ESR: Lesson - El Niño and La Niña (Topic 4.9) 📖

⏳ Estimated Reading/Watching Time: 9 - 11 minutes

Learning Objective

Describe the environmental changes and effects that result from El Niño or La Niña events (El Niño-Southern Oscillation).

 

Normal Pacific Ocean Circulation

Oceans, in general, play a significant role in shaping Earth’s climate and any changes that take place in normal ocean conditions will end up causing climatic changes elsewhere on the planet.

Under neutral conditions in the Pacific Ocean, trade winds combined with the Coriolis effect (deflection of the atmosphere caused by the spinning of the earth) cause water at the equator to flow from east to west.

An image of the southern pacific ocean labeled "normal year" The Walker circulation is operating normally, with cold water on the coast of South America and the trade winds moving from South America to Australia. Warm water is in the center and near Australia.
This graphic provides a visual of sea surface circulation during a normal year.

This sets up the Walker circulation which, in turn, causes warming of the waters in the western Pacific Ocean. There is increased precipitation in the warmed area and drier conditions in the eastern Pacific Ocean near the west coasts of North and South America.

A drawing of the pacific walker circulation during neutral conditions.
The Coriolis effect causes the Walker circulation in the Pacific Ocean. El Niño and La Niña affect the Walker circulation.

El Niño and La Niña are two climate patterns that break the normal climate patterns that exist in the Pacific Ocean, and they take place on an irregular basis. When either of these two opposing patterns sets in, they cause changes in climate patterns all over the planet.

Watch the video below to learn more about the basics of El Niño and La Niña:

El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO) cycle. ENSO is a climate pattern that breaks from the normal climate pattern usually found in the central and eastern tropical Pacific Ocean.

An ENSO event can be a warming event as in the case of an El Niño, or a cooling event as in the case of La Niña. In either case, the changes in temperature impact precipitation patterns and have a strong influence on weather patterns all around the world. The events typically last from nine to twelve months but sometimes can persist for years. The United States especially bears the brunt of some of the storms that can result from an El Niño or La Niña pattern.

 

Essential Knowledge

El Niño and La Niña are phenomena associated with changing ocean surface temperatures in the Pacific Ocean. These phenomena can cause global changes in rainfall, wind, and ocean circulation patterns.

 

El Niño Southern Oscillation (ENSO)

El Niño, a Spanish term for "Little Boy", is a phenomenon that happens when the central and eastern parts of the Pacific Ocean become warmer than usual by 1-3 degrees Celsius. This causes the trade winds that normally blow strongly from east to west to weaken and allow the warm waters to move eastward toward the western coasts of North and South America.

An image of the southern pacific ocean labeled "El Niño" The trade winds have reversed, pushing warm water from Australia to Peru, warming the East of the Pacific Ocean.
This graphic provides a visual of sea surface circulation during an El Niño year.

As a result, El Niño brings dry weather to the western Pacific Ocean and wet and stormy weather to the central and eastern Pacific Ocean. The short video below explains what happens during an El Niño year and some effects of an El Niño event:

 

El Niño also affects the Pacific jet stream, which shifts southward and creates wetter conditions in the southern United States and warmer and drier conditions in the northern United States.

A map of the world showing the changes to rainfall during El Niño events. Some notable changes are wet winters in the southern United States, drier conditions in Australia, southern Africa, and northern South America.
El Niño can cause shifts in rainfall patterns across the globe. For example, during an El Niño year, the southern United States is usually wetter than usual in the winter.
The map shows the United States and South America during an El Niño year. The Atlantic Ocean is blue, indicating fewer hurricanes and greater atmospheric stability. The northern Pacific, near California, is yellow, indicating more hurricanes. The Southern Pacific Ocean is red, representing warm and wet conditions.
El Niño can cause fewer hurricanes in the Atlantic, but more hurricanes in the Pacific Ocean.

 

El Niño also affects the Pacific jet stream, which shifts southward and creates wetter conditions in the southern United States and warmer and drier conditions in the northern United States. El Niño can also increase the intensity of storms and hurricanes in the affected regions.

 

 

 

 

 

 

 

La Niña

La Niña, which means "Little Girl" in Spanish, is a phenomenon that happens when the normal westerly winds in the equatorial Pacific Ocean become stronger and enhance the Walker circulation.

The La Niña climate pattern. It features a world map centered on the Pacific Ocean, depicted in shades of blue and red. A dashed line forms an oval over the ocean, with arrows indicating movement: three blue arrows curve westward and one red arrow curves eastward. The text explains that during La Niña, sea surface temperatures in the central and eastern Pacific Ocean become cooler than average and trade winds strengthen, pushing warm water westward.
This graphic provides a visual of sea surface circulation during a La Niña year.

This causes the sea surface temperatures to drop below average across the east-central equatorial Pacific Ocean.

A map of the world showing the changes to rainfall during La Niña events. Some notable changes are dry winters in the southern United States, wetter conditions in Australia, southern Africa, and northern South America.
La Niña can cause shifts in rainfall patterns across the globe. For example, during a La Niña event, the southern United States is usually drier than usual in the winter.
A map of the United States and South America during a La Niña year. The Atlantic ocean is yellow indicating more hurricanes. The northern Pacific, near California, is colored blue indicating fewer hurricanes. The Southern Pacific ocean is green with the description, "cool and dry conditions"
La Niña events cause more hurricanes in the Atlantic and fewer hurricanes in the Pacific Ocean.

The climate effects of a La Niña event are generally the opposite of those that happen during an El Niño event. The jet stream shifts further north, blocking the moisture from the Pacific Ocean to the more southern areas of the United States. The Southern U.S. becomes warmer and drier than usual, while the Northern U.S. and Canada experience colder, wetter weather.

Both El Niño and La Niña events alter the climate. In this course, we want to learn how each affects global weather conditions, as this can have an impact on crops, wildlife, ranching, and other natural and human activities. We also want to explore if these events are happening more or less often and if these changes may be related to global climate change.

 

Essential Knowledge

El Niño and La Niña are influenced by geological and geographic factors and can affect different locations in different ways.

 

AP Exam Tip

During a normal year, sea surface temperatures in the southern Pacific Ocean are warmer in the west, and trade winds blow from east to west.

During an El Niño year, sea surface temperatures are warmer than average in the southern Pacific Ocean and trade winds weaken or reverse (blow from west to east), allowing warm water to spread eastward.

During a La Niña year, sea surface temperatures in the southern Pacific Ocean are cooler than average in the central and eastern Pacific, and trade winds strengthen, pushing the warm water westward.

 

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