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El Niño: Unpredictable Christmas Child 

"There's a good chance 2002 will be an El Niño-year". 
El Nino, a sudden shift of warm sea surface water, changes the normal weather patterns above the Pacific. And that turns over the weather around the globe.  The Ocean researchers are alert: "there's something going on; something
El Niño-like" .

El Niño is as old as the world. The Inca's in Peru already knew of this phenomenon, and built their cities upon the hills, far away from the unpredictable water with its sudden flooding. Geologists found 13000 year-old traces of El Niños in Peruvian coastal villages. Written reports of El Nino effects in Peru go back to 1500.

But this Spanish "Christmas Child", was named "El Nino" by Peruvian fishermen; at the end of the 19th century. They discovered that once in odd years around Christmas time, the anchovy and sardine's suddenly vanished without trace.
The fish had moved; thousands miles to the south, to the Chilean coast, where the sea water was colder  -- and  more abundant in food that the warmer waters..
At the Peruvian and Ecuadorian coast, usually an Eldorado for fishermen, an enormous pool of warm water suddenly appeared, about 25 º C some 5 degrees warmer than normal.
The nets were empty, millions of seabirds and other animals  starved and the guano-gatherers; earning a living by collecting nitrate-rich dirt of cormorants, seagulls and penguins; searched in vain for this lucrative stuff.
El Niño had taken control for a while.

El Niño and the Ocean's seesaw.

Warm and cold ocean water don't mix easily: in fact that's the reason for the existence of  El Niño's. The dividing line between warm water layers (about 25 º C) and cold (15 º C and lower) is a sharp one; called a thermocline. In the Western Pacific; near Australia and Asia, this line lies far deeper that in the Eastern Pacific; near the Americas. The trade winds blows the warmer surface water to the west; it piles up there.  On the other hand, at the Eastern Pacific as the warm water is blown away; colder water will replaces it. The thermocline through the Pacific does not lie  horizontal. It's deeper in the west and shallower in the east. It behaves like a seesaw: with the movements of ocean water; caused by currents and wind, it swings to and fro. That swing is very slow, for the ocean is incredible tardy. Its a process that takes months'.


The Ocean and the atmosphere 

So the atmosphere and the ocean interact and influence each other. The trade winds also cause a change in temperature difference between the west and the east of the Pacific. Air rises more above warm water than above cold water. If the air rises and becomes colder as it enters higher altitudes, the water vapour in the air condenses and it rains: that is why in the Pacific it rains much more over the warm west near Asia than over the cold east near Peru and Ecuador. Also, the rising air in the west sucks in more air, and this is partly responsible for the strength of the trade winds.

If, for one reason of another, the trade winds slacken, it will cause the east to be less cold and the temperature difference between east and west will diminish. And if the difference in temperature lessens, the trade winds will lose strength. In this way there is a kind of circle of causes and effects that can reinforce each other: weaker trade winds give a smaller temperature difference, this gives weaker trade winds and so on.

Weaker trade winds means that less warm water is blown to the western Pacific; the thermocline will rise in the West. The warm water moves inchmeal to the east; the coast of America gets slowly but surely a thicker layer of warm water. This warm layer pushes the cold water to the deep; the thermocline sinks. 

And what's more: the thermocline dips often to the other side; - sinking in the east and rising in the west: its a self consolidating process. The thicker warm water layer in the east causes higher temperatures of the air above. This air has the tendency to rise -- and sucks in more air to the Eastern Pacific -- the opposite direction of the normal trade winds. Sometimes this is so strong that the trade winds will ease totally or reverse. -- and as that trade winds were responsible for moving the warm water to the western pacific -- the thermocline in the west will rise more and more

El Niño is born: thanks to the seesaw of the Ocean. 

The end of El Niño

The working of this seesaw also causes  El Niño's end -- or even the opposite situation La Nina. 

The deviations in the trade winds - caused by the higher-than-normal surface temperatures - are most strong in the middle of the Pacific Ocean, around the date line (longitude 180º ). The thermocline will also be deeper at that place as it is in the Eastern Pacific.
And this brings an El Niño to its end. All this warm water in the east, caused by the winds around the date line, must come from somewhere, and that is from regions a little north and south of the equator. 
The thermocline will rise in those off-equatorial regions. Its like a waterbed: when you push the water down at one place it'll rise somewhere. The shallow parts in the thermocline can move back to the equator. Probably, reflections of the shallow parts at the western coast are important for this process. On the contrary, the deep part in the east has a tendency to move poleward, leaving the region where it can influence El Niño development. 
Next, the shallow regions in the west travel eastward along the equator, and overcome the wind anomalies, and eventually the thermocline in the east will rise again.

