Extreme rains are increasingly common in Brazil – 04/25/2023 – Environment

Historical data and future projections indicate that extreme rains, such as those that hit the north coast of São Paulo during Carnival this year and caused 65 deaths, are not a passing or localized phenomenon. They are a variable of the new changing climate of the 21st century, driven by global warming, which has caused the average temperature of the planet to increase by almost 1.2ºC since the 1850s.

But the risk of disasters associated with storms is not the same for everyone or in every part of the country. It is higher in areas where most of the population is concentrated, such as in the large cities along the vast national coastline and in the Southeast and South regions, and among the poorest people who live in places more exposed to floods and landslides.

It is a complex scenario, in line with the information and forecasts of a vast scientific literature on the subject, summarized in the reports of the Intergovernmental Panel on Climate Change (IPCC), including the last one, released on March 20 of this year. As the average global temperature increases during this century, the IPCC projects increases in the total amount of rainfall in large parts of the Brazilian territory, such as in the Southeast and South regions, and a decrease in the Midwest, in a large part of the Northeast and in the east of the Amazon.

Sometimes, the same locality can be the target, in different months of the same year or in different years, of both extreme rains and severe droughts.

“Most disasters in Brazil are of the hydrological and geological type and are linked to too much or too little rain”, comments climatologist José Marengo, a specialist in climate change and risks at Cemaden (National Center for Monitoring and Alerts of Natural Disasters). . “These are land displacements or floods or major droughts in periods of prolonged drought that increase the risk of fires and water shortages.”

The behavior of rainfall in Brazil over the last six decades portrays this reality full of nuances. Historical data compiled by Inmet (National Institute of Meteorology) indicate that the average value of accumulated annual rainfall in areas located below the southern half of the states of São Paulo and Mato Grosso do Sul has increased in the last 30 years between 50 mm and 250 mm.

In this part of the national territory, a reduction in rainfall was recorded only at a small point in the west of Rio Grande do Sul, close to the borders with Uruguay and Argentina. There was also an increase in the average of the annual total accumulated rainfall in Espírito Santo, southern Minas Gerais, southwest Mato Grosso and some parts of the Amazon, especially in its western portion. Throughout the Northeast, in the center of Brazil and in the eastern part of the Amazon, there was a decrease in the average annual rainfall (see map). These values ​​stand out when comparing the average annual rainfall recorded between 1991 and 2020 with the previous 30-year period, between 1961 and 1990.

Since 1935, the WMO (World Meteorological Organization) has advocated the adoption of so-called climatological normals as reference values ​​for a given climate parameter, such as the monthly or annual amount of rainfall or the values ​​of average, minimum or maximum temperatures in a region. From the succession of values ​​recorded over periods of 30 years, the climatological normal of a parameter for a location is calculated. The formula aims to arrive at a value that serves as a basis, as a point of comparison, to know whether the current climate is changing in a certain location or has behaved as expected, that is, within the values ​​of the last normal.

An increase of 50 mm or 100 mm in the total accumulated rainfall in a city like São Paulo, where it rains on average, according to climatological normals, around 1,600 mm per year, may not seem like much. But this data needs to be put into perspective. One millimeter of rain recorded is equivalent to one liter of water over an area of ​​one square meter. Therefore, an increase of 50 mm in average annual rainfall means that 50 liters of water have fallen more than expected over a surface equivalent to a square with one meter on each side.

It is also necessary to take into account how an increase (or decrease) in rainfall is distributed over time. In some places, accumulated annual precipitation may decrease, but extreme rainfall increases. “A 50 mm concentrated rainfall episode can generate more problems and concerns than five 10 mm rains distributed on different days”, comments meteorologist Danielle Barros Ferreira, from Inmet.

Last year, Inmet released a survey that shows an increase in the occurrence of days of heavy and concentrated rain in three Brazilian capitals analyzed. Since the 1990s, the city of São Paulo, the most populous in the country, has been an illustrative case of this trend. Based on data always collected at the same meteorological station in the capital of São Paulo, located at Mirante de Santana, in the north zone, an Inmet team counted the number of days with high precipitation, with great potential to cause problems and eventually deaths. Days with three levels of extreme rain were counted: above 50 mm, 80 mm and 100 mm.

In the 1960s, there were 40 days with extreme rainfall, 37 with more than 50 mm and three with more than 80 mm. But no occurrence of daily rainfall exceeded 100 mm.

