Start of summer in Brazil sees record number of extreme rain events

Survey by National Disaster Surveillance and Early Warning Center (CEMADEN) highlights simultaneity of extreme weather episodes due to heavy rain in North, Southeast and parts of Center-West, and drought in South and parts of Northeast.

By Elton Alisson  |  Agência FAPESP – The start of the current summer season in Brazil saw a record number of extreme rain events, according to a survey by the National Disaster Surveillance and Early Warning Center (CEMADEN), which is based on São José dos Campos, São Paulo state.

In the period December 1-31, 2021, the center’s technical staff issued 516 warnings of geohydrological disasters such as mudslides, floods and flash floods for the 1,058 cities it monitors nationwide, and disasters actually occurred in 163 of these cases.

In the same period of 2020, it issued a similar number of warnings (539) but the number of occurrences was much smaller, totaling 103, or 60% less than a year later.

“We issued hazard warnings almost every single day in December 2020 and 2021. I don’t recall any other period when we’ve had so much activity,” Rafael Alexandre Ferreira Luiz, a technologist at CEMADEN, told Agência FAPESP.

“Considering the number of warnings issued, there were more extreme rain episodes in 2020 and 2021, and the same will probably be true of 2022, significantly heightening the risk of disasters in vulnerable areas.”

Besides the frequency and intensity of the rainstorms that have occurred in most of Brazil so far this summer, another factor that has drawn the attention of the center’s researchers is that unusually heavy rain in the North, Southeast and part of the Center-West has occurred simultaneously with drought or water shortages in the South and part of the Northeast (Pernambuco, Paraíba, Sergipe and Rio Grande do Norte states).

According to the researchers, La Niña is the main reason for the extreme rainfall seen in the states of Bahia, Minas Gerais, Piauí, Tocantins and Maranhão, and for the drought in the South. La Niña is an oceanic and atmospheric phenomenon that begins with the cooling of sea surface temperatures along the tropical west coast of South America. It occurs at intervals of between two and seven years, changing rainfall and temperature patterns worldwide. In Brazil, it causes heavier rain in the North and Northeast, and warmer and drier weather in the Center-West, Southeast and South. 

As for the huge amount of rain that has fallen recently in southern Bahia, the researchers blame a typical summer weather system known as the South Atlantic convergence zone (SACZ), an elongated axis of cloud, rain and wind stretching northwest to southeast, from the Amazon basin to the Southeast and South regions, acting as a strong convection band, and accounting for heavy rain above all in Minas Gerais, Espírito Santo and Rio de Janeiro states.

A rare configuration involving three SACZ episodes in a short period occurred in December 2021, mainly in southern Bahia, where precipitation greatly exceeded the historical average for the period. 

These SACZ episodes were also associated with low-pressure systems known as upper tropospheric cyclonic vortices (UTCVs), which are very frequent on the coast of the Northeast at this time of year and also contributed to the continuously heavy rainfall in southern Bahia.

“Typical weather patterns for the region wouldn’t have predicted rain at this time, let alone such huge amounts,” said José Marengo, a researcher at CEMADEN and principal investigator for the National Science and Technology Center for Climate Change (INCT-MC), one of the INCTs in the state of São Paulo funded by FAPESP in partnership with the National Council for Scientific and Technological Development (CNPq).

“The December rains in this region were abundant, irregular, and concentrated in three relatively short periods. Their impact was substantial owing to their volume and intensity,” Marengo said.

Three SACZs occurred in the Northeast at end-2021, causing heavy rain in southern Bahia. The first occurred on December 1-4, the second on December 7-11, and the third on December 23-27.

“The rainfall caused by the third SACZ episode wasn’t as heavy as that caused by the second, but the ground in southern Bahia was already waterlogged and there were flash floods and mudslides,” Marengo said.

The position of these three SACZs was unusual for the time of year, as was their rapid succession, according to Vinícius Sperling, a meteorologist at CEMADEN.

“These SACZs should have been in the Southeast, in Minas Gerais or São Paulo, but they were in the Northeast,” he said. “SACZs are rare in southern Bahia, and a series of two or three in a single month is even more unusual.”

Effects of climate change

For Marcelo Seluchi, CEMADEN’s General Coordinator, it is not possible to establish a direct correlation between climate change and the SACZs or these specific extreme weather events, but they may have been influenced overall by global warming.

“We can provide a meteorological explanation for every recent extreme weather event, such as the rainstorms in southern Bahia or the heatwave in the South of Brazil, but when we look at these events in conjunction with those of previous years, we can say that we’re being affected by climate change to some extent,” Seluchi said. “Extreme weather events are already frequent and will be increasingly so.”

The contribution of Working Group 1 (WG1) to the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC), published in August 2021, states that the intensity and duration of extreme weather events will increase even if global warming stabilizes at 1.5 °C. Under this scenario, heatwaves become more frequent, warm seasons last longer, and cold seasons are shorter. 

Under the 2 °C scenario, heatwaves more frequently reach the critical thresholds for agriculture and health. Limiting the average temperature rise to between 1.5 °C and 2 °C would mitigate the impact of extreme weather events because it would permit adaptation. Each additional half degree of global warming will cause a statistically significant increase in extreme temperatures, heavy rain and severe drought in some regions. Extremely heavy rain will intensify by about 7% for each additional degree, as a warmer atmosphere retains more moisture.

“Studies have shown that extreme precipitation episodes have proliferated in the last 40 to 50 years, particularly in the southeast of South America, and climate change projections for the coming decades point in the same direction,” said Marengo, who acted as a review editor for Chapter 3 of the report. 

“The climate is more irregular. This irregularity heightens the risk of adverse weather conditions and significantly increases the likelihood of geohydrological disasters in Brazil. However, the risk of disasters is also influenced by factors that have nothing to do with climate, such as urban planning and policies to reduce vulnerability. People should be helped to move away from areas with a high risk of flooding, landslides and so on. Governments and decision-makers generally play a key role in protecting people from future climate risks.”

Marengo also stressed the importance of CEMADEN and other surveillance centers in Brazil, which can work with civil defense bodies at all three levels of government to mitigate future impacts of climate change.