La Nina

While the ocean tries to find it normal balance; the thermocline in the east can rise above its normal level and a state develops which is known as La Niña, with colder than normal surface waters in the east, and stronger than normal trade winds. This mechanism, which consists of a quasi-instantaneous positive feedback, followed by a delayed negative feedback, is known as the delayed oscillator mechanism.

El Niño and the weather around the globe 

Everybody will agree: El Niño is a quite innocent name for such a phenomenon bringing trouble and disaster for huge parts of the world. Floods and heavy rain in South America's west coast to extreme drought and famine in Australia, India, Asia and Africa. Even the European weather is likely to be influenced by  El Niño.
But meteorologists think that El Niño's reputation is very blackened by the media -- and therefore by the public --. The phenomenon is almost a synonym for tornado's, hurricanes, downpours, floods, extreme droughts, abortive harvests and forest-fires. After almost every natural disaster the media  blame it on El Niño  -- even when it hasn't been an El Niño  year at all -- and when it isn't  El Niño; its the greenhouse-effect or global warming. Nonsense of course, El Niño is a natural phenomenon, it belongs to the earth and atmosphere. It isn't good or evil".
However during an El Niño Indonesia, the Philippines, Middle and South America's west coast are in severe danger. Due to the last El Niño Indonesia suffered a terrible famine and enormous forest-fires, which set up the whole area in a thick layer of smoke for months. Many floods and landslides in Middle America brought disaster and drew the attention of the whole world.

Positive effects.

But El Niño  also has positive effects; especially in North America. There are less hurricanes on the Atlantic and tornado's in the US. The North East of America is warmer than normal -- so less to burn -- for the polar jet stream can't get very far south during an El Niño.
California has more rain than normal, but it falls more equal - like real British drizzle - so there's not so much damage to the road. The Californian fishermen catch suddenly the most exotic tropical fish, in marvellous colours -- normally these fish do not come  so far north.

El Niño and the research

As El Niño brings so much trouble and disaster throughout the world; it'll be a good thing that an El Niño can be forecasted and eventually measures can be taken. And for oceanographers and meteorologists El Niño is a really fascinating phenomenon.
'It influences the worlds' weather in a striking organized order. But we do not understand the whole phenomenon properly. The ocean is slow, very dim. but it sure moves. The so-called Kelvin- and Rossby-waves - which somehow play a role in the El Niño development - move about 2 meters a second. You easily could walk with such a wave. The water moves, although very tardy - but it does - you can't stop an El Niño". 

Concerning speed the ocean may move at human pace; the amounts of water which is travelling across the Pacific; set going by El Niño,  goes beyond every imagination. "Within five to ten years we'll know more. Hopefully at that time we'll have climate-models that can predict better and more exactly.

The Oceans' buoys

All eyes are upon 70 TAO-buoys laid down in the Pacific Ocean. TAO stand for Tropical Atmosphere Ocean. They are posted since the eighties along the equator as the worlds first warning system for disruptions in the tropical ocean. They make daily measurements of water temperature - to 500 meter depth  - the pressure and humidity. These data are sent via a satellite-system to the Pacific Marine Environmental Laboratory in Seattle; and to other research-institutes throughout the US, Europe and Japan. "We have a good three-dimensional view of the ocean. The system is expensive and complicated a costs over a hundred million dollars a year for maintenance. But its worth it: we know much more about the El Niño-movements and every step brings us closer" .

The  El Niño indices

How strong is an  El Niño? You can measure it in two ways: The  Niño-3 index and the Southern Oscillation Index.
The one most used for the El Niño force is the Niño-3 index. It measures the water temperature in the Eastern Pacific ( the area between  - 5
º and + 5 º latitude and - 150º and 90º longitude). The Niño-3 index swings between - 2 º C and + 4º C; mostly between + 1º C  and - 1º C. During a heavy El Niño the sea water temperature can have a variation of 5 - 8 º C.

Except the warmer-than-normal sea water, there also is a lower-than-normal difference in air pressure between Tahiti and Darwin (Australia) during an El
This difference in pressure is called the Southern Oscillation Index, swinging
between - 3 and + 3. The Southern Oscillation (southern swing; term from Sir Gilbert Walker, an English meteorologist during the 1920's) is the change of pressure above the Pacific and the variation in the trade winds that's connected with it.
El Niño and the Southern Oscillation (ENSO) are the main authors of the climate changes across the globe.

Want to know more?

There are various El Niño-sites on the web; here are a few to help you on the way

Illustrations: NOAA/ENSO/KNMI web-sites
Quotations from  interviews with G.J. van Oldenborgh and G. Burgers; oceanography KNMI.

By Wijke Ruiter