A river that flows through the air

Winds from the North increase moisture in Brazil's Southeast and South

MARGI MOSS/BRASIL DAS ÁGUASRecycling: evaporation in the Amazon region puts part of the rainwater back into the atmosphereMARGI MOSS/BRASIL DAS ÁGUAS

On some days, a river of the magnitude of the Amazon crosses the skies of Brazil. Its source lies over the Atlantic Ocean, near the Equator; it becomes stronger over the Amazon Rainforest, and continues west to the Andes, where upon meeting this imposing rock face, it  deflects to the south. From there, this huge volume of water floats over Bolivia, Paraguay and the Brazilian states of Mato Grosso, Mato Grosso do Sul, Minas Gerais and São Paulo. Occasionally it reaches the states of Paraná, Santa Catarina and Rio Grande do Sul before returning to the ocean. Despite its size, nobody sees it, because this river has no banks and no fish: it is a metaphorical – but not a fictional – river, formed by a water vapor column some three kilometers high, hundreds of kilometers wide, and thousands of kilometers long.

Meteorology and hydrology experts know about this flying river – actually, currents of humid winds that have been called, technically speaking, low-level jet streams – since the early 1960’s, but only now have they started to develop a better acquaintance with the source of its water and  how it interacts with the planet’s surface, or with how it helps to form giant clouds that rise 15 kilometers high about the ground.

The earliest data from the Flying Rivers project, released on March 19 in São Paulo, confirm the composition changes that this current of humid wind goes through as it travels across Brazil. Most of the water vapor comes from the ocean, transported to the continent by the trade winds that blow from east to west – a small portion of this humidity condenses into clouds that turn into rainfall over the Amazon Region. But most of the vapor forming this invisible river comes from the actual woodlands. When passing over the world’s largest tropical rainforest, it incorporates the water that evaporates directly from the soil as well as the water that the plants draw out of the ground and release as vapor into the atmosphere. Some this Amazon Region water reaches the country’s south and possibly turns into rainfall.

To figure out the source of the water of this flying river, one of the many that stream across the Brazilian sky, a partnership arrangement was set up that is rather unusual in the world of science. It brought together researchers with a theoretical knowledge of hydrological and atmospheric phenomena, and an engineer and explorer used to experiencing them in his flights around the world. Thanks to Petrobras sponsorship, Gérard Moss, the engineer, carried out 12 flights over different areas of Brazil, starting in 2007. These were planned jointly with teams from the University of São Paulo (USP), the National Institute of Space Research (Inpe), the State University of Rio de Janeiro (Uerj) and the Brazilian Foundation for Sustainable Development (FBDS).

In each trip, Moss, Swiss born but a naturalized Brazilian, and his flight companion, Tiago Iatesta, collected on their single-engine plane samples of the water vapor that condensed on a tube chilled with dry ice. In one of these flights, under extremely favorable atmospheric conditions, Moss followed the flying river from the Amazon Region all the way to São Paulo and calculated that, in one particular segment, the amount of water flowing in this current was 3,200 cubic meters a second, more than in the São Franciso river. All the water carried along this air current in 24 hours would equal 115 days of the water consumption of the city of São Paulo, which has 11 million inhabitants.

Chemical signature
In slightly less than two years, Moss and Iatesta collected some 500 samples of water at heights ranging from 500 to 2,000 meters. These are now being examined in the Isotopic Ecology Laboratory of the University of São Paulo in Piracicaba by Reynaldo Victoria’s team. The analysis of a kind of chemical signature (the proportion of radioactive oxygen and hydrogen atoms) from the water samples and a comparison of this signature with samples of rain water and of  water from rivers throughout the entire country will allow the researchers to learn how the composition of this air mass changes as its advances over the continent. It should also clarify how much  humidity that results from evaporation in the Amazon Region adds to rainfall in the South and Southeast.

“These data fill a major gap in our knowledge about these humid currents and allow one to check the validity of the climate models developed for Brazil”, states Pedro Leite da Silva Dias, a researcher from the Institute of Astronomy, Geophysics and Atmospheric Sciences (AIG) at the University of São Paulo (USP) and head of LNCC, one of the partners in the project.

Years ago, IAG and Inpe groups took part in a study conducted jointly with Bolivia, Paraguay and Argentina, which measured  with equipment launched in balloons the temperature, humidity, pressure and speed of these humid air currents nicknamed flying rivers by climatologist José Marengo, from Inpe (see Pesquisa FAPESP, issue 114). “At that time, a plane collected water vapor samples only in other countries, because we hadn’t managed to get authorization to fly within Brazil”, Dias tells us.

It will take years to analyze all the more recent data collected. However, a preliminary assessment shows that the contribution of the Amazon region to the humidity that reaches the Southeast and South of Brazil is far from negligible. At Inpe, Demerval Moreira and Wagner Soares used mathematical weather forecast models to estimate how much water from the Amazon Region is carried by these currents into other regions. They concluded that on the days on which this flying river passes over the Amazon Region (about 35 days a year only) more humidity reaches the Brazilian Midwest, Southeast and South, increasing the likelihood of rainfall.

“When these winds pass over the Amazon region, they raise the humidity of the air in the city of Ribeirão Preto [State of São Paulo], for instance, by an average of 20 to 30 percent, increasing the rainfall potential”, tells us Dias. Sometimes, this increase can reach 60%. “We’re now trying to calculate how much of this humidity from the Amazon Region actually turns into rain”, he explains.

Moss is concerned about the pace of deforestation that he observes from his plane – and that may change the climate in the Amazon Region, with an impact that may affect the rest of the country. “The forest works like a cover: the plants and the soil retain rainwater, which then penetrates into the ground and is stored, before evaporating”, explains Dias. Without the rainforest, the humid winds that blow from the ocean over the continent may get to the south of Brazil faster, in two or three days, increasing the risk of storms. “Removing the forest would cut rainfall over the Amazon Region by 15 to 30 percent, according to several climate models, but increase rainfall in the South and in the River Plate basin”, Dias explains.

“The information currently available in Brazil”, states Eneas Salati, creator of the Isotopic Ecology Laboratory at USP in Piracicaba and one of the people who masterminded this project, “is more than enough to justify putting in place a policy of stopping deforestation and starting reforestation.”