El Niño & La Niña: phases, regions & global impacts
El Niño is the warm phase of the El Niño–Southern Oscillation and La Niña the cold phase. Both start as a temperature shift in the equatorial Pacific, then ripple out to weather on every continent.
An event is declared when the index holds past ±0.5°C for five consecutive overlapping three-month seasons: at or above +0.5°C is El Niño, at or below −0.5°C is La Niña, and anything in between is Neutral. The sections below cover where the Pacific is measured and what each phase does to global weather.
Global climate impacts (ENSO teleconnections)
The Niño index tracks more than ocean temperature; it reflects a shift in an atmospheric circulation called the Walker Circulation. As Pacific heat moves, the global jet streams move with it, producing “teleconnections” — weather effects felt far from the Pacific.
El Niño (the warm phase)
During an El Niño, trade winds weaken and warm water shifts eastward toward the Americas.
- North America / Western Hemisphere: the Pacific Jet Stream is pulled southward. This typically brings severe, wet storms and flooding to the Southern U.S. and Gulf Coast, while leaving the Pacific Northwest and Canada unusually warm and dry.
- Atlantic hurricanes: the shifted jet stream raises high-altitude wind shear over the Atlantic, which tears developing storms apart. El Niño years tend to have weaker, less active Atlantic hurricane seasons.
- Global systems: the displaced rainfall brings flooding to Peru and drought to Australia and Indonesia, hitting crop yields.
La Niña (the cold phase)
During a La Niña, trade winds strengthen, piling warm water toward Asia and pulling cold upwelling to the South American coast.
- North America / Western Hemisphere: the polar jet stream pushes north, bringing cold winters to the northern U.S. and Canada, heavy precipitation to the Pacific Northwest, and drought to the southern U.S.
- Atlantic hurricanes: Atlantic wind shear drops off. With little shear, storms intensify more easily, so La Niña years often see active Atlantic hurricane seasons.
- Global systems: Australia and Southeast Asia get heavy monsoon rains, while the cold upwelling off Peru feeds nutrient-rich water that supports the local fishing industry.
Interactions with other climate systems
- Pacific Decadal Oscillation (PDO): when a La Niña lines up with a cold PDO phase, the effects compound, deepening droughts in the American West.
- Global baseline temperatures: El Niño releases ocean heat into the atmosphere, which is why El Niño years often set global temperature records.
The Niño regions
The Pacific is monitored across four overlapping boxes. Niño 3.4 is the headline indicator; the others sharpen the picture of where and how an event is developing.
The foundational area for RONI. It captures the critical transition between the central and eastern Pacific, making it the most reliable indicator for global ENSO impacts.
Off the coast of South America, this region is the first to show warming during traditional El Niño events and heavily impacts regional fisheries and weather.
Historically the focus for ENSO monitoring, this region exhibits the most intense sea-surface temperature variations during peak events.
Warming in this western-central zone often signals ‘Modoki’ events, where climate impacts differ significantly from traditional eastern-centric El Niños.