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Global Weather Machine

  • By Mark Hoover
  • Posted 10.13.98
  • NOVA

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.

This NASA image of wind patterns over the Pacific Ocean gives a sense of the dynamism of global weather. Enlarge Photo credit: NASA

what is 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.

The major surface wind bands of Earth. Each hemisphere is divided into three belts. The path of a storm greatly depends upon the wind belt in which it is located. The easterly (west-blowing) trade winds of both hemispheres collide near the equator, in a region called the Intertropical Convergence Zone (ICTZ). Enlarge Photo credit: University of Illinois WW2010 Project

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.

The ITCZ on this satellite image is the band of bright clouds located just north of the equator. This zone is a prolific contributor of storms and clouds to the world's weather. Enlarge Photo credit: NASA

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.

As on Earth, Jupiter shows distinct wind bands generated by convection and rotation forces. Scientists have measured wind speeds in Jupiter's "Little Red Spot" reaching up to about 384 miles per hour—twice as fast as the winds of a Category 5 hurricane. Enlarge Photo credit: NASA

El Niño'S POWER

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.

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 direction.

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 east, which is why the Galapagos Islands and the coast of Peru are usually deserts.

The El Niño temperature anomaly of 1997-98 (appearing here as a red band in the Pacific Ocean) affected weather worldwide. Enlarge Photo credit: NASA / Image by R.B. Husar, Washington University; the land layer from the SeaWiFS Project; fire maps from the European Space Agency; the sea surface temperature from the Naval Oceanographic Office's

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 Australia.

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.

When a very strong El Niño strikes surface waters in the equatorial Pacific Ocean, warm water anomalies (red) develop in the Central Pacific. Winds that normally blow in a westerly direction weaken, allowing the easterly winds to push the warm water up against the South American coast. Enlarge Photo credit: NASA

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.

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.

This feature originally appeared on the site for the NOVA program Tracking El Nino.

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