The El Niño-Southern Oscillation (ENSO) is a climate pattern that shakes up the Pacific Ocean every few years. It's like a seesaw between warm El Niño and cool La Niña phases, causing changes in ocean temperatures, winds, and rainfall patterns.

ENSO's effects ripple across the globe, impacting weather, ecosystems, and human activities. From shifting fish populations to influencing crop yields, this climate dance has far-reaching consequences. Understanding ENSO helps us predict and prepare for its worldwide impacts.

El Niño-Southern Oscillation (ENSO)

Definition of ENSO phases

• ENSO is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean
  • Involves close interaction between the ocean and atmosphere
  • Occurs on average every two to seven years
• El Niño phase
  • Warming of the ocean surface temperatures in the central and eastern tropical Pacific Ocean relative to average conditions (eastern coast of South America)
  • Reduced upwelling of cold water along the coast of South America (Peru and Ecuador)
  • Weakening or reversal of the trade winds blowing from east to west across the tropical Pacific
• La Niña phase
  • Cooling of the ocean surface temperatures in the central and eastern tropical Pacific Ocean relative to average conditions
  • Enhanced upwelling of cold water along the coast of South America (Peru and Ecuador)
  • Strengthening of the trade winds blowing from east to west across the tropical Pacific

Processes behind ENSO development

• Normal conditions in the tropical Pacific
  1. Trade winds blow from east to west, pushing warm surface water towards the western Pacific (Indonesia and Australia)
  2. Cold, nutrient-rich water upwells along the western coast of South America (Peru and Ecuador)
  3. Warm air rises over the western Pacific, leading to convection and rainfall
• Development of El Niño conditions
  1. Weakening or reversal of the trade winds reduces the easterly wind stress on the ocean surface
  2. Reduced upwelling of cold water in the eastern Pacific allows the surface to warm
  3. Warm surface water moves towards the central and eastern Pacific, further weakening the trade winds
  4. Positive feedback loop between the ocean and atmosphere strengthens El Niño conditions (Bjerknes feedback)
• Development of La Niña conditions
  1. Strengthening of the trade winds increases the easterly wind stress on the ocean surface
  2. Enhanced upwelling of cold water in the eastern Pacific cools the surface
  3. Warm surface water is pushed further towards the western Pacific, reinforcing the trade winds
  4. Positive feedback loop between the ocean and atmosphere strengthens La Niña conditions

Global impacts of ENSO

• Weather patterns
  • El Niño
    • Increased rainfall in the eastern Pacific and along the west coast of South America (Peru and Ecuador)
    • Decreased rainfall in the western Pacific (Indonesia and Australia) and parts of the Amazon rainforest
    • Warmer temperatures in the eastern Pacific and western Americas
  • La Niña
    • Increased rainfall in the western Pacific (Indonesia and Australia) and parts of the Amazon rainforest
    • Decreased rainfall in the eastern Pacific and along the west coast of South America (Peru and Ecuador)
    • Cooler temperatures in the eastern Pacific and western Americas
• Ecosystems
  • Changes in ocean productivity due to variations in upwelling and nutrient availability
    • El Niño reduces upwelling and nutrient supply, decreasing marine productivity (plankton and fish populations)
    • La Niña enhances upwelling and nutrient supply, increasing marine productivity
  • Shifts in the distribution and abundance of marine species (tuna, sardines, and anchovies)
  • Impacts on terrestrial ecosystems due to changes in precipitation patterns (droughts and floods)
• Human activities and society
  • Agriculture
    • Crop yields affected by changes in rainfall and temperature (rice, wheat, and maize)
    • Droughts during El Niño can reduce agricultural productivity in Australia and parts of Asia
    • Flooding during El Niño can damage crops in South America
  • Fisheries
    • Fish populations and catch rates influenced by changes in ocean productivity
    • Peruvian anchoveta fishery heavily impacted by El Niño and La Niña events
  • Natural disasters and infrastructure
    • Increased risk of flooding and landslides during El Niño in the eastern Pacific (Peru and Ecuador)
    • Increased risk of droughts and wildfires during El Niño in Australia and Southeast Asia
    • Damage to infrastructure and displacement of populations due to ENSO-related extreme events

Other major climate oscillations

• North Atlantic Oscillation (NAO)
  • Atmospheric pressure oscillation over the North Atlantic Ocean
    • Measured by the difference in atmospheric pressure between Iceland and the Azores
  • Influences weather patterns in Europe, North Africa, and eastern North America
  • Positive NAO phase
    • Stronger-than-average pressure difference between the Icelandic Low and the Azores High
    • Increased westerly winds across the North Atlantic
    • Mild and wet winters in Europe, cold and dry winters in northern Canada and Greenland
  • Negative NAO phase
    • Weaker-than-average pressure difference between the Icelandic Low and the Azores High
    • Reduced westerly winds across the North Atlantic
    • Cold and dry winters in Europe, mild and wet winters in northern Canada and Greenland
• Pacific Decadal Oscillation (PDO)
  • Long-term ocean temperature variability in the North Pacific Ocean
    • Shifts between warm and cool phases on a multi-decadal timescale (20-30 years)
  • Warm PDO phase
    • Warmer-than-average sea surface temperatures along the west coast of North America (Alaska and California)
    • Cooler-than-average temperatures in the central North Pacific
    • Decreased upwelling and marine productivity along the west coast of North America
  • Cool PDO phase
    • Cooler-than-average sea surface temperatures along the west coast of North America
    • Warmer-than-average temperatures in the central North Pacific
    • Increased upwelling and marine productivity along the west coast of North America
  • Influences climate and ecosystem dynamics in the North Pacific and surrounding regions
    • Affects salmon populations and fisheries along the west coast of North America
    • Modulates the impact of ENSO on North American climate (constructive or destructive interference)