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News  El Niño is yawning
Four years ago, torrential rains battered the Southern US, mudslides struck in Peru - and the inhabitants of Canada's west coast saved up to 30% on their winter heating bills. The cause? El Niño, a huge temperature shift in the Pacific Ocean which spawns climate changes globally. Today, using satellite Earth observation data, scientists are detecting the early warning signs of a new El Niño event and predicting that it will develop over the next 3 to 6 months, bringing climate changes to countries thousands of miles from the western Pacific, birthplace of the event itself. "In normal years, there's a large area of warm water in the western Pacific and colder water at the eastern side. In El Niño years, that warm water shifts eastward, which has major effects on the atmosphere above the ocean and thus the climate of the nearby countries," explains Joel Picaut, of France's Centre National d'Etudes Spatiales.
"No one is yet quite sure of the mechanism that causes an El Niño," comments
Picaut. "It's clear that the movement of the warm water and the change in
the wind patterns are closely linked, but we do not yet know which is the
cause and which the effect." The warm water in the west causes convection in
the atmosphere and a relatively constant wind from east to west - the
trade winds. When the El Niño cycle begins, the trade winds reduce, no
longer pushing water toward the warm pool and the mass of warm water begins
to flow eastward, toward the Galapagos Islands and the Peruvian coast. "Over
a period of 1-2 weeks, we see so-called 'westerly wind bursts', in which the
direction of the trade winds reverses. At the equator, you see a very
interesting effect, where the wind and the Earth's rotation combine to
trigger 'Kelvin waves', which are like a wall of water 30 cm high moving
across the pacific at about 200 km per day. You can see the Kelvin waves
clearly and monitor their progress using radar altimeter and sea surface
temperature data," says Picaut. These fast-moving Kelvin waves are the
heralds of the more sedate progress of the main body of water, which travels
at only 30 km per day. In January, scientists detected such a Kelvin wave
near the International Date Line in the central Pacific.
"The ocean is the 'memory' of El Niño," says Picaut, "water has a heat
capacity 1000 times that of the air, so it's the ocean that drives the
event, and it's the ocean we must monitor to understand it." A 10-year
programme is underway using buoys and floating sensor packages to gather
ocean data from the sea itself. This data will help develop the next
generation of computer simulations of El Niño. "We've already created a
computer model which can help us predict an El Niño event from 6 months to a
year in advance," explains Picaut, "but you can't put nature into a
computer. We predicted the 1997 El Niño six months before it began, but we
were taken completely by surprise by how strong it was."
Once Envisat is in orbit it will continue to add a new frame each week to the onging film of the Pacific Ocean being taken by Earth observation satellites. This continuous monitoring provides early warning of an El Niño event as well as supplying climatologists with the data they need to learn more about El Niño, why it occurs and whether this phenomenon is on the increase due to global warming.
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