Figure 1
Just two decades ago, it was a phenomenon known mostly by Peruvian fishermen and climatologists, today it has become a household word. Though undeserving of all the blame it gets for virtually every incident of unexpected and severe weather we receive, it does have a major impact on our weather and seasons, no matter where we live in the world.

The term El Nino (Spanish for the Christ Child) was first used by fishermen off the West Coast of South America to refer to the warm ocean current that typically appears around Christmas time and lasts for several months. Fish tend to be less abundant in the warmer water so the fishermen typically return to port and use the time to repair their boats and nets and spend more time with their families. In some years, the warming becomes much greater and lasts longer with much more profound affects in the tropical Pacific, but worldwide. Over the years, the term El Nino has come to refer to these exceptional warm events.

The El Nino is a phase of the Southern Oscillation, one of many "see-saw" patterns observed in the global atmosphere. In the Pacific, the normal surface weather pattern is controlled by warm high pressure in both hemispheres located over the warm subtropical oceans north and south of the equator. These very dependable pressure systems cause the trade winds to blow towards the equator. Off the northwest coast of South America, these winds blow from the southeast and cause surface water to move west away from the land. The water is replaced by cold water from the deeper ocean, a phenomenon known as upwelling.

The deep, cold water is rich in oxygen and nutrients which causes plankton to bloom which attracts a great variety of sea life including fish, fur seal, sea lion and sea birds. Fishing fleets from many regions are attracted to this, one of the world’s richest fishing areas.

These trade winds are among the steadiest on earth, with changes that are usually slow and small. This is because the pressure patterns that cause the trade winds are among the most reliable and predictable on earth. Though changes are small and slow, they do play a key role in the El Nino phenomenon.

As early as the 1920s, it was noticed that when the pressure became unusually high in the eastern tropical Pacific, it tended to become unusually low in the west and vice-versa. It was also noticed that each phase of the oscillation lasted about a year. This see-saw pattern was termed The Southern Oscillation.

One of the phases is called the La Nina, the less famous cousin of the El Nino. When pressures rise in the eastern Pacific relative to the western Pacific, the trade winds become stronger than normal and the upwelling is enhanced. Even cooler than normal ocean waters result and the east winds near the equator help move this cold water westward along the equator. This cool water phenomenon is called the La Nina. It is the opposite, less well-known cool phase of the Southern Oscillation.

Figure 2
The El Nino is the warm water phase. During El Nino years, pressures rise in the western Pacific and fall in the east. The easterly trade winds weaken (and usually in the western half of the Pacific actually reverse to become westerly). As the winds diminish, warmer sea-water surges eastward and sea levels rise in the eastern Pacific and fall in the western parts of the Pacific basin. As the warm water accumulates in the eastern Pacific, it pushes the cold water down. The weakened upwelling is unable to tap colder water and the sea and air temperature rises accelerate.

Figure 3
The El Nino can have a disastrous affect on marine life which in that area thrives on the cold, nutrient rich water. The fish, sea bird, seal and sea lion populations were greatly diminished by the strong El Ninos of 1982/83 and 1997/98.


El Ninos or La Ninas events occurred in about 80% of the winters in the last 40 years. In other years, the Southern Oscillation is considered neutral. When the La Nina and the El Nino occur, they have been shown to have significant effects not only on areas closest to the affected waters but World-wide.

In the El Nino, the warming of the water often causes heavy rains in normally dry areas of South America near the coasts of Ecuador and Peru. In these areas in the Great El Nino of 1982/83, over 100 inches of rain fell in a 6-month period, transforming coastal deserts into a lush grassland peppered with lakes and abundant new animal life. Offshore, the opposite occurred. The fishing industry suffered the loss of the anchovy harvest and sardines moved south to chillier Chilean waters.

That winter, typhoons and hurricanes in the Pacific left their normal tracks and hit Hawaii and Tahiti. Droughts and disastrous fires occurred in Indonesia and Australia. In the U.S., powerful storms battered California and all across the southern states. All told the damage that winter worldwide exceeded $8 billion.

