Download About El Nino and La Nina

About El Nino and La Nina
In 1997, it brought more energy with it than a million Hiroshima bombs, and it's resurfaced again in 2007. El Nino is a
weather cycle that happens every few years but the El Nino of '97-'98 was one of the most devastating. It killed nearly
2,100 people worldwide and left $33 million US in property damages in its wake. As if that wasn't bad enough, El Nino
was followed by one of the worst La Nina cycles in recent history - the very next year. How a weather system can wreak
so much havoc? Read on to find out!
El Nino and La Nina - Equatorial Weather Patterns
El Nino is a weather pattern that passes through the
Equatorial Pacific Ocean every couple of years (it
tends to appear every three to eight years). El Nino is
associated with warmer ocean currents but that's not
all that happens when this system passes through. Air
pressure changes during El Nino causing higher
temperatures and, when that is combined with
warmer ocean currents, evaporation takes place at a
greater speed. All these elements combined lead to
drastic changes in normal weather patterns and can
cause abnormal amounts of rain. These rainstorms take place within the Equatorial Pacific Ocean - causing massive
flooding for the countries in that area, including the United States, Peru and other parts of South and Central America.
While the Eastern Pacific area is being pummeled by extra rain, areas like Australia, Indonesia and India deal with severe
drought. This is because much of the moisture that is normally dispersed around the world is evaporating too quickly and
staying within the Eastern Pacific Ocean.
El Nino and La Nina - Role Reversal
Just like everything else in the world, El Nino has an opposite too. La
Nina is a weather system that hits the Pacific Ocean between
December and March. La Nina is as unpredictable as El Nino but
usually shows up every three to eight years, following an El Nino
system. While there are mild winters and flooding associated with El
Nino, the complete opposite is true when La Nina arrives; La Nina
brings harsh winters with drought and abnormally cold temperatures.
El Nino and La Nina - The History
There was a time when these two weather patterns were responsible for
thousands of deaths. During the El Nino cycle in 1789, 600,000 people
died in one region of India due to severe drought. Thanks to modern
day science, we are now able to predict when we will see these
weather systems pass through. This doesn't mean we can necessarily
lessen the physical damage they cause, but early warnings may help to
save lives in the future. The El Nino system making its way through
the Pacific Ocean in 2007 is expected to bring some of the hottest temperatures on record along with it.
El Nino and La Nina - Fun Facts
Both El Nino and La Nina mean, The Child, in Spanish (one is the male version and one is the female version). El Nino
usually refers to the baby Jesus because the weather patterns often begin to surface around Christmas time.
