A SINK BECOMES SOURCE

Droughts kill billions of trees in the Amazon like no other extreme event

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FROM JUNE, meteorological signs foretold that a drought would hit the Amazon region. Droughts in the rainforest are typically fuelled by high sea surface temperatures in the North Atlantic Ocean on the northeast and in tropical Pacific Ocean on the northwest.

Scientists particularly watch out for high sea surface temperature in the tropical Pacific, as it marks the onset of El Niño, the warm phase of a recurring climate pattern El Niño-Southern Oscillation. Many drought episodes in the Amazon have occurred during intense El Niño conditions such as those recorded in 1926, 1983, 19971998, and 2010. These conditions weaken the "Walker Circulation"-an atmospheric circulation in the Pacific Ocean over the tropics. Walker Circulation resembles a loop consisting of rising air in the west and sinking air in the east. During an El Niño, the rising moist air, which is meant to bring rainfall, does not reach the continent of South America. Instead, there is more sinking and dry air moving towards the land, setting the stage for a drought.

This year, the World Meteorological Organization declared the onset of El Niño in the tropical Pacific on July 4, but said that the conditions were moderate. Then what caused such a severe drought in the Amazon?

JOCHEN SCHÖNGART

Amazon has seen more droughts, floods in the past 15 years than in a century

The Amazon basin is in transition to an anthropogenic disturbance-dominated regime