The red and white color of the eastern tropical Pacific in the upper image (from Nov. 1997) indicates a higher than normal ocean level due to the piling up of warm ocean water during El Niño. The lower image (Feb 1999) shows the ocean level during La Niña when there is cool water from upwelling in the eastern tropical Pacific. Data is from TOPEX/Poseidon and Jason-1 and shows the height of the ocean surface.
Credit: NASA JPL

Floods and mudslides in Ecuador, the droughts and wildfires in Australia, and extreme California rainstorms – could all of these events be triggered by the same thing? Yes, they can. It’s the El Niño-Southern Oscillation (ENSO), a combination of changes in the ocean and atmosphere that affect weather in many areas of the world.

Normally, trade winds move water at the ocean surface from the eastern tropical Pacific towards the western Pacific. This creates an upwelling of cold nutrient-rich water off the coast of Peru and Chile, which supports a diversity of marine life. The western Pacific is in a low pressure system and has wet weather. The eastern Pacific, in a high pressure system, is dry. But every 3 to 7 years, the atmosphere and ocean change during El Niño and La Niña events – the two extremes of ENSO.

During El Niño, air pressure rises in the western Pacific and falls in the central and eastern Pacific. Without the strong pressure gradient, the trade winds weaken. Without the trade winds, there is less upwelling near the coast of South America and the warm and nutrient-poor water of the western Pacific spreads east, piling up water in the eastern tropical Pacific. Not as much marine life can survive in the warm water as can in the cool nutrient-rich waters. The warm water in the Pacific evaporates faster than cool water, leading to more water vapor in the air which can cause high humidity and storms in areas that are usually dry.

During La Niña, the cold phase of ENSO, the trade winds grow stronger across the Pacific because the low pressure over the western Pacific strengthens, as does the high pressure over the central and eastern Pacific. This causes more upwelling of ocean water off the coast of Peru and Chile, making the surface water of the eastern tropical Pacific unusually cold.

Both El Niño and La Niña events can have far-reaching effects on the weather. Intense rainstorms and flooding, extreme droughts, the strength of the Atlantic hurricane season, and winter storms in many areas of the world are affected by ENSO events. ENSO may also have an impact on the North Atlantic Oscillation as it has an effect on the Arctic troposphere. These impacts are called teleconnections.