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El Nino is a complex phenomenon which we are only now beginning to understand. But what is known to the scientific community about El Nino, we have decided to share with you. Before we talk about what El Niņo conditions are, we must show what normal conditions in the Pacific Ocean are.
Under normal weather, there is a high pressure system in the tropics in the eastern Pacific. Over Indonesia, to the west, as predicted by Walker, there is a corresponding low-pressure system. Winds, as discussed before, blow from a high-pressure system to a low-pressure system. This is done to achieve equillibrium in the system. Therefore, under normal conditions, strong easterly trade winds blow from east to west in the Pacific Ocean. These winds blow so strongly because of the steep air pressure gradient between the east and west. These winds are so strong that there is actually more warm water present in the west since it has been piled up by the winds' force. If you measure the ocean near New Guinea, you will see that it is usually 20 inches higher than in the water near Ecuador. The temperature in western Pacific is also normally higher, usually about 12-14o F.
Why is this so?
The strong equatorial trade winds push the water towards western Pacific. Therefore, deep, cold water from below then rises to replace the water in the eastern Pacific near South America. This is a phenomenon called upwelling. Sun rays mostly warm the surface waters of the ocean, therefore, deeper water is usually much cooler. Thus, when this deeper and colder water replaces the surface water, the temperature in the eastern Pacific ocean is usually about 12-14oF less. Warm surface waters and cold, deeper water layers in the oceans are separted by a layer of water known as the thermocline, where temperature changes very rapidly. Below the thermocline is cold, stratified water in which temperature falls steadily with depth. Above the thermocline, by contrast, is warmer water which is not in stratified temperature layers. The thermocline is closer to the surface of the ocean in the east. This cooler layer of water also contains many nutrients which help increase the number of animals in the food chain since it directly increases plankton population.
One effect of the build-up of warm water in the west is a corresponding cold undercurrent flowing eastwards, and rising to the surface in 'upwellings' off the South American coast near Peru and Ecuador. The Peruvian fisheries are totally dependent on these upwellings, which carry with them rich nutrient material from the sea-bred on which surface fish feed. These upwellings are driven by a combination of temperature, atmospheric pressure, wind, water currents, and the rotation of the Earth. How they interrelate, and the mechanisms that drive them, are largely unknown.
The warm water in the west produces rain clouds which form over New Guinea and start the wet season. While this air rises, the drier and cooler water from the South American water rush in to fill in the empty space left by the rising air, which brings rain to the west. This cycle strengthens the trade winds which start the process all over again.
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