Brazilian downpours oust familiar drizzle

The garoa provides a bath for a macaw, but not for much longer around São Paulo.
Image: By Weslei Santos 2015 via Wikimedia Commons

Misty rain is giving way to fear of flash floods as Brazilian downpours cause chaos in the country’s biggest city.

SÃO PAULO, 19 August, 2017 – Climate change has put an end to the romantic garoa for which São Paulo, South America’s biggest city, was once famous, as fierce Brazilian downpours move in. The gentle misty rain has been replaced by flash floods and violent deluges, with human victims and economic costs.

Less than half a century ago São Paulo’s famous garoa was much used in song and verse to paint an enticing picture of the city. There was even a popular band called the Demons of the Garoa.

Now that gentle rain is just a memory: instead, people frequently run the risk of being trapped in streets suddenly turned into raging torrents; cars are flattened by falling trees; hillside shanty dwellings are swallowed up by mudslides. The city has also got much hotter.

A new study published in the August number of the International Journal of Climatology has confirmed that rainfall patterns in the southeast region of Brazil, where São Paulo is located, have changed substantially.

After analysing meteorological data for the region over the last 74 years, a group of scientists from São Paulo University (USP) found an increase both in the frequency of rainy days and in the volume of rain. Professor (retired) Maria Assunção Faus da Silva Dias, of the Institute of Astronomy, Geophysics and Atmospheric Sciences at USP, said the aim of their research was to verify if the forecasts about changing rain patterns were becoming a reality.

“We discovered that where it rains a lot it will rain more, and where there is drought there will be more drought”

In previous studies, climatologists had foreseen that one of the main effects of climate change would be the exacerbation of extreme effects, including an increase in the frequency and intensity of storms and severe droughts.

The data they used was extracted from 36 meteorological stations in the southeast of Brazil. They checked the quantity of days without rain, with a small amount (less than 5 mm), or with extreme rain.

“Looking at the pattern of rain in the last decades, we can project tendencies. We discovered that where it rains a lot it will rain more, and where there is drought there will be more drought,” explained Professor Faus da Silva Dias, in further confirmation of a frequently predicted consequence of climate change. 

The team concluded that climate change has altered the rain pattern in the region, with an increase in rainfall in the state of São Paulo and a reduction in the states of Rio de Janeiro and Espirito Santo.

Strong storms

In these two states, located to the northeast of São Paulo on the Atlantic coast, they found a reduction in the frequency of rainy days and in the volume of rain, but a concentration of strong storms in fewer days. Days with light rain were less frequent.

The effects of the changing rain patterns have been felt more in highly urbanised regions, like São Paulo. In the metropolis, home to 20 million people, so-called heat islands have led to a substantial rise in temperatures. Over the last 70 years, the temperature in São Paulo’s urban region has increased by about 4°C (39°F) – equivalent to the forecast global rise for the next century.

Besides putting an end to the garoa, this has contributed to the increase in extreme downpours. The air from the colder regions around the city converges on this heat bubble, provoking intense storms.

According to Professor Faus da Silva Dias there is another factor that could be playing into the change in rainfall patterns. This is an alteration in the Zone of Convergence of the South Atlantic, a band of rainclouds that usually extends from the Amazon to the southeast, reaching the ocean. “One of the hypotheses is that, with climate change, this zone of rain has moved slightly further south”, she says. – Climate News Network

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