It is no exaggeration to state that the rain in the city of São Paulo every summer flows into the Tiete River, then into the Parana River and finally to the River Plate, until finally this huge volume of water ends its journey in the South Atlantic. This grandiose mass of water can cause substantial changes in the circulation and the physical and chemical characteristics of the waters of the continental platform, to the point of interfering in fishing productivity and in the climate of the coastal regions. Due to their lower salinity and also because rainwater is lighter than sea water, the River Plate waters remain on the surface and form a low salinity plume – a strip of fresh water 50 kilometers wide and 150 kilometers long in the midst of the sea, stretching over 1,500 kilometers, from the estuary of the River Plate to Cabo Frio, on the southeast coast of Brazil.
Brazilian, Uruguayan, Argentine, Chilean and American oceanographers that are part of an international research group named The South Atlantic Climate Change Consortium/SACCC have been working for more than ten years to understand the relationship between air, land and water in the South Atlantic region under the influence of this low salinity plume formed by the waters flowing from the River Plate. They have concluded that the River Plate basin acts – albeit at a proportion that is five times lower – like the Amazon River, which every second pours into the Atlantic Ocean a volume of water equal to the volume of Guanabara Bay. In the ocean, thanks to the winds and the Earth’s rotation, this mass of water flows to the North along the continental platform for long distances until it finally becomes fully mixed with ocean waters. The interaction between the continental and ocean waters has consequences that are still unknown in the process of climate changes. However, studies based on mathematical models strongly suggest the possibility of changes in the rainfall pattern, whereby it rains more in some places and less in others in Brazil, from the Amazon Region to the pampas.
By comparing satellite images with data from oceanographic expeditions, and by resorting to mathematical models, the researchers concluded that two factors contribute decisively to the increased outpouring from the River Plate and its effect on the circulation of the sea water: the rainfall volume on the hydrographic basin drained by the Parana and Paraguay Rivers and the South Atlantic winds pattern. The sea water turmoil caused by the River Plate’s waters is far stronger than had ever been expected, both in terms of its complexity and in relation to the geographical area affected by this outflow. Basically, this volume of water interferes in two systems – both of them related to the climate – in the South Atlantic. “Every time an intense El Niño phenomenon occurs, the amount of rainwater that reaches the Plate can double, because it rains more over the central part of South America,” explains Edmo Campos, a researcher from the Oceanographic Institute, who has coordinated this research consortium since it was established in 1996. “If this is coupled with an atypical wind pattern, in which the winds blow from the southwest, all the fresh water is pushed towards the Brazilian coast.”
Sometimes, the Plate’s fresh water plume creates phenomena that are not readily explained. Campos says that in the seventies, another professor – the late Yasunobu Matsura – from the Oceanographic Institute had already detected a low salinity plume of water in the Santos basin on the coast of the State of São Paulo. “He did not know what it was, but suggested that the disappearance of schools of sardines at that time was probably related to an unknown mass of water.”
Might it have been the mass of fresh water from the Plate, pushed north by the winds? This is what Campos believes. In his opinion, the low salinity water plume might have resulted from the combination of more water from the River Plate and the wind pattern, which could have interrupted the flourishing of nutrient-rich waters known as CWSA (central water from the South Atlantic). This body of water originates in the southernmost tip of Argentina and flows northwards, counterclockwise, in the subtropical region of the South Atlantic. It normally flows deeper midway and then resurfaces around the region of Cabo Frio, bringing up from the bottom of the sea the nutrients that maintain the food chain. More fresh water on the surface, as a result of a stronger El Niño, might prevent the upflow of this nutrient-rich water, resulting in less food for fish that live in shallower coastal waters.
The extension of the Plate’s plume is a seasonal phenomenon: it tends to be greater in the winter than in the summer. It can have a strong impact upon the inhabitants of the coast. “During the winter, these originally cold waters flow northwards, robbing warmth from the atmosphere and changing the local atmospheric circulation. This can alter the rainfall pattern, which has an indirect impact on a range of social and economic activities,” says Campos. As the Enso (El Niño – Southern Oscillation) tends to become stronger because of global warming, the climate in southern South America may change substantially, with more rain in some areas and droughts in others. The researchers are beginning to pay more attention to the pampas, where farmers could be severely affected by these rainfall pattern changes.
The oceanographers want to gain a deeper understanding of mechanisms to capture the carbon, one of the phenomena associated with global warming in the South Atlantic. Global warming is caused by excess carbon in the atmosphere. Given that a higher quantity of this gas is in the air, could it be that the bomb to capture the carbon, that is the ocean, has become somewhat clogged? The seas are good reservoirs of carbon because they exchange gases with air: oxygen exits while carbon gas enters.
There are signs that this gaseous balance might directly affect the productivity of the ocean’s biomass – for example, the production of phytoplankton, the organisms that produce oxygen by photosynthesis and that are transported by the ocean currents. Researchers have found there is direct interference of stronger warming in the south of South Africa, in a region of the Atlantic that is close to the spot from where 16th century European navigators circumvented the globe on their way to the Indies. In the area near the Cape of Good Hope, the waters from the Indian Ocean and the Atlantic mix. If this fails to take place as expected, the temperature and the salinity of the South Atlantic and the rainfall pattern of the continental regions of South America and Africa will be affected.
If the upcoming studies confirm another conclusion – the displacement of the Intertropical Convergence Zone/ITCZ, a region that links the South Atlantic and the North Atlantic, and that is crucial to regulate the climate in Brazil – the relationship between global warming and the Amazon and Northeast regions of Brazil could become more complex. According to Campos, changes in weather patterns around South America could lead to more rainfall in Brazil’s Northeast, while the Amazon Rain forest could turn into a sparser and less humid jungle.
A rainy Northeast?
This argument is based on the analysis of an index that lists sea temperatures on the surface of the Tropical Atlantic, referred to as Atlantic gradient mode, previously know as dipole. When this index is positive, the surface temperatures of the Tropical North Atlantic are usually higher than normal, while those of the Tropical South Atlantic are colder. The opposite occurs when the index is negative. Campos points out that the amount of rainfall in the Northeast of Brazil is closely related to this index: there is more rain when the dipole index is negative. This group believes that the relationship between these two regions of the Atlantic Ocean is more important than El Niño, which occurs in the Pacific Ocean, to determine the rainfall pattern in the Northeast of Brazil and in a region of Africa called Sahel, which lies between the Sahara Desert and the more fertile lands to the south.
A better understanding of the behavior of these weather phenomena could help improve weather forecasts and prevent tragedies, such as the one that occurred in March 2004. A small cyclone began to form in the South Atlantic; because of the high amount of heat on the ocean’s surface, this cyclone gained strength and turned into hurricane Catarina, the first ever to hit the Brazilian coast. As the researchers and technicians had no efficient ocean monitoring system, they were unable to predict that the amount of heat was more than enough to feed the cyclone until it had become a hurricane. In addition to destroying property and causing two deaths, the hurricane forced three thousand people to flee their homes when the strong winds came. “A more accurate monitoring system would have been crucial to keep closer track of these phenomena,” says Campos. As the data collected by Campos’s team indicates, global warming will produce more heat on the surface of the South Atlantic, while extraneous episodes such as the Catarina hurricane could become more frequent in the forthcoming years.
Large-scale oceanographic survey on the southeast continental platform of South America (LAPLATA); Modality: Individual Research Grant; Coordinator: Edmo J. D. Campos – IO/USP; Investment: R$ 130,776.43 (FAPESP), US$ 178,000.00 (United States Office of Naval Research) and US$ 900,000.00 (Inter-American Institute for Global Change Research)