Academic Reading — Test 62

Reading passage

The El Nino-Southern Oscillation, commonly abbreviated to ENSO, is among the most influential patterns of natural climate variability on the planet. It describes a coupled interaction between the surface waters of the tropical Pacific Ocean and the atmosphere above them, and it operates on a cycle that recurs irregularly every two to seven years. Although the phenomenon is centred on the Pacific, its effects on rainfall, temperature and storm activity are felt across much of the globe. Scientists generally divide the cycle into three phases: a warm phase known as El Nino, a cool phase called La Nina, and a neutral state in which conditions sit between the two extremes. Understanding how these phases redistribute rainfall has become a central concern for meteorologists, farmers and water managers alike. In a neutral year, strong easterly trade winds blow across the tropical Pacific from the Americas towards Asia. These winds push warm surface water westwards, so that the sea around Indonesia and the western Pacific stands noticeably warmer and higher than the sea off the coast of South America. Where the ocean is warmest, the air above it heats, rises and forms towering clouds that release heavy rain. This is why the western Pacific is typically humid and wet, while the eastern Pacific, where cooler water wells up from the deep, remains comparatively dry. The contrast in sea-surface temperature between the two sides of the basin is therefore the engine that drives the region's normal pattern of rainfall. During an El Nino event, the trade winds weaken and may even reverse direction. The warm pool of water that usually gathers in the west slides eastwards across the ocean, carrying the zone of heavy rainfall with it. As a result, regions that are normally dry, such as the coast of Peru and Ecuador, can experience torrential downpours and flooding, while the western Pacific is left unusually parched. Indonesia, eastern Australia and parts of South-East Asia frequently endure drought during these years, and the reduced rainfall can devastate crops and increase the risk of bush fires. The displacement of rainfall is rarely modest; entire agricultural seasons can fail because the rains arrive in the wrong place at the wrong time. La Nina represents the opposite swing of the cycle. The trade winds strengthen beyond their usual force, piling warm water even more firmly into the western Pacific and drawing colder water to the surface in the east. Consequently, the wet conditions of the western basin become wetter still, and countries such as Indonesia and Australia may be subjected to exceptionally heavy rain and flooding. Meanwhile, the eastern Pacific grows drier than normal. Because the two phases tend to produce contrasting outcomes in the same locations, a region that suffers drought under El Nino may face flooding under La Nina, leaving communities little time to recover between the extremes. The atmospheric component of the cycle is measured using an index based on the difference in air pressure between Tahiti, in the central Pacific, and Darwin, in northern Australia. When the pressure is comparatively high at Darwin and low at Tahiti, the system tilts towards El Nino; when the reverse holds, La Nina is more likely. By tracking this index alongside ocean temperatures, forecasters can often anticipate an emerging event several months in advance. Such early warning is valuable, for it allows governments to prepare reservoirs, advise farmers on which crops to plant, and ready emergency services for floods or droughts. Even so, no two events are identical, and the precise strength and timing of each remains difficult to predict with confidence. The consequences of ENSO extend well beyond rainfall. The shifting warm pool influences the formation of tropical cyclones, the productivity of fisheries and even the spread of certain diseases that thrive in wet conditions. Off the coast of South America, the cold, nutrient-rich water that normally supports vast shoals of anchovy is replaced during El Nino by warmer water, causing fish stocks to collapse and harming the communities that depend on them. Researchers are also examining whether a warming climate might change the frequency or intensity of these events, though firm conclusions remain elusive. What is certain is that the Pacific, far from being a passive expanse of water, acts as a powerful regulator of weather for billions of people, and that the rise and fall of its temperatures continue to reshape where the world's rain falls.