Typically, the level of ocean water around the world is higher in the western Pacific and lower in the eastern, near the Western coast of South and North America. This is due primarily to the presence of easterly winds in the Pacific, which drag the surface water westward and raise the thermocline relatively all the way up to the surface in the east and dampen it in the west. During El Nino conditions, however, the easterlies move east, reducing the continuing interaction between wind and sea, allowing the thermocline to become nearly flat and to plunge several feet below the surface of the water, allowing the water to grow warm and expand.
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With the help of the National Oceanic and Atmospheric Administration’s weather satellites, tracking shifting patterns of sea-surface temperatures can be made easier. Normally, a “pool” of warm water in the western Pacific waters exists. Under El Nino conditions, this “pool” drifts southeast towards the coast of South America. This is because, in a normal year, there is upwelling on the western South American coastline, and cold waters of the Pacific rise and push westward. However, during an El Nino year, upwelling is suppressed and as a result, the thermocline is lower than normal. Finally, thermocline rises in the west, making upwelling easier and water colder.
Air pressures at sea level in the South Pacific seesaw back and forth between two distinct patterns. In the high index phase, also called “Southern Oscillation”, pressure is higher near and to the east of Tahiti than farther to the west near Darwin. The east-west pressure difference along the equator causes the surface air to flow westward. When the atmosphere switches into the low index phase, barometers rise in the west and fall in the east, signaling a reduction, or even a reversal the pressure difference between Darwin and Tahiti. The flattening of the seesaw causes the easterly surface winds to weaken and retreat eastward. The “low index” phase is usually accompanied by El Nino conditions.
The easterly winds along the equator and the southeasternly winds that blow along the Peru and Ecuador coasts both tend to drag the surface water along with them. The Earth’s rotation then deflects the resulting surface currents toward the right (northward) in the Northern Hemisphere and to the left (southward) in the South Hemisphere. The surface waters are therefore deflected away from the equator in both directions and away from the coastline. Where the surface water moves away, colder, nutrient-rich water comes up from below to replace it which is called upwelling. The winds that blow along the equator also affect the properties of upwelled water. When there is no wind, the dividing layer between the warm surface water and the deep cold water would be almost flat; but the winds drag the surface water westward, raising the thermocline nearly all the way up to the surface in the east and depressing in the west. The resulting changes in sea-surface temperature will have an effect on the winds. 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
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condense to form clouds and raindrops. As a result, this part of the ocean stays indubitably free of clouds during normal years and the rain in the equatorial belt is mostly confined to the extreme western Pacific. However, when the easterlies weaken and retreat eastward during the early stages of an El Nino event, the upwelling slows and the ocean warms. The moist air above also warms. It would produce deep clouds which make heavy rain along the equator. The change in ocean temperatures thus causes the major rain zone over the western Pacific to shift eastward. In this way, the dialogue between wind and sea in the Pacific can become more and more intense.
Normally, each area of the globe follows a fairly predictable pattern and receives only that amount of rainfall that it is accustomed to receiving. However, conditions are quite different during El Nino. During normal years, when the winds blowing east along the equator are blowing at full strength, this strip of ocean stays free of clouds and the rain in the equatorial belt largely confined to the extreme Western Pacific, near Indonesia. But when the easterlies weaken and retreat eastward during El Nino years, the moist air above the ocean becomes buoyant enough to form clouds, and the clouds produce heavy rains along the equator. These rains are only some of the many weather changes that occur all over the globe during an El Nino event. Many other weather changes have resulted in great amounts of damage to the area. In 1982-1983, the El Nino resulted in 100 inches of rain falling during a six month period on Ecuador and northern Peru. The rain transmogrified the coastal desert into a grassland mottled with lakes. That same El Nino also caused typhoons to hit Hawaii and Tahiti. The monsoon rains that fell over the central Pacific, instead of on the Western side, led to terrible droughts and forest fires in Indonesia and Australia. Also, winter storms struck southern California and caused a lot of flooding across the southern United States, while northern regions of the USA received unusually mild winters and a lack of snow. Obviously, El Nino events have quite an effect on global weather patterns. Hopefully, as scientists develop better models, they will soon be better able to understand and make predictions about this curious event.
Normally, the thermocline is quite high in the eastern Pacific. Stirring by the wind mixed the nutrient-rich water below with the surface water. In the presence of sunlight, phytoplankton can produce chlorophyll, a tiny green plant substance. In turn, this substance feeds zooplankton, which in turn feeds higher members of the food chain. During El Nino conditions, the water level rises in the east and lowers in the west, forcing many changes to happen among the plant and animal life. Sea birds in the east must leave their nests, abandoning their young and searching for food which is not there, because the critical upwelling which causes the plankton and other lower members of the food chain to be produced is not there. Water temperature is above normal, and tropical fish are displaced poleward or migrate, along with the anchovy and sardines. On land, the effects produced a great amount of rainfall, making the desert lands into a grassland with lush vegetation and abundant life. Grasshoppers come, fueling toad and bird populations, and the increase in rainfall produces lakes which fish come to inhabit, fish that had migrated upstream during floods produced by the rain and become somehow trapped. In some flooded coastal cities, shrimp production set records. So too did the number of mosquito-borne malaria cases.