Equatorial Currents and El Niño

Definition

"El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific having important consequences for weather and climate around the globe."—NOAA.

The more exact technical definition is:
El Niño exists when the 5-month running mean of sea-surface temperature anomalies in the region 5°N - 5°S, 120°W - 170°W exceeds 0.4°C for six months or longer. (Trenberth, 1997).

What Happens During an El Niño?

The best description of the El Niño is found on the What is an El Niño? web page published by the NOAA Pacific Marine Environmental Laboratory.

During Normal Conditions
Trade winds push water on the equator westward, piling it up against Asia. The warm water in the west Pacific warm pool fuels convection in the atmosphere, which leads to rain warming the atmosphere when latent heat is released. The heating drives a convective loop, the Walker Circulation, which helps maintain the trade winds. The convection is also a major driver of the global atmospheric circulation and the Hadley Cell.

In the west:

1. Sea surface temperatures are warm,
2. Air pressure is low, and trade winds blow strongly from the east,
3. The pool of warm water is deep, and
4. Heavy rain falls in the far western tropical Pacific and over New Guinea and northern Australia.

Average rain rate in January 2000 measured by the Tropical Rain Measuring Mission satellite. Click on the image for a zoom. From Tropical Rain Measuring Mission office at the NASA Goddard Space Flight Center.

In the east:

1. Sea surface temperatures are cold on the equator,
2. Air pressure is high,
3. The atmosphere is very stable, and
4. Coastal areas of Peru and Ecuador are cool and dry with stratus clouds.



Two states of the equatorial pacific. Top: Normal or La Nina conditions with heavy rain in the west. Bottom: El Niño conditions with heavy rain in the central equatorial Pacific. Colors give temperature of the ocean surface, red is hottest, blue is coldest.
From: NOAA Pacific Marine Environmental Laboratory.

During El Niño Conditions
If the trade winds weaken, or reverse in the west, the warm water can surge eastward on the equator, moving the eastern edge of the warm pool toward the central equatorial Pacific, the red area in the image above. The rain follows the warm water eastward. Because rain heats the atmosphere and drives the atmospheric circulation, the main heat source driving the atmospheric circulation moves eastward. As the heat source (the rain) moves eastward toward the central equatorial Pacific, changes in the convection weakens the trade winds in the east, leading to a strengthening of the El Niño conditions.

In the west,

1. The pool of warm water is shallower.
2. Air pressure is low, and trade winds are weak, or sometimes reversed.
3. Warm sea-surface temperatures extend far into the central equatorial Pacific.
4. Heavy rain falls in the central and eastern tropical Pacific.
5. Little rain falls on New Guinea and northern Australia.

Average rain rate in January 1998, an El Niño year, measured by the Tropical Rain Measuring Mission satellite. Notice that the rain has shifted from the western Pacific and eastern Indian ocean to the central Pacific. Click on the image for a zoom. From Tropical Rain Measuring Mission office at the NASA Goddard Space Flight Center.

In the east,

1. Sea surface temperatures are warm, up to 3°C warmer than normal,
2. Air pressure is low,
3. The atmosphere is warm and unstable, and
4. Rain falls on coastal areas of Peru and Ecuador.

Warm water surges up the east and west coasts of the Americas, leading to above average rains in California.

Sea-surface temperature anomalies during the strong 1997 El Niño. Note the warm temperatures along the west coasts of the Americas. Click on image for a zoom.
From US Navy Fleet Numerical Meteorological and Oceanography Center.

The oscillation of surface atmospheric pressure between the western and eastern Pacific was first noticed by the meteorologist Sir Gilbert Walker in the early decades of the 20th century. He found that that pressure fluctuations throughout that equatorial Pacific are highly correlated with pressure fluctuations in many other regions of the world. He found that the two strongest centers of the variability are near Darwin, Australia and Tahiti. The fluctuations at Darwin are opposite those at Tahiti, and resemble an oscillation. Furthermore, the two centers had strong correlations with pressure in areas far from the Pacific. Walker named the fluctuations the Southern Oscillation. El Niño is often referred to as the El Niño Southern Oscillation, or ENSO.

End of El Niño

There are several reasons for the end of El Niño. All lead to a strengthening of the trade winds, which pushes warm water back toward Asia. Rain follows the warm water, the Walker Circulation strengthens, further weakening El Niño. Soon, within a month or two, the circulation returns to normal.

La Niña
After the end of El Niño, the trades may continue to strengthen, leading to La Niña conditions with stronger than normal trade winds and colder than normal water at the surface in the eastern equatorial Pacific.

La Niña conditions in the Pacific.
From: NOAA Pacific Marine Environmental Laboratory.