El Nino is also known as ENSO (El Nino-Southern Oscillation). Southern Oscillation is a term used to describe the way
air pressure changes during an El Nino system.
The longest lasting El Nino system lasted for four years. They usually only last about 12 to 18 months.
Source: http://www.kidzworld.com/article/2620-el-nino-and-la-nina-systems
1997 年，厄尔尼诺带来的能量超过
100 万颗广岛原子弹，2007 年，它
又发生了。厄尔尼诺是每隔几年发
生的气候循环，但 1997-98 年的厄
尔尼诺是其中最具破坏性的之一。
随着它的到来，全世界 2,100 人丧
生，造成 3,300 万美元的财产损失。
似乎这还不算太糟糕，在最近的记
录中，其中一场最严重的拉尼娜循
环随着厄尔尼诺而来——恰好就
在明年。你对一种气候现象如何造
成这样严重的灾害好奇吗？请继
续阅读找到答案！
厄尔尼诺和拉尼娜——赤道附近的气候模式
厄尔尼诺每几年（通常每 3—8 年一次）经过赤道附近太平洋的气候模式。厄尔尼诺与更热的洋流有关，但不是
这种气候现象发生的全部原因。当厄尔尼诺造成更高的温度时，空气压力发生变化，当与更高的洋流温度结合时，
水以更快的速度蒸发。所有这些因素结合起来，导致正常的气候模式变化，并且反常的降雨量。这些暴风雨在赤
道附近的太平洋地区降临，给那个地区的包括美国、秘鲁和其它中、南美洲在内的国家造成大规模洪水泛滥。当
东太平洋地区受到过多雨水的连续袭击时，象澳大利亚、印度尼西亚和印度之类的地区正在应对严重干旱。这是
因为许多在全世界正常分布的湿气蒸发太快，并且滞留在东太平洋地区。
厄尔尼诺和拉尼娜——作用的相反
正象世界上其它事物一样，厄尔尼诺也有与其相反的事物。拉
尼娜是在 12 月至次年 3 月间袭击太平洋的气候现象。拉尼娜
象厄尔尼诺一样不可预测，但通常每过 3—8 年，紧随厄尔尼
诺出现。暖冬和洪水泛滥与厄尔尼诺相关联，而当拉尼娜来临
时，完全相反的现象真正出现；拉尼娜带来伴随干旱的令人难
受的冬天和反常的寒冷气候。
厄尔尼诺和拉尼娜——历史
在过去的的时间里，两种气候模式曾是造成成千上万人死亡的
原因。在 1789 年的厄尔尼诺期间，印度一个地区有 60 万人由
于严重干旱而死亡。由于现代科学的发展，现在我们能预测这
些气候现象发生的时间。这并不意味着我们能必然地减轻它们
所造成的物质损失，但预警可能有助于在未来挽救生命。预计
在 2007 年经过太平洋一路前进的厄尔尼诺将带来破记录的最
高气温。
厄尔尼诺和拉尼娜——有趣的事实
厄尔尼诺和拉尼娜西班牙语的意思都是小孩（一个是小男孩，一个是小女孩）
。因为这种气候模式通常在圣诞节
前后出现，厄尔尼诺通常指刚出生的耶苏。
厄尔尼诺也指“恩索”（厄尔尼诺—南方涛动）。南方涛动是描述在厄尔尼诺现象发生期间空气压力变化的方式的
术语。
最长的持久的厄尔尼诺现象持续了 4 年。这种现象通常持续 12—18 个月。
厄尔尼诺现象概括
[编辑本段]
厄尔尼诺现象又称厄尔尼诺海流，是太平洋赤道带大范围内海洋和大气相互作用后失去平衡而产生的一种气候现象，就是沃克环流
圈东移造成的。正常情况下，热带太平洋区域的季风洋流是从美洲走向亚洲，使太平洋表面保持温暖，给印尼周围带来热带降雨。
但这种模式每 2—7 年被打乱一次，使风向和洋流发生逆转，太平洋表层的热流就转而向东走向美洲，随之便带走了热带降雨，出
现所谓的“厄尔尼诺现象”。
“厄尔尼诺”一词来源于西班牙语，原意为“圣婴”。19 世纪初，在南美洲的厄瓜多尔、秘鲁等西班牙语系的国家，渔民们发现，每隔
几年，从 10 月至第二年的 3 月便会出现一股沿海岸南移的暖流，使表层海水温度明显升高。南美洲的太平洋东岸本来盛行的是秘
鲁寒流，随着寒流移动的鱼群使秘鲁渔场成为世界四大渔场之一，但这股暖流一出现，性喜冷水的鱼类就会大量死亡，使渔民们遭
受灭顶之灾。由于这种现象最严重时往往在圣诞节前后，于是遭受天灾而又无可奈何的渔民将其称为上帝之子－－圣婴。后来，在
科学上此词语用于表示在秘鲁和厄瓜多尔附近几千公里的东太平洋海面温度的异常增暖现象。当这种现象发生时，大范围的海水温
度可比常年高出 3－6 摄氏度。太平洋广大水域的水温升高，改变了传统的赤道洋流和东南信风，导致全球性的气候反常。
厄尔尼诺现象的基本特征是太平洋沿岸的海面水温异常升高，海水水位上涨，并形成一股暖流向南流动。它使原属冷水域的太平洋
东部水域变成暖水域，结果引起海啸和暴风骤雨，造成一些地区干旱，另一些地区又降雨过多的异常气候现象。
厄尔尼诺的全过程分为发生期、发展期、维持期和衰减期，历时一般一年左右，大气的变化滞后于海水温度的变化。
在气象科学高度发达的今天，人们已经了解：太平洋的中央部分是北半球夏季气候变化的主要动力源。通常情况下，太平洋沿南美
大陆西侧有一股北上的秘鲁寒流，其中一部分变成赤道海流向西移动，此时，沿赤道附近海域向西吹的季风使暖流向太平洋西侧积
聚，而下层冷海水则在东侧涌升，使得太平洋西段菲律宾以南、新几内亚以北的海水温度升高，这一段海域被称为“赤道暖池”，同
纬度东段海温则相对较低。对应这两个海域上空的大气也存在温差，东边的温度低、气压高，冷空气下沉后向西流动；西边的温度
高、气压低，热空气上升后转向东流，这样，在太平洋中部就形成了一个海平面冷空气向西流，高空热空气向东流的大气环流（沃
克环流），这个环流在海平面附近就形成了东南信风。但有些时候，这个气压差会低于多年平均值，有时又会增大，这种大气变动
现象被称为“南方涛动”。60 年代，气象学家发现厄尔尼诺和南方涛动密切相关，气压差减小时，便出现厄尔尼诺现象。厄尔尼诺发
生后，由于暖流的增温，太平洋由东向西流的季风大为减弱，使大气环流发生明显改变，极大影响了太平洋沿岸各国气候，本来湿
润的地区干旱，干旱的地区出现洪涝。而这种气压差增大时，海水温度会异常降低，这种现象被称为“拉尼娜现象”。
http://baike.baidu.com/view/1705.htm
It rose out of the tropical Pacific in late 1997, bearing more energy than a million Hiroshima bombs. By the time it had
run its course eight months later, the giant El Niño of 1997-98 had deranged weather patterns around the world, killed an
estimated 2,100 people, and caused at least 33 billion [U.S.] dollars in property damage.
