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Variations in climate and weather can significantly impact our daily lives in subtle ways. We are irrevocably linked to our ecosystem in events resulting from extremely cold winters, crop failure from drought, or emergency conditions such as flooding, heat waves, or forest fires. These events can cause higher heating bills and may lead to higher food prices. Many of our valuable resources are wasted due to lack of preparedness. Effective planning requires study and analysis of the climatic variabilities. The largest climatic anomalies are those associated with the El Niño phenomenon. The El Niño is related to the Southern Oscillation, which is a seesaw of air pressures on the eastern and western halves of the Pacific Ocean. Normally the atmosphere above the eastern South Pacific is dominated by a persistent high-pressure zone, while a low pressure zone dominates the western portion. These two systems are coupled; when the pressure rises in the east, it falls in the west and vice versa. To measure this coupling, meteorologists take the pressure at Easter Island, about 2,700 mi west of South America, and subtract it from the pressure at Darwin in northern Australia. From this difference, they calculate the Southern Oscillation Index. Under normal conditions, this difference in pressure drives the trade winds from east to west along the equator (the easterlies). At the same time, high above the ocean surface, this wind circulation is completed, as it continues to blow around from west to east. This convection of air is called the Walker Circulation, after Sir Gilbert Walker, who first identified it in the 1920's. Every 2 to 7 years, the Southern Oscillation Index becomes negative. The east end of the pressure seesaw goes down, the west end goes up, and the Walker Circulation collapses and sometimes even reverses direction. With the collapse of the winds comes the characteristic warm flow of water to the east, and the normally cold waters on the South American coast warm by 2o to 8o C. All of these elements combine to form the phenomenon called El Niño, which means "The Child" in Spanish, for its tendency to arrive around Christmas (in reference to "The Child Jesus"). The phenomenon is characterized by a dwindling, and sometimes even a reversal, of the trade winds, which causes unusual warming of the tropical eastern Pacific.
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There are historical records of an El Niņo event which took place in 1567. Peruvian fishermen along the westernmost shores of South America were the first to give a name to the climatic anomaly. In an normal year, the waters they fished were cold and flowed from south to north. However, every two to seven years, the waters reversed their flow and warmed up. The phenomenon usually began to peak around the Christmas holiday; therefore, the sailors named the odd weather "El Niņo". In the early 20th century, the concept of El Niño gradually spread through the world's scientific community. At the beginning of the 20th century, scientists believed this strange phenomenon occurred independently of other weather patterns. During the 1920's, while scientists in South America were busy documenting the local effects of El Niño, a British scientist, Sir Gilbert Walker, separately noted that climate anomalies occured around the world every few years. As Sir Walker sorted through world weather records, he recognized some patterns of rainfall in South America, and associated them with changes in ocean temperatures. He found a connection between barometer readings at stations on the eastern and western sides of the Pacific (Tahiti and Darwin). He noticed that when pressure rises in the east, it usually falls in the west, and vice versa. He coined the term Southern Oscillation to emphasize the ups and downs of this east-west seesaw effect. He also realized that, under certain barometric conditions, the Asian monsoon seasons were often linked to drought in Australia, Indonesia, India, and parts of Africa, and mild winters in western Canada. He was the first person to claim that there was a connection between the monsoons in India and unusually mild winters in Canada. In the late 1960's, Jacob Bjerknes, a Norwegian meteorologist, proposed that El Niño was just the oceanic expression of a large-scale interaction between the ocean and the atmosphere, and that the climate anomalies could be better understood as atmospheric teleconnections emanating from the warm-water regions of the mid-Pacific along the equator. Starting at about 1975, oceanographers and meteorologists began to combine their efforts to expand and refine the Bjerknes hypothesis by systematically studying the El Niño and the Southern Oscillation together in what is now referred to as "El Niño-Southern Oscillation," or ENSO. The 1982-83 El Niño has been widely recognized as the most severe of the century. North America experienced wildly unusual weather throughout 1983; Australia experienced massive drought and devastating brushfires; it was one of the worst periods for drought in the sub-Sahelian countries, and the monsoons failed in the Indian Ocean. Total damages were estimated at between 8 and 13 billion dollars, and 2,000 lives were lost. After the 1982-83 event, there was a minor respite, followed by another event in 1986-87, and in an unusual break with tradition, the period 1990-95 was the longest El Niño event in the 130 years of record.
