El Niño and La Niña are phenomena in which changes in sea-surface temperatures in the tropical Pacific cause climate variations on a global scale. They are closely related to abnormal weather in Japan, such as cool summers, warm winters, and extremely hot weather.
Basic Knowledge
El Niño: Normally, trade winds blow warm Pacific seawater westward. When these trade winds weaken, the warm water flows back eastward, causing sea-surface temperatures from off the coast of Peru to the central Pacific to become higher than average. It occurs irregularly, generally in cycles of 2〜7 years.
La Niña: In contrast, the trade winds strengthen, sea-surface temperatures in the western Pacific rise, and temperatures in the eastern Pacific fall. Its effects are almost the opposite of those of El Niño.
Effects on Japan: Years with El Niño tend to bring cool summers and warm winters, while years with La Niña tend to bring extremely hot summers and severe winters (however, yearly variations exist, so this is not certain).
Definition of abnormal weather: The Japan Meteorological Agency defines abnormal weather as “a phenomenon occurring at a particular place and time with a frequency of once in 30 years or less.”
El Niño (A phenomenon in which sea-surface temperatures in the equatorial eastern Pacific remain higher than average)
La Niña (A phenomenon in which sea-surface temperatures in the equatorial eastern Pacific remain lower than average)
Trade winds (Prevailing winds that blow from east to west in the tropics. Weaken during El Niño)
Walker circulation (Atmospheric circulation over the equatorial Pacific. Weakens during El Niño)
Teleconnection (A statistical relationship between the climates of distant regions)
Abnormal weather (A weather phenomenon occurring once in 30 years or less)
Further Study (Background and Significance)
El Niño–Southern Oscillation (ENSO: El Niño–Southern Oscillation) is a global climate variation mode maintained by the interaction between the ocean and atmosphere. When sea-surface temperatures off Peru rise, rising air shifts eastward, altering the entire atmospheric circulation over the Pacific (Walker circulation). This influence spreads through teleconnections to precipitation and temperature anomalies around the world.
During El Niño years, dry conditions and droughts are more likely in India and Australia, while severe flooding is more likely along the coast of Peru in South America. In Japan, the rainy season tends to last longer and summers tend to be cool (such as the exceptionally cool summer of 1993).
Recent global warming may be affecting the strength and frequency of El Niño, raising concerns that abnormal weather in the future may become more frequent and severe. The WMO (World Meteorological Organization) continues to monitor ENSO and use it for forecasts six months ahead.
- El Niño = high sea-surface temperatures in the equatorial eastern Pacific
- La Niña = low sea-surface temperatures in the equatorial eastern Pacific
- Cause = changes in the trade winds
- El Niño → Japan tends to have cool summers and warm winters
- La Niña → Japan tends to have extremely hot summers and severe winters
- Cycle = 2〜7 years (irregular)
- Abnormal weather = frequency of once in 30 years or less
Points to Note (Common Confusions)
① During El Niño, the eastern Pacific becomes warmer because the trade winds weaken and warm water flows eastward. ② La Niña is the opposite, with a cooler eastern Pacific. ③ Japan's “cool summer” during El Niño is a probabilistic tendency and does not occur every time. ④ ENSO, Walker circulation, and teleconnections are parts of one connected mechanism.
Practice
- When El Niño is occurring, what happens to sea-surface temperatures off the coast of Peru?
- What summer tendency is common in Japan during a La Niña year?
- State the frequency criterion in the Japan Meteorological Agency's definition of abnormal weather.