Isaias Ipanaqué Silva knew none of that. All he and the other peasant farmers in the Peruvian hamlet of Chato Chico
could see was that after weeks of incessant rain the adjacent Piura River had not stopped rising. The rainfall itself was no
surprise. Every three to seven years, for as long as anyone could remember, the same rainfall had arrived after a pool of
hot seawater the size of Canada appeared off the west coast of the Americas. The ocean would heat up right around
Christmastime, so fishermen called the phenomenon El Niño, for the Christ Child. Then that titanic storm source would
pour vast amounts of precipitation onto Peru’s normally arid northwestern coast.
But few had ever seen this much rain—five or six inches a day in some places.
Finally, on February 15, 1998, the river broke its banks. The sodden ground could hold no more, and water swept into the
riverside homes of Chato Chico. The swirling torrent was first knee-deep and soon chest high. “Suddenly we were
surrounded from all directions,” Ipanaqué Silva says. “It took all the little animals. Then my house just fell down
completely.”
Hundreds of families splashed frantically through the muddy flood to save what they could. In most cases, says another
villager, Rosa Jovera Charo, “we just grabbed clothes for the children.” Everything else—chickens and goats, pots and
pans, religious icons and personal treasures—washed away. Compared with other places in Peru and around the world,
the residents of Chato Chico were fairly lucky. Some were evacuated on barges, a few in helicopters, to a barren but dry
refugee camp in the desert. Nearly all survived.
That was not the case some 60 miles [100 kilometers] to the south, in a 3-acre [1.2-hectare] pocket of one-room houses
called Motse outside the city of Chiclayo. “We thought that the water couldn’t come here,” says Flora Ramirez, “but we
lost practically everything.” Ramirez’s neighborhood was overrun in a matter of minutes. “They strung ropes from one
house to another to rescue people,” recalls Manuel Guevara Sanchez. “Some spent three days on the roof. Those who
knew how to swim brought them food.” When the flood finally receded, they could begin to count the dead: ten out of a
village of just 150.
The runoff from the floods poured into the coastal Sechura Desert. Where there had been nothing but arid hardscrabble
waste for 15 years, suddenly—amazingly—lay the second largest lake in Peru: 90 miles [145 kilometers] long, 20 miles
[30 kilometers] wide, and ten feet [three meters] deep, with occasional parched domes of sand and clay poking up eerily
from the surface.
In other areas the water simply pooled. The mosquitoes that thrived in these places caused rampant malaria—some
30,000 cases in the Piura region alone, three times the average for its 1.5 million residents.
Peru was where it all began, but El Niño’s abnormal effects on the main components of climate—sunshine, temperature,
atmospheric pressure, wind, humidity, precipitation, cloud formation, and ocean currents—changed weather patterns
across the equatorial Pacific and in turn around the globe. Indonesia and surrounding regions suffered months of drought.
Forest fires burned furiously in Sumatra, Borneo, and Malaysia, forcing drivers to use their headlights at noon. The haze
traveled thousands of miles to the west into the ordinarily sparkling air of the Maldive Islands, limiting visibility to half a
mile [0.8 kilometer] at times.
Temperatures reached 108°F [42°C] in Mongolia; Kenya’s rainfall was 40 inches [100 centimeters] above normal;
central Europe suffered record flooding that killed 55 in Poland and 60 in the Czech Republic; and Madagascar was
battered with monsoons and cyclones. In the U.S. mudslides and flash floods flattened communities from California to
Mississippi, storms pounded the Gulf Coast, and tornadoes ripped Florida.
