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

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