In early 2014, the Cantareira system, the main reservoir that feeds São Paulo, hit less than 10% of its capacity because of intense heat and drought, forcing Brazil’s most populated region to severely restrict its water consumption. Scientists working to understand the phenomenon say droughts like this can happen again and are likely to hit the region even harder.
Marine heat waves—periods of extreme ocean temperatures—are associated with droughts, which are linked to extreme heat not only over land, but also in the ocean.
A study led by Regina Rodrigues, from the Department of Oceanography at the Federal University of Santa Catarina, Brazil, sheds light on this mechanism. It was published last year in Nature Geosciences and will be presented at AGU’s Fall Meeting on 15 December.
Analyzing sea surface temperature data from NOAA and atmospheric data from the European Centre for Medium-Range Weather Forecasts, Rodrigues’s team found that persistent high-pressure systems over eastern South America inhibit the formation of clouds over the region leading to drought. “The lack of clouds, in turn, allows more solar radiation to reach the sea surface, causing both land and marine heatwaves,” she said. As a result, in early 2014 the western South Atlantic Ocean also experienced a persistent and intense marine heatwave.
A new study in Scientific Reports shows that this mechanism also occurs in other parts of the world. “We found that most subtropical extreme marine heatwaves were also triggered by persistent atmospheric high-pressure systems and anomalously weak wind speeds,” said Rodrigues, who also co-authored the study. Weak winds can intensify marine heat waves since they reduce the ability of the ocean to cool down.
Heat Waves and Droughts Go Hand in Hand
Droughts and heat waves are likely to become more frequent. Using data from 1982 to 2016, Rodrigues and her team observed that the frequency, duration, intensity, and extension of these phenomena have increased.
João Geirinhas, a researcher at the University of Lisbon, found similar results in another study that is currently in press. His team looked at the increase in the frequency of droughts and heat wave events between 1980 and 2018. “Droughts and heat waves go hand in hand since the former can cause the latter,” he said. Geirinhas will present the work at AGU’s Fall Meeting on 15 December.
Geirinhas’s study finds the concurrence of droughts and heat waves spiked after 2010. São Paulo, for example, had a peak in the mid-1980s (with a less than 30% chance of a marine heat wave and drought happening simultaneously), but that peak was surpassed in the mid-2010s (when there was an almost 50% chance of a concurrence).
“Climate change can make these effects stronger and longer lasting,” Geirinhas said.
Human activity can contribute to conditions influencing marine heat waves and drought. Wilson Feltrim, coordinator of the Climatology Laboratory at the Federal University of Paraná, warned that deforestation can contribute to the phenomena. “The loss of trees decreases rainfall year after year. What happened in 2014 can become more frequent due to deforestation and climate change,” he added.
To Feltrim, the studies from Rodrigues and Geirinhas are complementary. “While Rodrigues looks at the genesis of the phenomenon, Geirinhas looks at the intensification in its occurrence,” said the researcher, who did not take part in either study.
Maria Assunção Dias, a senior professor of atmospheric sciences at the University of São Paulo who was also not involved in either study, agreed with Feltrim. “These studies hand different pieces of a puzzle that fit together to help explain an absolutely unprecedented event in our lives as researchers.” The changes that we are seeing in the climate probably have not been witnessed by the human species before, she added.
“It might have happened millennia before, but we didn’t have the data to understand it and weren’t here to see,” said Dias.
—Meghie Rodrigues (@meghier), Science Writer
14 December 2020: This article has been corrected to clarify the work of Regina Rodrigues.