By the time the debris settled and the collective misery was tallied, the devastation had in some respects exceeded even
that of the El Niño of 1982-83, which killed 2,000 worldwide and caused about 13 billion dollars in damage.
And that’s not the end of it. It is not uncommon for an El Niño winter to be followed by a La Niña one—where climate
patterns and worldwide effects are, for the most part, the opposite of those produced by El Niño. Where there was
flooding there is drought, where winter weather was abnormally mild, it turns abnormally harsh. La Niñas have followed
El Niños three times in the past 15 years—after the 1982-83 event and after those of 1986-87 and 1995. Signs of another
La Niña began to show up by June 1998.
Over the years, the appearance of La Niña has been less predictable than that of El Niño, and fewer of its effects have
been recorded. But both patterns are now far better understood than ever before. That is because the most recent El Niño
will be the first to be remembered for more than just a litany of disasters. The 1997-98 El Niño marked the first time in
human history that climate scientists were able to predict abnormal flooding and droughts months in advance, allowing
time for threatened populations to prepare. The U.S. National Oceanic and Atmospheric Administration (NOAA) first
announced a possible El Niño as early as April 1997; Australia and Japan followed a month later. By summer detailed
predictions were available for many regions.
In northern Peru warnings allowed many farmers and fishermen to make the best of El Niño’s effects. Grass grew on
land that is usually barren, and farmers raised cattle. Rice and beans could be planted in areas normally too dry to
support them; fishermen were able to plan for shrimp harvests in coastal waters, generally too cold for the shellfish.
“The potential uses of advance information are almost limitless,” says Michael H. Glantz of the National Center for
Atmospheric Research (NCAR) in Boulder, Colorado, pointing out how governments and industries around the world
can make planning for El Niño and La Niña pay off. For example, Kenyan coffee growers find their product in greater
demand when droughts affect coffee harvests in Brazil and Indonesia. Palm oil production in the Philippines typically
declines during El Niño, as does the squid catch off the California coast. Countries that anticipate these developments
can fill the gaps and prosper.
At the very least, preparation can save lives. Even in poverty-ridden Peru constructing storm drains and stockpiling
emergency supplies saved hundreds of lives during 1997 and ’98. Forewarnings brought timely international aid to such
places as Papua New Guinea, where highland populations were threatened with starvation after frost and drought
combined to destroy subsistence crops. Many affected areas could prepare for floods and fires, population migration, and
the spread of disease.
There are written records of El Niño’s effects in Peru at least as far back as 1525, and researchers have found geologic
evidence of El Niños in Peruvian coastal communities from at least 13,000 years ago. “We know the Inca knew about
them,” says Adm. Giampietri Rojas of Peru’s Institute of the Sea. “They built their cities on the tops of hills, and the
population kept stores of food in the mountains. If they built on the coast, it was not near rivers. That’s why so many of
their dwellings are standing today.” But it was not until about 25 years ago that the rest of the world began to pay
attention to El Niño. And after the surprise devastation of 1982-83, climate experts intensified efforts to understand how
the process works globally. Governments invested in equipment to monitor the particular conditions in the Pacific that
trigger El Niño. Perhaps the most important effort was the development of the TAO (tropical atmosphere/ocean) array of
70 moored buoys to span the equatorial Pacific. Completed in 1994, the TAO buoys are now the world’s premier
early-warning system for change in the tropical ocean. They monitor water temperature from the surface down to 1,600
feet [500 meters], as well as winds, air temperature, and relative humidity.
The data collected by the buoys are transmitted to polar-orbiting satellites and then to NOAA’s Pacific Marine
Environmental Laboratory in Seattle. Supplemented with temperature measurements taken by research ships, the data
help create a comprehensive portrait of the upper ocean and lower atmosphere.
Meanwhile the TOPEX/Poseidon satellite, a U.S.-French mission begun in 1992, orbits Earth at a height of 830 miles
[1,300 kilometers], measuring sea-surface elevation and relaying information about ocean circulation, including the
enormous rhythmic sloshings called Kelvin and Rossby waves that travel back and forth across the entire Pacific.
Thanks to the TAO buoys, the TOPEX/ Poseidon satellite, and a variety of other tools, climate scientists now have
information of unprecedented range and accuracy, which has enabled them to confirm and expand their theories about
what occurs both during normal weather patterns and during sea changes that herald the periodic—and
inevitable—arrivals of El Niño and La Niña.