The Great El Nino of 1982/83 sparked considerable research into the relationships between these El Ninos and La Ninas and weather patterns and anomalies of temperatures and precipitation World-wide. Some clear and surprising relationships were found.

In El Nino years, during the winter season, researchers found it typically was warmer and drier than normal weather in the northwestern and north central states while it tends to be wetter and cooler than normal in the southeastern states. Wet weather also typically follows in southeastern Brazil and Argentina, and parts of east central Africa and the islands of the central tropical Pacific. Meanwhile abnormally dry weather and sometimes serious droughts occur in much of the western Pacific from Australia to Indonesia, in parts of northern South America and Central America, in the interior of India and southeast Africa. During the El Nino Northern Hemisphere summers, the tropical activity is enhanced in the eastern Pacific but suppressed in the Atlantic.

In the La Nina, in general the opposite patterns are observed. It is dry and warm in the southeastern U.S., cold and wet (often snowy) in the Pacific Northwest into Southwestern Canada. It is wet in much of Australia and Indonesia, India, southeast Africa, northern South America and the southern Caribbean. Dry weather is experienced in the islands of the central Pacific, east central Africa and southern Brazil and Argentina. During La Nina summers, tropical activity is suppressed in the eastern Pacific but enhanced in the Atlantic and Caribbean. Both the East Coasts of the United States and of Central America are vulnerable to strong Atlantic hurricanes.

The Great El Nino of 1997/98 lived up to all the hype it received. Devastating droughts and forest fires affected Indonesia, parts of Brazil and Mexico. Record rainfall fell in California and Florida while a heat wave and drought desiccated Texas. The huge area of water water in the pacific helped warm the weather around the entire world, making 1998 the warmest year since record keeping began, according to NOAA (the United States National Oceanic and Atmospheric Administration).

A strong La Nina event followed in 1998/99 bringing with it, bitter cold weather in Alaska, all-time record rains and snows in the Pacific Northwest, heavy snows in the Great Lakes and a very active tornado season. Drought and forest fires replaced flooding rains in Florida. Around the world, the La Nina brought an end to the drought in Indonesia, Australia, Mexico, Central America, and Brazil and heavy rains and floods to Columbia and Bangladesh.

Figure 4
These changes are just part of the continuing cycle of changing weather that we have experienced all our lives. However, it now has a name…El Nino.


El Nino was a term originally used by 19th century fisherman to describe a southward moving current of warmer water off the coast of Peru and Ecuador that occurred each year around or shortly after Christmas. The term El Nino means "the Infant Boy" referring to "the Christ Child". In some years, the warming is more extensive and can last a year or two. Today we use El Nino to describe these large-scale, long-lasting "warm" events. El Nino is characterized by large-scale weakening of the trade winds and warming of the surface layers in the eastern and central equatorial Pacific waters. These events occur irregularly every 2 to 7 years (averaging every 3 to 4 years).

A Warm Event (or alternately Warm Phase of ENSO or alternately ENSO Warm Event) is an El Nino or an anomalous warming of the waters in the eastern and central tropical Pacific. Warming of the waters there is often accompanied by a relative cooling of the waters in the western tropical Pacific. ENSO is an acronym that stands for El Nino/Southern Oscillation. The term is used to describe the full range of the Southern Oscillation including both the warm (El Nino) and cold (La Nina) events. Technically, it includes both the oceanic (warm or cool water) component and the atmospheric (Southern Oscillation) component of the phenomena. Sometimes the term has been incorrectly used to refer only to the warm events or El Ninos.

La Nina (in Spanish meaning "the Infant Girl") is in many respects the opposite of El Nino. It has also been called El Viejo ("the Old One") and the Anti-El Nino, but this term has fallen into disfavor, as it would imply an "anti-Christ". This cool relative of El Nino is characterized by a large scale strengthening of the trade winds and a cooling of the water in the eastern and central tropical Pacific due to upwelling of cool water from beneath. Like El Ninos, La Ninas often begin in the summer of the Southern Hemisphere (December to February) and may last a year or two. Like the El Nino, the La Nina has occurred irregularly between 2 and 7 years, averaging once every 4 or 5 years.

