Atmospheric & Oceanic Interactions (DP IB Geography)
Revision Note
El Niño and La Niña
During normal conditions, Trade Winds blow in a westerly direction:
Warm water moves from the South Americas towards the Asian continent
Through upwelling, colder water rises to the top of the oceans to replace this warm water
El Niño and La Niña interrupt these normal conditions
Image showing normal conditions (without El Niño and La Niña)
El Niño-Southern Oscillation (ENSO) and La Niña
El Niño Southern Oscillation (ENSO) is the change in warm water movement in the central-equatorial Pacific Ocean
It affects atmospheric circulation, resulting in global temperature and rainfall changes
The Southern Oscillation refers to changes in atmospheric pressure in the tropical Pacific
There are three periods of ENSO:
Neutral
El Niño
La Niña
In the neutral phase, air sinks at the Eastern Pacific, whilst warm and moist air rises at the Western Pacific
El Niño is the weakening of trade winds, causing warm air to rise in the Eastern Pacific, and cooler air to sink in the Western Pacific
Warm surface water moves in an easterly direction towards the American continent
The Pacific jet stream moves further south
El Niño occurs when ocean temperatures rise 0.5°C above average
May occur every 2-7 years
As ocean temperatures rise, e.g. off the Peru coast, thermal expansion occurs, resulting in sea level rise. As air rises and pressure is low, more rainfall occurs
As ocean temperatures cool e.g. off the Australian coast, rainfall reduces, resulting in drier conditions
La Niña is the opposite of El Niño
During La Niña, trade winds are very strong, and even more warm water moves towards Asia
The Pacific jet stream moves further north
Occurs when ocean temperatures are below average
Occurs every 3-5 years
Results in higher rainfall over Indonesia, and less rainfall over the central tropical Pacific Ocean
Upwelling causes cold water to move to the surface
Each period of El Niño or La Niña can last between 9-12 months
El Niño is more common that La Niña
Images showing the process of El Niño and La Niña
Impacts of El Niño and La Niña
Effects of El Niño and La Niña
El Niño and La Niña affect the climate, environment and the economy
El Niño and La Niña can bring positives and negatives
Climate change could worsen the effects of El Niño
Table showing the climatic, environmental and economic effects of El Niño and La Niña
Period | Climatic | Environmental | Economic |
---|---|---|---|
El Niño | Weather in the US and Canada is dry and warm Some areas of the world have higher rainfall, which can cause flooding, e.g. South America In India, El Niño can negatively affect the monsoon season, reducing rainfall and causing droughts Northern European winters are drier and colder Southern European winters are wetter UK summers are dry and hot Australia and Indonesia can experience heavy droughts | Changes in climate patterns can affect ecosystems and wildlife Coral bleaching can occur, which affects marine life | Damage to infrastructure occurs due to flooding, wind, rainfall, and fires Poor weather conditions can cause agricultural supply chain issues and increase costs Food insecurity increases, particularly in subsistence agriculture |
La Niña | Southern US experiences drier weather North-western US and Canada experience higher rainfall and flooding Winters are colder in the north and warmer in the south Hurricanes are more intense in areas like the Caribbean The monsoon season in India is more intense The UK is much wetter Australia and Indonesia can experience higher rainfall | Waters are nutrient-dense from upwelling, benefitting ecosystems and the marine food chain | In India, the intense monsoon season is a boost for agriculture and the economy Damage from intense flooding is costly in areas like Australia In South America, the fishing industry booms due to upwelling of nutrient-dense waters |
Examiner Tip
Always remember to use a case study/example to back up the points you make in your answers. You’ll get higher marks for providing evidence.
Examples of geographic impacts of El Niño and La Niña
1997–1998 El Niño
One of the strongest El Niño periods recorded
At the beginning of 1997, trade winds weakened and reversed in the west/central equatorial Pacific, resulting in warm sea temperatures
Droughts hit Indonesia and the Philippines:
These droughts resulted in agriculture shortages in Indonesia
Food prices increased dramatically
Intense forest fires affected Indonesia, causing dangerous smog levels
Peru experienced intense flooding and rainfall:
The area of Tumbes in Peru had 16 times more rainfall over the year than usual
Caused massive infrastructure damage e.g. homes, health centres, roads, bridges
Damaged crops and killed livestock, causing a hike in food prices
Disease outbreaks e.g. malaria and diarrhoea
Drastic differences in US weather:
The US West Coast and the South had high rainfall and flooding
Storms battered this area, resulting in loss of life
Economic damage from cyclones e.g. crops and infrastructure
Hit tourism and recreation industries
The US Midwest experienced hot weather – this year became “the year without a winter”, one of the warmest winters recorded
2010–2012 La Niña
At the beginning of 2010, there were visible signs of El Niño, before eventually moving to a neutral phase
In July, La Niña began, with colder ocean temperatures at the central/eastern tropical Pacific
Australia:
Record rainfall across Australia, bringing severe flooding across parts of the country
Damaged crops
Flooding of mines
Cyclones e.g. Cyclone Yasi
South-west Australia experienced one of the driest years ever recorded
Prices of commodities exported from places like Australia rose around the world
Heavy rainfall landed in Colombia:
Resulted in flash flooding and landslides
Resulted in deaths and injuries
Damage to infrastructure e.g. sanitation systems
Damage to livelihoods caused displacement in Colombia
Displacement resulted in increased conflicts
Disease spread e.g. diarrhoea, dengue, malaria, skin infections and even risk of rabies
Intense droughts hit East Africa:
A humanitarian emergency was declared in the Horn of Africa as food insecurity increased
Food and fuel prices rose
Livestock died from lack of water, reducing local farmer’s incomes
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