Weather is so variable that it’s hard to call any situation normal. But in most years climate in the equatorial
Pacific is governed by one generally dependable pattern. Sunlight heats the uppermost layer of seawater in the
western ocean around Australia and Indonesia, causing huge volumes of hot, moist air to rise thousands of feet
and creating a low-pressure system at the ocean’s surface. As the air mass rises and cools, it sheds its water
content as rain, contributing to monsoons in the area.
Now much drier and far aloft, the air heads east, guided by winds in the upper atmosphere, cooling even more
and increasing in density as it travels. By the time it reaches the west coast of the Americas, it is cold and heavy
enough that it starts to sink, creating a high-pressure system near the water’s surface. The air currents then flow
as trade winds back toward Australia and Indonesia. This giant circulatory loop, moving from west to east in the
upper air and from east to west at low altitudes, is called the Walker circulation, for Sir Gilbert Walker, the
British scientist who studied the process in the 1920s.
As the trade winds blow westward over the Pacific, they push the warm top layer of the ocean with them, causing
the hottest water to pile up around Indonesia, where, because of both wind action and thermal expansion, the sea
level is usually about 18 inches [46 centimeters] higher than it is off the west coast of Mexico. All along the eastern
Pacific, and especially off Ecuador and Peru, colder subsurface water wells up to replace the sheared-off top layer,
bringing up a bevy of nutrients from the deep ocean. That chemical bounty sustains an enormous food web and
makes the coastal waters off Peru one of the world’s most prolific fisheries.
El Niño changes all that. For reasons that scientists still do not comprehend, every few years the trade winds
subside or even disappear. The usual air-pressure pattern reverses itself in a phenomenon called the southern
oscillation, making barometer readings higher in Australia than they are in the central Pacific. The resulting
pattern—known as ENSO, for El Niño/Southern Oscillation—involves only one-fifth of the circumference of the
planet. But it transforms weather around the globe.
Without the trade winds the top layer of the eastern Pacific does not move west. It stays in place, getting hotter
and hotter, swelling as it warms. Eventually it hits the threshold for what meteorologists call deep convection—the
point at which the steamy surface air blasts into the upper atmosphere. (In some places during 1997-98, sea levels
off South America were 10 inches [25 centimeters] above normal and surface temperatures reached almost 86°F
[30°C].) When that happens, water in the upper atmosphere condenses and falls as torrential rain on the west
coast of the Americas.
This, in turn, reduces the salinity of the coastal seas, where deepwater upwelling has already declined or stopped.
Marine life that customarily thrives off Ecuador and Peru, including economically essential anchovy populations,
heads south in search of cooler, richer waters—to the great benefit of fishermen in Chile. Off North America
exotic warmwater species suddenly appear farther north. In 1997, apparently for the first time, a fisherman
caught a marlin in the ordinarily chilly seas off Washington State. Californians started pulling in bonito and
albacore tuna, species normally found only far offshore. Other tuna were netted in the Gulf of Alaska.
Because El Niño moves the rains that would normally soak the western Pacific toward the Americas, such places as
Australia, Indonesia, and India may experience severe drought. According to historical records, 600,000 people died in
just one region of India from the epic droughts of the 1789-1793 El Niño.
In Africa the altered wind, heat, and moisture patterns of El Niño portend drought—generally in the east and extreme
south. In particular, cooling of the southwestern Indian Ocean customarily strengthens a high-pressure area that keeps
rainfall from reaching the south.
Meanwhile, back in North America, the jet streams that travel 5 to 8 miles [8 to 13 kilometers] above Earth’s surface
shift dramatically. The polar jet stream tends to stay farther north over Canada than usual; as a result, less cold air moves
into the upper United States. In fact, northern-tier states saved an estimated five billion dollars in heating costs during the
1997-98 El Niño.
At the same time upper-level tropical winds reverse themselves, blowing the tops off cyclones forming in the
mid-Atlantic and usually reducing the number of hurricanes that strike land in the U.S. by half—from an average of two
a year to one or none, according to studies at Colorado State University and Florida State University. One study indicates
that El Niño also generally reduces tornadoes in the southern Plains states.
An unexpected crop of sardines off the coast of Chile? Tuna in the Gulf of Alaska? Lower heating bills in the U.S.?
Fewer hurricanes in the Atlantic?
Enter La Niña. During a La Niña event, an abnormal cooling in the eastern Pacific produces conditions more or less the
opposite of those created by El Niño—nature’s way, perhaps, of rectifying the heat imbalance that El Niño represents. As
with El Niño, the effects of La Niña are most pronounced from December to March.
