E-Learning Media Center

Weather Effects: El Nino... what is it?  What does it mean?

El Nino is a disruption of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe. Periodically, the flourishing fish populations commonly found off the west coast of Peru South America are replaced by the sight of dead fish littering the water and beaches. Unusual weather conditions occur around the globe as jet streams, storm tracks and monsoons are shifted. Such disarray is caused by a warm current of water that appears every three to seven years in the eastern Pacific Ocean called El Nino. This module introduces El Nino, conditions are responsible for its occurrence, plus the impact it has on the rest of the world.

• What is El Nino and how does it relate to the tropical Pacific?
• Where did the name El Nino come from? When was it discovered?
• Global Consequences of El Nino

What is El Nino and how does it relate to the tropical Pacific?

El Nino is an intermittent disruption of the climate system centered in the equatorial Pacific that has effects on short-term climate around the Pacific basin. To understand El Nino, it is necessary to understand the normal trade-wind system in the tropical Pacific.  The sun heats the equatorial regions more strongly than the rest of the globe, so air tends to rise from the surface there, replaced by inflow from the subtropics. The Corriolis effect turns these inflows to the right in the

Several hurricanes hit Florida in 2004 partially due to the warmer ocean waters caused by El Nino. Hurricane Frances, September 2004

northern hemisphere and to the left in the southern, resulting in the great trade-wind belts that blow equator ward and westward over the width of the the tropical Pacific. This sets up the coupled ocean-atmosphere interaction in the tropical Pacific in which the winds determine the water temperature but the water temperature also determines the winds, in a chicken and egg situation. In this system, we can start a description at any point in the cycle. For example, we observe that there is cool water in the east and warm in the west . The winds blow towards the warm water, since that heats the atmosphere and makes the air rise, then other air flows in to fill the gap. (These are the trade winds, that the Spanish used to sail from their colonies in South America to the Philippines). Because of the pressure of the trades, sea level at Indonesia is about 1/2 meter higher than at Peru. At the same time the trade winds act on the ocean as well. The westward winds along the equator push the warm water (heated by the sun) off to the west, drawing up the thermo cline and exposing the cooler water underneath in the east. This upwelling cools the eastern surface water, and we have returned to the starting place of the description.

During El Nino events, this entire system relaxes. The trade winds weaken, particularly west of the Dateline, and the piled-up water in the west sloshes back east, carrying the warm pool with it. The region of rising air moves east with the warm pool, and so does the pumping of heat and moisture into the upper atmosphere, distorting the usual paths of the jet streams, which eventually causes the changes in weather around the world. With weakened trade winds, the upwelling in the east correspondingly weakens; as the warm pool moves east the upwelled water is also not as cool as during normal periods. When eastern SST becomes warm the east-to-west temperature contrast is small, and so the trade winds weaken even further, leading to a complete collapse with essentially flat conditions across the entire equatorial Pacific.

We don't know what initiates El Nino.

There are two main theories at present. The first is that the event is initiated by the reflection from the western boundary of the Pacific of an oceanic Rossby wave (type of low-frequency planetary wave that moves only west). The reflected wave is supposed to lower the thermo cline in the west-central Pacific and thereby warm the SST by reducing the efficiency of upwelling to cool the surface. Then that makes winds blow towards the (slightly) warmer water and really start the event. The nice part about this theory is that the Rossby waves can be observed for months before the reflection, which implies that El Nino is predictable.

The other idea is that the trigger is essentially random. The tropical convection (organized large scale thunderstorm activity) in the rising air tends to occur in bursts that last for about a month, and these bursts propagate out of the Indian Ocean (known as the Madden-Julian Oscillation). Since the storms are geotropic (rotating according to the turning of the earth, which means they rotate clockwise in the southern hemisphere and counter-clockwise in the north), storm winds on the equator always blow towards the east. If the storms are strong enough, or last long enough, then those eastward winds may be enough to start the sloshing. But specific Madden-Julian Oscillation events are not predictable much in advance (just as specific weather events are not predictable in advance), and so to the extent that this is the main element, then El Nino will not be predictable.

The most severe effects of El Nino are found close to the equator.  But definitely affects the whole world. There is no doubt that the entire ocean-atmosphere system is interconnected, and each El Nino event occurs on the background of existing conditions, which includes the positions of the mid-latitude jet streams as an important component.

In a complicated system like this, each feature fills a role in the grand scheme of things. We cannot say exactly what the role of El Nino is, but we do observe that these events drain the west Pacific of heat that is built up over several years by the trade winds. In any case El Nino does not exist in isolation, and any changes in it would reverberate around the whole system in unpredictable ways. Further, as part of the natural environment of the Pacific basin that the animals, fish, birds and plants have adapted to over the millennia (we know that El Niño's have been occurring as far back as we can see), it is not clear that stopping El Nino would even be desirable. Even if it were possible to make El Nino disappear, we have no idea what the result would be. Very possibly we wouldn't like it!
Back to top

Where did the name El Nino come from? When was it discovered?

The name El Nino (referring to the Christ child) was originally given by Peruvian fisherman to a warm current that appeared each year around Christmas. What we now call El Nino seemed to them like a stronger event of the same type, and the usage of the term changed to refer only to the irregular strong events. It wasn't until the 1960s that it was widely realized that this was not just a local Peruvian occurrence, but was associated with changes over the entire tropical Pacific and beyond.

The name El Nino now refers to the warm phase of a large oscillation in which the surface temperature of the central/eastern part of the tropical Pacific varies by up to about 4°C, with associated changes in the winds and rainfall patterns. The complete phenomenon is known as the El Nino/Southern Oscillation, abbreviated ENSO. The warm El Nino phase typically lasts for 8-10 months or so. The entire ENSO cycle lasts usually about 3-7 years, and often includes a cold phase (known as La Nina) that may be similarly strong, as well as some years that are neither abnormally hot nor cold. However, the cycle is not a regular oscillation like the change of seasons, but can be highly variable in strength and timing. At present we do not fully understand what causes these changes in the ENSO cycle.
Back to top

Precipitation anomalies during El Nino in Summer: The twists and turns in the ongoing dialogue between ocean and atmosphere in the Pacific can have a ripple effect on climatic conditions in far flung regions of the globe. This worldwide message is conveyed by shifts in tropical rainfall, which affect wind patterns over much of the globe. Imagine a rushing stream flowing over and around a series of large boulders. The boulders create a train of waves that extend downstream, with crests and troughs that show up in fixed positions. If one of the boulders were to shift, the shape of the wave train would also change and the crests and troughs might occur in different places.

Dense tropical rain clouds distort the air flow aloft (5-10 miles above sea level) much as rocks distort the flow of a stream, or islands distort the winds that blow ovser them, but on a horizontal scale of thousands of miles. The waves in the air flow, in turn, determine the positions of the monsoons, and the storm tracks and belts of strong winds aloft (commonly referred to as jet streams) which separate warm and cold regions at the Earth's surface. In El Nino years, when the rain area that is usually centered over Indonesia and the far western Pacific moves eastward into the central Pacific, as shown on p. 17, the waves in the flow aloft are affected, causing unseasonable weather over many regions of the globe.

Winter: The impacts of El Nino upon climate in temperate latitudes show up most clearly during wintertime. For example, most El Nino winters are mild over western Canada and parts of the northern United States, and wet over the southern United States from Texas to Florida. El Nino affects temperate climates in other seasons as well. But even during wintertime, El Nino is only one of a number of factors that influence temperate climates. El Nino years, therefore, are not always marked by "typical" El Nino conditions the way they are in parts of the tropics.
Back to top