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El Nino Explained

THURSDAY OCTOBER 01, 2009

La Nina is the normal situation

Consider the Pacific as being in a rectangle, with a left and a right side. On the left is Indonesia and Australia and NZ. On the right is South America.

Cold water comes up from the depths of Antarctica and travels up beside Peru. It brings nutrients up to the surface and the fishing is good. In the air above is warmer equatorial high pressure. It pushes down onto the waters, onto the cooler currents emerging from below. When you push down on water in the bath it slides out from under your hand, and in the bath having nowhere else to go, it causes a ripple wave which goes to the other end of the bath. It is the same with the Pacific. Higher pressure goes to lower pressure. With the land of S and N America blocking the right side, the current is forced (by the high pressure in the air) to go LEFT.

 

As the water goes left, so does the air. Why? Because the surface of the ocean is joined to the bottom of the air. Now the water is an easterly current going to the west, and so is the air, and this produces easterly winds. Easterly winds bring moisture from the sea and by the time they arrive on the Australian coast they are rain-laden.

 

Mostly, the rain that hits the eastern seabord of Australia is from easterlies. Winds go from high to low pressure too, and there is lower pressure over Australia than over S America. The combination of cooler easterlies and lower pressure brings rain. This is the NORMALsituation. But it doesn't last. The movement of water to the left eventually makes the sealevel higher on the left, just as a very slow-moving ripple wave would in a bath. When it gets to a critical height difference, about 62cms, it wants to go the other way. Slowly the water starts flowing to the RIGHT. Higher pressure now is on the LEFT, above Australia. And higher pressure flows towards lower pressure.

This is warmer water warmed by the sun near Indonesia where it has been locked in, unable to dissipate north because of the land mass of Asia and unable to disspate to the south because of Australia. The water is joined to the air and produces westerlies. The warmer water arrives at Peru and is called El Niño.

The very reason it was originally called El Niño is that just after Xmas 1982 a bunch of warm water arrived that took fishermen by surprise. The fishing dropped off and an algae bloom covered the fishing area. The event was called El Niño, which means Xmas baby, or bundle of trouble we could do without. But the El Niño is just a flattening out, a returning of the waters. The normal situation lasts about 3 years. When the normal situation reaches its maximum point it is called La Niña. El Niños last about 1-2 years.

The El Nino is only the stabiliser.

It is best to remember that all that is really going on is a tipping of the sea, like sloshing from side to side in a big dish from right to left, then left to right to get the sealevels back again. This is called the Southern Oscillation. It is very predictable and has been going for millions of years. The whole cycle is about 4.5 years. There is also a Northern Oscillation. Meteorologists see the right-to-left motion and declare a La Niña condition. Then, at the end of a year, if it brought rain and easterlys they declare a La Nina episode.  Scientists observe changes in sea temperatures at Darwin and Tahiti, which are the places the changed sea temperatures from either direction first show up. When the sea temperature goes up they know the switch has occurred and waters are now travelling to the right. Six months to a year after that it will probably build to an El Niño. When the sea temperatures go cooler they say "possible" La Nina, and so La Nina has come to mean cooler temperatures, rain to eastern Australia, easterlies and drought relief, sometimes flooding. For the western side of Australia a La Nina can be drying, because the eastern sea-air loses all its moisture by the time it gets across to the west. The El Niño has come to mean warmer temperatures and westerlies, wet in the west but dry in the east. It is the same in NZ, where the east side dries out as the westerlies drop rain on the seaward side of the Southern Alps and not in the east.  The last El Niño was 2006-7. The next is expected to start up by about November 2009 and become fully in control by about 2010. Until then we are (as of December 2008) in La Niña -neutral mode.

The reversal of currents is due to changing lunar declinations as the moon changes latitudes over a 9-year cycle. At 4.5 years a midpoint is reached, 23deg latitude, which is when the moon's declination matches the tilt of the earth and sea currents stall. Up the western side of S America the current shuts off. If in a bath your hand stops pushing the surface water down the ripple will come back the other way. With the pressure now off, Pacific sealevels begin to revert because the sea is no longer being driven along from east to west. As the stabilisation sets in and picks up momentum a new west to east current takes over, by which time the moon has moved from its midpoint. The current reversal is also aided by the solar cycle. El Niños kick in immediately after a solar minimum.  The next time you hear El Niño or La Niña you will know they don't mean as much as the Southern Oscillation Index (SOI).

That's all there is to it. It has nothing to do with global warming, climate change, what make of lightbulbs we should be using or whether or not enough of us catch buses. Nor does El Niño slow or speed up global warming, anymore than a wave on the ocean can control a current beneath. It is merely one of the giant cycles that rule the planet. Did ancient man notice it? Absolutely. A team lead by Don Rodbell, from New York's UnionCollege, digging in the Andes in 1999 found a continuous geological record and evidence that ancient civilisations 5000 and 8000 years ago planned for and used the El Niño rains to boost crop production. Scientists have also identified El Niño signatures hundreds of thousands of years old in coral growth rates, tree rings and polar ice cores. We need not fear El Niño any more than we should fear day or night.

 Ken Ring

 


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