Since the 1990s, at least 60 days of extreme rainfall have been recorded every decade. Between 2011 and 2020, an even more worrying sign appeared. There was a small reduction in the number of days with rainfall greater than 50 mm, but a significant increase in the number of episodes of concentrated rainfall at the other two levels. Comparing the results of the last decade with the previous one (2001 to 2010), there was a slight reduction in the incidence of days with rain above 50 mm (from 53 to 47), but a significant increase in days with rain above 80 mm (from 9 to 16 occurrences) and 100 mm (from 2 to 7).

“In the capital of São Paulo, heavy rains tend to become more frequent”, comments Ferreira. A similar pattern of intensification of extreme rainy days was also observed in two capitals located at opposite ends of the country, Belém, in Pará, and Porto Alegre, in Rio Grande do Sul.

The intensity of the torrential rains on the north coast of São Paulo last February seems to be an outlier even when taking into account the current scenario of uncertainties and climate extremes. At the turn of February 18th to 19th, several cities in the region received unprecedented levels of rain in less than 24 hours or that, according to climatological norms, should only occur once every 100 years.

The two most eloquent cases occurred in the neighboring municipalities of Bertioga and São Sebastião, where the precipitation in 24 hours was equivalent, according to the historical average, to what should have rained over the months of January and February. In Bertioga, 683 mm of rain fell, the highest rainfall recorded in the country in such a short period of time. In São Sebastião, it rained a little less, 627 mm, according to Cemaden data.

Despite having caused great material damage and having displaced around 2,000 local residents in various parts of the north coast, 64 of the 65 deaths verified in the region occurred in São Sebastião. In that municipality, more people were in risk areas, subject to the effects of landslides caused by the waters that descended the slopes of the Serra do Mar. In Bertioga, there were many flooded areas, but human occupation, marked by the presence of medium and high standard condominiums, is further from the mountain slopes and there were no deaths.

“It was a frightening situation”, recalls meteorologist Pedro Leite da Silva Dias, from the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP). “Until we lost electricity and internet access around 1am on the 19th, I followed the formation and spread of rain on the website of the European Meteorological Center [ECMWF] and by data from Cemaden.”

Alongside his wife Maria Assunção Faus da Silva Dias, also a meteorologist and now a retired professor at the IAG-USP, Dias was staying at a friend’s house on a beach in São Sebastião affected by the rains and landslides, but where there were no deaths.

Simultaneously with the arrival of an intense cold front in the south, the formation of a small cyclone on the coast between the city of Ubatuba and Paraty in the state of Rio de Janeiro, substantially increased the intensity of rainfall between February 18th and 19th. This cyclone, small but disastrous, reports Dias, appeared clearly in the images of the radar of Cemaden and of satellites of the United States.

Heavy rains are frequent and expected in the region of São Sebastião in the summer, the season with the highest rainfall in much of the country. However, specialists point out that there was a perfect, and perverse, synchrony that led to the trapping — anchoring, in the jargon of the area — of water-laden clouds over a stretch of the north coast for a period of many hours.

“Part of that rain could have fallen on the sea or moved to other points, but a series of known factors acted for it to have stayed there for a long time”, says researcher Pedro Camarinha, a specialist in climate change and disasters, who works in the situation room at Cemaden.

Another portion of the moisture directed towards the north coast came from increased evaporation of water in the Atlantic Ocean, which was at least 1ºC warmer than normal. To complete the picture, surface winds blew in a direction that interacted with the relief of Serra do Mar. This intensified the so-called orographic effect and fed back the formation of rains for hours.

“I cannot say, with absolute certainty, that a rain of this magnitude is due to global warming, but it is compatible with this scenario”, ponders Dias.

In the case of the north coast of São Paulo, the particularities of the municipality of São Sebastião enhanced the climate that resulted in the tragedy. There, a good part of the poor population lives in risk areas, close to or on the slopes of the mountain range, there is great social inequality and a scarcity of action plans to mitigate the effects of extreme rainfall.

“I made a ranking of the most vulnerable municipalities in São Paulo to disasters related to landslides in my doctoral thesis, which I defended in 2016 at Inpe [Instituto Nacional de Pesquisas Espaciais]”, says Camarinha. “Taking into account multiple climatic and non-climatic factors, São Sebastião was classified as the municipality in the most critical situation both in the present time and in the coming decades.”