What are the causes and effects of ENSO?

• interpret satellite images and maps to draw conclusions about the physical processes producing ENSO;
• explain how, when, and where ENSO events occur;
• give examples of how ENSO events affect humans in different places; and
• use geographic information to develop national plans and investments to prepare for ENSO events in Peru and other places.

1. How does a change of energy in one sphere of the Earth System affect another sphere?
2. How the energy transferred among the various spheres of the Earth System?
3. How is matter shared within the various spheres of the Earth System?
4. What other parts of the Earth System may experience the effects of El Nino and how?  
5. What phenomena are associated with changes in different spheres of the Earth System? What variables appear to be correlated?

See NASA Mission Geography's Background on ENSO here.

Part 1. What is ENSO?

Atmospheric and oceanic variability affect the weather. Weather and climate change through complex links between the oceans and the atmosphere. Sources of variability in weather and climate around the world are changes in water currents, atmospheric pressure, and temperature in the oceans, especially the Pacific Ocean. These changes in the Pacific are often referred to as the El Niño Southern Oscillation, or ENSO. Two types of changes are referred to as El Niño and La Niña.

El Niño is the name given to the occasional warming of surface waters in the central and eastern equatorial Pacific Ocean. Under normal conditions, easterly trade winds blow from east to west along the equator and push warm surface water to the western tropical Pacific, where it piles up near Indonesia and the Australian continent. The persistent easterly trade winds are key ingredients of ENSO because they push warm water toward the western Pacific. This gives that area the warmest ocean temperatures on Earth. Usually above 28 degrees C (82 degrees F), parts of this pool are sometimes as warm as 31.5 degrees C (89 degrees F). Because this large pool of warm water is pushed towards the western Pacific, the atmosphere above the ocean is heated, causing favorable conditions for convection and precipitation. In fact, the persistent oceanic heat surrounding Indonesia and other western Pacific islands lead to frequent thunderstorms and some of the heaviest rainfall on Earth.

As the easterly trade winds push warm surface water against the western boundary of the Pacific Ocean, colder, nutrient-rich water comes up from below along the coast of South America to replace it. This is called upwelling. Upwelling helps fish and other animals thrive. In addition, as the warm surface water moves westward, the layer dividing the warm surface water and deep cold water, known as the thermocline, is raised. Because warm water contains more volume than colder water, and because trade winds push the water westward, the sea level is higher on the western side of the Pacific. In fact, the sea level in the Philippines is normally about 60 centimeters (23 inches) higher than the sea level on the southern coast of Panama.

In the upper levels of the atmosphere, the winds blow from west to east, completing a large-scale atmospheric circulation known as the Walker Circulation, named after Sir Gilbert Walker who studied variations in the tropical Pacific atmosphere during the 1920s.

In an El Niño year, which typically occurs every three to seven years, the normal trade winds diminish and the warm pool of water in the western Pacific is free to move back along the equator toward the east and the South American continent. The sea level drops in the west and rises in the east as warm surface water surges along the equator in the form of a pulse, or a Kelvin Wave. In addition, the upwelling of cold water along the South American coast decreases, reducing the supply of nutrients to fish and other animals.

This displacement of warm water affects the atmosphere. The convection and precipitation that previously occurred in the western Pacific shifts with the warm pool to the central and eastern Pacific and usually result in heavier than normal rains over areas such as northern Peru, Ecuador, and other areas in tropical South America. In the western Pacific, the mechanism for precipitation is shut off, and Indonesia and Australia will often experience drought conditions while an El Niño persists.

In a La Niña year, unusually cold ocean temperatures occur in the equatorial Pacific, which is the opposite of an El Niño. Generally, during a La Niña, the easterly trade winds increase in intensity, more upwelling occurs, and the water temperatures along the equator are reduced. This usually results in less cloudiness and rainfall for South America and more rainfall over Indonesia, Malaysia, and northern Australia.

Part 2. How is ENSO measured?

In order to both understand how ENSO works more fully and predict its effects, better measurements of ocean conditions and climate change are needed. Scientists have begun a number of different measurement projects to identify El Niño and La Niña. For example, space-borne sensors are helping to monitor variations of surface wind, sea level, and sea surface temperature along the equator and the west coasts of the American continents. Sea level measurements. NASA satellites can measure sea level for the entire Pacific Ocean within 5 centimeters (2 inches). Images from these satellites can tell us where the warm water is located in the Pacific Ocean because it takes up more volume, thus raising the level of the ocean. Upwelling measurements. Another way of measuring ENSO using satellites is by examining upwelling. Upwelling currents carry nutrients with them, which leads to phytoplankton growth and chlorophyll blooms. Satellites can detect the pigments of the phytoplankton from space. Ocean-based measurements. Another method of identifying when an ENSO event is starting is through measurements taken in the water of the Pacific Ocean. Scientists have created an extensive system of floating and moored buoys, tide gage stations, and ship-based observation systems throughout the Pacific Ocean.

