El Niño Southern Oscillation (HL) (DP IB Environmental Systems & Societies (ESS))

El Niño Southern Oscillation (ENSO)

• The El Niño Southern Oscillation (ENSO) is a natural climate phenomenon

• It is characterised by fluctuations in wind patterns and sea surface temperatures across the tropical Pacific Ocean

• ENSO cycles consist of two extreme phases:

  • El Niño (a warming phase)

  • La Niña (a cooling phase)

    • Neutral or transitional conditions occur between these two extremes

• These changes in the Pacific Ocean can influence weather and climate patterns not only locally in the Pacific region but also across the globe

• ENSO events are irregular and difficult to predict in terms of frequency and intensity

  • They usually occur every 2-7 years

• During normal conditions, trade winds blow in a westerly direction:

  • Normally, these winds push warm surface water toward the western Pacific (near Asia and Australia)

  • This normally causes colder water to rise to the top of the ocean near the coast of South America to replace this warm water

    • This cooler, nutrient-rich water rises through upwelling

• El Niño and La Niña interrupt these normal conditions

El Niño phase

• El Niño occurs when the east-to-west trade winds weaken or reverse

  • This is due to a weakening or reversal of the Walker circulation

• During El Niño, warmer-than-usual sea surface temperatures develop in the central and eastern tropical Pacific

• This results in reduced upwelling of cold, nutrient-rich water off the coast of north-western South America

  • This particularly affects Peru and Ecuador

  • Reduced upwelling decreases marine productivity in these regions

  • This negatively impacts local fisheries and food chains

Effects of El Niño

• Global weather impacts: El Niño can cause extreme weather events around the world

  • Drier conditions in Australia and Southeast Asia, leading to droughts and wildfires

  • Heavy rainfall and flooding in the western coast of the Americas, particularly Peru and parts of the USA

  • Warmer winter temperatures in Canada and the northern USA

  • Decreased monsoon rains in India and parts of Southeast Asia

La Niña phase

• La Niña is the opposite phase of El Niño and occurs when the Walker circulation (the east-west atmospheric circulation) strengthens

  • Stronger trade winds push warm water even further west

  • This causes cooler water to dominate the eastern and central Pacific

  • This leads to increased upwelling of cold, nutrient-rich water off South America

  • This increases marine productivity in these areas

Effects of La Niña

• Global weather impacts: La Niña also affects weather patterns globally, but generally in the opposite way to El Niño

  • Increased rainfall in Australia and Southeast Asia, leading to floods

  • Drier conditions in South America and parts of the USA

  • Harsher winter conditions in Canada and the northern USA

Impacts of ENSO on marine ecosystems

• Both El Niño and La Niña events have a significant impact on marine productivity

  • During El Niño, the lack of nutrient-rich upwelling causes a decline in plankton, leading to fewer fish and affecting entire marine food chains

  • La Niña promotes nutrient upwelling, boosting marine life and improving fisheries

• The cycle can also affect coral reefs

  • Warmer waters during El Niño leading to coral bleaching events

  • This damages ecosystems in places like the Great Barrier Reef in Australia

Secondary impacts of ENSO

• Agriculture:

  • Crops can be damaged due to droughts (El Niño) or flooding (La Niña), leading to food shortages

• Health:

  • Changes in climate can increase the spread of diseases such as malaria in tropical regions where wetter conditions provide breeding grounds for mosquitoes

• Economic impacts:

  • Natural disasters caused by ENSO events can result in economic losses, especially in countries that depend heavily on agriculture and fishing