APES Topic 4.9: El Niño and La Niña

Gyres Overview: Gyres are large-scale ocean circulation patterns driven primarily by global wind patterns. * Northern Hemisphere: Gyres rotate in a clockwise direction. * Southern Hemisphere: Gyres rotate in a counterclockwise direction.
Trade Winds ( $0^{\circ}$ to $30^{\circ}$ ): These winds blow from East to West ( $E \rightarrow W$ ). This movement pushes equatorial currents from the eastern side of ocean basins toward the western side.
Westerlies ( $30^{\circ}$ to $60^{\circ}$ ): These winds blow from West to East ( $W \rightarrow E$ ), pushing mid-latitude currents toward the eastern edges of ocean basins.
Upwelling Zones: These are specific areas of the ocean where surface winds blow warm surface water away from a landmass (typically the coast of a continent). * Mechanism: As the warm surface water is pushed away, colder, deeper, and more nutrient-dense water is drawn upward to replace it. * Biological Importance: Upwelling brings oxygen ( $O_2$ ) and essential nutrients to the surface, supporting high primary productivity and creating highly productive fishing grounds.

Global Connection: This system acts as a global "conveyor belt" that connects all the world's oceans, mixing salt, nutrients, and temperature regulated across the planet.
The Process of Circulation: * Warm Water Movement: Warm water from the Gulf of Mexico travels toward the North Pole. * Cooling and Evaporation: As this water moves toward the poles, it cools and undergoes evaporation. * Density Increase: Because salt does not evaporate with the water, the remaining water becomes saltier. This increased salinity, combined with the colder temperature, makes the water more dense. * Sinking: The dense, cold, salty water sinks at the poles and spreads along the ocean floor. * Rising: This deep water eventually rises back into shallow, warm ocean currents in upwelling zones.

Definition: ENSO is a periodic pattern of shifting atmospheric pressure and ocean currents in the Pacific Ocean, specifically occurring between South America and Australia/Southeast Asia (the Tropical East Pacific).
Oscillation Characteristics: The system regularly shifts or "oscillates" between three distinct phases: Normal conditions, El Niño, and La Niña.
Geographic Focus: The primary observable changes occur along the coast of South America and across the equatorial Pacific.

Wind Patterns: Standard Trade Winds blow from East to West ( $W \leftarrow E$ ).
Western Pacific (Australia/SE Asia): * Trade winds pool warm surface water off the coast. * Conditions are characterized as normally wet with significant precipitation.
Eastern Pacific (South America): * Characterized by cool surface waters. * Normal conditions are relatively dry. * Steady upwelling occurs, providing excellent conditions for the fishing industry.

Atmospheric Change: Trade winds weaken or sometimes reverse direction.
Western Pacific (Australia/SE Asia): * The region experiences dry conditions and less rain than usual. * Can lead to drought. * Fishing remains good in these regions relative to the shift.
Eastern Pacific (South America): * Temperature: The surface waters become significantly warmer. * Precipitation: The region experiences very wet conditions and frequent flooding. * Oceanic Impact: There is a distinct lack of upwelling because the warm water is not being pushed away from the coast. * Economic Impact: The suppressed upwelling leads to poor fishing harvests due to the lack of nutrient-rich deep water.

Atmospheric Change: Trade winds become significantly stronger than normal.
Western Pacific (Australia/SE Asia): * The region receives even wetter conditions than normal, often leading to flooding.
Eastern Pacific (South America): * Atmospheric Conditions: Stronger trade winds cause extreme dry conditions and potential drought. * Oceanic Impact: Stronger winds cause "extreme upwelling" as more surface water is displaced. * Economic Impact: The enhanced upwelling results in exceptionally good fishing conditions due to the massive influx of nutrients.

Global Patterns: ENSO events cause global changes to rainfall, wind patterns, and ocean circulation.
Factors of Influence: These phenomena are influenced by both geological and geographic factors and affect different global locations in distinct ways.