Changes adrift on the South Atlantic

By Elton Alisson
Agência FAPESP – The southern half of the Atlantic Ocean is receiving increasing volumes of water from the Indian Ocean, whose waters are not only warmer but also saltier than the waters from sub-Antarctic regions.
This newly discovered process could cause changes in the composition of the waters of the South Atlantic Ocean – which flow northward to the North Atlantic – and affect the atmospheric temperature of sub-Arctic regions.
The phenomenon is occurring because the waters of the South Atlantic that flow toward the North transport and release heat into the atmosphere at higher latitudes. As they become saltier and, consequently, heavier, they will tend to sink more rapidly before reaching the high latitudes of the North Atlantic. This process could reduce the temperature of the ocean’s surface and the atmosphere in sub-Arctic regions.
These findings, discovered previously with the aid of numerical models, were confirmed by an observational study conducted by an international group of researchers, including Brazilians, that has just been published in the online edition of the Journal of Geophysical Research: Oceans (JGR).
The study is the first based on data collected on the Alpha Crucis – the oceanographic ship FAPESP acquired in 2012 for the Oceanography Institute at Universidade de São Paulo (IO - USP). The ship is part of the international project for analysis of heat circulation in the area – South Atlantic Meridional Overturning Circulation (SAMOC).
This international research effort involves researchers and institutions from the United States, France, Brazil, South Africa, Argentina, Russia and Germany. The participation of Brazilian researchers is funded by FAPESP through a Thematic Research Project conducted under the auspices of the FAPESP Research Program on Global Climate Change and an agreement forged between the Pernambuco Research Foundation and the French National Research Agency (ANR).
“The objective of SAMOC is to monitor the meridional flows and thermodynamic properties of bodies of water in the vertical section along latitude 35°S, which begins at the Chuí region in South America and extends to South Africa, and in what we call SAMBA [SAMOC Basin-wide Array],” said Edmo Campos, a faculty member at IO-USP and coordinator of the project on the Brazilian side.
Sea fronts
According to Campos, this geographic line represents a border region through which the mixture of waters from the Indian and Pacific Oceans enters the subtropical region of the South Atlantic; from there, a significant portion flows on to the North Atlantic as part of the Meridional Overturning Circulation (MOC).
Upon monitoring and detecting signs of variation in this region, it is possible to forecast changes in the temperature of the North Atlantic in the coming decades, indicates Campos.
“The South Atlantic transports heat to the North Atlantic at a rate of 1.3 petawatts, which represents a quantity of energy equivalent to that produced by more than 200,000 Itaipu power plants working at full steam,” explained Campos. “Any small change in this process of transporting heat could set off a series of consequences for the planet’s climate.”
According to the researcher, because of the importance of the South Atlantic and the fact that this is one of the oceanic regions with the fewest maritime observations, a series of international efforts to monitor this region have been undertaken in recent years.
SAMOC is one of these efforts. Brazilian, Argentine and North American researchers assumed responsibility for monitoring the western part of the SAMBA line. Researchers in South Africa and France are studying the eastern region. A collaboration of , and the United States with other countries, including Brazil, is currently planning the implementation of the monitoring system for the central part of the line.
“Our objective is for Brazil to take leadership and cover the entire extension of this latitude [from Chuí to South Africa] in cooperation with other countries to guarantee monitoring of the SAMBA line and obtain information to infer variations in the heat transportation system between the oceans that can eventually have impacts on a regional and global scale,” reported Campos.
The first experiments involving Brazilian participation in the project were conducted in late 2009 during a survey voyage by the hydrooceanographic vessel Cruzeiro do Sul, which was acquired by the Brazilian Navy in partnership with the Ministry of Science, Technology and Innovation (MCTI).
In December 2012, Brazilian researchers conducted a series of experiments during the first international voyage made by the Alpha Crucis.
The first Alpha Crucis voyage
In this first voyage, the vessel left the Port of Santos headed toward 34.5 °S (located 1,400 kilometers from the Brazilian coast). From this point and throughout the length of this latitude, the vessel returned to the Brazilian coast on the border with Uruguay.
During this trajectory over a 17-day period, the researchers on board collected several types of oceanographic data, such as current measurement, temperature, oxygen concentration and water salinity, through a series of instruments available on the vessel.
Additionally, researchers installed a series of conductivity, temperature, oxygen and flow sensors and echosounders with pressure and marine current speed sensors on the sea floor along the western extremity of the SAMBA line.
Called Current, Pressure Inverted Echo-Sounders (CPIES), the echosounders emit a sound signal in the direction of the ocean’s surface. Using the time the signal takes to reach the surface and return to the sea floor, researchers can infer the density and temperature of marine currents and, therefore, estimate the speed with which they transport heat through the vertical cross-section established by these sensors, explained Campos.
“The instruments installed during the maiden international voyage of the Alpha Crucis take continuous measurements; the data are stored and can be collected via satellite through messengers that are released periodically and upon reaching the surface transmit data to be collected by acoustic transducers on oceanographic vessels, which pass near the equipment and extract the catalogued data for analysis,” explained Campos.
“With the first voyage of the Alpha Crucis, we managed to collect a much greater quantity of information than we had and to conduct analysis on the variables in the heat transfer in the South Atlantic to the North Atlantic, as shown in the article published in JGR,” he affirmed.
According to the researcher, the analyses – which also include historical data and the results of numerical models – indicate changes in the South Atlantic. These changes, according to him, are in accordance with the previous hypotheses that the volume of water that the ocean receives from the Indian Ocean is increasing and changing the concentration of salt in its water mass. The basis for this conclusion is that the waters of the Indian Ocean have a higher quantity of salt (salinity) and a higher temperature than those found at 34.5°S in the Atlantic.
This increased concentration of salt in the South Atlantic could change the flow of water to the North Atlantic and the process of heat transfer with the atmosphere, warned Campos.
“Minimal changes in the temperature or the concentration of water change the process of heat transfer from the surface of the ocean to the atmosphere, and the response of the climate could even be catastrophic,” he affirmed.
“The climate depends on how the ocean exchanges heat with the atmosphere and how it redistributes this temperature to the rest of the planet,” stressed Campos.
The researcher stressed that because the data collected cover a period of only 20 months. It is still not possible to obtain signs of climate change based solely on these observations, as these observations would have to be obtained over much longer periods – decades, for example.
The study, however, represents one of the first contributions toward better understanding how heat transfer occurs in the South Atlantic and varies over shorter scales of months and years, he pondered.
“Our objective is to obtain these data for periods much longer than a few years through other planned voyages with the Alpha Crucis,” said Campos.
“At any rate, the study is already a practical result based on data collected on voyages undertaken by the Alpha Crucis, which contributed significantly to the observations in the South Atlantic,” he evaluates.

The article Temporal variability of the Meridional Overturning Circulation at 34.5°S: Results from two pilot boundary arrays in the South Atlantic (doi: 10.1002/2013JC009228), by Campos et al, can be read by subscribers of the Journal of Geophysical Research: Oceans at onlinelibrary.wiley.com/journal/10.1002/(ISSN)2169-9291.