A Cool Event (or alternately Cool Phase of ENSO or alternately ENSO Cool Event) is a La Nina or an anomalous cooling of the waters in the eastern and central tropical Pacific. Cooling of the waters there is often accompanied by an accumulation of warm water in the western tropical Pacific. The Southern Oscillation is an inter-annual see-saw in sea-level pressure across the tropical Pacific closely linked with El Nino and La Nina. It was first documented and named by Sir Gilbert Walker in the 1930s. He noted there was an inverse relationship between surface air pressures at two sites: Darwin, Australia and the South Pacific Island of Tahiti. High pressure at one site is almost always concurrent with low pressure at the other and vice versa. It represents a standing wave or "see-saw", a mass of air oscillating back and forth in the tropical and sub-tropical Pacific. In Walker's Southern Oscillation Index (SOI), the pressures at both locations are "normalized" (relative to normal) in order to account for normal seasonal differences.

In El Ninos, unusually high pressure typically develops in the western tropical Pacific while pressures in the eastern tropical Pacific become unusually low (a negative SOI). This causes a weakening in the trade winds, which can reduce the cool water upwelling. This leads to a subsequent warming of the waters in the eastern tropical Pacific. These changes in pressures, winds and water temperatures cause the tropical showers that normally are found in the western tropical Pacific to shift east to the east central Pacific and in extreme events all the way to the coast of Peru.

In La Ninas, pressures rise in the eastern Pacific while they fall in the western Pacific (a positive SOI). This causes an increase in the trade winds, enhanced upwelling and a cooling of the waters in the eastern Pacific. These changes cause a suppression of showers in the eastern tropical Pacific and a further enhancement of the normal rainfall in the western tropical Pacific.

The Multivariate ENSO Index (MEI) Climate Diagnostics Center developed Multivariate ENSO Index (MEI) to provide a new comprehensive data set for measuring ENSO that consider both the atmospehatmosphericeric and oceanic components of this coupled phenomenon. ENSO is the most important coupled ocean-atmosphere phenomenon to cause global climate variability on inter-annual time scales. The Multivariate ENSO Index (MEI) is based on the six main observed variables that are different during the two phases of ENSO in the tropical Pacific. These six variables are:
  • Sea-level pressure
  • Both components of the surface wind
    • East-West (zonal)
    • North South (meridional)
  • Sea surface temperature
  • Surface air temperature
  • Total cloudiness fraction of the sky

Positive values of MEI represent the warm ENSO phase or El Nino, while the negative values of the MEI represent the cold ENSO phase or La Niña.

AnimationSea Surface Temperature Anomalies are departures from normal of ocean temperatures. Usually measured at the surface by ships, fixed buoys, satellite and even aircraft. Sea surface temperature anomalies in the tropical Pacific accompany both El Nino and La Nina and are typically opposite in sign. In El Nino, the water in the eastern and central Pacific is warmer than normal while water in the western Pacific tends to be colder than normal. In La Ninas, the eastern and central tropical Pacific waters are colder than normal while waters in the western Pacific are warmer than normal.

Trade Winds are generally the most dependable and steadiest winds on the earth. The trades are found in the subtropics between a belt of clear skies and high pressure called the subtropical high (centered in the oceans around 30 to 40 N and S) and the zone of low pressure, clouds and showers near the equator called the Intertropical Convergence Zone. The trade winds blow from the northeast in the Northern Hemisphere and southeast in the Southern Hemisphere. The trade winds are often referred to as the Easterlies.

In La Ninas, the trade winds blow stronger than normal, which enhances the cold water upwelling near the South American coast and carries the resulting cold surface water westward along the equator. These strong trades also act to pile up warm water in the western Pacific.

In El Ninos, the trade winds weaken and may reverse in the western Pacific. This allows warm water to "slosh" eastward in the Pacific Basin. The weakened trade winds reduce the upwelling of cold water near the South American Coast and water warms at the surface. When the warm water arrives from the west, sea levels rise and the depth of the warm water layer near the surface deepens. The warm water appears as a characteristic plume extending west along the equator to near or past the International Dateline.

