Rainfall in the past

Study estimates surface temperatures of the South Atlantic in the last 12,000 years and their connection to precipitation over the South American continent

Maps of average temperatures on the ocean surface. In the larger map, pink denotes warmer areas; in the smaller, more detailed map,  higher temperatures are shown in red

NASA'S EARTH OBSERVATORYMaps of average temperatures on the ocean surface. In the larger map, pink denotes warmer areas.NASA'S EARTH OBSERVATORY

The drought affecting the state of São Paulo in 2014 has surprised everyone, and it is endangering the water supply in the state capital and surrounding municipalities. Earlier forecasting might have facilitated measures to keep the reservoirs better supplied. The work of an international group of researchers could help scientists make more accurate calculations of variations in rainfall and moisture in South America and lead to quicker action in the future. The study, published in Scientific Reports in June 2014, combined estimates of temperatures in the past with mathematical modeling to reconstruct the temperature on the surface of the South Atlantic over the last 12,000 years. In addition to establishing a more accurate picture of the climate during that period, the study could help scientists understand the dynamics between ocean temperatures and moisture on the continent.

The team of four scientists headed by Ilana Wainer of the Oceanographic Institute of the University of São Paulo (IO-USP) observed that an increase in temperature in the northern South Atlantic near the Equator was associated with a higher volume of rainfall in the area that is now the Brazilian Northeast, and less rainfall in the Southeast, over the last 12,000 years. Conversely, the Northeast experienced more severe droughts and the Southeast had more rainfall when the temperature of the South Atlantic was higher.

The best explanation for the climatic variation during those 12,000 years, the researchers say, was a South Atlantic temperature distribution pattern similar to what is being observed today, with periods in which the temperature of the surface water was higher to the north, and other periods when it was higher to the south. The researchers use the term South Atlantic Subtropical Dipole to refer to the temperature distribution pattern in which the ocean appears to have a warmer pole and a colder one—with the occasional inversion. “If it existed during those 12,000 years, this phenomenon could have had a significant effect on rainfall distribution on the continent,” says meteorologist Luciana Figueiredo Prado, coauthor of the paper, who is a doctoral student under Ilana Wainer at IO-USP.

This conclusion is somewhat surprising. Prior to that time, the variation in the volume of rainfall in South America was attributed mainly to the influence of the El Niño phenomenon—temperature fluctuations on the surface of the Pacific that occur over short periods of 15 to 18 months. But some studies had already shown that El Niño does not fully explain the variations in the present-day rainfall patterns in South America. Some of the variation (about 20%) appears to stem from changes in the surface temperature of the South Atlantic. For this reason, even though the 2014 drought in São Paulo is linked to El Niño, Wainer believes that it is not the whole story. “We showed that conditions in the South Atlantic are also important for determining the precipitation scenarios in South America,” the researcher says. “And this should not be ignored.”

 in the smaller, more detailed map, higher temperatures are shown in red

Oceanography, Climate and Cryosphere Laboratory / IO-USPIn the smaller, more detailed map, higher temperatures are shown in redOceanography, Climate and Cryosphere Laboratory / IO-USP

History in the sediments
The team used two earlier studies as a starting point for reconstructing the temperature variations in the South Atlantic over the last 12,000 years. In one study, Maria Alejandra Gómez Pivel of the Federal University of Rio Grande do Sul and her colleagues at IO-USP had collected samples of marine sediments taken at a depth of 827 meters in the Santos Basin. The sediments contain fossils of single-celled creatures—the Foraminifera—which reproduce in varying quantities depending on the water temperature, and are precipitated to the ocean floor when they die. These fossils enabled the researchers to estimate the surface temperatures on the western edge of the South Atlantic in the last 13,000 years.

These results, published in 2013 in Palaeogeography, Palaeoclimatology, Palaeoecology, were combined with the results of another paper published in 2005 in Paleoceanography. In that study, researchers in the U.S. had collected marine sediments at a depth of 1,992 meters off the coast of Namibia in Africa, and dated them using another technique. By measuring the ratio of the chemical elements calcium and magnesium in the Foraminifera fossils, they obtained indirect references for temperature values on the surface of the ocean in that region over the last 21,000 years.

Wainer and Prado, collaborating with one researcher in France and another in the U.S., used these measurements to create the prior history of the South Atlantic Subtropical Dipole in the last 12,000 years—or a close proximity.

After reconstructing the ocean temperature patterns, the researchers did a climate simulation that indicated the distribution of the rainfall during that period. To compare the results of the model with actual conditions, they looked for indirect records of the rainfall history on the continent obtained by other groups. The most common way to obtain these records is through the study of speleothems—stalactites and stalagmites—that form inside caves as a result of deposition of sediments dissolved in rainwater. The larger the volume of rainfall in a given period, the thicker the layer formed on a speleothem. By comparing the precipitation data with the simulations, they were able to build a model capable of reconstructing the ocean-atmosphere dynamics of the past and predict what they might be in the future.

The researchers now want to refine the model by increasing the number of samples of marine sediments analyzed. To collect the sediment, they plan to use the Alpha-Crucis, the state of São Paulo’s new oceanographic vessel. “The goal is to understand these drought and excess rainfall events on the South American continent, taking into account the temperature variation on the surface of the South Atlantic and the way in which these changes in temperature alter moisture and wind transport,” says Wainer. “We also plan to incorporate external factors into the analysis that have rarely before been explored, such as vulcanism.”

1. Investigation of the evolution of the sub-surface ocean in the South Atlantic for the last millennium: impact on climate change (No. 2013/02111-4); Grant mechanism Scholarships abroad; Principal investigator Ilana Coaracy Wainer (IO-USP); Investment R$59,257.59 (FAPESP).
2. Air-sea interaction in the South Atlantic region: mechanisms of climatic variability during the last millennium (No. 2013/11496-7); Grant mechanism Doctoral scholarships abroad; Grant recipient Luciana Figueiredo Prado (IO-USP); Investment R$81,928.18 (FAPESP).

Scientific article
WAINER, I. et alReconstruction of the South Atlantic Subtropical Dipole index for the past 12.000 years from surface temperature proxyScientific Reports. 13 jun. 2014.