Global Weather Machine (continued)
How does El Niño take over such a huge system? It begins with an effect
due to the vastness of the Pacific Ocean itself, an effect intimately
related to the birth of an El Niño. In the
Pacific near the equator, the prevailing winds blow from east to west, as
cool air sinking from higher latitudes toward the equator gets whipped
sideways by the Coriolis force. We know these as the tradewinds, which
sailors of old could always depend upon to blow steadily in the same
In the tropical Pacific, these west-blowing tradewinds push steadily
against the sea for thousands of miles. The warm water on the surface is
literally blown sideways, and the water piles up in the west, creating a
pool thousands of miles across. This leads to a heat imbalance: as more
and more warm water is stripped from the east and moved west, cold waters
from deep in the ocean near South America are drawn up to take its place.
This cool water inhibits evaporation and the creation of rain clouds in
the east, which is why the Galapagos Islands and the coast of Peru are
usually deserts. Just the opposite effect happens in the west, near
Australia: intense rain cloud formation occurs as warm moist air, heated by
the warm sea, rises and condenses into clouds. These clouds carry the
drenching rains of the monsoons upon which the entire region of Indonesia
and Southeast Asia depends. The huge volumes of rising warm air create a
vacuum as they move upward, which draws cooler air from the east to replace
it, strengthening the tradewinds and reinforcing the entire cycle. Another
two-lane convection highway is created, but instead of between the equator
and the poles, this one is between coastal South America and the region of
Clouds and warm moist air billow upward from a heated sea. In this NASA Space Shuttle photo of a section of the tropical Pacific, clouds condense in the rising air mass created by contact between the ocean and the atmosphere.|
Here's where it gets interesting, the crux of the mystery of El Niño. This
cycle should be self-perpetuating. But it's not. For unknown reasons,
every few years, something hidden in the machinery causes the west-blowing
tradewinds to slacken in the Pacific. The warm waters, which have been
held by the winds in a pile 5 feet above sea level in the west, begin to
flow back across the sea, drawn down by gravity, like a river breaching a
levee. This massive surge of heated water shoots across the ocean and
repositions itself near South America. East becomes
west. Because of the heated water, all of the rainmaking that normally
would happen in the west now happens in the east, and the convection cell
reverses flow, which means rising warm air in the east sucks in the air
from the west, and the tradewinds actually reverse their direction.
Because the water in the west is now comparatively cool, rainmaking stops.
The monsoons fail in Indonesia, but unending rains begin in Peru. The
Child has arrived.
In its new, temporary headquarters off South America, the warm pool's heat
again creates a huge mass of warm moist air, which bulges into the zones of
prevailing winds at the surface as well as high in the air. Like a car
dumped in a stream, this foreign obstruction creates ripples and waves
"downstream" in the vast air rivers that circulate the Earth. These ripples cascade outward, pushing and disturbing the
midlatitude jetstreams which sweep weather across the temperate zones. Off
the west coast of North America, the bulging effect is pronounced. Pacific storms which
normally would remain in the tropics now have an open door to the west
coast, as the jet stream lurches north. California, Mexico, and even
British Columbia brace for an onslaught of winter rain.
Warmer water off the coast of South America. See the full Jan. 4, 1997 - Oct. 2, 1997 animation (2MB):
QuickTime | AVI
After lurching north, the jet stream (like everything else in the system)
tries to compensate for its too-far north motion by diving south, usually
over the Rocky Mountains. It then snakes north again, creating the classic
El Niño pattern. Because the jet stream represents a boundary
between cold northern air and warm moist southern air, meteorologists are
able to make general predictions for weather in an El Niño winter.
For starters, Pacific storms form farther east than usual. The northward
bulge of the jet stream then conducts these abnormal storms into California
and Mexico. Meanwhile, normal winter storms that would otherwise be
steered through Washington and Oregon now veer northward toward the coast
of Alaska, eventually being guided east into Canada. The west coast gets
drenched; the Canadian Rockies get record snowfalls.
By strengthening east-blowing winds in the Caribbean, El Niño also creates
a favorable environment for storms to develop in the Gulf of Mexico, and
the displaced jetstream lets these storms pass up into the southeast of the
United States. Florida and the southeastern states have a cool, abnormally
rainy winter. A similar strengthening of the east-blowing winds in the
Southern Hemisphere during its winter season brings massive storms to
southern Brazil, Chile and Argentina.
Winds at the jetstream level (small arrows)
change their course between normal (top) and El Niño (bottom) winters. A winter bulge caused
by El Niño over North America's west coastkeeps temperatures above normal in
the orange region, and steers storms that would otherwise pass northward toward the
coast of Alaska (heavy arrow), while admitting more southerly storms north to Mexico
You can also see on the diagram that in the midwest and northeast, the
jetstream's strange dip and rise keeps colder Canadian air stuck in Canada,
far north of its usual winter position. Acting as a boundary between this
cold northern air and mild southern air, the displaced jetstream lets
Chicago and New York enjoy a relatively warm, if somewhat wet, winter.
Because the rain machine in the west stops working, southeast Asia and
Australia suffer devastating droughts. Meanwhile, North and South America
get drenched, because the rain machine in the east is working overtime.
Farther downstream in the great wind bands that circle the globe, El Niño
continues to create havoc. By using the bands of prevailing winds as
avenues along which to transmit its disruptive waves, El Niño eventually
influences weather in Africa, the North Atlantic, even the Middle East.
Teleconnected to distant regions by the Earth's rivers of air, El Niño
invades the global weather machine.
See El's Niño reach across the globe.
Photos: (1) University of Illinois WW2010 Project; (2) NASA/JSC; (3) NASA/JPL; (4) University of Washington.
Find out more about El Niño's origins.