Part 3. How do El Niño and La Niña affect the weather?

The large oceanic and atmospheric changes caused by El Niño and La Niña have a profound effect on Earth’s weather. This is because ENSO occurrences have strong teleconnections to other weather patterns. Teleconnections are atmospheric interactions between widely separated regions. One way of thinking of teleconnections is that changes in the ocean and atmosphere in the Pacific can have a ripple effect on climatic conditions in other parts of Earth. This worldwide message is conveyed by shifts in tropical rainfall and 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 would occur in different places. Scientists are studying the relationships between ENSO events and weather around the globe to determine whether links exist. Understanding these teleconnections can help in forecasting droughts, floods, tropical storms, and hurricanes. Based on ENSO patterns and on measurements of the general circulation of the atmosphere and oceans, scientists are predicting abnormally wet, dry, warm, or cold conditions for different regions at different times. 

How does ENSO affect weather patterns in North America? During an El Niño, there is a tendency for higher than normal temperatures in western Canada and the upper plains of the United States. This is because the low-pressure system in the Pacific draws up warm air into Canada, some of which filters into the northern United States. Another low-pressure system draws cold moist air into the southern United States, bringing lower than normal temperatures. The same low-pressure system in the southern United States is also responsible for increases in precipitation during an El Niño, especially in those areas close to the Gulf of Mexico.

Part 4. How are human activities affected by ENSO? ENSO has many effects on human activities. The economic impacts of the 1982-1983 El Niño, perhaps the strongest event in recorded history, are conservatively estimated to have exceeded $8 billion worldwide from droughts, fires, flooding, and hurricanes (Table 1). Virtually every continent was affected by this strong event. An estimated 1,000 to 2,000 deaths were blamed on El Niño and the disasters that accompanied it. In addition, the extreme drought in the Midwest Corn Belt of the United States during 1988 has been tentatively linked to the “cold event,” or La Niña, of 1988 that followed the ENSO event of 1986-1987. The effects of El Niño and La Niña vary according to the strength of the episodes and the geographic distribution of weather changes. Some areas experience heat waves and droughts, while others have torrential rains and flooding. These specific weather events occur within wet, dry, warm, and cool zones associated with the episodes.

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The World in Spatial Terms

• Standard 1: How to use maps and other geographic representations, tools, and technologies to acquire, process, and report information from a spatial perspective

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Human Systems

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Environment and Society

• Standard 14: How human actions modify the physical environment
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• Standard 18: How to use geography to interpret the present and plan for the future

One of the most well-known climate patterns that we have come to recognize and better understand is El Niño. Every three to seven years during the months of December and January, the balance between, wind, ocean currents, oceanic and atmospheric temperature and bioshpere breaks down, resulting in a severe impact on global weather.

In a normal year the trade winds blow westward and push warm surface water near Australia and New Guinea. When this warm water builds up in the western Pacific-Ocean, nutrient-rich cold waters are forced to rise up from the deeper ocean just off of the west coast of South America. This colder nutrient-rich water fosters the growth of the fish population.

During an El Niño event the trade winds weaken. Warm, nutrient-poor water is not pushed westward and comes to occupy the entire tropical Pacific Ocean. The cold water is not forced to the surface and the coastal waters of Peru and Ecuador are unusually warm. This warmer water has a devastating impact on their fishing crops which rely on cool waters to thrive. The region also experiences an extremely higher than average amounts of rainfall.

While the impact of an El Niño is most dramatic off of the coast of Western South America its impact is felt in weather around the world. A severe El Niño will enhance the jet stream over the western Pacific and shift it eastward, leading to stronger winter storms over California and southern United States, with accompanying floods and landslides. In contrast, El Niño may also cause severe droughts over Australia, Indonesia, and parts of southern Asia. Further, while El Niño is known to lower the probability of hurricanes in the Atlantic, it increases the chances of cyclones and typhoons in the Pacific.

Oceanography from Space

El Niño is one example of how observing the ocean from space leads to significant insights. Researchers use data from NASA Earth observing satellites to create telling images of how El Niño events form in the ocean, and the factors that may impact its strength and duration in a given climate cycle.

