Now that you are very well aware of what normal conditions in the Pacific are, we can now explain what happens during an El Niño. If you still unsure about some of the normal conditions, please go back to the previous page.

The strength of the Trade Winds declines - The strength
of the trade winds declines which then reduces the force that
the winds are blowing from east to west

As we have previously stated, El Niño have a frequency of about three to five years. Even though scientists do not know the exact workings of an El Niño, they do have a pretty good idea of what occurs.

At one point, when the air pressure gradient declines as part of the southern oscillation, the southeast trades weaken. Slackening trade winds allow the warm surface water that had been driven westward to drift slowly eastward along the equator and then southward off the Peruvian coast. El Niño is in progress.

During an intense El Niño, the southeast trade winds eventually shift direction and become equatorial westerly. This phenomenon is technically knows as El Niño-Southern Oscillation or ENSO, but many people refer to it as simply El Niño.

Upwelling Decreases Strength - Upwelling, which was
such a strong force during normal conditions, now loses
strength and the great temperature difference is not more

The warm water near the South America coast is no longer warm because upwelling has decreased. Because of this, the water now increases its temperature by 3.5-5.5^o F. This has fatal effects on the living organisms in the food chain since less less nutrient rich water is now brought to the surface. Planktons are decreased in numbers, thus, bringing the rest of the food chain down with them.

The thermocline now falls almost 500 feet due to this effect. Sea levels and sea surface temperatures off the South American coast rise, and the themorcline is pushed down to a much deeper level.  Upwellings then occur entirely above the thermocline and this cuts off the supply of nutrient-rich water from the ocean floor. to the surface.

Deprivied of nutrients, plankton in the coastal waters of the south-eastern Pacific began to die.  Their population collapse affects the open-ocean [pelagic] fish that feed on them and that are the basis of the fishing industries of Ecuador, Peru, and Chile.  The fish shoals move in search of more nutritious waters.   Their location is upredictable and often cut out of range of the fishing vessels.   Cathces usually drop while El Niño conditions previal.

Go to our impacts section to learn more about this and other effects of El Niño.

Warm air creates clouds beginning the wet season -
The warm water in the west becomes water vapor which
rises and then condenses and form clouds

In normal conditions, rain clouds used to be over Australia. However, now they move towards the coast of South America bringing heavy rains and floods to Peru and Ecuador. This movement also has potential to bring drought to Australia. The equatorial trade winds now decrease greatly in strength and the winds to the west now start to change direction. Then begin to blow the opposite way and these winds start to become stronger than the trade winds.

Bjerknes had generalized brilliantly by saying that the continual Southern Oscillation, not just the event of 1957, is both the cause and the consequence of continually changing sea surface temperature patterns. This meant that El Niño, rather than an event that occurs sporadically, is but one phase of a cycle. In essentially the same the way that seasonal cycle in an oscillation between winter and summer, so the Southern Oscillation is a fluctuation between El Niño and a complementary state, which has been given the apposite name La Nina. La Nina features exceptionally strong trade winds and unusually low sea-surface temperatures (SST) in the central and eastern tropical Pacific. ENSO is not always followed by La Nina.

Thus, El Niño and the Southern Oscillation are just different faces of the same climate fluctuation that involve the ocean and the atmosphere. Bjerknes came to this realization in the 1950s; scientists qualified this only after the devastating effect of the 1982-1983 El Niño. However, research is far from over. For example, the persistence of warm El Niño conditions during the early 1990s was not anticipated by any El Niño models. Records of the 1920s and 1930s show the complete absence of an El Niño. Scientists are currently exploring the reasons for the variable intensity and irregular visits of El Niño.