In La Niña years the easterly winds from the Americas are stronger than usual. That drives more than the normal amount
of warm sea-surface water westward, in turn causing larger than normal volumes of deep, chilly water to rise to the
surface and producing a “cold tongue” that extends 3,000 miles [4,800 kilometers] along the Equator from Ecuador to
Samoa.
With so much warm water flowing toward Asia, the Pacific’s mighty heat engine remains firmly anchored in the west,
causing heavier monsoon rains in India, higher than average precipitation in Australia, and wetter than normal conditions
as far west as southern Africa. The huge air masses and cloud banks associated with the hot zone also change the path of
the jet streams, which move high-altitude air from west to east across the ocean.
The polar jet stream, which in an El Niño year stays high in Canada, moves farther south, driving frigid air down into the
U.S. Winters are colder, especially in the northwestern and upper midwestern states. The subtropical jet stream that
blows across Mexico and the Gulf during El Niño events weakens during La Niña; consequently, far less rain falls in the
Gulf and southeastern states. Drought is common in the desert Southwest. Hurricanes in the tropical Atlantic encounter
no westerly wind resistance and therefore are twice as likely to strike the U.S. The 1998 La Niña hurricane season was
the deadliest in the past two centuries.
As experts use increasingly reliable data to comprehend the forces and patterns of these periodic weather cycles, they are
making better predictions of at least the broad contours of the cycle. There are two major ways of forecasting large-scale
weather events such as El Niño, and climate scientists use both.
One method is statistical. Analysts pore over past weather records to determine what kind of conditions have the highest
probability of occurring simultaneously. For example, lower barometric pressure and higher sea-surface temperatures in,
say, Tahiti usually mean more rain for Ecuador and less for northern Brazil. This technique yields results even if the
analyst has no idea how the two coexisting conditions are related, and traditionally forecasters have preferred its
reassuring mathematical solidity.
But statistical procedures provide very little information about what cause-and-effect relationships may be producing
various climate conditions. Moreover, statistical analysis can only determine the likelihood that past conditions will
recur—and no two El Niños or La Niñas are the same.
With the advent of supercomputers, scientists have taken advantage of an alternative method of prediction called climate
modeling. In this method, software incorporates the fundamental laws of oceanic and atmospheric physics into a
simulated world where weather changes over time. Researchers then feed tens of thousands of specific pieces of
information about the real world into the model and see how accurately the computer-generated results resemble what
actually happens.
In theory, models can reveal the unique or idiosyncratic conditions that will result from a given climate pattern and then
fast-forward to see how events related to that pattern will unfold. In practice, most of the results have proved too broad
or uncertain to predict weather on even a large regional scale, much less in a local range of 100 to 200 miles [150 to 300
kilometers].
So, historically, statistical predictions have been somewhat more accurate than computer-generated models—until now.
The 1997-98 El Niño “was one in which the full climate models were more successful than statistical predictions for the
first time,” according to NCAR climate analyst Kevin Trenberth. “The tropical Pacific,” says Trenberth, “appears to be
predictable for a year or so in advance.”
In fact, “to a certain extent we underplayed what the models were telling us,” says Ants Leetmaa, director of NOAA’s
Climate Prediction Center. He believes that if scientists had relied on the models more and the statistical evidence less,
their 1997-98 predictions would have been even more accurate.
As encouraging as the model results have been, there is still room for improvement. For example, most of the best
models created in advance of the 1997-98 El Niño predicted much smaller monsoons in India than actually occurred and
far less rain than actually fell in southeastern Africa and Australia. Kenya and Somalia had heavy and prolonged rains
that provoked an epidemic of waterborne Rift Valley fever and dengue fever, among other maladies. “The big question is
why,” says Leetmaa. “That’s the challenge for the future.”
It would be far easier to tune the climate models if scientists were able to look through centuries of records. But “we just
don’t have hundreds of years of data,” Leetmaa explains. And even if they were available, “data sets aren’t going to give
us the full answer. But analyzing the data in combination with computer-simulated experiments is where we’re going to
make progress.”
Greater distribution of monitoring equipment would also increase the accuracy of climate-pattern prediction. No
observation networks have been established yet for the equatorial Atlantic and Indian Oceans. Since part of the
variability among El Niños and their regional impacts can be attributed to activity in these ocean basins, the need for
improved data reporting in these areas seems clear. Especially, experts note, if El Niños are becoming more ferocious.
There is a consensus among climate scientists that El Niños have become more frequent and progressively warmer over
the past century. Beyond that there is little agreement, particularly about whether human activity might be exacerbating
their effects.
