Academic Reading — Test 150

Reading passage

The tropical Pacific Ocean is rarely still in the climatic sense. Across its vast expanse, a recurring see-saw of warming and cooling reshapes weather not only along the equator but across continents thousands of kilometres away. Scientists call this coupled phenomenon the El Nino-Southern Oscillation, or ENSO, a term that joins two observations once thought separate. The first, El Nino, describes a periodic warming of surface waters in the central and eastern Pacific. The second, the Southern Oscillation, refers to a swing in atmospheric pressure between the western and eastern halves of the basin. Only in the twentieth century did researchers recognise that the ocean and the atmosphere were behaving as a single, linked system, and that neither half could be understood in isolation. Under normal conditions, often labelled neutral, the trade winds blow steadily from east to west along the equator. These winds drag warm surface water towards Indonesia and northern Australia, piling it up so that the sea surface in the western Pacific stands measurably higher and warmer than in the east. The accumulated warmth feeds towering clouds and heavy rainfall over the western basin, while the eastern Pacific, off the coast of South America, remains comparatively cool and dry as deeper, nutrient-rich water rises to replace the displaced surface layer. This upwelling sustains some of the world's most productive fisheries. The whole arrangement is held in balance by the strength of the trade winds, which both respond to and reinforce the temperature contrast across the ocean. During an El Nino event, this balance breaks down. The trade winds weaken, and sometimes reverse, allowing the pool of warm water to slide eastward across the basin. As the warmth migrates, so does the zone of intense convection and rainfall that normally sits over the western Pacific. Regions such as eastern Australia, Indonesia and parts of South-East Asia frequently experience reduced rainfall and a heightened risk of drought, while the usually arid coasts of Peru and Ecuador may be struck by unaccustomed downpours and flooding. The eastern upwelling falters, warmer water suppresses the rise of nutrients, and fish stocks that depend on the cold current can decline sharply, with severe consequences for coastal communities. El Nino events are not regular; they tend to recur every two to seven years, and their intensity varies considerably from one occurrence to the next. The opposite phase, known as La Nina, can be thought of as an intensification of normal conditions rather than their reversal. The trade winds strengthen beyond their usual force, the western Pacific grows even warmer and wetter, and the eastern Pacific becomes cooler still. Australia and Indonesia may then receive above-average rainfall, sometimes enough to cause widespread flooding, while drought tightens its grip on the western coast of South America. Because El Nino and La Nina represent opposite extremes of the same oscillation, with neutral conditions lying between them, forecasters monitor a suite of indicators to judge which phase is developing. Among these are sea surface temperatures in defined regions of the central Pacific and an index based on the pressure difference between Tahiti and Darwin in northern Australia. The reach of ENSO extends well beyond the Pacific rim. Through atmospheric connections that meteorologists call teleconnections, an event in the tropical ocean can alter rainfall and temperature in regions as distant as eastern Africa, southern Asia and the Americas. The Indian monsoon, on which hundreds of millions of people depend for their crops, has historically tended to weaken during strong El Nino years, although the relationship is not perfectly reliable and other influences can override it. This variability is precisely why prediction remains difficult. Forecasters can now anticipate the onset of an event several months ahead using networks of ocean buoys, satellites and computer models, yet the exact timing, strength and regional consequences of any given episode are still uncertain. Understanding ENSO matters because the populations affected are enormous and often poorly equipped to absorb sudden swings between flood and drought. Agriculture, water supply, fisheries and public health are all sensitive to the shifts in rainfall that the oscillation drives. A reliable seasonal forecast, even an imperfect one, allows farmers to choose hardier crops, governments to store water or grain, and aid agencies to position resources before a crisis arrives. As the climate continues to warm, an open question is whether El Nino and La Nina events will become more frequent or more severe; the evidence so far is mixed, and researchers continue to debate it. What is certain is that the restless Pacific will go on shaping the lives of people far from its shores.