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The main reason for the El Nino is difficult to ascertain as it involves the full complexity of ocean-atmosphere interaction on a global scale. The origin cause of El Niño is not understood, however, scientists have a pretty good understanding of how it evolves once it has begun, and this provides a useful ability to make forecast six to nine months ahead for some regions. During normal, or non-El Niño years, the winds at the surface of the Pacific Ocean near the equator blow from the northeast and the southeast, converge and move to the west. These winds are called the tradewinds because they blow so regularly that trade ships can depend on them to transport their cargo across the ocean. These winds meet in a band parallel to the equator creating what is known as a convergence zone. The tradewinds drive the ocean currents, causing a flow of surface water in the same direction, from east to west, along the equatorial Pacific. The movement of wind causes the warm surface water to build up in the western Pacific. This region of warm surface water is known as the western Pacific warm pool. The winds also cause stirring in the upper layer of the ocean, which causes warm surface water to be mixed into the deeper ocean. This upper layer, known as the mixed layer, is fairly uniform in temperature, salinity and density. The region which separates the warmer water of the mixed layer from the colder deep ocean water is called the thermocline. The warm-water buildup and the strong surface winds over the western tropical Pacific create a thermocline of approximately 100 m or more in depth below the surface. In the eastern Pacific, the cold deep ocean waters are upwelled off the coast of South America to replace the wind blown warm surface water. This produces a shallow thermocline, approximately 10-50 m deep. These upwelled waters are rich in nutrients and produce some of the richer fishing waters in the world. In the warm pool region of the western Pacific, the warm surface waters and the strong tradewinds cause large amounts of water to evaporate from the surface of the ocean. The evaporation of the warm, moist air causes the air to rise and creates an atmospheric low-pressure system at the surface. This low-pressure system is known as the Indonesian Low. The rising air encounters lower temperature and pressure at higher altitudes leading to the formation of intense rain producing cumulus clouds. Further cooling of the rising clouds produces heavy rainfall over the western Pacific convergence zone. After the water condenses and rains out, the dry air continues to travel aloft. Some of the air mass travels back towards the east and accumulates over the eastern Pacific. The accumulation of air sinks creating an atmospheric high pressure system over the eastern tropical Pacific. Small amounts of rainfall are experienced in the eastern Pacific because the sinking air prevents the formation of strong cloud systems. The difference between the surface pressure in the eastern and western Pacific is called a pressure gradient. At the equator, wind naturally moves across the pressure gradient from a region of high pressure to a region of low pressure. This east-west circulation cell is completed as the air mass continues to move westward along the ocean surface where it again acquires moisture from the evaporating ocean. This pattern of air movement from east to west along the surface and from west to east aloft is known as the Walker Cell circulation, and is the east-west component of the large-scale tropical circulation.
The north-south component of the large-scale tropical airflow is the Hadley Cell circulation. The Hadley Cell circulation is comprised of two circulation cells which transport heat through the atmosphere from the tropical regions to higher latitudes. As the tradewinds converge, they create a band of low pressure in the tropics called the Intertropical Convergence Zone (ITCZ). Warm moist air rises along the ITCZ carrying with it latent heat. As the air encounters lower temperatures at higher altitudes, it cools and the moisture condenses releasing latent heat. The moisture condensation can eventually lead to cloud formation and rainfall. After the moisture condenses and rains out, the dry air continues to travel at high altitudes transporting heat to approximately 30o N and S latitude, where it begins to sink. As the dry air sinks, it compresses and heats up due to increased pressure. The air near the surface returns to the equatorial region gathering moisture along the way completing the Hadley Cell circulation. During El Niño, several changes occur in the ocean and the atmosphere to the dynamic and thermodynamic processes described above. Oceanographers and meteorologists do not yet know what initiates an ENSO event, but they are able to identify and describe the changes associated with ENSO. These changes impact atmospheric circulation patterns and can alter climate conditions across the globe. When the easterlies weaken and retreat eastward during the early stages of El Niño event, thus reducing the upwelling of cool waters in the eastern Pacific. This process allowing the pool of warm water in the west to drift eastward toward South America. As the central and eastern Pacific warms, atmospheric pressure gradient along the equator weaken, and the trade winds diminish even more. The moist air above the ocean also warms. It becomes buoyant enough to form deep clouds which produce heavy rain along the equator. The change in ocean temperatures thus cause the major rain zone over the western Pacific to shift eastward. Related adjustment in the atmosphere cause barometers to fall over the central and eastern Pacific and rise over Indonesia and Australia resulting in further weekening and eastward retreat of the easterlies.