In the past 98 years there have been 23 El Niños and 15 La Niñas. Of the century’s ten most powerful El Niños,
four—the four strongest—have occurred since 1980. But no one knows whether this indicates a trend or is simply a
meaningless random clustering.
And no one can know at this point. Even a hundred years of precise rainfall and temperature observations in the Pacific
might not be sufficient to confirm a major tendency one way or the other. Moreover, many experts now suspect that El
Niños—and indeed many oceanic weather patterns—may alternate in form and severity on a timescale of decades or
even centuries. “By and large,” says NOAA’s Leetmaa, “the El Niño patterns look a lot like the overall changes in U.S.
rainfall and temperature patterns from decade to decade.” But no matter what’s happening, “the bottom line is the past 20
years are different from the previous 30.”
It is difficult to imagine how the global warming observed over the past hundred years, which amounts to about
one-tenth of a degree Fahrenheit [one-twentieth of a degree Celsius] a decade, could have much effect on the stupefying
volume of water in the equatorial Pacific. But it is plausible, some scientists believe.
”El Niño moves heat,” says Tom Karl, one of NOAA’s veteran climate experts, “both in terms of water temperature and
in atmospheric convection. This heat is transported out of the oceans and the tropics during the peak of El Niño as global
temperatures increase. As the heat is released, the whole El Niño cycle begins again, with less cloudiness in the tropics
and with the oceans absorbing more heat. With global warming there is more heat available. So the cycle may be
shortened because the recharge time is shorter or because the release of heat is less efficient.”
Whatever the future may bring, the world need never again be taken completely off guard by El Niño or La Niña. Due to
the unprecedented foresight that climate science has made possible, the ocean’s thermal moods may not seem so
unpredictable and diabolical, but rather an ordinary part of life on the planet. “We have to realize that it’s something
natural that’s going to happen again and again,” says Capt. Hector Soldi Soldi, a hydrographics expert with the Peruvian
Navy. “And we have to be ready for that.”
Even Isaias Ipanaqué Silva—now living in one of the spare refugee camps in northern Peru, where homes are no more
than four woven-straw walls with a plastic tarp for a roof—knows it. He and his neighbors walk three miles each way,
every day, to farm the riverside fields that lay right next to their homes before El Niño swept their hamlet away. “We
can’t go back,” he says, sad but resigned. “It will happen again. If God wants to save us next time too, we say thanks.
But right now, this is where we will stay.”
http://www.nationalgeographic.com/elnino/mainpage2.html
For many years, coastal residents of Peru had noticed a strange feature of the eastern Pacific Ocean waters that border
their home. This region of tropical yet relatively cool water is host to one of the world's most productive fisheries and a
large bird population. In the first months of each year, a warm southward current usually modified the cool waters. But
every few years, this warming started early (in December), was far stronger, and lasted as long as a year or two.
Torrential rains fell on the arid land; as one early observer put it, "the desert becomes a garden." Warm waters flowing
south brought water snakes, bananas, and coconuts from equatorial rain forests. However, the same current shut off the
deeper, cooler waters that are crucial to sustaining the region's marine life.
This is El Niño, "the Christ child," so named because of its frequent late December appearance. Once thought to affect
only a narrow strip of water off Peru, it is now recognized as a large-scale oceanic warming that affects most of the
tropical Pacific. The meteorological effects related to El Niño and its counterpart, La Niña (a cooling of the eastern
tropical Pacific), extend throughout the Pacific Rim to eastern Africa and beyond.
El Niño is normally accompanied by a change in atmospheric circulation called the Southern Oscillation. Together, the
ENSO (El Niño-Southern Oscillation) phenomenon is one of the main sources of interannual variability in weather and
climate around the world. Since recognizing some 25 years ago that the oceanic and atmospheric parts of ENSO are
strongly linked, scientists have moved steadily toward a deeper understanding of ENSO. Climate forecasters have taken
the first steps toward predicting the onset of El Niño and La Niña events months in advance. Still, much remains to be
learned about these children of the tropics.
Hurricanes
By definition, a hurricane is fierce rotating storm with an intense centre of low pressure that
only happens in the tropics. In south-east Asia they're known as typhoons and in the Indian Ocean,
tropical cyclones.
Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called "tropical
depressions" (热带低气压). Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically
called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called hurricanes or
typhoons or tropical cyclones.
