What causes the rainy season to start in the Amazon, Congo, and the Great Plains

Rong Fu was puzzled. Rains come to the southern Amazon rainforest, pounding their water drops through the canopies of the Brazil nut tree, the acai palm, the spiny Tacuma palm, the rubber tree, the ungurahui tree, and a thousand other tree species. These rains come in October. This is sooner than what conventional climate science would expect. Usually the rain comes because of the inter-tropical convergence zones (ITCZs) that bring moisture seasonally. Usually that happens in January. The Amazon rains on the other hand, were arriving several months earlier. The question of what caused the onset of the rainy season was of import, because if the onset could vary in time, then that would impact the food production for millions of people.

Born in China, Rong Fu studied meteorology at Beijing University, then came to the US, where she worked as a professor at the University of Arizona, Georgia Institute of Technology, and UCLA. Over a period of two decades, during her time at each university, she would work out a different part of the onset-of-rain puzzle, each time with different collaborators. Her methods were based in analyzing observational data for patterns that gave clues to weather’s mechanisms.

The first observation Rong Fu made was that there was an increase of the water vapor in the Amazonian air in October. This, she surmised, was correlated to the onset of the rains. She proposed that the water vapor condensed into clouds, which would release latent heat into the surrounding air. The heated air would then rise higher into the upper atmosphere, in a process called deep convection, creating thunderstorms. Because the air had moved upward, this created a low pressure system below. Air from the oceans would then rush in to fill that gap, bringing with it moisture from the ocean. The more the thunderstorms convected air upwards, the more the winds would change their direction to start blowing in from the ocean, altering the large scale atmospheric circulation. Multiple thunderstorms thus could help kick start the rainy season. [1]

[In the wet season the winds blow from the Atlantic ocean towards the southwest. In the dry season the winds blow towards the northwest]

Several years later Rong Fu would make the additional hypothesis that the extra Amazonian October humidity came from forest evapotranspiration. The forests release water that helps generate the thunderstorms that bring in the ocean air. [2,3]

To help verify her hypothesis, Rong Fu needed evidence that the October humidity was coming from evapotranspiration. At that time the biologist Ranga Myeni, from Boston University, was studying the seasonality of trees in evergreen forests. It had been traditionally thought that in the tropics forests would grow more during the wet season, because trees had more water to drink. But, using remote sensing methods, Myeni and his colleagues found that the leaves were growing more at the end of the dry season, as October rolled in. This meant that the forests were growing more, and evapotranspiring more in October, as Rong Fu has hypothesized. When Rong Fu heard about Myeni’s results she got very excited. She contacted him telling him ‘This is the missing piece; this is exactly what I didn’t know before.’

Years later when she came to UCLA, Rong Fu thought to use a technique, invented by Eneas Salati, that could figure out where water vapor came from by tracing its chemical signature. She found John Worden, from the nearby Jet Propulsion Labs of Caltech, who was a specialist in using satellite imagery to study these chemical signatures.

In the 1970’s the Brazilian meteorologist Eneas Salati was studying precipitation recycling (aka the small water cycle), and trying to figure out a way to quantify scientifically how much of the rain had come from the ocean, and how much from the forests. He came up with the genius idea to use isotope tracers to do this. Oxygen and hydrogen in water molecules can take the form of different isotopes, some heavier than others. Ocean water vapor has lighter water molecules, because when ocean water evaporates the lighter ones rise first. Vegetation evapotranspired water will have a greater proportion of heavier water molecules because the suction of the plant sucks up water of all types. By analyzing the weight of the isotopes Salati could determine whether the water vapor was from trees or from the ocean.

In 2017, using methods that had evolved from Salati’s work, Rong Fu, John Worden, and their colleagues looked at the satellite data of the water isotopes, and showed that the a large percentage of the humidity and the rain in October was made of heavier water molecules. [4] This provided further proof that the extra humidity was indeed coming from the forests.

This forests-bring-rainy-season theory helped explain why the dry season in the Amazon had been getting longer over the decades. It points at a culprit for this loss of rain - which is the bulldozing of the Brazilian rainforest to make way for agriculture.

Brazil’s farms produce corn, soybeans, cotton, orange juice, coffee, sugar, meat, ethanol, and other foods to the tune of $230 billion. About half that is for the nation’s use, and about half that is for exports. Agriculture is a quarter of Brazil’s economy.

Brazil’s efforts to grow its agricultural system by chopping down forests was counterproductive. Time magazine wrote “The irony is that the tree-clearing strategy has thrown Brazilian agriculture into a self-inflicted crisis. The country’s rain forests are the key to preserving the wet, stable climate that makes things grow.”

One of the solutions that would help with rain preservation, is to use agriculture systems that keep and utilize trees on the farm, systems like agroforestry. Agroforestry is a solution the Time article also promotes - “growing trees alongside crops is like installing an air conditioner and sprinkler system, or, in the words of one prominent farmer, planting water”. Word about agroforestry is slowly trickling into Brazilian culture. The popular soap opera Pantanal in Brazil, had a character convince their dad to convert their farming methods to agroforestry for the sake of sustainability. The shift to agroforestry will not be easy though, it requires a larger labor force and a change in the supply chains of the agricultural industry. But it may turn out to be a necessary shift, if the loss of rains is to be avoided.

There are many climate scientists studying the impact of deforestation on the Amazonian rain - Antonio Nobre, Francina Dominguez, Roni Avissar, Markus Petters, Abigail Swann, Charles Jones, Anastasia Makarieva, Quentin Lejeune etc. The climate models have to take many factors into account - how deep the roots of trees grow, how much aquifer water tree roots bring up to then evapotranspire, how the frictional roughness of the trees slows the wind, and how deforestation heats up the land and alters atmospheric circulation patterns. Currently, different climate model set their parameters a little differently, which results in some variations on the generally agreed upon narrative of how the loss of rain is happening. The Amazon climate is still a lively field of research.

