El Niño-Southern Oscillation and Global Weather

The normal Pacific and El Niño

In a normal year, the trade winds blow from east to west along the equator. They push warm surface water toward the western Pacific, near Indonesia. That water piles up there, and sea level is a little higher in the west.

In the east, near Peru, the surface water that is pushed away is replaced by cold, nutrient-rich water from below. This is upwelling. It feeds huge fish populations.

During El Niño, the trade winds weaken. The warm water flows back east, and the central and eastern tropical Pacific become warmer than average. Upwelling weakens, and the rain-producing zone moves east.

The atmosphere changes too. This see-saw in air pressure between the eastern and western tropical Pacific is the Southern Oscillation. During El Niño, pressure rises over the western Pacific (Indonesia) and falls over the eastern Pacific (Tahiti), which weakens the trade winds. Because the ocean and atmosphere change together, the whole pattern is called ENSO.

La Niña and how NOAA classifies ENSO

La Niña is the opposite phase. The trade winds blow stronger than normal, more warm water is pushed west, and the central and eastern tropical Pacific are cooler than average. Together, El Niño, La Niña, and the neutral state between them are called the El Niño-Southern Oscillation (ENSO).

Scientists measure ENSO with an anomaly: the difference between the observed temperature and the long-term average for that place and time of year.

NOAA's Climate Prediction Center watches the Niño 3.4 region of the tropical Pacific. A sustained anomaly of $+0.5\,^\circ\text{C}$ or higher means El Niño. A sustained anomaly of $-0.5\,^\circ\text{C}$ or lower means La Niña. Anything in between is neutral.

Global weather effects

Warm water heats the air above it and drives evaporation. Where the warm pool sits, thunderstorms form and heavy rain falls. During El Niño the warm pool and its rain move east:

• Peru and Ecuador: heavy rain and flooding.

• Indonesia and Australia: drier, with a higher risk of drought and fires.

• Western Pacific: sea level drops a bit, while it rises near Peru as warm water slides east.

The effects reach North America through the jet stream. In El Niño winters, the subtropical jet stream strengthens and shifts south across the Pacific. It carries moist air and storms into California, the Gulf Coast, and Texas, so these regions are often wetter and cooler. The northern US and the Pacific Northwest tend to be drier and warmer because storms pass to the south.

La Niña flips the pattern. The jet stream shifts north, so the southern US is often drier and warmer, with more drought risk in Texas, while the Pacific Northwest gets more rain and snow.

Ocean temperature and ecosystems

Warm water affects more than weather. Near Peru in the eastern Pacific, sea level rises as warm water arrives, because warm water expands and the water that was piled up in the west flows back east. In the western Pacific near Indonesia, sea level falls. Evaporation also increases over the warmer surface, adding water vapor to the air and more rain nearby.

Ocean heat also stresses coral reefs. Corals live with tiny algae that give them food and color. If the water stays about $1\,^\circ\text{C}$ above the usual maximum monthly mean for weeks, the corals expel the algae. The coral loses its color and looks white. This is coral bleaching. Bleached corals are starving, and if the heat lasts, they die.

Losing coral hurts the whole reef. Fish and other animals lose food and shelter, biodiversity drops, and coastlines lose a natural wave barrier.

Back to the start: warm water in the central Pacific shifts the jet stream, which sends storms across Texas and the Gulf Coast.

Summary