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El Niño: Causes and Effects

In Spanish, El Niño means the little one. It was Spanish-speaking South American fishermen who first noticed and named this “unseasonable” weather that crept upon them occasionally in the month of December. As fishermen, what they noticed immediately was the lack of fish during an El Niño. Also, when it was supposed to be the dry season, the rain would fall, and the waters were warmer than usual. It was just something that occasionally happened.

Eventually, scientists in the United States who were curious about U.S. changes in basic atmospheric weather patterns began to link our warmer- or colder-than-usual winters, dryer- or wetter-than-usual summers, bigger and more frequent storms, and other weather anomalies with the reported El Niños of South America. Also, scientists drew links between more global consequences of unseasonable weather. As the world's scientists became more knowledgeable and better able to gather data, they began to analyze their data to discover the cause for El Niño to better understand its effects and to fit many loose ends of the global weather puzzle together.

Their initial research revealed how the atmospheric wind forces and the ocean temperatures work together, one either driving or reacting to the other. Understanding this interaction became the key to finding the cause for El Niño. The studies was focused on several areas. The 30° North (N) - 60° N latitude is important, as the western shores of the U.S. receive the prevailing westerlies. These are not particularly strong and can easily be dominated by more powerful forces. Also included in the chain of interactive areas of focus is Hawaii at 21° N. The island of Hawaii sits in the middle of the Pacific, quite a distance from the mainland. Sometimes, however, a Pineapple Express of torrential warm rains can explode on the northwest region of the United States. El Niño influences this pattern. The University of Hawaii (due to its unique location and geology) has developed an outstanding program of oceanic and weather research helpful in unlocking some of the mysteries of water temperature and the impact of El Niño in the region.

However, the neighborhood and apparent source of El Niño includes the distance from the eastern shores of the Pacific Ocean (particularly the coast of South America — mainly Peru) to the western shores of the Pacific Ocean (Malaysia). The latitudes of this primary focus include the distance from the equator at 0° to 30° S, within the southern trade winds that are the prevailing easterlies. Because this is the area from which the mysteries of El Niño were unraveled 0°, we start with the Equator.

Wind Forces

Among the primary wind forces affecting El Niño are the pressure gradient force and the Coriolis force.

Pressure gradient force. Solar radiation (incoming energy from the sun) is most intense at the Equator. Moving away from the Equator, north and south, the radiation is spread over a larger area, mainly because of the earth's curve and the subsequent increased distance from the sun. At the equator, the earth approaches a 90° angle much of the year. Above and below that center point, the angles are greater. Because the rays must penetrate through more atmosphere from a greater distance, the heat is less intense. These differences in distance from the primary energy source to the surface of earth create uneven heating. The land, surface water and atmosphere in the tropical areas around the 0º heat quickly and remain heated. Because of the increased evaporation caused by the heat, the air stays humid. This is what causes the air to rise, creating a low-pressure area. At 30º N and 30º S, the air is both drier and cooler at the same time. Therefore, the air pressure is higher and the air drops. At 60º N and 60º S (the subpolar latitudes), the air is cold and moist and, as in the tropics, the air rises, creating another low pressure area. There are the polar caps that have a cover of extremely cold desert air. This polar air is an alternate band of high pressure, and because the air is very cold it sinks.

The resulting alternate bands of atmosphere set up convection currents. These are circular patterns of heat exchanges in which the high pressure blows toward the low pressure—either north or south, depending on the latitude. The force in which the high pressure flows toward the low is called the pressure gradient force (PGF). The greater the differences, the stronger PGF winds blow. This action might look like two young foxes chasing each other's tails, except that with the atmospheric winds an intervening force knocks air currents off their directly north or south routes. It is called the Coriolis force.

Coriolis force. As the moving air gains speed, it is deflected by a force called the Coriolis. This phenomena of physics is caused by the earth's rotation. When the PGF meets the Coriolis force, the resulting path is a curve to the right in the northern hemisphere and to the left in the southern hemisphere. It's somewhat similar to the baseball pitcher's curve ball in which one sees him throw the ball at an apparent trajectory, yet it somehow seems to cut a new curve off the anticipated line. The atmosphere no longer goes directly north or south, but instead the Coriolis force causes the winds in these latitudinal bands to move in a northeasterly, northwesterly, southeasterly or southwesterly direction—each latitude band alternating direction. Convergent zones lie in the subpolar regions at 60°S and N and at the Equator. These are zones that receive atmosphere from both the north and the south. For example, the northern trade winds above the equator blow southeasterly toward the equator as the southern trade winds blow northeasterly toward the equator, both sides moving toward each other. There is an airlift here that causes precipitation and an absence of ocean surface winds; this area is called the doldrums (a word now synonymous with depression).

