When Hurricanes Hardly Ever Happen

A huge mass of warm water has built up off the coast of Peru and Ecuador over the past two months, signalling another episode in the periodic - but unpredictable - series of disruptions to the normal circulation of the tropical Pacific Ocean.

It is less than three years since El Nino - the name given to the current of warm water - last appeared. It lasted with little interruption from 1991 to 1994 - an unprecedentedly long period.

In previous decades, El nino has typically lasted for a year or two and recurred every four to seven years. So far climatologists have had little success in predicting its ebb and flow.

 

El Nino has its greatest effect on countries bordering the Pacific, although many parts of the world are likely to feel its impact over the coming year.

The western coastline of the Americans will be exceptionally wet, with a risk of flooding from California to Chile.

On the other side of the Pacific, Australia is threatened by severe drought.

Archaeological evidence from Peru suggests that El Nino has been occurring since about 3,000BC.

The Spanish name, meaning the boy or more specifically the baby Jesus, was originally given by local fishermen who often noticed the first appearance of warm water around Christmas.

The ultimate cause of the phenomenon is a mystery. El Nino is part of a complex interaction between ocean and atmospheric circulation in the tropical Pacific, known as the Southern Oscillation.

In the ocean, the overall effect is something like water sloshing up and down a bath that is hot on top and cold underneath, says Mike Davey, an El Nino specialist at the UK Meteorological office.

One immediate cause of El Nino is a reversal of the prevailing trade winds that normally blow from east to west across the Pacific, just south of the Equator.

When El Nino is absent, the winds pile up warm surface water in the region of Indonesia and Australaisa, while cold water wells up from deeper levels off south America. As a result, the sea surface is normally about 8 degree C Cooler (and half a meter lower) in Ecuador than in Indonesia.

The cold water is laden with nutrients from the ocean depths and it supports rich fisheries.

But it gives the South American coast a generally dry climate, while rising air over the warmer water on the other side of the ocean drenches Indonesia in plentiful rainfall.

When El Nino occurs, the trade winds slacken or even reverse direction. As a result, the circulation no longer supports the upwelling of cold water off Peru and Ecuador - and the fish die of starvation of migrate south to Chile.

Ocean surface temperatures rise by several degrees, feeding moisture into clouds that soak the Peruvian coast.

Meanwhile, the changing wind directions reduce rainfall on the other side of the Pacific. There is a strong correlation between El Nino and severe drought in Australia and Indonesia.

At the opposite extreme of the Southern Oscillation is La Nina (the girl). Then the easterly trade winds intensify and even colder water wells up off the south American coast.

The meteorological effects of La Nina are opposite to those of El Nino: flooding in eastern Australia and drought in Peru. But this phenomenon is not so common; La Nina last appeared strongly in 1988-89.

El Nino affects the global atmospheric circulation and influences weather will away from the Pacific. Most but not all of the effects are negative.

One good thing is that El Nino reduces the number of destructive hurricanes in the Caribbean region, says Mark Saunders, a climatologist at University College, London.

This is because high-level westerly winds blow more strongly from the Pacific into the Atlantic, shearing the tops off storm clouds before they can develop into full-scale hurricanes.

According to the US National Oceanic and Atmospheric Administration, the worldwide economic cost of the 1982-83 El Nino - the most intense on record - was at least $8billion ( 5 billion).

Peru was hit hardest, with economic output cut by 12 per cent through destructive flooding and the collapse of fish stocks.

No one knows whether global warming - caused by human activities, particularly the burning of fossil fuels - will produce more frequent and more intense El Ninos, though some climatologists believe so on the basis of computer modelling.

However, there is definitely a converse effect, at least in the short term: El Nino temporarily increase the average global temperature by pumping more heat out of the tropical Pacific.

It would not be surprising if this effect, on top of a general warming trend, made 1997 and/or 1998 the warmest years for the world as a whole since records began in the last century.

Clive Cookson