El Ni帽o, sometimes called the Earth鈥檚 heartbeat, is a dramatic but mysterious climate system that periodically rages across the Pacific. Its intensity is such that it affects temperatures, storm tracks and rainfall around the world. Droughts in Africa and Australasia, tropical storms in the Pacific, torrential rains along the Californian coast and lush greening of Peruvian deserts have all been ascribed to the whim of El Ni帽o. Until recently it has been returning about every three to five years. But recently it has become more frequent 鈥� for the first time on record it has returned for a fourth consecutive year 鈥� and at the same time a giant pool of unusually warm water has settled down in the middle of the Pacific and is showing no signs of moving. Climatologists don鈥檛 yet know why, though some are saying these aberrations may signal a worldwide change in climate.
The problem is that nobody really seems sure what causes the El Ni帽os to start up, and what makes some stronger than others. And this makes it particularly hard to explain why it has suddenly started behaving so differently.
El Ni帽o means 鈥渢he little boy鈥� or 鈥渢he Christ child鈥� in Spanish, and is so called because its warm current is felt along coastal Peru and Ecuador around Christmas. But the local warming is just part of an intricate set of changes in the ocean and atmosphere across the tropical Pacific, which covers a third of the Earth鈥檚 circumference.
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In the absence of El Ni帽o and its cold counterpart, La Ni帽a, conditions in the tropical eastern Pacific are the opposite of those in the west: the east is cool and dry, while the west is hot and wet (Figure 1).
In the east, it鈥檚 the winds and currents that keep things cool. It works like this. Strong, steady winds, called trade winds, blowing west across the Pacific drag the surface water along with them. The varying influence of the Earth鈥檚 rotation at different latitudes, known as the Coriolis effect, causes these surface winds and water to veer towards the poles, north in the northern hemisphere and south in the southern hemisphere. The surface water is replaced by colder water from deeper in the ocean in a process known as upwelling. A wind that blows from the southeast along the coasts of Peru and Ecuador also causes upwelling along these shores, and the upshot is a cold tongue of surface water stretching from the coast of South America west along the equator towards the dateline.
The cold surface water in turn chills the air above it. This cold dense air cannot rise high enough for water vapour to condense into clouds. The dense air creates an area of high pressure so that the atmosphere over the equatorial eastern Pacific is essentially devoid of rainfall.
In normal years, the situation in the far western Pacific is quite the opposite. The waters around Indonesia are warm, warming the air above. This warm, moist air rises, creating an extensive system of low pressure and fierce tropical rainstorms. The system migrates from Indonesia in July to the waters around northern Australia in January, causing the monsoons.
As the low pressure system migrates, air pressure across the Pacific seesaws. When the pressure falls in the west, it usually rises in the east and vice versa. The fluctuation is known as the Southern Oscillation.
So much for what happens under normal circumstances. El Ni帽os happen when the tropical low pressure system in the western Pacific overshoots and heads east from Australia towards the central Pacific and the dateline (Figure 2). No one knows why this happens, but when it does it causes havoc 鈥� a patch of ocean the size of the US becomes the centre of a surging tropical storm system that can change atmospheric circulation around the globe.
What鈥檚 more, it causes changes in the trade winds that in turn allow the warm water to move even further eastward. As the massive low pressure system moves into the central Pacific the trade winds drop because the high-pressure/low-pressure driving force across the Pacific is reduced. The cold tongue thus retreats back towards the east, and this allows the warm pool to surge even further east. In fact it becomes a feedback loop: as the eastern Pacific warms the trades die down, and as they die down the water warms. This is the time of El Ni帽o when eastern sea surface temperatures rise from 1 to 5 掳C. In the east schools of fish dive for cooler depths and unaccustomed rain soaks the deserts of Peru. In the west it sucks moisture out of Asia, ushering in drought from the rainforests of Borneo to the wheat fields of Australia, and even through the Indian Ocean to East Africa.
Baffling developments
But there is a natural, albeit delayed shutdown mechanism which prevents the warming going on forever. The key is a mismatch between the responses of the atmosphere and the ocean. Although the air adjusts rapidly to changes in sea surface temperatures, the ocean adjusts more slowly to changes in the air, because the ocean responds to past as well as present winds. The memory of past winds is stored in deep waves along the thermocline 鈥� an abrupt interface between warm surface water and the cold water below. One such wave, which forms as El Ni帽o begins, exacerbates the warming. As the surface water warms it expands, forcing the thermocline deeper into the ocean. The deepening thermocline spreads east in a wave until it reaches the South American coast, where it drives the cold water even deeper. This depresses upwelling, which in turn dampens the trades and further warms the ocean.
But another wave that forms at the same time ultimately acts to suppress the warming. Changes in the winds over the central Pacific give rise to a similar 鈥減lanetary scale鈥� wave along the thermocline which travels towards Asia. When it reaches the ocean鈥檚 western boundary it is reflected back into the middle of the Pacific, causing the thermocline to rise from the depths as it progresses. On the wave鈥檚 return to the dateline, over half a year later, it raises the thermocline, counteracting the first wave and cooling the ocean, according to Max Suarez, an oceanographer at NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland. El Ni帽o ends and La Ni帽a begins. The deep waves explain the oscillation between El Ni帽o and La Ni帽a, one following the next in a slightly erratic tango through time.
