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