Science: Captured in space: the deadly course of El Nino

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Every picture tells a story - and this one is of the change last year in sea currents which caused droughts in Indonesia, western Africa and central America, heavy rains in east Africa and Canada, and, perhaps, the mild winter we're generally experiencing. El Nino is back - and this time it's really big. Charles Arthur, Science Editor, explains.

The purple band running across the Pacific Ocean in the Earth pictured in the foreground is not a cloud, or a wind pattern: it is a change in the height of the ocean, by about 18cm (7in), detected by a satellite known as Topex.

That sounds modest. Yet the effects of that comparatively minor change (which is independent of the tides) are enormous, for when it stretches across thousands of miles of the Pacific Ocean, then it is a sign that the climate phenomenon known as "El Nino" is in full swing - something that happens every three to seven years.

The foreground sphere shows sea surface levels on the Pacific Ocean in 1997-98; that in the background for 1994-95, when there was no El Nino. This year's "event" (as climatologists call it) is the strongest ever recorded, perhaps the biggest for 150 years, and that livid bruise across the largest ocean in the world is its signature.

The effects have been dramatic. Earlier this month people were throwing snowballs in Jerusalem. China, meanwhile, was in the grip of the heaviest drought for 30 years, while parts of Australia were recovering from devastating floods.

El Nino - literally the "boy child", as in Jesus, since its effects usually become apparent around Christmas time - meant that the usual pattern of westward trade winds over the Pacific died off. Several eastern Pacific hurricanes turned north and hit Mexico. Fishing in Peru and other South American countries was affected because the altered sea currents that bring warmer water drive fish away from those shores, towards colder, nutrient-rich areas.

But what is the picture actually showing? Though you might think the oceans are level, variations in the temperature of huge volumes (such as oceans) can make them expand or contract with time, making the height at "sea level" vary from place to place. From its orbit 1,336km (830 miles) above the Earth's surface, Topex measures sea level along the same path every 10 days, to an accuracy of a couple of inches. Overall, the difference between the highest and lowest parts of the ocean is just over 2m (6ft) - from about 120cm below the mean to 100cm above it. (A greater area is distorted upwards, hence the smaller variation in that direction.)

On the picture, the redder an area is, the higher the water level. The purple streak is like a contour line on a ground map - something that indicates the direction water will flow in. "It's like pressure systems in the atmosphere," says David Cotton, a satellite oceanographer at the Southampton Oceanography Centre. "The currents flow around the contour."

In a normal year (like the background picture), there's no such contour, and the westward trade winds push warm surface water towards Australia and New Guinea. That creates a cycle which then pushes colder water deep down and back towards western South America. Because it comes from the ocean floor, that cold flow is rich in nutrients, and fish thrive on it; and the Peruvian fishermen are happy.

But in other years, something happens and it all turns sour. The heavy rains normally tied to the movement of that westward-moving warm water move instead to the central Pacific Ocean.

The result: drought in Indonesia (where the rampant forest fires that polluted a chain of countries went out of control) and Australia (which has been dry and prone to bush fires since June) where rainfalls have more than halved; a weaker monsoon in the Indian subcontinent; and altered jet streams over North and South America.

In December thousands of seal pups started dying off California as their normal food - the fish fed by those colder waters - failed to appear. Near Los Angeles, scientists had to watch sea lions and northern fur seals starving - because they are forbidden by law from intervening in what (usually) appears to be a process of natural selection. But on the mainland, conservation groups made rescue efforts to feed the pups: they consider this severe El Nino to have been caused - at least partly - by man-made global warming.

The devastating weather that paralysed Canada earlier this month could also be the fault of that strip of water across the world. It has even been blamed for an increase in the number of reported cases of malaria in Kenya over the past three months, attributed to unusually heavy rains - brought on by the weather system.

Yet this is not some new occurrence. "El Nino has been happening for thousands of years," says Michael Davey, a climate scientist at the Hadley Centre in Bracknell. "We have some idea of how they occur: there are various natural cycles between the tropical oceans and the climate.

"There are different reasons for El Nino to arise. The tropical climate system is unstable: if it goes into El Nino, then it will go back to the previous state in time. El Nino events are inherently unstable. Yet the trade winds seem to accelerate the instability. And there is also the chaotic nature of the whole climate system."

Could this year's event be the fault of global warming? "It's too early to blame that," Davey says. "There's no good evidence for it. We have had big ones in the past when there was no clear evidence of man-made climate change."

In that case, might the global warming which scientists reckon we are causing lead to more El Nino events? "That's a serious question that people are looking to answer. But the models used to look at global warming scenarios haven't reached the sort of sophistication where you can answer that."

It's even possible that the comparatively mild winter that we're having in Britain is due (at least in part) to the rotten weather being visited on other parts of the world - though the World Meteorological Organisation, in a round-up of effects of this season's El Nino, comments: "Experts do not claim with confidence that impacts of El Nino reach the European continent." Davey adds, "It isn't responsible for everything."

Knowing that El Nino is inherently unstable offers some relief as well. The trade winds eventually take over (inspired by the Earth's rotation), starting the normal cycle up again. By the end of this year, that huge stripe will be gone. But the questions will remain. "If it did turn out that global warming did change El Nino for the worse, that would be very important," Davey says. "But we don't know yet."

You can learn more about the Topex satellite at the Web page at http://topex- www.jpl.