This at least is the theory, and it has worked pretty well over the past century, with El Ni帽o occurring about every three to five years and La Ni帽a in between. But there have been some baffling developments in recent years. For one thing, El Ni帽o has returned three times in the past four years. For another, since 1976 El Ni帽o has dominated relative to the cooler phase (La Ni帽a). There has been only one significant La Ni帽a, but five El Ni帽os, including an extremely severe one in 1982-83 that caused damage costing $8 billion dollars (see 鈥淓l Ni帽o鈥檚 global signature鈥�). Moreover, a huge pool of warm water has settled down near the dateline in the central Pacific.
Many scientists no longer view these phenomena as aberrations. Rather, they feel it represents a distinct trend. 鈥淭his is a new regime. What is happening is unprecedented in the last 100 years,鈥� says Ants Leetmaa, a climatologist at the National Meteorological Centre in Camp Springs, Maryland. Tim Barnett at the Scripps Institution of Oceanography in La Jolla, California, agrees. 鈥淚t looks like we鈥檝e shifted to another climate regime.鈥�
But to assess the kind of change that is occurring scientists must first have a baseline of what 鈥渘ormal鈥� is. And in the case of El Ni帽o, that baseline is extremely short: a 112-year record of atmospheric pressure variation over the Pacific, and detailed observations of sea surface temperatures for little more than a decade. 鈥淲hether a similar period has occurred in the last millenium or not, we don鈥檛 know,鈥� says Mark Cane, a climate modeller at Lamont-Doherty Earth Observatory in New York.
鈥淓l Ni帽o鈥檚 behaviour in the last few years has caused us great problems,鈥� says Neville Nicholls, a meteorologist in Australia鈥檚 Meteorology Research Centre. 鈥淚t indicates that we鈥檙e a long way off from complete understanding.鈥�
Feast or famine
Yet it is important to understand the changes if scientists are to be able to forecast the climate effects of El Ni帽os with any degree of accuracy. This is not just an academic task 鈥� accurate forecasts can spell feast or famine in many tropical countries around the world. Forecasting efforts have focused on El Ni帽o whose effects are generally more severe than those of La Ni帽a.
A worrying possibility is that the changes may be due to greenhouse warming. If so, the recent fluctuations may be an early glimpse of worse things to come. Most climatologists expect the planet to warm as greenhouse gases such as CO2 accumulate in the atmosphere. These gates trap heat that would otherwise radiate into space. Since the late 1800s, fossil fuel burning has caused a 25 per cent increase in the concentration of CO2 in the atmosphere. Over the same period global temperatures have risen by 0.6 掳C. This includes a steep jump over the past 20 years of about 0.4 掳C, according to Nicholas Graham, an oceanographer at Scripps. Simultaneously, the central Pacific sea surface temperature increased by half a degree.
快猫短视频s admit that it is difficult to prove whether global warming is at work in the tropical Pacific. 鈥淚t鈥檚 a real chicken and egg problem,鈥� says Nicholls. 鈥淚f you have consistent El Ni帽os, then you end up producing a warmer world. So are more El Ni帽os causing global warming, or is global warming causing more El Ni帽os? I鈥檇 hesitate to even guess.鈥�
Not everyone is so cautious. 鈥淢y personal feeling is the planet has changed 鈥� I think we鈥檙e seeing the impact of global warming,鈥� say Leetmaa. 鈥淏ut it鈥檚 not a big signal out there. A half a degree Celsius is barely detectable.鈥�
Graham also has a hunch that global warming is at work. He addressed the question indirectly, using a computer model to examine how the transfer of water between the atmsphere and the ocean 鈥� the hydrological cycle 鈥� has changed in the tropics over the past 20 years. He realised that the hydrological cycle seemed to have intensified over this time period, and reasoned that this might be responsible for the warming. The idea is that evaporation of water requires heat energy, so higher sea surface temperatures (SSTs) encourage more evaporation. Later, when water vapour condenses and rain falls from storm clouds, latent heat is released, warming the air.
Graham primed his computer model with the SSTs from 1970 to 1988, and his simulated atmosphere evolved in much the same way as the real atmosphere had 鈥� the temperature rose, and evaporation and rainfall over the tropical ocean increased, closely matching actual records (Science, in press).
Based on these findings, Graham concludes that the increases in SST could well have caused the intensification of the hydrological cycle, explaining the warming. Admittedly this does not explain what caused the sea temperatures to rise in the first place, but Graham also points out that various other computer models, which incorporate the effects of increased atmospheric CO2, have predicted rises in SST similar to those seen in the Pacific in recent years.
