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The map of the winds

Air currents lead humidity or smoke from the Amazon to the Prata basin

Between December of 2002 and February of 2003, the scientists who participated in the Brazilian stage of the SALLJEX  Project (South American Low-Level Jet Experiment) launched almost 700 balloons, similar to those used to decorate children’s parties, into the skies above the Amazon. Some of them had carried sensors in a small box, which measured atmospheric pressure, temperature, air humidity and the speed of the winds.

The data analysis – stored in computers in Brazil, Bolivia, Argentina and Paraguay – is detailing out the characteristics and the trajectories of the so-called South American Low-Level Jet, or SALLJ, as well as pointing out the phenomena that help to set them off. Identified in the 1960’s, the jets are born in the Northern Region and cross the country in a southerly direction, establishing a direct relationship between the winds that blow in the Amazon to the south, east of the Andes, and the rains that fall in the Prata basin, a vast area that, as well as São Paulo and the southern Brazilian states, covers Uruguay, the north of Argentina and Paraguay.

“These jets are like flying rivers, which carry humidity from the north to the south”, explains José Antonio Marengo Orsini, from the Weather Forecast and Climate Studies Center (CPTEC) of the National Institute for Space research (INPE) and the coordinator of the work. “The jets locate themselves in the lower bands of the atmosphere, those of up to 3 kilometers in altitude, and travel with velocities that can reach as high as 50 kilometers per hour”, adds Maria Assunção Faus da Silva Dias, a researcher at the CPTEC and a member of the study group. “When they arrive at the Prata basin”, completes Carolina Vera, from the University of Buenos Aires, another who participated in the project, “the jets are one of the details responsible for the occurrence of heavy rains, especially during the summer”.

Marengo, Assunção and Carolina were members of a team of around 50 researchers from eight countries: Brazil, Argentina, Paraguay, Uruguay, Bolivia, Chile, Peru and the United States. The SALLJEX Project integrates into the Variability of American Monsoon System (VAMOS), sponsored by the International Program of Climate Variability and Predictability (CLIVAR), associated to the World Meteorology Organization. For  Marengo, this work helps to estimate the possible impacts caused by the deforestation of the Amazonian Rainforest concerning the climate in the southern portion of Latin America, as well as contributing to improving weather forecast for these areas.

The origin of the low level jets is associated with the Trade Winds coming from the Atlantic Ocean, which invade the Brazilian territory at the upper part of the Northeastern Region. When they arrive at the Amazon basin they absorb a lot of water steam, released  by the leaves of the forest by way of transpiration. Then at the frontier of the state of Acre with Bolivia they encounter the foothills and then mountains of the Andes. The mountains function simultaneously as an accelerator and a barrier, since they boost the circulation speed of the jets and deviate them in a direction towards the south. The jets then pass over the states of Mato Grosso, Mato Grosso do Sul and São Paulo. On their arrival at the Prata basin, they interact with the relief and with cold fronts coming from the South Pole, bringing about the so-called Mesoscale Convective Complexes. These extremely dense clouds, which can reach up to 18 kilometers in altitude and 1,000 kilometers in diameter, can have a life cycle that lasts as long as 36 hours.

Normally formed during the night and principally during the summer, these clouds are responsible for storms and for electrical discharges that occur in the south of the country and in the north of Argentina and Paraguay. “Thanks to the low level jets, when they begin to blow there, it’s just as well to prepare for very heavy rains around the region”, comments Pedro Leite da Silva Dias, a professor at the Astronomy, Geophysics and Atmospheric Sciences Institute (IAG) of the University of Sao Paulo (USP) and a participant in the project. Dias points out that the influence of the jets is more evident during the summer when the humidity is intense; in winter, the drier season, the impact tends to diminish.

The jets that represent the sources of rains, nevertheless, could serve as a means of unchaining elements that are not so welcome. “The problem is that the jets can also transport smoke from forest fires”, alerts Marengo, the main author of the scientific articles that detailed out these research results, published in the magazine Climate Dynamics in January of this year and in the Journal of Climate in June of 2004. “With deforestation increasing”, he says, “it’s supposed that a reduction in the contribution of water vapor from the Amazon vegetation to the atmosphere appreciably affects the transport of humidity to the Prata basin, with direct consequences on the rainy seasons, although as yet it has not been possible to quantify this change”. The alert makes sense. Between 2003 and 2004, the INPE registered the second highest rate of deforestation of the Amazon Rainforest since the start of accompanying this situation began back in 1988. There have been 26,130 square kilometers of trees destroyed, an area similar to that of the state of Alagoas.

