Amazon rainforest triggers its own rainy season

The southern Amazon rainforest triggers its own rainy season, a new study has revealed. The phenomenon is a result of water vapour from plant leaves – and it helps explain why deforestation is linked to reduced rainfall in the region.

Two factors control the timing of the rainy season in most tropical regions – monsoon winds and a belt of converging trade winds around the equator known as the Intertropical Convergence Zone (ITCZ).

The southern Amazon experiences both factors in December or January. However, the region’s rainy season starts several months earlier. Now, a study conducted by NASA has unveiled the forces that could be at the root of this early rainfall.

The study used water vapour data from NASA’s Tropospheric Emission Spectrometer on the Aura satellite, along with other satellite measurements, to show that the clouds that build over the southern Amazon at the end of the dry season are formed from water rising from the forest.

During the transition from the dry to wet season, transpired water becomes a significant moisture source for the atmosphere – in particular for the middle troposphere, where the increasing water vapour 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,” commented paper co-author John Worden of NASA’s Jet Propulsion Laboratory.

To unlock the findings, Worden developed a technique that distinguishes between hydrogen and its heavier isotope deuterium, which combines with oxygen to make heavy water. Lighter isotopes evaporate more easily than heavier isotopes, so water vapour that has evaporated into the atmosphere has less deuterium than liquid water.

However, water that is transpired by plants has the same amount of deuterium as water that’s still in the ground. The plant sucks water out of the ground no matter which isotope the water contains.

The new study adds evidence to the theory that deforestation 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 would not occur until the ITCZ arrived.

Photo credit: Joseph King/ CC BY-NC-ND 2.0

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