Science: The ocean warmed and then came the rain: The Mississippi floods defied predictions about global weather patterns, writes Bill Burroughs

ANYONE who seeks an explanation for the floods that have caused widespread damage in the Mississippi Valley in the United States during the past few weeks need look no further than the southern Pacific Ocean.

Over the past decade or so, a consensus has grown up among meteorologists that there are predictable cyclical variations in the temperature of the surface waters of the equatorial Pacific, which are linked with extremes of weather around the globe. The only problem is that no one predicted the floods in the Mississippi valley.

There was a significant warming of large areas of the Pacific in 1991 and 1992, and the consensus was that this would be followed by a cooling in 1993. One consequence of this was a forecast that the central US would experience a severe drought this summer. Instead, rain and flooding have caused billions of dollars of damage. This could be explained by a renewed surge of warming in the tropical Pacific.

A record-breaking warming of the Pacific in 1982 and 1983 triggered meteorologists' interest. For centuries, fishermen in Peru had been aware that, in certain years, warm water spread southwards along the coast, shutting off cold, up-swelling, nutrient-rich water and dramatically reducing fish stocks. Because this occurred around Christmas, they called the event El Nino (The Child) to associate it with the Nativity.

These periodic changes are part of a great shift in the weather patterns in the Tropics. The warm water first appears off the coast of Peru and during the next year spreads across the Pacific. At the same time, atmospheric pressure over the eastern Pacific falls and rises over Australia and the Indian Ocean. With that, the easterly trade winds along the Equator reverse.

Sir Gilbert Walker, an eminent British meteorologist, noted in the Twenties: 'When pressure is high in the Pacific Ocean, it tends to be low in the Indian Ocean from Africa to Australia, and vice versa.' He termed this behaviour the Southern Oscillation. For this reason, meteorologists seek to combine these far-flung effects and refer to El Nino Southern Oscillation (ENSO).

It is relatively easy to see the connection between shifts in the weather in equatorial regions and El Nino. The area of heavy rainfall that is normally found over Indonesia moves eastwards to the central Pacific. The heaviest rainfall over Amazonia moves west of the Andes, bringing torrential rains to the arid coast of Peru and Ecuador. The region of ascending air over Africa is replaced by a descending motion connected with the frequent droughts in the Sahel and southern Africa of the past two decades.

At higher latitudes the connections are more complicated. There seems to be clear evidence that El Nino delays and reduces the strength of the Indian monsoon and causes widespread drought across Australia. Over North America, El Nino years are associated with cold winters in the east.

More dramatically, in El Nino years the global average temperature is markedly higher. The scale of all these effects is such that considerable efforts have been made to forecast El Nino accurately. This has involved developing computer models of how the atmosphere and ocean interact. These have to address the basic conundrum about how the system can switch from one extreme to the other. During an El Nino, the more extensive warm water strengthens westerly winds along the Equator, which in turn drive more warm water towards South America. Conversely, during non-El Nino conditions, the cold water that extends westwards across the Pacific reinforces the easterly winds, which drive more cold water away from South America.

The key appears to lie in the surface layer of the ocean. In the tropical Pacific this consists of a layer of warm water some 100m (330ft) deep, on top of cold deep water. Disturbances caused by changes in the wind fields along the Equator create waves that travel in this surface layer east and west across the Pacific basin. Those travelling eastwards are relatively fast-moving and can cross the ocean in two to three months. Those moving westwards take between nine months and several years to make the journey.

Computer models suggest that the slow waves in the Pacific may interact over the years so that the sea's surface temperatures oscillate back and forth in a quasi-cyclical manner over a period of three to five years. Although this process is not strictly predictable, it did seem to justify extrapolations forward for a few years.

When they successfully predicted the warming in 1991 and 1992, modellers' euphoria rose. Some questioned early records of El Nino lasting two to three years (as in 1939 to 1941), arguing that the event was inconsistent with their models.

None of the models predicted the warming in 1993. So the modellers will have to go back to their drawing boards, while those who seek evidence of predictable cycles in the weather will see yet more confirmation for the one certain rule: the moment a cycle is identified with sufficient confidence to be used to make a forecast, it disappears.