An El Niño Animation

The NOAA Climate Diagnostics Center has an online Animation Of An Idealized El Niño/La Niña Cycle in the Pacific showing anomalies of sea-surface height (the grid in the animation) and anomalies of sea-surface temperature (the color of the grid). The weakening of trades in the western equatorial Pacific causes warm water in the upper layer of the equatorial region to move eastward, leading to higher sea level and warmer water in the eastern equatorial Pacific. The wave of higher sea level (called a Kelvin wave) reflects off South America, and returns to the west at latitudes north and south of the equator.

El Niño animation of anomalies of sea-surface height (vertical scale, in centimeters) and anomalies of sea-surface temperature (color scale in degrees Celsius) in the tropical Pacific during an idealized El Niño/La Niña cycle..
Produced by Joe Barsugli at the NOAA Cooperative Institute For Research In Environmental Sciences.

Some Consequences of the El Niño/La Niña Cycle

A complete El Niño cycle results in a net heat discharge from the tropical Pacific toward higher latitudes. At the end of the cycle the tropical Pacific is depleted of heat, which can only be restored by the slow accumulation of warm water in the western Pacific by normal trade winds. Consequently, the time scale of the Southern Oscillation is given by the time required for the accumulation of warm water in the western Pacific. Its release is triggered by fluctuations in the tropical atmosphere.—Klaus Wyrtki 1985.

Changes in Rainfall

As surface temperature patterns change at the sea surface, rain patterns also change. Rain tends to stay centered over the warmest water. Below is an image showing the difference in rain patterns in the Pacific in El Niño years compared with La Niña years.

This plot shows the change in rainfall in the Pacific during a typical El Niño year compared with a typical La Nina year using all data from 1979 to 2001 collected by the Global Precipitation Climatology Project by Scott Curtis and Robert Adler. Notice that the area of high rainfall in the equatorial Pacific shifts eastward by more than 65° during an El Niño event. Thus the major heat source for driving the atmospheric circulation also shifts east, causing rain elsewhere around the world to change.
From Curtis and Adler (2002), for more information read Curtis and Adler (2003).

Not All El Niños Are The Same

El Niño can be strong or weak. Some have small changes in water temperature in the Niño 4 region, but heavy rains in the central tropical Pacific, others have larger changes in temperature with less of a change in rain. The El Niños of 1981–1982 and 1997–1998 were very strong, sometimes called the El Niños of the century. The El Niño of 1957–1958 had weaker temperature anomalies than the El Niño of 1972–1973, but it produced much large changes of weather patterns. The El Niño of 1976–1977 was weak. The El Niños of 1982–1983 and 1997–1998 were strong.

The NOAA Climate Prediction Center publishes a table of the Oceanic Niño Index starting in 1950 showing the anomaly of sea-surface temperature in the nino 3.4 area (5°N - 5°S, 120° - 170°W). Cold temperature in blue indicates a La Niña, red indicates an El Niño. The larger the anomaly, the larger the influence of the Pacific on global weather patterns.

El Niño Multivariate Index since 1950 showing the variability of El Niño/La Niña. The index is based on six main observed variables over the tropical Pacific. These six variables are: sea-level pressure, zonal (east-west) and meridional( north-south) components of the surface wind, sea-surface temperature, surface air temperature, and total cloudiness fraction of the sky. Red is El Niño, blue is La Niña.
From NOAA/Climate Analysis Branch Multivariate ENSO Index (MEI) web page.

Additional Information

There is more information, at different levels of technical content, available on the web.

1. Begin with the introduction paragraphs to Chapter 14 on El Nino. You can read as much as you like, but it gets a little technical toward the end.
2. Teachers may want to see a simplified version for El Nino Basics for Kids and Adults at the Jet Propulsion Laboratory.
3. The National Academy of Sciences also has a good description of El Niño and La Niña: Tracing the Dance of Ocean and Atmosphere.
4. Remote Sensing Systems web page for TMI data. has many useful maps of ocean temperature, water vapor in the atmosphere, surface wind speed, liquid water in clouds, and rain rate. Click on Pre-rendered Images Monthly to bring up links to the maps. Use the scroll-down menu to select a month, then click on Update Display. Click on any of the images to bring up a much larger image. Here is the map of rain for January 1998, which you can compare with the similar map above.
5. NOAA has many maps of sea-surface temperature anomalies for the world and for selected regions, from 1996 to the present.
6. NOAA's Physical Sciences Division map room has the latest maps of sea-surface temperature and temperature anomalies.

References

Curtis, S. and R. Adler (2002). ENSO Related Precipitation Anomalies from the Tropics to the Extratropics. CLIVAR Exchanges 7 (1): 8–9, 13.

Curtis, S. and R. F. Adler (2003). Evolution of El Niño-precipitation relationships from satellites and gauges. Journal of Geophysical Research 108 (D4): 9-1 – 9.8.

Trenberth K.E. (1997). The definition of El Niño. Bulletin of the American Meteorological Society 78 (12): 2771–2777.

Revised on: 15 August, 2008