What is El Niño and How it Affects the Weather

The weather is extreme in some places and beautiful in other places. Is it because of El Niño? What exactly is El Niño and what causes it?

Meteorologist use computer models and look for patterns to forecast the weather. There can be normal patterns and abnormal patterns. As meteorologists have learned more about these patterns, they found two distinct patterns in the Pacific Ocean. These are called El Niño and La Nina.

The Trade Winds

The oceans have normal wind patterns, with names like the trade winds, the doldrums, the easterlies and the westerlies. These normal wind patterns guided mariners centuries ago and maintain the climate and weather patterns. Any disruption in these patterns can cause a direct change in weather.

The trade winds are a normal flow of wind over the oceans. In the equatorial regions of the Pacific Ocean, the sun heats up the sea surface temperature (SST). Warm air rises and cooler air fills in the area. The warmer air moves towards the north in the northern hemisphere and south in the southern hemisphere and then curves back down again towards the equator. In the latitudes between 30 degrees north and 30 degrees south, these winds are called the trade winds and blow from an easterly direction (from the east to the west).

Looking at a map of the southern hemisphere, these normal trade winds would normally blow from Peru and Chile to the west towards Australia and Indonesia. The wind blowing to the west causes the hot surface water to also blow west. The water temperature can be as much as 8 º C warmer and 1 / 2 meter higher in Indonesia than in Ecuador or Peru. 

What Causes an El Niño

Fishermen along the west coast of South America have noticed El Niño conditions for centuries. Scientists look at what is called the Southern Oscillation Index (SOI) for signs that an El Niño might be starting. The Southern Oscillation Index is the difference between the surface pressure (barometer readings) between Tahiti and Darwin, Australia. Many times you will also see this abbreviated as ENSO (El Niño Southern Oscillation).

When looking at a chart of the SOI over the years, a negative value for a longer than normal time period usually indicates that an El Niño occurred that year, or could be starting. An El Niño year and a negative SOI means that the pressure at Darwin is higher than average and the pressure at Tahiti is lower than average. The differences between high-pressure areas and low-pressure areas can determine wind speed and direction. The trade winds can stall or actually change direction, blowing from west to the east, the opposite of a normal trade wind direction.

The hottest waters on the planet are in the Pacific Ocean near Indonesia. When the trade winds stall, this warm pool of water can move east towards the Americas on waves called Kelvin waves. These waves bring much warmer than average sea temperatures to the west coasts of both South America and North America. This movement of warmer water also caps and prevents what is known as upwelling, which prevents the colder water from deeper down moving up towards the surface.

El Niño usually peaks around Christmas time during the fishing season in Peru and it is believed that is where the name comes from. El Niño in Spanish means baby boy or the Christ Child.

Warm waters in yellow and red near South America

How El Niño Changes Weather Patterns and Animal Life

The wind, the oceans and the currents, when normal are a perfect loop for weather and the food chain for fish, animals and humans. But because of El Niño, the warmer surface water near South America caps the upwelling of the deeper colder waters. This disrupts the nutrients from the lower depths of the ocean from coming up to feed plankton. The plankton can die without these nutrients from the deeper depths of the ocean. Birds and fish that depend on the plankton for life cannot find plankton and have to leave the area to find food. Fish like sardines and anchovies disappear from the coast of South America and the fisherman suffer. During the strong 1982/1983 El Niño, 25% of the sea lion population died along the Peruvian coast. These conditions can also occur along the coastal areas of the Western United States.

The warmer water causes heat and moisture to rise into the atmosphere, which then travels throughout the currents of the atmosphere. More heat and moisture causes more convection (thunderstorms) and can cause large amounts of rain to fall in deserts while normally rainy areas can be in severe droughts.

The jet stream carries storms along its path and the El Niño can enhance and change the normal pattern of the jet Stream. A typical El Niño can cause a dip or trough in the jet stream. This dip can send moisture laden storms into California and the southern United States while sending warmer than usual air further north into Alaska, British Columbia and New England.. Sometimes the El Niño can cause the normal monsoons of India to not appear, causing droughts in India affecting crops.  

In the United States, the effects of El Niño are usually felt during the winter and is characterized with warmer than average temperatures in the Northwest and Midwest states. Summer storms can be more numerous and severe in the High Plains and Midwestern states.

The End of an El Nino and the Start of La Niña

An El Niño can last from 12 to 18 months and once in a while, the end of an El Niño can lead right into a La Niña, which is the opposite of an El Niño. It is cooler than average sea surface temperature of the Pacific Ocean along the equator. La Niña also affects the weather in the Unites States and globally.


With all of this data, scientists can look back at records and with a fair degree of accuracy predict if an El Niño year is starting. This can help prepare for flooding, droughts and severe storms in certain regions. These predictions can also predict if the hurricane and typhoon seasons might be less or more active in different regions.

There are some consistencies and inconsistencies with each El Niño. In the 1997 El Niño, New England had a mild winter and in the 2009/2010 El Niño, they were buried in snow. Factors other than El Niño can change yearly global weather. There is a link below to a list of El Niño years, you can look at the list and see how that years El Niño affected your local weather [1].

Scientists are starting to look at a possible relationship between an El Niño year and health related issues. One of the strongest El Niño years was 1918/1919, which was also the same year as the Spanish flu pandemic that killed more than 20 million people worldwide [2].

© Copyright Sam Montana 2010


[1] Previous El Niño Years

[2] El Niño link to Spanish flu 1918 epidemic

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