IV. Climate Phenomena

1. El Niño

The term El Niño comes from Spanish, meaning "Christ Child". Peruvian fishermen used this term to describe the phenomenon of abnormal sea temperature and ocean current changes in sea areas near the tropical Pacific region during Christmas time.

Under normal climate conditions, the atmospheric pressure of the east Pacific is higher than the west. This difference of atmospheric pressure generates the tropical easterlies and drives the east Pacific Ocean current westward. After the westbound ocean currents are heated by the sun they will gather in the central and western Pacific. In the east Pacific, the low temperature sea water in the deep ocean will flow up to replace the westbound ocean currents. This upwelling current is rich in nutrients, and therefore will attract a lot of fish. This is the cause behind the booming success of the fishery industry in Peru and other nations in the region. The excrement from sea birds accumulated here while looking for the fish also serves as a main source of fertilizer for the agricultural industries in the region.

During El Niño, the atmospheric pressure in the east Pacific lowers while the pressure in the west increases. This sudden difference in atmospheric pressure reduces the strength of the Tropical easterlies even turning them into westerlies. The ocean current of the east Pacific will therefore no longer flow west, but rather east and collect in the eastern Pacific region after temperatures have risen by heating from the sun. This in turn results in higher east Pacific temperature and lower temperatures in the west. The warmer sea water in the east Pacific prevents the deep sea water in the area from rising. As a result the number of fish in the current is also reduced and with it the number of sea birds, creating substantial damage to both the fishing and agricultural industries in the region.

The characteristics of El Niño are a reverse of sea temperature differences between the east and west Pacific. An accompanying effect to the atmosphere is the east-west oscillation of the pressure fields. When the sea temperature is higher in the east Pacific, the pressure of the atmosphere will be higher in the west. Conversely if the sea temperature is higher in the west, then the atmospheric pressure will be higher in the east. Since meteorologists indicate the difference of these two atmospheric pressures by measuring the pressures at Tahiti in the south Pacific and Darwin in Australia, they call this oscillation of pressure fields "Southern Oscillation". Since El Niño and Southern Oscillation are closely interrelated atmospheric and oceanic phenomena, they are sometimes abbreviated as ENSO.

The process of abnormal climate changes caused by El Niño can be explained by the following simple physical process. When sea temperature rises, it heats up the atmosphere like the fire in a stove heating up water in a pot. During El Niño, when the sea temperature of the east Pacific rises abnormally, the atmosphere above the ocean surface will be heated by the water vapor from the ocean and the air flows upward. After the effects of convection it will become clouds and rain. To balance the rising effect in the east Pacific region, the air in the west Pacific region with lowered sea temperature will therefore flows downward, increasing the air pressure at the ground surface and therefore suppressing rain. The effects the changes of climate during an El Niño can be summarized as follows: The increase of sea temperature, the rise of air and the reduction of ground surface pressure in the east Pacific region, will increase the chances of rain and of floods. In the west Pacific the situation is reversed, which can result in droughts especially in Indonesia, the Philippines, and north Australia.

El Niño phenomenon takes place once every two to seven years. El Niño usually has a life span of 1.5 to 2 years, which is divided into an early stage, mature stage, and decay stage. In the past 20 years there have been four significant occurrences of El Niño, in which the one in 1982-83 created the greatest difference of sea temperature and caused the most serious damage. According to estimation, the droughts and fires induced which occurred in this period in South Asian countries and Australia caused a total damage of US$3.5 billion. On the other hand, the floods also caused American countries in the eastern Pacific region US$2.5 billion in damage. These are by no means small damage.

Due to the severe damage El Niño can cause to human life and property, weather centers in the world are all devoting themselves to El Niño-related research. The Central Weather Bureau has also promoted the "short-range climate forecast" research project. In its current stage, the focus is on analyzing and evaluating the characteristics of climate change that El Niño causes to Taiwan, as well as the predictability of such. The utmost goal is to establish a forecast operation on short-range climate changes in the Taiwan region.

If you want to know more about El Niño, please refer to these books or websites:

1. Ming-Chin Wu, 1986: "El Niño/Southern Oscillation", Science Monthly VoL17, No.2, 107-112.
2. SCIENCE 1983: Vol222, No. 4629, 1189-1210.
3. S. George Philander, 1990: El Niño, La Niña, and the Southern Oscillation, Academic Press, pp. 293.
4. Climate Prediction Center, NOAA, USA ( http://www.ncdc.noaa.gov )