The Atlantic Ocean holds the key to western Amazon rainfall

By Samuel McGlennon

This post is an excerpt of a piece on the website for the Center for International Forestry Research.
View the full article here

In 2010, catastrophic fires ravaged huge tracts of the western Amazon, a region of rainforest that until just a few years earlier was considered beyond the reach of serious drought. Those flames followed the major fires of 2005, which were also caused by extreme drought. Both of these conflagrations imperilled communities and livelihoods, sending massive pulses of carbon into the atmosphere.

With each destructive fire, the forests of the western Amazon become more susceptible to drying and further burning.

And that is a big problem according to Lou Verchot, one of the authors of a new study and Director, Forests and Environment Research at the Center for International Forestry Research (CIFOR).

“Many climate mitigation initiatives involve removing carbon dioxide from the atmosphere and storing it on land in trees and forests.  There are many initiatives to restore degraded forests or expand agroforestry as carbon sequestration measures, for example,” he says.

“Fire is a huge risk to these investments and one fire can undo a decade or more of work.  If we can predict fire risks, we can factor these elements into carbon sequestration schemes and improve their performance.”

Piecing Together the Puzzle 

Verchot along with other scientists from CIFOR and the  International Research Institute for Climate and Society (IRI) have found part of the solution may lie thousands of kilometers away from the Western Amazon – in the Atlantic Ocean.

The amount of rain that falls on the land during its dry season can be predicted from sea surface temperatures in the Atlantic Ocean during the preceding months, according to a 2011 study.

“2005 and 2010 were very peculiar droughts. They were very, very intense… so we started looking at the pull of the Atlantic,” says lead author Kátia Fernandes, associate research scientist at IRI.

“We looked at the frequency of dry events within a decade and found that greater frequency of droughts, or dry years, are related to oceanic patterns,” Fernandes says.

The oceanic pattern in question is the relative warmth of the North in relation to the South Atlantic Ocean. When the northern ocean is warmer, the moisture-bearing Intertropical Convergence Zone, a band of clouds along the equator, is drawn northwards away from the rainforests which reduces moisture transported from the Atlantic to the continent. That means less rainfall in the western Amazon.

What is only now being fully appreciated – via the CIFOR study – is the extent to which decade-long cycles in sea temperatures are synchronised with dry season rainfall in the western Amazon.

Using statistical techniques, the researchers were able to quantify the relationship between decadal cycles in sea temperature and western Amazon rainfall.

They show that when the northern Atlantic Ocean is the warmer of the two oceans, there is a 66 percent probability of three or more years per decade with ‘dry’ dry seasons. The relationship also works in reverse, with three or more ‘rainy’ dry seasons likely (77 percent) per decade when the southern Atlantic is warmest.

For the last decade, the northern Atlantic has been the warmer of the two oceans, which Fernandes says likely “added to the severity” of at least the most recent western Amazon drought.

Science for Decision Makers

Better understanding the factors influencing western Amazon rainfall is not being done for precision’s sake alone. As Fernandes says, it can be difficult for decision-makers to grapple with the findings of climate or hydrological models that focus on long-term trends and make forecasts for ‘the end of the 21st century’.

Read the rest of the story here.