Temperature

NOAA's AVHRR and NASA's Aqua satellite have provided scientists over 25 years of sea surface temperature data. The vast tropical Pacific Ocean receives more sunlight that any other region on Earth. Much of this sunlight is stored in the ocean in the form of heat. During an El Niño water temperatures in the Pacific Ocean may rise on average 3 - 5 degrees above average. This happens as the water in seas around Indonesia, referred to as the Pacific Warm Pool are not forced westward due to weakened east to west trade winds. This pool of warmer water expands westward toward North and South America. Scientists have observed that the size and the frequency of an El Niño is impacted by the fluctuations in the Pacific Warm Pool.

Topography

Currents and tides influence topography, as does temperature. Water expands as it gets warmer, and the lack of cold water dependent nutrients make it less dense. This expanded, less dense water results in a rise in sea level, observable from space. Ocean surface height may rise as much as 6 to 13 inches above normal in some ocean regions during an El Niño.

Winds

The QuikSCAT satellite tracks vector winds, and the Jason and Topex/Poseidon missions track their effects. Typically, the Pacific trade winds blow from east to west, dragging the sunlit warm waters westward where they accumulate in Pacific Warm Pool mentioned above. Weakened east to west trade winds during an El Niño event result in the Pacific Warm Pool waters expanding westward. NASA's QuikSCAT satellite data have shown how trade wind irregularities of less than a few months in duration leading up to an El Niño may have dramatic results on the weather events to follow.

Currents

Currents, or circulation within ocean is influenced primarily by two physical factors, the sinking and rising of warming and cooling water, and movement due to the forcing of the surface waters due to wind. The interactions of ocean water temperature and the strength and direction of winds create currents within the ocean that define the strength and duration of the El Niño. NASA satellite data in combination with data collected from buoy systems and ships provide scientists an expanding understanding of the relationship between ocean currents, weather and climate.

Salinity

Ocean water salt content or salinity is a key variable in understanding the ocean's capacity to store and transport heat. Salinity and temperature combine to dictate the oceans' density. Greater salinity, like colder temperatures, results in an increase in ocean density with a corresponding depression of the sea surface height. In warmer, fresher waters, the density is lower resulting in an elevation of the sea surface. These ocean height differences are related to the circulation of the ocean. Beginning in 2009 the Aquarius mission will take regular measurements of the changes in ocean surface salinity. By knowing the how changes in salinity impact the physical processes in the ocean scientists will be able to create computer models that may more accurately predict El Niño episodes.

Sea Ice

Sea ice modulates planetary heat transport by insulating the ocean from the cold polar atmosphere, and also by modulating the thermohaline circulation of the world ocean. Moreover, the high albedo of snow-covered ice further insulates the polar oceans from solar radiation and introduces another positive feedback in the climate system. The El Niño and its related Southern Oscillation appear to affect regional ice distributions around Antarctica. Understanding this connection between the Southern Oscillation and southern ocean climate and the sea ice cover will substantially improve our understanding of global climate. El Niño episodes affect the Weddell and Ross Seas, areas that are regarded as key sources of cold and dense bottom water that influences global ocean circulation. The strongest links were observed to be in the Amundsen, Bellingshausen and Weddell Seas of the west Antarctic. Within these sectors, higher sea level pressure, warmer air temperature and warmer sea surface temperature are generally associated with the El Niño phase.

Future

By combining data sets from various satellite and in situ measurements we have been able to learn a great deal about how the physical properties of the ocean interact to create climate patterns. Scientists will continue to create models that combine data allowing them to better understand and predict complex ocean processes.

By using data collected from discrete measurements of physical properties of the ocean scientists have learned a great deal about El Niño and how it is impacted by the ocean system. Scientists are just beginning to have data of a time span sufficient to allow them to begin to predict climate patterns longer than the 3 to 7 year El Niño pattern. The ability to study the El Niño climate pattern and create models to simulate conditions, has helped us better predict its impact on our climate and weather.

El Niño is only one of the climate anomalies that scientists have observed;others include the North Atlantic Oscillation, the Atlantic Intertropical Convergence Zone oscillation, the Pacific Decadal Oscillation. Together with El Niño these systems are believed to be responsible for well over fifty percent of the climate variability on Earth. As models are created to simulate the individual patterns they will be combined to create a global model of climate change. If scientists get to the point to where they understand all of these climate cycles they may be able to predict major weather patters months in advance.

• Physical oceanographer - This scientist researches waves, tides, currents, transmission of light and sound in the ocean, and ocean-atmosphere relationships
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