Residents of southeast Brazil are recovering from an unprecedented multi-day stretch of torrential rain and destructive flooding at a time of year when frequent rainfall is already the norm. The worst downpours occurred late last week in association with a subtropical system tracking well off the Brazil coast.
Flooding has taken at least 54 lives in the region and displaced some 30,000 people, the Associated Press reported on Tuesday. States of emergency were declared for more than 100 communities in the state of Minas Gerais, and adjacent Espírito Santo was also hard hit. “Landslides & poor infrastructure [have] exacerbated the incurred damage,” tweeted Steve Bowen (Aon).
One of the hardest-hit areas was the city of Belo Horizonte, Brazil’s sixth largest. Located about 250 miles (400 km) north of Rio de Janeiro, the metro area has about 2.5 million people. This month is already the single wettest month on record for Belo Horizonte. The total rainfall through January 27 of 809.7 mm (31.88”) is more than 250% of the January average of 329.1 mm (12.96”), according to a statement from Brazil’s national meteorological institute, INMET.
Between 9 a.m. local time on Thursday and Friday, January 23–24, Belo Horizonte racked up 171.8 mm (6.76”) of rain—the city’s largest amount for any 24-hour period in records dating back to 1910.
Showers and thunderstorms are routine in southeast Brazil during the summer wet season. However, these downpours tend to come and go rather than lingering. The region sits too close to the equator to get drenched by prolonged midlatitude storms, and tropical or subtropical cyclones in the South Atlantic are so infrequent that they weren’t even tracked as such until 2011. (The list of storm names now maintained and assigned by the Brazil Navy Hydrographic and Navigation Center is not part of the World Meteorological Organization’s naming system for tropical cyclones in ocean basins around the world.)
Most tropical or subtropical cyclones in the South Atlantic spin harmlessly well away from the coast. There are rare exceptions. Catarina—the first hurricane-strength tropical cyclone on record in the South Atlantic—swept onto land northeast of Rio de Janeiro in late March 2004, causing as many as 11 indirect fatalities and some $350 million in damage (USD 2004).
Last week’s devastating rains in southeast Brazil were fueled by a subtropical cyclone named Kurumí, the first named system in the South Atlantic to occur in January. Briefly a minimal subtropical storm, Kurumí funneled moisture into the region from well offshore. Even as Kurumí moved away from the coast, a pronounced ribbon of moisture extended inland all the way to central and even northern Brazil.
“It is not every day that you see such a satellite image,” tweeted @MetSul.
Rainfall has since returned closer to normal across southeast Brazil. Showers and thunderstorms will be widely scattered later this week but may increase next week, with daily amounts of 5 to 20 mm (0.20” – 0.80”) possible.
A wet climate? Yes. This wet? Not usually
The 24-hour record in Belo Horizonte may seem on the low side for a place that gets more than 60” of rain in a typical year. In the eastern U.S., most cities are less rainy on average than Belo Horizonte yet have seen higher one-day totals. Some parts of southeast Brazil have higher records, especially in the Petropolis area just north of Rio, where moist onshore flow is pushed up hillsides extending higher than 800 meters (2600 feet).
A list compiled by meteorologist Jérôme Reynaud includes several 24-hour totals exceeding 400 mm from the Petropolis area on March 17-18, 2013, including 474.7 mm (most of it in 15 hours) on 17-18 March 17-18, 2013, at the Instituto Estadual do Ambiente station.
Even these peak values are surprisingly modest by global standards. “These are lower numbers than I would have expected for a tropical/subtropical coast with (often) hills nearby,” said Blair Trewin (Australian Bureau of Meteorology). “Another hilly east coast in the tropics and subtropics—the coast of New South Wales and central and southern Queensland in Australia—has had numerous daily totals exceeding 400 mm, with two instances of over 700 mm (even discounting events associated with current or former tropical cyclones). There is credible historical evidence of two totals exceeding 500 mm in the 1840s, before the start of official observations, at sites within what is now metropolitan Sydney.”
The biggest climate-change threats to southeast Brazil are drought and heat
One of the hallmarks of human-induced climate change is a revving up of the hydrological cycle. Many areas across the planet are already seeing intensified rainfall extremes, while areas from California to Zimbabwe are getting longer dry seasons as well as drought impacts exacerbated by hotter temperatures.
The world watched in horror late last year as massive wildfires—the worst in more than a decade, following years of progress in fire reduction—struck the Amazon of northern Brazil. As critical as the Amazon is to regional and global climate and to biodiversity, local climate impacts may actually hit many more people in densely populated southeast Brazil. A ferocious drought from 2014 to 2016 led to severe water restrictions in São Paulo, the world’s 12th-largest urban area.
A 2017 study in Theoretical and Applied Climatology, using a high-resolution (5-km) variant of the Eta model, agreed with earlier work in projecting a hotter and largely drier future for southeast Brazil as this century unfolds, with the trends especially severe if greenhouse emissions continue to increase.
