Circulation Patterns and El Nino State Dynamics in the Equatorial Pacific

Introduction to Equatorial Pacific Circulation Patterns

  • Circulation patterns in the Equatorial Pacific are intrinsically tied to changes in sea surface temperature (SST).
  • These patterns are connected to the ENSO (El Nio-Southern Oscillation) discussed in previous segments.
  • Understanding normal conditions is essential to predicting and identifying El Nio states.

Normal Conditions in the Equatorial Pacific

  • Trade Winds: Under normal conditions, strong trade winds are a dominant feature of global atmospheric circulation.
  • Oceanic Response and Temperature Gradients:
    • Water originates from Antarctica and moves along the coasts of South America and Peru.
    • As the water reaches the Equator, it begins to warm.
    • This warm water pools on the western side of the Pacific, along the coasts of Australia and Indonesia.
    • Some water returns through a circulation process known in oceanography as a gyre, a pattern of currents circulating around an ocean basin.
  • The Walker Circulation:
    • This is the vertical atmospheric circulation pattern that indicates normal conditions.
    • Warm sea surface temperatures cause air to warm and rise, leading to evaporation off the ocean.
    • As the air rises to the top of the troposphere, it cools and condenses, forming continuous thunderstorms.
    • These storms create the lush rainforests found in Australia and Indonesia.
    • After rising, the air travels along the top of the troposphere and eventually sinks off the coast of South America where it is cool.
    • The sinking air meets the surface trade winds and continues the cycle.
  • Upwelling:
    • Off the coast of South America, deep ocean water that has not seen sunlight for a long time rises to the surface.
    • This process creates a large pool of cold water.
    • Upwelling brings essential nutrients from the ocean floor to the surface.
    • Marine life, specifically fish, depend on this cold water and the continuous food source provided by the nutrient-rich upwelling.

The 3.4 Region and Monitoring

  • The 3.4 region is a specific area in the Equatorial Pacific used to measure sea surface temperatures.
  • Monitoring Tools: Buoys are stationed in this region and far out at sea to measure SST.
  • Predictive Role: Temperature changes in the 3.4 region often occur before effects are felt in coastal Peru, providing a warning system for fishermen and local authorities.
  • The Three-Month Average:
    • Scientists use a three-month average of SST data to define a full-fledged El Nio.
    • This is necessary because the ocean can occasionally "slosh" back temporarily without the trade winds permanently weakening.
    • If conditions return to normal immediately after a quick flashback, it is not classified as El Nio. This prevents unnecessary alarm.

El Nio: Atmospheric and Oceanic Disruptions

  • Trade Wind Weakening: During El Nio, the trade winds become weak or may stop entirely.
  • Countercurrents: In some cases, the trade winds may даже reverse, blowing from west to east.
  • Sea Surface Topography:
    • Warm water is no longer pushed westward by trade winds.
    • Warm water piles up into a "hill" or "bumpy" surface in the middle of the ocean, higher than normal sea level.
    • A strong countercurrent then causes this warm water to slosh back toward South America.
  • Biological and Environmental Impacts:
    • Upwelling Cessation: When the warm water hits the South American continent, it is forced downward. This stops the upwelling process entirely.
    • Fisheries: The loss of upwelling destroys the nutrient supply, leading to significant distress for fish populations.
    • Weather Extremes:
    • South America: Experiences heavy rain and severe flooding that the region is not adapted for.
    • Australia/Indonesia: Thunderstorms cease; the rainforests dry out, leading to devastating wildfires.

Historical Context and Data Comparisons

  • 1997–1998 Event: This was a major El Nio event characterized by the warm pool of water moving from Australia to Peru.
  • 2015–2016 Event: Data indicates this event is very similar in scale and behavior to the 1997–1998 event.
  • Cross-sectional data confirms that during these events, the cool pool off the coast of Peru is replaced by the warm water originally stationed near Australia.

La Nia: Exaggerated Normalcy

  • Definition: La Nia is characterized by the normal Walker circulation behaving in an exaggerated or vigorous manner.
  • Atmospheric Conditions: The trade winds become much stronger than usual.
  • Upwelling Effects:
    • Upwelling becomes extremely intense ("goes crazy").
    • This results in a massive pool of exceptionally cold water stretching across the Equatorial Pacific.
  • Global Impact: These intensified conditions shift global circulation patterns, resulting in widespread effects on weather around the world.