1 mile per hour = 0.869 international nautical mile per hour (knot)
1 mile per hour = 1.609 kilometers per hour
1 mile per hour = 0.4470 meter per second
1 knot = 1.852 kilometers per hour
1 knot = 0.5144 meter per second
1 meter per second = 3.6 kilometers per hour
One of the most powerful of all weather systems, hurricanes are powered by the heat energy
released by the condensation of water vapour. However, the conditions have to be exact for a hurricane
to form, with the sea's surface temperature being above 26.5°C.
How hurricanes happen
Air above warm tropical water rises quickly as it is
heated by the sea. As the air rises it rotates or spins
creating an area of low pressure, known as the eye of the
storm. The eye can be clearly seen on satellite pictures,
and is usually eerily calm.
The hurricane only moves slowly at speeds of
20-25mph bringing torrential rain and thunderstorms
and very strong winds. However, they also cause
flooding on low lying coastlines with a phenomenon
known as a 'storm surge'.
Storm surge
Storm surge is the onshore rush of sea or lake water caused by the
high winds associated with a landfalling cyclone and secondarily by
the low pressure of the storm.
This is caused by the intense low pressure at the eye
of a hurricane, combining with the effect of strong
winds. The sea rises 1cm for every millibar of pressure if the pressure is 930 millibar, the sea surge will be
about 80 cm. Hurricanes can raise the seas surface by as much as 4m.
The hurricane winds push the surge along in front of its path. When this surge hits low-lying
coasts, the effects can be devastating. In addition to the sea surge, flooding can also result from
torrential rain falling from the storm clouds.
Once it reaches the mainland, a hurricane may cause widespread damage for a few days, but with
no warm water to supply heat, they quickly die out.
The "eye" is a roughly circular area of comparatively light winds and fair weather found at the center of a severe
tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is
little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and
warmest temperatures aloft - the eye temperature may be 10°C [18°F] warmer or more at an altitude of 12 km [8 mi]
than the surrounding environment, but only 0-2°C [0-3°F] warmer at the surface (Hawkins and Rubsam 1968) in the
tropical cyclone. Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are approximately 30-60
km [20-40 mi] in diameter (Weatherford and Gray 1988).
The eye is surrounded by the "eyewall", the roughly circular ring of deep convection which is the area of highest surface
winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as
a result of many moderate - occasionally strong - updrafts and downdrafts. The eye's warm temperatures are due to
compressional warming of of the subsiding air. Most soundings taken within the eye show a low-level layer which is
relatively moist, with an inversion above - suggesting that the sinking in the eye typically does not reach the ocean
surface, but instead only gets to around 1-3 km [ 1-2 mi] of the surface.
The exact mechanism by which the eye forms remains somewhat
controversial. One idea suggests that the eye forms as a result of
the downward directed pressure gradient (['greidiənt] 梯 度 )
associated with the weakening and radial (辐射状的) spreading of the
tangential ([tæn'dʒenʃ(ə)l]切线的) wind field with height (Smith,
1980). Another hypothesis suggests that the eye is formed when latent heat release in the eyewall occurs, forcing
subsidence (下沉) in the storm's center (Shapiro and Willoughby, 1982). It is possible that these hypotheses are not
inconsistent with one another. In either case, as the air subsides, it is compressed and warms relative to air at the same
level outside the eye and thereby becomes locally buoyant. This upward buoyancy approximately balances the
downward directed pressure gradient so that the actual subsidence is produced by a small residual force.
Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eyewall
convection (对流). Convection in tropical cyclones is organized into long, narrow rainbands which are oriented in the
same direction as the horizontal wind. Because these bands seem to spiral into the center of a tropical cyclone, they are
sometimes called "spiral bands". Along these bands, low-level convergence is a maximum, and therefore, upper-level
divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface,
ascends through these bands, diverges aloft, and descends on both sides of the bands. Subsidence is distributed over a
wide area on the outside of the rainband but is concentrated in the small inside area. As the air subsides, adiabatic
warming takes place, and the air dries. Because subsidence is concentrated on the inside of the band, the adiabatic
warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is
lighter than cold air. Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase
due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eyewall
form (Willoughby 1979, 1990a, 1995).
Thus the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the
eyewall and to a forced descent caused by the moist convection of the eyewall. This topic is certainly one that can use
more research to ascertain which mechanism is primary.
Some of the most intense tropical cyclones exhibit concentric ([kɔn'sentrik]同中心的) eyewalls, two or
more eyewall structures centered at the circulation center of the storm ( Willoughby et al.
1982,Willoughby 1990a ). Just as the inner eyewall forms, convection surrounding the eyewall can
become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the
subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The
pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the
intensification of the outer eyewall, and the cyclone itself weakens for a short period of time.
http://www.nhc.noaa.gov/