Satellite eyes focus on El Nino
At the end of 1997, torrential rains battered California, mudslides struck in Peru – and many inhabitants of southern Canada saved up to 30% on their winter heating bills. The cause? El NiÒo, a huge temperature shift in the Pacific Ocean which spawns climate changes globally. El NiÒo is a regular event, and using satellite earth observation data, scientists are becoming able to predict its onset months in advance, and are beginning to understand the mechanisms which cause it.
‘In normal years, there’s a large area of warm water in the western Pacific, and colder water at the eastern side. In El NiÒo years, that warm water shifts eastward, which has major effects on the atmosphere above the ocean, and thus the climate of nearby and distant countries,’ explains Joel Picaut, of France’s Institut de Recherche pour le DÈveloppement.
Imaging from space to measure sea level elevation, surface winds and surface temperatures, reveals that the trade winds near the Equator pile up a mass of water, warmed by the sun on its journey across the Pacific, which covers an area the size of Europe. This so-called ‘warm pool’ off the Philippine coast is in fact a plateau in the sea some 50 cm high, and with a temperature of approximately 29*C – 5*C above average. In contrast, the water off the Galapagos Islands on the other side of the Pacific is normally a mere 22*C. When an El NiÒo event takes place, all this changes.
‘No one is yet quite sure of the precise mechanism that causes an El NiÒo,’ 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 NiÒo 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 Ecuadorian and Peruvian coasts. ‘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 250 km per day. You can see the Kelvin waves clearly and monitor their progress using radar altimeter and in situ temperature data,’ says Picaut. These fast-moving Kelvin waves are the heralds of the more sedate progress of the main body of water, which sweeps from the Western Pacific at only 50km per day.
‘We don’t yet know if the wind bursts and Kelvin waves are fundamental to the El NiÒo mechanism, or whether they are simply an effect on top of the underlying transfer of warm water from west to east,’ comments Picaut. Despite the mystery surrounding the origins and mechanism of El NiÒo, scientists are making significant strides in the ability to predict its onset. ‘Although El NiÒo is cyclical, it’s not like clockwork, and the strength of the effect varies dramatically from one event to the next. We had a very strong El NiÒo in 1982/83, another in 86/87, a series of small ones in 91/92 and 93 and a huge one in 1997,’ comments Picaut. ‘In 1999/2000, we were actually in ‘La NiÒa’, which is the opposite effect.’ La NiÒa causes an increase in the trade winds, and more cooling of the waters around the Galapagos Islands.
‘The tropical ocean is the ‘memory’ of El NiÒo,’ says Picaut, ‘water has a heat capacity 1000 times that of the air, and it takes time to move an enormous quantity of water over half the Pacific, so it’s the ocean that drives the event, and it’s the ocean we must monitor to understand it.’ An international programme is underway using research vessels, moored buoys, floating sensor packages, and tide gauges to gather ocean data from the sea itself. This is complemented by satellite measurements, such as those of ERS, soon to be followed by Envisat, due for launch in June 2001. This data will help develop the next generation of computer simulations of El NiÒo. ‘We’ve already created several computer models which can help us predict an El NiÒo 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 NiÒo six months before it began, but we were taken completely by surprise by how strong it was.’
Prediction may be an inexact science, but any warning is better than none at all. In the US, the prediction led to bulldozers moving in along the Californian coast to create better sea defences against the El NiÒo-charged Pacific rollers. With sufficient warning, countries like Ecuador and Peru, which will bear the brunt of rains 10-40 times heavier than normal, can prepare to deal with flooding by improving watercourses and moving people from the areas most threatened by flash floods and mudslides. ‘If you know El NiÒo is on the way, you can be ready,’ sums up Joel Picaut.
