El Nino-Southern Oscillation

The dominant year-to-year climate fluctuation, a coupled swing of the tropical Pacific ocean and atmosphere between warm El Nino and cool La Nina states.

Definition

The El Nino-Southern Oscillation is a recurring coupled ocean-atmosphere mode in the tropical Pacific, alternating between warm El Nino and cool La Nina phases that reorganize sea-surface temperatures, trade winds, and rainfall.

Scope

This topic covers the El Nino-Southern Oscillation, the coupled ocean-atmosphere mode centered in the equatorial Pacific that produces the largest interannual climate variations on the planet. It treats the Bjerknes feedback that drives it, the oceanic processes such as thermocline adjustment and equatorial waves that set its timescale, the warm El Nino and cool La Nina phases, the global teleconnections that carry its influence, and its evolving predictability and behavior under a warming climate.

Core questions

• What feedback couples the tropical Pacific ocean and atmosphere to drive El Nino?
• What oceanic processes determine the timing and duration of events?
• How does the oscillation influence weather across the globe?
• How predictable is it, and how might warming change it?

Key theories

Bjerknes feedback

Weaker trade winds warm the eastern Pacific, which reduces the east-west temperature gradient and further weakens the winds, a positive feedback that drives the system toward El Nino, with the reverse producing La Nina.

Delayed oscillator and ocean memory

Equatorial ocean waves redistribute warm water and adjust the thermocline with a delay, providing the negative feedback that turns the warm phase back toward the cool phase and sets the multi-year timescale.

Mechanisms

Normally strong trade winds pile warm water in the western Pacific and lift the cold thermocline in the east; when the winds weaken, warm water spreads eastward, suppressing upwelling and amplifying the warming through the Bjerknes feedback to produce El Nino. Equatorial oceanic waves then adjust the thermocline and eventually reverse the anomaly, while the warmed Pacific shifts atmospheric convection and launches teleconnection patterns that alter weather worldwide.

Clinical relevance

Because El Nino and La Nina shift rainfall, droughts, floods, and storm patterns across many regions, monitoring and forecasting the oscillation is central to seasonal climate prediction and to managing risks in agriculture, water, and disaster preparedness.

History

Walker identified the Southern Oscillation in atmospheric pressure in the 1920s, and Bjerknes in 1969 unified it with the oceanic El Nino into a single coupled phenomenon; the deployment of the tropical Pacific moored buoy array from the 1980s and 1990s enabled real-time monitoring and skillful seasonal forecasts.

Debates

Response of the oscillation to global warming

Whether El Nino events will become stronger, more frequent, or more variable as the climate warms remains uncertain because models disagree on changes in the underlying feedbacks.

Key figures

• Jacob Bjerknes
• George Philander
• Gilbert Walker
• Michael McPhaden

Seminal works

• bjerknes1969
• philander1990

Frequently asked questions

What is the difference between El Nino and La Nina?

El Nino is the warm phase, with a warmer-than-normal eastern tropical Pacific and weakened trade winds, while La Nina is the cool phase, with a colder eastern Pacific and stronger winds.

Why does an event in the Pacific affect weather worldwide?

Shifts in tropical Pacific convection alter the atmospheric circulation and launch wave patterns, called teleconnections, that change temperature and rainfall in distant regions.