What brings the rainy season to the Congo rainforest and to the USA’s Great Plains

Having studied the worlds largest rainforest in the Amazon, Rong Fu then turned her attention to the world’s second largest rainforest in the Congo, which is home to mahogany, ebony, agba, iroko, sapele, rubber and coral trees, and the cinchona, rauwolfia, and philodendron congo rojo plants. While the Amazon has quite a few climate scientists studying it, the Congo rainforest has been comparatively unstudied. It was thought at the time that the onset of the rainy season in Congo rainforest rain had to do with the ocean moisture. But by examining satellite photos for the isotopic makeup of the water molecules, Rong Fu, teaming with John Worden once more, found again that it was the evapotranspiration from the rainforest that was triggering the start of the rainy season. The evapotranspiration in the Congo played an even larger role that it had in the Amazon, with 80% of the moisture in the air originating from the vegetation. [5]

Rong Fu also studied the onset of rain in the USA. The Great Plains are flatlands of prarie, steppe and grassland, spread across the central US, encompassing Kansas, Nebraska, North Dakota and South Dakota, and spilling over into neighboring states. Here farms of corn, sorghum, barley, alfafa, wheat, and oats produce a quarter of America’s crops, while livestock farms produce four tenths of America’s beef. Rains are very important to the production of the Great Plains farms, especially as the Ogallala aquifer becomes dangerously, agriculturally overdrawn. The extreme drought of 2012 devastated crops and livestock.

Extreme dry years like 2012 had not been predicted by traditional climate models, and Rong Fu sought to figure out why. In Rong Fu’s eyes the drought is basically a delay of the onset of the rainy season. Traditional climate models had focused more on sea surface temperature changes influencing the onset of the rainy season. Rong Fu though found that the onset of the rainy season depended on the amount of moisture present in the US Southwest in the months before. [6,7,8] The moisture hopped (aka precipitation-recycling/small-water-cycle) from the Southwest to the Great Plains. When Rong Fu included the Southwest moisture data (which included soil moisture), she was able better able to predict when extreme drought would occur. The 2012 drought in the Great Plains thus had its origin in the lack of moisture in the Southwest that year, a conclusion that was also independently arrived at by the atmospheric scientists Julio E. Herrera-Estrada and Francina Domingues using different methods. [9]

This then brings up the question, since the amount of Southwest vegetation affects the amount of moisture in the Southwest, - how much is the logging industry and land development in the Southwest affecting the rain east of it in the Great Plains? How much is the damming and overuse of Colorado River water, which has lead to the drying up of the end of the river, the loss of seasonal overflowing of river banks, and the subsequent lessening of vegetation in the floodplains, affecting the rain in the Great Plains? The question also arises of how much of a role vegetation plays in the onset of the rainy season in the Southwest.

Rong Fu has been solving the puzzle of what brings the onset of the rainy season in the Amazon, in the Congo, and in the Great Plains, over the course of her career, coming to the conclusion that forests and land cover play a significant role. Summarizing her work she says “For a long time we have known that the climate is important to the ecosystem, but the ecosystem is also extremely important for the climate”

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References

[1] Fu, Rong, Bin Zhu, and Robert E. Dickinson. "How do atmosphere and land surface influence seasonal changes of convection in the tropical Amazon?." Journal of Climate 12, no. 5 (1999): 1306-1321.

[2] Li, Wenhong, and Rong Fu. "Transition of the large-scale atmospheric and land surface conditions from the dry to the wet season over Amazonia as diagnosed by the ECMWF re-analysis." Journal of Climate 17, no. 13 (2004): 2637-2651

[3] Fu, Rong, and Wenhong Li. "The influence of the land surface on the transition from dry to wet season in Amazonia." Theoretical and applied climatology 78 (2004): 97-110

[4] Wright, Jonathon S., Rong Fu, John R. Worden, Sudip Chakraborty, Nicholas E. Clinton, Camille Risi, Ying Sun, and Lei Yin. "Rainforest-initiated wet season onset over the southern Amazon." Proceedings of the National Academy of Sciences 114, no. 32 (2017): 8481-8486

[5] Worden, Sarah, Rong Fu, Sudip Chakraborty, Junjie Liu, and John Worden. "Where does moisture come from over the Congo Basin?." Journal of Geophysical Research: Biogeosciences 126, no. 8 (2021): e2020JG006024

[6] Erfanian, Amir, and Rong Fu. "The role of spring dry zonal advection in summer drought onset over the US Great Plains." Atmospheric Chemistry and Physics 19, no. 24 (2019): 15199-15216

[7] Zhuang, Yizhou, Rong Fu, and Hongqing Wang. "Large‐scale atmospheric circulation patterns associated with US Great Plains warm season droughts revealed by self‐organizing maps." Journal of Geophysical Research: Atmospheres 125, no. 5 (2020): e2019JD031460

[8] Wang, Gaoyun, Rong Fu, Yizhou Zhuang, Paul A. Dirmeyer, Joseph A. Santanello, Guiling Wang, Kun Yang, and Kaighin McColl. "Influence of Lower Tropospheric Moisture on Local Soil Moisture-Precipitation Feedback over the US Southern Great Plains." EGUsphere 2023 (2023): 1-16

[9] Herrera‐Estrada, Julio E., J. Alejandro Martinez, Francina Dominguez, Kirsten L. Findell, Eric F. Wood, and Justin Sheffield. "Reduced moisture transport linked to drought propagation across North America." Geophysical Research Letters 46, no. 10 (2019): 5243-5253