Associated with El Nino is a change in the atmospheric pressure system, known as the Walker circulation, across the Pacific ocean known. Normally the pressure in the east is higher, causing the trade winds to blow westwards across the Pacific. If a Nino comes to the east, the air above it warms, and its atmospheric pressure drops. The reduced pressure difference, causes the trade winds to relax, and sometimes even to start blowing in the opposite direction -- a phenomenon known as the Southern Oscillation. Thus climatologists often refer to ENSO, the El Nino-Southern Oscillation, since the two are intertwined. Indeed, they are so intertwined that they form a "chaotic" system, meaning that some small trigger event (which is usually impossible to identify) may start the whole system moving in the direction of an ENSO.
The effect of the weakening trade winds is that warm surface water, which is normally piled up in the western Pacific (near Indonesia and Australia) by the winds, starts to slosh back across towards the centre.
The ocean temperature affects the atmosphere above it. Above warm water the air rises. As it rises it cools, and eventually the moisture from it condenses into rainclouds. In an El Nino year, therefore, tropical storms which would normally hang over Indonesia and Australia move, with the warm surface water, across to drench the centre of the Pacific. Meanwhile the warm currents off the South American coast lead to unusually heavy rains and flooding in Peru, Ecuador and Chile.
The changed weather patterns over the Pacific divert the jet streams, high in the atmosphere, which direct the weather fronts into North America, and thence to the rest of the world. Although these effects get harder and harder to disentangle as they move eastwards, a typical El Nino year brings strange weather patterns all around the globe. Among them are drought in north-east Brazil, wet weather in the Gulf of Mexico, drought in southern Africa, extra rain in parts of central Europe and the Middle East, and drought in Indonesia and northern Australia.
This weather has many side effects, some direct, and some less so. Crops may suffer from too much or too little rain (or rain at the wrong time). Flooding damage is particularly large. The North Atlantic experiences, in fact, fewer hurricanes. Unusual sea temperatures may cause algal blooms in the oceans, which are suspected to help the spread of diseases such as cholera. Some places that experience unusually wet weather see a boost in the malarial mosquito population. The anchovy stocks off Peru fall, affecting the livelihoods of the fishermen -- and of the marine birds who feed on them and then provide Peru with another economic resource, in the form of their droppings, which are used as fertiliser. The strong Nino developing this year, of course, has a famous side-effect: the delayed rains in Indonesia have allowed the fires started for land-clearing purposes to burn on and on, shrouding the entire region in smoke. The damage from this remains to be assessed.
Predicting a Nino is, as mentioned, not easy. Since the 1982-83 Nino, the last great one, which is estimated to have caused around $13 billion of damage worldwide, various climate scientists have tried to use computer modelling to reproduce, in a simplified simulacrum of the earth's oceans, the effects they see in nature. There are a variety of computer models, relying on different assumptions. The Cane-Zebiak model, developed by two researchers at Columbia University in New York, is the longest-running, begun in 1986. It has been successful at predicting the mild Ninos that have occurred in between, but did not predict the onset of the current Nino, which climatologists think may yet turn out to be the most powerful one this century. That was first seen coming in November by other computer models, but it was not until around May that the intensity of the forthcoming upset became apparent.
A bewildering variety of models is now trying to forecast how the current Nino will play out. Most seem to agree that it will probably continue at least until spring 1998. Some predict a corresponding cold episode, La Nina (which is like a Nino inverted, with colder than normal surface temperatures in the eastern Pacific), following soon afterwards. Ninas are less well understood, but also have climatic effects around the globe.
Most areas that have been affected by a Nino in the past already know broadly what to expect. Preventive measures can be taken, such as flood barriers, changing of crops and planting times, vaccinations, stockpiling of emergency water or food supplies and action against mosquitoes. With experience, the computer models will improve, and scientists hope to be able to offer specific forecasts for certain regions which will allow them to plan more precisely. However, thanks to the chaotic relationship between the oceans and the atmosphere, it seems unlikely that a Nino will ever be predictable more than a year or so in advance.
Gideon Lichfield The Economist (London/UK) (gideonlichfiled@economist.com)
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