world? It is
because El Niño is inextricably bound up in a
Global Weather Machine.
Photos/Images: (1) NASA Goddard Space Flight Center; (2) NOAA; (3) © T.W. Swetnam and A.C. Caprio; (4) NASA.
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