Smoke and dust
The impact of the forest fires, one of the main strategies used for the expansion of agricultural frontiers, and very well known: threatens the extinction of species of animals and plants and creates soil erosion, which then gives even less protection. The smoke and the gases liberated – such as carbon monoxide (CO) and ozone (O3) – concentrate themselves in the air and turn the climate drier and produce higher temperatures. Because of the low level jets, it is possible to say that even those who live in the regions of the South and Southeast of the country and even in neighboring countries, are not free of these consequences. Although the smoke emission is localized, its impact is global. Through the forest fires, the jets become less swollen and, instead of water vapor they help to transport dust and pollutant gases to the Prata basin.

The alterations in the climate of the Southern Region can have significant consequences. According to the INPE, in January of 2002 the average quantity of rainfall in the states of Rio Grande do Sul and de Santa Catarina was 100 to 150 millimeters; in 2003, during the same period, the average was maintained – with the difference that, in a long territorial band located in the south of Rio Grande do Sul, this value fell to between 50 and100 millimeters. The following year the two states registered rainfall totaling 50 to100 millimeters, it being that in the northwest of Rio Grande do Sul, the quantity of rain only reached from 25 to 50 millimeters. The situation improved in January of this year, when Rio Grande do Sul and Santa Catarina returned to having rainfall quantities of between 100 and 150 millimeters. It is true that the low level jets are not the only considerations responsible for rains, which are also associated with cold air masses that come up from the South Pole and sea currents, beyond that of El Niño that warms the waters of the Pacific ocean.

Less rainfall
“The economy of the Prata basin basically depends on agriculture and cattle raising, which for their part depend on the rainy seasons”, says Tercio Ambrizzi, a professor at the IAG of USP who participated in the project. This concern is also justified. The harvest in the Southern Region for 2003/2004 was approximately 49 million tons of grains, but the forecast is that this will fall to 45 million in 2004/2005. The specialists speculate that the alteration in the profile of the rains, caused by the natural variability of the climate and by human action, especially the forest fires of the Amazon, could be one of the main reasons responsible for this fall in productivity, since the growing area has remained stable.

This type of humidity transportation began to be studied some four decade ago, when the American William Bonner established the relationship between the low level jets born in the Gulf of Mexico and the humid climate of the central prairies of the United States. Afterwards the German Gordon Gutman, who lived in Argentina, identified similar winds that blew along the Andes, but it was the Tanzanian Hassan Virji, who had settled in the United States, who demonstrated the existence of these jets to also occur in South America, back at the beginning of the decade of the 1980’s.

Twenty years after, on the 19th of January of 2003, in Santa Cruz, in Bolivia, an aircraft loaned by the National Oceanic and Atmospheric Administration of the United States, used along with the data collection balloons, detected low level jets in elevated quantities, traveling at a speed of approximately 40 kilometers per hour. The next day they were measured reaching 50 km/h. A day later enormous clouds covered the skies of Argentina and Paraguay – these were the Mesoscale Convective Complexes. Heavy storms reached these two countries on the 22nd and 23rd of January. “We accompanied all of the process, right from the formation of the jets in the Amazon to the storms in the Prata basin”, celebrated Maria Assunção.

But it’s not always so that the low level jets appear in the bulletins of weather forecasts. The problem does not lie in the resolution of meteorological models but in the fact of having few weather observation stations in the Northern Region of the country. The World Meteorological Organization recommends a weather station every 500 kilometers, but in the Amazon this distance can reach 1,000 kilometers. For this reason, very often the jets pass unperceived and the storms that occur in the Prata basin are not forecast with very much antecedence.

The Project
Brazilian Component of the Low Level Jet Field Experiment to the East of the Andes: Interactions on Meso and Large Scale between the Amazon Basin and the Prata Basin (Salljex-Brazil)
Thematic Project
José Antonio Marengo Orsini – CPTEC/INPE
R$ 1,150,742.09 (FAPESP)