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New study by Prof. Jonathon Wright published in PNAS shows that the southern Amazon rainforest makes its own rainy season


On 21 July 2017, an international collaborative study co-led by Jonathon Wright at Tsinghua University and Rong Fu at the University of California-Los Angeles was published online in the Proceedings of the National Academy of Sciences USA. This study provides the strongest observational evidence to date that rainforest transpiration does not merely respond to the seasonal cycle of rainfall but also plays a central role in initiating the local wet season.
Rainforest health depends on the amount of rainfall and the length of the dry season. In the southern Amazon, both rainfall amount and dry season length are close to the threshold for sustaining rainforests. Conversion of rainforest to farmland and a trend toward longer dry seasons have left this region increasingly vulnerable to climate change and raised the spectre of a tipping point beyond which the remaining rainforest can no longer be sustained. Given the importance of Amazonian rainforests to global biodiversity, carbon cycling, and atmospheric hydrology, such a tipping point not only threatens regional ecosystems and agriculture, but could also disrupt other aspects of the global Earth system. An increase in the frequency of extreme droughts within the past 15 years has heightened the urgency of understanding the root causes behind rainfall seasonality in this region.
This study brought together an international team of researchers with expertise in climate dynamics, satellite remote sensing, climate modeling, and data analysis to address the question: are rainforests in the southern Amazon an active or passive participant in the seasonal cycle of rainfall? The idea that rainforest transpiration initiates the wet season has been debated for more than a decade, but observational evidence was limited and key mechanisms were unclear. In this study, measurements from multiple satellites are used to show that rainforest transpiration initiates the onset of the wet season in the southern Amazon (Figure 1). The evolution of rainfall from the peak of the dry season through wet season onset (day 0) over the southern Amazon is shown at upper left. Vertical profiles of atmospheric moisture content (upper right) illustrate that shallow cumulus clouds that develop over the rainforest pump moisture into the atmosphere. This moisture pump prepares the way for the frequent strong storms and heavy rainfall of the wet season, but where does the moisture come from? The answer to this question is provided by satellite observations of the isotopic composition of water vapor above the rainforest (lower right), which function as “fingerprints” that can distinguish the contributions of rainforest transpiration from those of ocean evaporation. When used to track the evolution of moisture sources, these ratios show that rainforest transpiration initiates and primes the atmospheric moisture pump. This pumping action changes the winds to bring moisture from the nearby ocean into the southern Amazon, where it is turned into rain that sustains the local environment.

Figure 1. The transition from the dry season to the wet season over the southern Amazon.

The results of this study show that enhanced rainforest photosynthesis during the late dry season (green lines at lower left) causes the transition to the wet season to occur much earlier than it would without the rainforest, but this situation may be changing. The transition to the wet season now occurs about a month later than it did in the 1970s. The conversion of rainforest land to agricultural use has likely contributed to this delay. Aerosol pollution produced by forest fires (grey bars at lower left) may also affect the efficiency of the atmospheric moisture pump and change the timing of the transition. This pollution is most severe during the transition season. Understanding how these processes may change in the future is complicated by the difficulty of accurately representing rainforest transpiration and shallow cumulus clouds in global models. The central importance of these processes for initiating wet season onset over the southern Amazon suggests a useful litmus test for models aiming to project the future evolution of climate in this region.
Jonathon Wright is an Associate Professor at Tsinghua University and the first author of the paper. Dr. Wright joined the Department of Earth System Science in January 2012. His research group focuses on understanding the movement and cycling of water and energy in the atmosphere. This study was supported at Tsinghua University by the Young Thousand Talents Plan and the NSFC Research Fund for International Young Scientists.