El Nino and La Nina EXPLAINED!

by Ken Ring
[source: Predict Weather.com]

Monday 22nd September 2008

Sometimes these words are tossed around with gay abandon. "This is a typical La Niña pattern", some TV weather anchors will say about almost every bunch of rain clouds passing over. But real meteorologists only call off on El Niño or La Niña when a year is over, in hindsight.  That's not much use if you're a farmer trying to plan ahead. El Niño and La Niña do have a simple explana-tion. There are only two basic points to remember.

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.

Figura 1
Fig. 1

Cold water comes up from the depths of Antarctica and travels up beside Peru. It brings nutri-ents up to the surface and the fishing is good. In the air above is warmer equatorial high pres-sure. 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.

Figura 2
Figure 2

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.

Figura 3
Fig. 3

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 combi-nation of cooler easterlies and lower pressure brings rain. This is the NORMAL situation. 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.

Figura 4
Fig. 4

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.

Figura 4
Fig. 5

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 Niño 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. But whether or not a La Niña or El Niño is occurring is not known until afterwards. Meteorologists see the right-to-left motion and declare that this is a La Niña condition. Then, at the end of a year, if it brought a lot of rain and easterly winds they declare that it was a La Nina episode. Same with El Niño. Meteorologists observe the 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 tem-perature 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.

What causes El Niño? The moon. The reversal of currents is partly due to changing lunar decli-nations which is 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 usually 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). Meteorologists love to plot things on graphs and talk in points and numbers, e.g. "a strongly and consistently positive SOI pattern (e.g. consistently above about +6 over a two month period) is related to a high probability of above the long-term average (median) rainfall for many areas of Australia, especially areas of eastern Australia (including northern Tasmania) - La Niña. Conversely, a 'deep' and consistently negative SOI pattern (less than about minus 6 over a two month period, with little change over that period) is related to a high probability of below median rainfall for many areas of Australia at certain times of the year - El Niño". At the moment (as of December 2008) the SOI is in neutral.

Figura 6

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 Union College, 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. The challenge is not to fear El Niño any more than we should fear day or night. As Bob Dylan says, it is life and life only.

Beyond the SOI

What happens in other oceans?

As might be expected, there are oscillations everywhere and sealevel inbalances caused by currents butting up against land masses with nowhere to go except in the other direction, building sea-level inbalances within each system that eventually restore themselves periodically by tipping back.

In the same manner as the SOI, the Northern Oscillation Index (NOI ) in the "normal" situation, operates north of the equator but while it correlates to the SOI, varies as to the shape of land masses involved. In the case of the NOI, cold currents coming from the north pole scoot down alongside N America, reach the latitude of Mexico and become subject to high equatorial pressure, thence can only move west along the equatorial band. The NOI would be roughly a mirror image of the SOI, as if a mirror was placed upright on the equator and viewpoint was from the S Pole.

Figura 7

(from http://www.google.co.nz/imgres?imgurl=http://www.kidsgeo.com/images/ocean-current.jpg&imgrefurl=http://www.kidsgeo.com/geography-for-kids/0145-ocean-currents.php&h=295&w=400&sz=16&tbnid=GiauEgVqJ6yX8M::&tbnh=91&tbnw=124&prev=/images%3Fq%3Docean%2Bcurrents&hl=en&usg=__DK95FlHwpFccXr73zeVUBMaK8jk=&sa=X&oi=image_result&resnum=6&ct=image&cd=1)

El Ninos north of the equator are represented by weaker trade winds.

In the Indian Ocean colder currents come up the West Australian coast and form another anticlockwise system between West Australia and the Madagascar coast that is almost a repeat of the SOI between S, America and Australasia. South of the equator normal oceanic currents run anticlockwise. Another such system happens in the S Atlantic between S Africa and the east coast of S America but because of ocean size the two Pacific systems, SOI and NOI dwarf the others. In the northern hemisphere we see clockwise systems, one the NOI and the other in the N Atlantic , circulating warmer current/air from the Caribbean, up the east side of N America and across to Spain then down the west coast of N Africa, with a branch that pushes north taking warmer waters and W/SW winds to the UK and Ireland, known as the Gulf Stream. The winds influence the weather. To the UK SWs bring warmth, cloud, fog and drizzle. S and SES are warm, turning to rain when winds come from the west. An anticyclone over Scandanavia is a long fine spell for Britain.

Figura 8

(from http://www.google.co.nz/imgres?imgurl=http://www.kidsgeo.com/images/ocean-current.jpg&imgrefurl=http://www.kidsgeo.com/geography-for-kids/0145-ocean-currents.php&h=295&w=400&sz=16&tbnid=GiauEgVqJ6yX8M::&tbnh=91&tbnw=124&prev=/images%3Fq%3Docean%2Bcurrents&hl=en&usg=__DK95FlHwpFccXr73zeVUBMaK8jk=&sa=X&oi=image_result&resnum=6&ct=image&cd=1)

The oscillations are all linked to lunar declination and recycle accordingly. Otherwise we could ask what is driving these fluctuations, which are in effect giant tidal systems? It would indeed be weird if the giant moon-sun system(mainly moon by 2.5:1) which drives the giant ocean tides all over the globe, fails to have any involvement in current oscillation.

Ken Ring
New Zealand

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