In spite of this, some scientists suspect that the Pacific changes are part of a natural fluctuation that occurs over decades, one which has perhaps not been observed or recorded before. Here, too, the problem may lie with defining what is normal. So-called 鈥渘ormal鈥� SSTs are based on the average temperature from 1951 to 1980. But if climate fluctuates over decades, that 30-year average may be too narrow a window. 鈥淢ay be we need 90 years 鈥� We could be in one regime for 10 to 15 years, then, for all we know, we could be in a different, warm regime for the next 10 or so years,鈥� says David Halpern, an oceanographer at the California Institute of Technology. There are other reasons to try to pin down the longer-term behaviour of El Ni帽o. 鈥淭he interdecadal timescale is the most important to mankind,鈥� says Tim Barnett. 鈥淐ivilizations are set to handle one bad year, but they鈥檙e not equipped to handle 5 to 10 years of straight drought.鈥�
However, no one has yet found clear patterns in El Ni帽o on a timescale of decades, although scientists are uncovering clues that such patterns exist. Robert Allen, an atmospheric scientist at the CSIRO, Australia鈥檚 national research organization, says that from the 1870s until shortly after the turn of the century, strong El Ni帽os appeared regularly. Then, with the exception of a major event from 1939-42, they died down until the 1950s. Since the 1950s a more robust pattern has re-emerged. 鈥淭here seems to be climate change on a scale of decades, and the ENSO, which is interannual, a shorter fluctuation, is somehow included in it and affected by it,鈥� he says.
But proving that change is at work over longer timescales remains problematic, not least because of the computer power that it requires. El Ni帽o forecasts just a year in advance require 100 to 200 times as much computer power as do weekly weather forecasts. 鈥淚f you want to stretch the timescale to 10 years, you鈥檙e really stretching our technical capabilities in terms of computer power and the models,鈥� says Leetmaa.
In the end, the secret to El Ni帽o鈥檚 recent palpitations may lie outside the tropical Pacific after all, in a comprehensive picture of world climate. 鈥淚t is becoming more and more evident that there isn鈥檛 one single encapsulated pattern that will explain everything,鈥� says Stefan Hastenrath, a meteorologist at the University of Winconsin.
Fortunately there is plenty more research under way. April marks the end of a 10-year international programme to study El Ni帽o, and participating scientists will gather in Melbourne, Australia, to sum up their progress and chart new directions.
The international research community is also launching a 15-year follow-up programme called the Climate Variability and Prediction Program, which will extend the scope of interannual climate research beyond the Pacific Ocean to the Indian and Atlantic Oceans, and beyond the tropics to the mid-latitudes. And rather than limiting its focus to the ocean and atmosphere, it will also embrace the land, snow cover and sea ice.
This inclusive approach breaks with the past. Most previous work on the tropical Pacific was based on the assumption that El Ni帽o could be predicted without reference to the rest of the world. 鈥淲e had a simpler vision of the world a few years ago 鈥� we weren鈥檛 so bothered by the trend thing,鈥� says Leetmaa. Now, it seems that the trend thing has to be reckoned with.
El Ni帽o鈥檚 global signature
El Ni帽o and the Southern Oscillation change atmospheric circulation on a global scale. Each El Ni帽o episode is unique, but there are sometimes similar weather patterns, called teleconnections. The 1982-83 El Ni帽o was this century鈥檚 worst with economic damage costing $8 billion worldwide and thousands of deaths. Its effects were felt across all the world鈥檚 continents.
Monsoon rains fell over the central Pacific Ocean instead of to the west. Consequently, droughts plagued the Pacific rim countries. Forest fires in eastern Borneo burned a total area greater than that of Switzerland. Eastern Australia suffered its worst drought in recorded history; Melbourne choked on thousands of tonnes of earth blown in by a vast dust storm. Killing droughts also afflicted Indonesia, India, Sr Lanka, China, Africa and Brazil.
Meanwhile, heavy rainfall caused freak tropical storms on central Pacific islands. Typhoons hit Hawaii and Tahiti. Island birds populations were decimated as their nests in sand bars were flooded by heavy rains.
Along the western coast of South America El Ni帽o suppressed the normal upwelling of cold nutrient-rich water that feeds phytoplankton production. Deprived of their food, anchovies that teem off the Peruvian coast perished. The anchovy catch in 1983 was 1% of what it had been 10 years before. Sardines swam south into Chilean waters. Without fish to eat, seabird populations were decimated. Hunger also claimed one-quarter of the year鈥檚 fur seal and sea lion adults and all the pups.
Meanwhile coastal Peru received up to 3 metres of rain within six months, transforming arid desert into a lush land where water washed away bridges and roads. Rainfall soaked other areas as well, among them southern Brazil, parts of the US bordering the Gulf of Mexico, and southern China, where floods crippled the wheat crop. Winter storms smashed into coastal California, causing $1 billion worth of damage.
Accurate forecasting of El Ni帽o can help farmers decide on what crops to plant, and give policy makers time to adjust plans in water supply, hydroelectric generation, fisheries, food imports and a host of other activities. Australia, Peru, Ecuador, Brazil, India, China and Ethiopia all use El Ni帽o forecasts to aid critical decision making.
One example of the potential use of El Ni帽o forecasts came last year, when Columbia University鈥檚 Mark Cane, working with Roger Buckland of the Southern Africa Development Community, reported in Nature that from 1973-1990 maize yield in Zimbabwe fluctuated almost exactly in step with El Ni帽o and the Southern Oscillation.