The point at which the PGF and Coriolis forces equalize and travel along in parallel lines is a pattern called geostrophic winds. The geostrophic winds which blow across the ocean have a fairly high velocity because of the inadequacy of their mountain- and landform-induced frictional force. Those parallel lines (the resulting path) that find resistance on land are the isobars that meteorologists use to indicate the direction of the weather. They show high and low pressure air and indicate direction of airflow in curves around land and water forms. However, the prevailing geostrophic winds in the El Niño latitude of 0° – 30° S are the trade winds that blow persistently and without resistance in a northeasterly direction across the Pacific from the west coast of South America to Malaysia. Highly significant is that the ocean currents below them follow the same pattern in parallels of equal temperature lines called isotherms.

Ocean Forces

Among the water forces primarily associated with El Niño are water layers and upwelling.

Water layers. Flying from North America over the Pacific Ocean, the observer has scant idea of the ocean's immense power and its enormous effect on the world's weather. The three significant attributes that might be missed from aesthetic observations are the salinity of the ocean, its nutrition, and its temperature. Elements that are dispersed in various proportions within three distinguishable layers. These ocean layers vary as much as a layer cake of three different flavors might vary, except that the depths of the layers in the ocean are fluid and in a constant state of change. More important is that the layers of the ocean contain the main ingredient for El Niño: a variety of temperatures.

The oceans top layer is called the mixed layer as it is mixed by surface winds and wave action. This makes it the most uniform in its chemistry and temperature. The depth of the mixed layer can vary from less than 100 feet to a depth of 500 feet. It is euphotic in that it allows the penetration of light and the process of photosynthesis. Even though photosynthesis can take place in the mixed layer, this layer does not contain the most nutrients. It has the least salinity and less density due to precipitation and the run-off of fresh water from streams and rivers. However, it is the most vulnerable to evaporation because it can exchange with the atmosphere and because it is readily warmed by the radiant energy of the sun. As the trade winds prevail in their northeasterly direction along the coast of Peru, it is the lighter mixed layer of water that gets pushed steadily toward Malaysia.

The thermocline, the mixed-layer and the deep-water transitional layer, represents a steep decline in temperature, thus its name. In the tropic region, the thermocline temperature ranges from about 25° C (77° F) at its top to 5° C (41° F), the point at which the third, deep-water layer begins. The thermocline can begin at a depth of less than 100 feet, though its normal range is between 150 to 200 feet, depending on the condition of the winds, the location, the time of the year, or the presence of conditions such as El Niño. Other than its declining temperature, the thermocline is defined by its nutrient level. Though photosynthesis does not take place in this layer, it contains most of the ocean's nutrients. It is rich in plankton and has diversity of species and mineral nutrients.

These three ocean layers can be visualized as a stew, with the most nutritious tidbits sunk to or just drifting above the bottom and the lighter juices on top. It takes some “stirring up” to bring the good stuff up to where it can be harvested. The wind forces do just that in the process called upwelling.

Upwelling. The world's oceans get stirred up by the winds, increased salinity (which can cause water to sink), and water temperature. In the tropical latitudes where El Niño begins, the trade winds do the work. The action of the prevailing easterly trade winds drags the warm mixed layer away from the Peruvian shores and east to Malaysia. The warm water that has moved actually raises the level on the western side of the Pacific by as much as 19 inches, about half a meter. Some report that this can be up to 3 feet. Also, as the warm waters on the western Pacific shores gather, they push deeper down to displace and thin out the thermocline and deep water layers. As the mixed layer moves from the Peruvian coastline, the thermocline is thrust to the top and swells up along the coast, taking to the surface with them the rich nutrients from the colder waters below. This process is called upward nutrient transport.

There are five major fisheries of the world: Namibia (off the southwest coast of Africa), Mauritania (off the northwest coast of Africa), Somalia (off the northeast coast of Africa), and the coasts of California and Peru. Each of these fisheries depends on forces such as upwelling for its livelihood. In Peru, the most important fish are anchovies. They are used to supply a huge food meal industry for the world's livestock. Anchovies have historically been abundant when the upwelling produces the nutrients necessary to support them. But every three to seven years, the conditions change and the reversal can produce hardship or devastation.


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