Реферат: El Nino Essay Research Paper El Nino
El Nino Essay, Research Paper
El Nino has been a reoccurring phenomenon for centuries. Man has only started to realize how much of the worlds weather is effected by it. The term El Nino refers to an irregular warming of the seas surface. During the last 40 years there have been 10 significant El Nino occurrences. Most affecting the coast of South America. Water temperatures increase along the coast as far as the Galapagos islands. Weak events will raise the water temperature 2 to 4 degrees Celsius and will have minor impact on fishing. However strong events such as the 1982-83 event will disrupt climate conditions around the world as well as local conditions. It has been linked to floods and droughts all over the world. Hurricanes and tropical storms are also altered in their numbers by El Nino. Therefore it would be very helpful for people if El Nino could be predicted and prepared for in some form.
During a El Nino cycle there are many biological changes. Due to a depressed thermocline there is less photosynthetic activity resulting in a decrease in the primary life forms that form the beginning of the food chain. The warmer waters that are brought by these changing cycles hold less dissolved oxygen forcing fish to go deeper or venture elsewhere. Due to a lack of data during El Nino occurrences it is not fully known if fish populations are depleted solely due to exposure to El Nino. A decrease in their growth and reproductive success has been observed by many surveys in coastal waters.
The link between climatic effects around the world and El Nino is now well established. It has taken many years of studying to understand how the pieces of the puzzle, from ocean currents to winds and heavy rains fit together. During the 1920s a scientist was on assignment in India trying to predict the Asian monsoons. As he sorted through his records he discovered a connection between barometer readings at stations on the eastern and western sides of the Pacific. He noticed that when pressure rises in the west it usually falls in the east and vice versa. He coined this term Southern Oscillation. When it is on its high index state pressure is high on the eastern side of the Pacific and low on the western side (figure 1). The east west pressure contrast drives easterly surface winds. When it switches into low index (figure 2) the easterly surface winds weaken. The biggest changes are over the western Pacific. West of the dateline the easterlies usually completely di!
sappear during low index years, but east of the dateline they only weaken.
The surface winds that move the ocean currents are a major controller in weather. The easterly winds that blow along the Ecuador and the southeasterly winds that blow along the Peru and Ecuador coasts both tend to drag surface water with them. The Earth?s rotation then deflects the resulting surface currents northward in the Northern Hemisphere and southward in the Southern Atmosphere. The surface waters are therefore deflected away from the equator in both directions and away from the coastline. When the surface water moves away, colder nutrient-rich water comes up from below to replace it. This is known as upwelling. This nutrient rich water is the beginning of the food chain as phytoplankton establish themselves in these waters.
When the easterlies are blowing at full strength, the upwelling of cold water along the equatorial Pacific chills the air above it, making it too dense to rise high enough for water vapor to condense and form clouds. As a result, this strip of ocean stays free of clouds during normal years and rain in the equatorial belt is largely confined to the extreme western Pacific near Indonesia.
But when the easterlies weaken and retreat eastward during early stages of an El Nino event, the upwelling slows and the ocean warms. The moist air above the ocean also warms. It becomes light enough to form deep clouds which produce heavy rain along the equador. The change in ocean temperatures causes a major rain zone over the western Pacific to shift eastward. Related adjustments in the atmosphere cause barometers to drop over the central and eastern Pacific and rise over Indonesia and Australia. This results in a further weakening and eastward retreat of the easterlies. In this way small changes in the ocean and wind currents continue to amplify each other until a full blown El Nino event occurs.
Dense tropical rain clouds alter the air flow 5 to 10 miles above sea level (ex. Rocks distort the flow of a stream). These clouds create waves in the airflow that in turn determine the positions of monsoons, the storms tracks and belts of strong winds above the earth that separate warm and cold regions of the earth?s surface.(referred to as jet streams). The impacts of El Nino upon climate in temperature show up most clearly during wintertime. Most El Nino winters are mild over western Canada and parts over the United States, and wet over the southern United States from Texas to Florida. El Nino affects temperature climates in all other seasons as well. But even during wintertime El Nino is only one of a number of factors that influence temperature climates. El Nino years are not always marked by “tropical” El Nino conditions the way they are in the tropics.
Scientists noticed that a monsoon season with low index conditions are often marked by droughts in Australia, India, Indonesia and parts of Asia. Although scientists did not realize the magnitude of what they were discovering they knew they had discovered only a small portion of this phenomenon. It would require a knowledge of wind patters above ground level (which were not recorded at that time) to continue to gain knowledge. In the following decades researchers added new pieces to the emerging picture of Southern Oscillation.
The march of the seasons is quite predictable. In contrast El Nino recurs at irregular intervals ranging from two years to a decade, and no two events are ever exactly alike. For example the 1982-83 El Nino caught scientists by surprise because unlike previous EL Ninos it was not proceeded by a period of stronger easterlies on the equator. To further confuse scientists this event also set in unusually late in the calendar year. In order to guard against the possibility of being surprised by another El Nino scientists continue to document as many past events as possible by piecing together bits of evidence from many different sources such as sea surface temperatures, daily observations of atmospheric pressure and rainfall and regarded as the strongest this centuries by many “experts”. I t was not predicted and not even recognized by scientists during its early stages. The economic impact were large. The Equador and Peru fishing industries suffered heavily when there a!
nchovy harvest failed and their sardines unexpectedly moved south into Chilean waters. Up to a 100 inches of rain fell in Equador and northern Peru. This transformed the coastal desert into a grassland dotted with lakes. The new vegetation attracted swarms of grasshoppers which fueled explosions in the toad and bird populations. The new lakes also created a temporary habitat for fish that had migrated up stream from the sea and had become trapped. Many of them were harvested by local residents as the lakes dried up. In some of the flooded coastal estuaries, shrimp production records were set, but so did the number of mosquito-borne malaria cases.
Further to the west abnormal wind patterns shifted typhoons off their usual tracks and sent them to the islands of Hawaii and Tahiti, which are unaccustomed to such severe weather. They also caused the monsoon rains to fall over the central Pacific instead of the western side which caused droughts and forest fires throughout Indonesia and Australia. Many winter storms battered southern California and caused flooding throughout the southern United States. Overall the loss to the world economy as a result to climate changes resulted in 8$ billion.