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On average an El Niño event occurs every 2-7 years but only irregularly, and not as predictably as the astronomically controlled tides. The return period El Niño is varied; its intensity and duration are also varied and difficult to predict. Typically, it lasts anywhere from 14 to 22 months, but it can be much longer and much shorter. Every other events tend to be stronger or weaker, with the strong events occurring at 8-15 yr intervals. It decays when there is no longer enough warm water to sustain the cycle. There are some exceptional cases; a wave of warm water from the 1982 El Niño survived for 12 yr, measuring only 8 in high in 1994 and traveling at about 5 mph. El Niño often begins early in the year and peaks between the following November and January, but no two events behave in the same way. The intervening weak and moderate events do not typically bring such disastrous consequences. The event of 1982-83 was unusually strong, equaled only by an event in the late 1800's and the more recent 1997-98 event. Big events like 1982-83 and 1997-98 occur only a few times every century. Evidence of the occurrence of El Niño events goes back hundreds of years, in the form of tree ring analysis, sediment or ice cores, coral reef samples, and even historical data from early settlers.
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The ocean and atmosphere disturbances created by an El Niño event can have devastating impacts on the environment and human life. Changes in the "normal" weather patterns can bring floods to the driest lands on earth, creating lakes where normally exist desert plains. At the same time, El Niño disturbances can bring drought conditions to the wettest regions of the globe, the rainforests, bringing drought stress and sometimes huge fires to the forests. The effects are global in nature and extend as far north and south as Canada and New Zealand and over the entire planet from east to west. These effects are felt on many species of life from phytoplankton to humans. Changes in the normal ocean circulation patterns can cause major disturbances in the marine biosphere as sea surface temperature changes and their associated effects touch all forms of life. The weakening of the easterly trade winds during El Niño affects the climate. As the upwelling slows, the sea surface temperature rises and warms the moist air above the ocean. The eastward displacement of the atmospheric heat source changes the global atmospheric circulation. The rainfall follows the warm water eastward and results in flooding in Peru and drought in Indonesia and Australia. The change in weather patterns during an ENSO event alters regions of high and low pressures around the globe. Descending air of atmospheric circulation cells creates high pressure centers at the surface. The high surface pressures prevent areas of precipitation from moving into its region. When these abnormal high pressure patterns persist they lead to drought conditions, depriving the area and its ecosystem of rainfall. Droughts generally occur in the western Pacific during ENSO events, an area of normally rich in rainfall. During El Niño, years droughts were observed in many other regions of the world, including southeastern Africa, India and the northeastern region of the South American continent. During the 1982-83 ENSO event Australia was hit particularly hard by drought. After several months without rains, brush fires and dust storms ravaged eastern Australia. Droughts were also endured in parts of Indonesia and the Philippines where rainforests dried out and subsequently ignited into flames. Summer monsoons from the Indian Ocean where blocked from reaching the India subcontinent by a high pressure system. This brought hardship to the people of southern India who rely upon the rains for drinking water. In the end of the 1982-83 ENSO, South Africa and Botswana was experienced severe drought without rainfall for months with devastating effects on the wildlife and people of the region. The shifting precipitation patterns associated with El Niño, events bring heavy rains to areas not accustomed to such amounts. The rains can bring much needed water to dry regions like Peruvian coast. The heavy precipitation associated with strong ENSO events can create havoc causing major flooding and sometimes avalanches if the precipitation falls as snow in higher mountainous areas. Some of the devastating impacts of flooding include loss of human lives, destruction of homes, damage to crops and natural vegetation, loss of livestock and the spreading of diseases. In 1982, during the strongest ENSO on record, Peru recorded its greatest rainfall amounts in over 200 years. The rains drenched northern Peru and Ecuador creating massive floods along the Piura River and in the Quayaquil area. The flood swollen rivers wiped out the prime banana and rice growing regions, swept away vital bridges and flooded towns, overflowing toilet pits and causing virulent diseases to spread thought the areas. The rains soaked hillsides and brought