Tropical Easterlies
The Trade Winds from both hemispheres converge on the equatorial region. The tropical Easterlies result. The easterlies tend to be strongest in cold La Nina events. The easterlies can reverse and become westerlies in warm El Nino events, at least in the western half of the Pacific Basin. The strength of the easterlies are monitored closely as changes might signal a change of phase of ENSO. ITCZIntertropical Convergence Zone (ITCZ) is the belt of clouds and showers that oscillates seasonally around the equator. It results from a convergence of the trades winds from both hemispheres. The relative strength and level of activity associated with the ITCZ varies considerably in the El Nino and La Nina years. In El Ninos, the ITCZ is anomalously active in the eastern and central Pacific but anomalously weak in the west. In La Ninas, the opposite is observed.

Hadley CellHadley Cell is the three-dimensional circulation of air in the tropics and subtropics. Air rises in the Intertropical Convergence Zone (ITCZ) and then moves poleward at high altitudes it converges in the subtropics with air moving equatorward from higher latitudes and sinks in the Subtropical High belt. The air then moves equatorward as the trade winds only to converge and rise again at the ITCZ. This circulation helps produce the clouds and rainfall and low pressure of the ITCZ and the sunshine, high temperatures and high pressures of the subtropical high zone.

The Walker Circulation is another three-dimensional circulation in the tropical Pacific associated with El Nino and La Nina. It was proposed by the renowned meteorologist Jacob Bjerknes in 1969 and named in honor of Sir Gilbert Walker who first documented the El Nino phenomenon. In El Nino the air sinks in the western areas moves eastward and rises in the central (and in stronger El Ninos the eastern) Pacific only to return westward at higher altitudes. The result is an eastward displacement of the clouds and showers, In La Nina, the pattern reverses as air sinks in the eastern Pacific and moves westward to rise in the western Pacific. In La Nina, the cloudiness and showers are confined mostly to the western tropical Pacific.

Upwelling is the rising of cold water from the deeper ocean to the surface caused by the removal of surface water by the wind. When the action of the wind causes surface water near the land to move away from the coast, it must be replaced by water from beneath. Since deeper water is usually colder water, the result is that the upwelling produces a cooling of the surface. The trade winds near the South American coast act to produce upwelling. This upwelling is enhanced in La Ninas because the trade winds are stronger than normal. The upwelling is diminished in El Ninos as the trade winds diminish.

AnimationThe Thermocline is the oceanic equivalent of the inversion in the atmosphere. It is the boundary between well-mixed warm water near the surface and deeper colder water. It is normally about 40 meters (130 feet) deep in the eastern Pacific but varies between 100 and 300 meters (330 to 660 feet) deep in the western Pacific. The thermocline varies considerably across the tropical Pacific in the El Nino and La Nina. In ENSO events, the thermocline is directly proportional to the departure from normal of sea level. A deeper than normal thermocline is usually associated with above normal sea levels. A shallower than normal thermocline meanwhile is typically associated with below normal sea levels

In El Ninos, the thermocline is much deeper than normal in the eastern Pacific (the warm water layer is deeper than normal) while it is usually shallower than normal in the western Pacific.

In La Ninas, the thermocline is shallower than normal in the Eastern Pacific and deeper than normal in the west.

Mean Sea Level Height is not really constant. It can vary due to the action of the wind. Across the Pacific Basin it varies nearly 2 feet (23 inches) on average between the western Pacific (the Philippines) and the eastern Pacific (Panama coast) between the two phases of ENSO. During La Ninas, the enhanced trade winds push water westward along the equator and pile it up in the western areas. In El Ninos, as the trade easterlies weaken, water sloshes eastward and sea levels rise in the eastern Pacific and fall in the western Pacific. This drop in sea level can cause exposure damage to the sensitive coral in the western Pacific.

Convection is a process of heat transfer involving vertically moving air currents. It is most favored in unstable air where low level warm air becomes buoyant and rises. Cooler, heavier air from above sinks to replace it. Convection results in clouds and showery type precipitation. In La Ninas, the convection is confined mainly to the western Pacific. In El Ninos, the convection in the tropical Pacific shifts eastward which has an effect on atmospheric jet streams and storm tracks.