New Study Shows the Amazon Makes Its Own Rainy Season

A new study gives the first observational evidence that the
southern Amazon rainforest triggers its own rainy season using
water vapor from plant leaves. The finding helps explain why
deforestation in this region is linked with reduced rainfall.

The study analyzed water vapor data from NASA’s Tropospheric
Emission Spectrometer (TES) on the Aura satellite, along with
other satellite measurements, to show that at the end of the
dry season, clouds that build over the southern Amazon are
formed from water rising from the forest itself. The research
is published in the journal Proceedings of the National Academy
of Sciences (PNAS).

It’s been a mystery why the rainy season begins when it does in
the Amazon south of the equator. In most tropical regions, two
factors control the timing of the rainy season: monsoon winds
(a seasonal change of direction in prevailing winds) and the
Intertropical Convergence Zone (ITCZ), a belt of converging
trade winds around the equator that shifts north or south with
the seasons. The southern Amazon experiences both of these. But
they don’t occur till December or January, while the rainy
season currently starts in mid-October — two or three months
earlier. So what does set off the increase in rainfall?

Scientist Rong Fu of UCLA, a leader of the new research
efforts, published a paper in 2004 suggesting that increased
evaporation of water from leaves — a process known as
transpiration — might be the cause. “We didn’t have hard
evidence,” she said. “We speculated that the moisture came from
vegetation because satellite measurements showed the vegetation
became greener at the end of the dry season.”

Greener plants are a likely indicator of increased plant growth
and transpiration, but not a definitive one. Also, color
measurements can’t show how much water vapor is moving from the
plants to the atmosphere or whether it’s rising high enough in
the atmosphere to make clouds and rain. So the speculation
remained just that, until now.

John Worden of NASA’s Jet Propulsion Laboratory in Pasadena,
California, developed a data analysis technique for TES that
enabled Fu, study first author Jonathon Wright (Tsinghua
University, Beijing) and colleagues to pinpoint the moisture
source. The technique distinguishes between hydrogen and its
heavier isotope deuterium, which combines with oxygen to make
heavy water. Lighter isotopes evaporate more easily than
heavier isotopes. That means water vapor that evaporated into
the atmosphere has less deuterium than liquid water. For
example, water vapor that evaporated from the ocean has less
deuterium than water that’s still in the ocean.

Water that is transpired by plants, on the other hand, has the
same amount of deuterium as water that’s still in the ground —
the plant sucks water out of the ground like a straw, no matter
which isotope the water contains. That means water vapor
transpired from plants has more deuterium than water vapor
evaporated from the ocean.

This difference is the key that allowed the scientists to
unlock the rainy-season mystery. The two isotopes have
different spectral “signatures” that can be measured from space
by the TES instrument. The measurements showed that, during the
transition from dry to wet season, transpired water becomes a
significant moisture source for the atmosphere, and in
particular for the middle troposphere, where the increasing
water vapor provides the fuel needed to start the rainy season.

“What we showed is that during the dry season water from
vegetation is pumped into the middle troposphere where it can
turn into rain,” said Worden, a coauthor on the new paper.

The finding raises another question: Why do plants start to
grow and transpire more during the dry season, before there’s
an increase in rain? That’s still a subject of research, Fu
said. “This may be the way the forests optimize their growth.
In the late dry season, plants still get sunshine, and they
could anticipate the coming rainy season because they are
adapted to the seasonality of the rain.”

That seasonality has been changing in recent decades, however.
The rainy season in the southern Amazon now starts almost a
month later than it did in the 1970s. There’s evidence that if
the Amazon dry season becomes longer than five to seven months,
the forest will no longer receive enough rain each year to keep
trees alive, and the region will transition from forest to
grassy plains. Over a large fraction of the southern Amazon,
the dry season is now only a few weeks shorter on average than
this transitional threshold. There has already been some
irreversible damage to the forest. The loss of a major
Amazonian forest ecosystem could increase Brazilian droughts
and potentially disrupt rainfall patterns as far away as Texas.

The reasons for the delayed onset of the wet season are not
completely understood, but the new study adds evidence to the
idea that deforestation is playing a role. Reducing the trees
available to produce moisture would naturally reduce the
forest’s cloud-building capacity. If deforestation slowed the
increase in transpiration to the point that it could no longer
trigger a rainy season, rains wouldn’t begin till the ITCZ
arrived at the end of the year.

The finding highlights how closely connected the rainforest
ecosystem is with climate, Fu said. “The fate of the southern
Amazon rainforest depends on the length of the dry season, but
the length of the dry season also depends on the rainforest.”

The PNAS paper is titled “A rainforest-initiated wet season
onset over the southern Amazon.” Wright is the first author.
Besides Fu and Worden, additional coauthors are from Google
Inc., Mountain View, California; the Dynamic Meteorology
Laboratory (LMD), Paris, France; and the University of Texas at

News Media Contact

Alan Buis
Jet Propulsion Laboratory, Pasadena, California

Written by Carol Rasmussen
NASA’s Earth Science News Team