El Niño and Tropical South American Drought

El Niño increases drought risk in tropical South America by changing tropical air circulation and reducing rainfall over parts of the region. It is the warm phase of the El Niño-Southern Oscillation (ENSO), marked by unusually warm surface waters in the central and eastern equatorial Pacific and associated atmospheric changes. Its influence reaches the Amazon through atmospheric teleconnections, meaning that changes over the Pacific affect weather far away.

How Pacific warming suppresses rainfall

Under normal conditions, equatorial Pacific trade winds help maintain warm water and strong rising air over the western Pacific. This forms part of the Walker circulation, a tropical east-west overturning circulation.

• During El Niño, the trade winds weaken, and warm water, deep convection and tropical rainfall shift towards the central and eastern equatorial Pacific.
• The resulting reorganisation of tropical circulation can favour subsiding air over parts of tropical South America, including the Amazon basin.
• Descending air warms and lowers relative humidity, making deep cloud formation and rainfall less likely.
• When rainfall deficits persist, soils and rivers receive less water, increasing the likelihood of meteorological, agricultural and hydrological drought.

Why drought severity varies

El Niño raises drought probability; it does not produce an identical drought everywhere. Rainfall responses vary with the event's strength and location, the season and the part of tropical South America considered.

• Drought risk is especially relevant to northern tropical South America, but impacts are not uniform across the continent.
• Tropical Atlantic sea-surface temperatures also influence moisture transport and rainfall over the Amazon, so Pacific conditions alone cannot explain every drought.
• Rainfall deficits can be intensified by higher evaporative demand during hot conditions, accelerating soil-moisture loss where water remains available.

How forest feedbacks amplify the impact

Amazon forests return water to the atmosphere through evapotranspiration, helping sustain regional moisture recycling. Severe drought can weaken this contribution and increase ecosystem stress.

• Water stress can reduce photosynthesis and tree growth, while prolonged or severe drought can increase tree mortality.
• Dry vegetation becomes more susceptible to fire, particularly where human ignition and forest disturbance are present.
• Reduced carbon uptake, tree mortality and fire emissions can weaken the forest's carbon sink; this does not mean that every El Niño turns the entire Amazon into a net carbon source.