(Photograph omitted)

Start your day with The Independent, sign up for daily news emails
News
peopleIt seems you can't silence Katie Hopkins, even on Christmas Day...
Arts and Entertainment
Left to right: Stanley Tucci, Sophie Grabol and Christopher Eccleston in ‘Fortitude’
tvSo Sky Atlantic arrived in Iceland to film their new and supposedly snow-bound series 'Fortitude'...
Life and Style
fashion
Arts and Entertainment
tv
PROMOTED VIDEO
ebooks
ebooksA year of political gossip, levity and intrigue from the sharpest pen in Westminster
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Account Manager

£20000 - £35000 per annum: Recruitment Genius: This full service social media ...

Recruitment Genius: Data Analyst - Online Marketing

£24000 - £35000 per annum: Recruitment Genius: We are 'Changemakers in retail'...

Austen Lloyd: Senior Residential Conveyancer

Very Competitive: Austen Lloyd: Senior Conveyancer - South West We are see...

Austen Lloyd: Residential / Commercial Property Solicitor

Excellent Salary: Austen Lloyd: DORSET MARKET TOWN - SENIOR PROPERTY SOLICITOR...

Day In a Page

Isis in Iraq: Yazidi girls killing themselves to escape rape and imprisonment by militants

'Jilan killed herself in the bathroom. She cut her wrists and hanged herself'

Yazidi girls killing themselves to escape rape and imprisonment
Ed Balls interview: 'If I think about the deficit when I'm playing the piano, it all goes wrong'

Ed Balls interview

'If I think about the deficit when I'm playing the piano, it all goes wrong'
He's behind you, dude!

US stars in UK panto

From David Hasselhoff to Jerry Hall
Grace Dent's Christmas Quiz: What are you – a festive curmudgeon or top of the tree?

Grace Dent's Christmas Quiz

What are you – a festive curmudgeon or top of the tree?
Nasa planning to build cloud cities in airships above Venus

Nasa planning to build cloud cities in airships above Venus

Planet’s surface is inhospitable to humans but 30 miles above it is almost perfect
Surrounded by high-rise flats is a little house filled with Lebanon’s history - clocks, rifles, frogmen’s uniforms and colonial helmets

Clocks, rifles, swords, frogmen’s uniforms

Surrounded by high-rise flats is a little house filled with Lebanon’s history
Return to Gaza: Four months on, the wounds left by Israel's bombardment have not yet healed

Four months after the bombardment, Gaza’s wounds are yet to heal

Kim Sengupta is reunited with a man whose plight mirrors the suffering of the Palestinian people
Gastric surgery: Is it really the answer to the UK's obesity epidemic?

Is gastric surgery really the answer to the UK's obesity epidemic?

Critics argue that it’s crazy to operate on healthy people just to stop them eating
Homeless Veterans appeal: Christmas charity auction Part 2 - now LIVE

Homeless Veterans appeal: Christmas charity auction

Bid on original art, or trips of a lifetime to Africa or the 'Corrie' set, and help Homeless Veterans
Pantomime rings the changes to welcome autistic theatre-goers

Autism-friendly theatre

Pantomime leads the pack in quest to welcome all
The week Hollywood got scared and had to grow up a bit

The week Hollywood got scared and had to grow up a bit

Sony suffered a chorus of disapproval after it withdrew 'The Interview', but it's not too late for it to take a stand, says Joan Smith
From Widow Twankey to Mother Goose, how do the men who play panto dames get themselves ready for the performance of a lifetime?

Panto dames: before and after

From Widow Twankey to Mother Goose, how do the men who play panto dames get themselves ready for the performance of a lifetime?
Thirties murder mystery novel is surprise runaway Christmas hit

Thirties murder mystery novel is surprise runaway Christmas hit

Booksellers say readers are turning away from dark modern thrillers and back to the golden age of crime writing
Anne-Marie Huby: 'Charities deserve the best,' says founder of JustGiving

Anne-Marie Huby: 'Charities deserve the best'

Ten million of us have used the JustGiving website to donate to good causes. Its co-founder says that being dynamic is as important as being kind
The botanist who hunts for giant trees at Kew Gardens

The man who hunts giants

A Kew Gardens botanist has found 25 new large tree species - and he's sure there are more out there