thousands of homes tumbling down in the mudslides. In Australia, near the end of the 1982-83 ENSO event, and directly after having suffered through devastating drought, torrential rains swept through and flooded large portions of eastern Australia. Tens of thousands of sheep and cattle were stranded in shallow water or on hilltops surrounded by water. Flooding occurred in many other regions of the world, including Brazil, Europe, China, Cuba and the United States. In the United States much of the precipitation fell as snow in the western mountains, delaying the floods until the following spring melt. Higher sea surface temperatures in the eastern Pacific provide fertile ground for the development of hurricanes and tropical storms. The eastern Pacific normally has much cooler water, but during El Niño events warm water invades the area. Hurricanes are fueled by warm water evaporating off the surface of the ocean. In 1982 Tahiti and its neighboring islands of French Polynesia experienced their first hurricanes in 75 years. Six storms pounded the islands within a five month period decimating entire villages on the Tuamotu Archipelagos with high winds and waves generated by the storms. These rough seas and increased sea heights combined to make feeding difficult for marine mammals that feed near the shores. Young marine iguanas of the Galapagos Islands were noted by scientists to have smaller sizes than normal. In the same period, 18 storms with wave heights exceeding 6 meters slammed into the coast of the southwestern United States damaging over 10,000 coastal homes, uprooting giant kelp beds offshore and causing coastal erosion that virtually eliminated entire beaches. Upwelling of deep ocean currents off the coast of Peru create one of the most productive fishing waters in the world. The upwelled ocean currents bring nutrient rich waters to the surface. When ENSO conditions prevail, the upwelling of nutrient rich waters is slowed and sometimes even stopped if the event is strong enough. The reduced upwelling decreases the amount of planktonic diatoms which are the base of the food chain. Without plankton, higher level life-forms starve or migrate out of the region. Marine species such as zooplankton, macaral, anchoveta, bait fish and squid perish from the lack of nutrients, disrupting fishing industries. Bird species, which depend on these fish as food sources, also starve or migrate. Often this leads to nest abandonment by nesting birds. In 1983, up to 85% of the seabirds on the Peruvian coast died. During this same time, the Galapagos and Christmas Islands, home to 17 million seabirds, were nearly completely abandoned due to the lack of a food source. On Christmas Island this entailed a total reproductive failure for the year. Sea mammals are also affected by the lack of fish. Sea lions and fur seals have been noted to have markedly reduced birth rates in the years following an ENSO event. Albatross, boobies, penguins, cormorants and marine iguanas are other species that are adversely affected by the lack of food. A compounding problem with El Niño, is the timing of the events. The warmest sea surface temperatures and its impacts occur when many mammals are breeding. At a time when food supplies should be highest, El Niño, diminishes food supplies by reducing upwelling and dampening primary food production. Loss of food has particularly detrimental effects on nursing females and their young or unborn. Territorial male mammals suffer due to their inability to move with the migrating food sources. In spite of the destructive nature of El Niño, some marine creatures do benefit from the disturbances brought upon the phenomenon. As a result of the 1982-83 El Niño, scallops accelerated their growth and reached enourmous densities. Purple smails and octopuses became more common and the shrimp fishery reached its higest level. This is most likely the result of increased run-off from the rivers. which provide a greater abundance of nutrients and decreased predation from a dispersed fish population. Coral reefs are a notably sensitive ecosystem. The temperature increases create thermal stresses which kill endosymbiotic algae which live in the coral. The algae protects the coral from the sun's damaging ultraviolet (UV) radiation. Without the algae, corals suddenly turn white because they are left unprotected against the UV radiation from the sun. The whitening of the coral is known as coral bleaching. Within two to four weeks of the bleaching, the coral skeletal structure is destroyed and the plant dies. The 1982-83 El Nino nearly wiped out many eastern Pacific coral communities. Scientist estimated coral mortality was close to 98% of the total population. Damage was also observed during this period in reefs in the central and western Pacific, Japan, the Persian Gulf and the tropical western Atlantic with mortalities reaching 70-90% in some of the regions. The damage to the coral reefs is particularly devastating as recovery for the reefs occurs over time periods of decades to centuries.
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