Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 5:05 PM GMT on January 14, 2011
Torrential rains inundated a heavily populated, steep-sloped area about 40 miles north of Rio de Janeiro on Tuesday and Wednesday, triggering flash floods and mudslides that have claimed at least 611 lives. Rainfall amounts of approximately 300 mm (12 inches) fell in just a few hours in the hardest-hit regions, Teresopolis and Nova Friburgo. Many more people are missing, and the death toll is expected to go much higher once rescuers reach remote villages that have been cut off from communications. The death toll makes the January 2011 floods Brazil's worst single-day natural disaster in its history. Brazil suffers hundreds of deaths each year due to flooding and mudslides, but the past 12 months have been particularly devastating. Flooding and landslides near Rio in April last year killed 246 people and did about $13 billion in damage, and at least 85 people perished last January during a similar event.
Figure 1. Flooding at Sao Jose do Vale do Vale do Rio Preto in Brazil, photographed on Thursday, January 13, 2011.
Role of near-record sea surface temperatures in Brazil's flood
This week's heavy rains occurred when a storm system crossing from west to east over southern Brazil drew in a moist southerly flow air off the Atlantic Ocean over southern Brazil at the surface. At higher levels, the storm drew in very moist air from the Amazon. Sea surface temperatures along the Brazilian coast are at near-record warm levels, which likely contributed to the heavy rains. Record rains are more likely when sea surface temperatures over the nearby moisture source regions are at record high levels. This occurs because increased amounts of water vapor evaporate into the atmosphere from a warm ocean compared to a cold one, due to the extra motion and energy of the hotter water molecules. According to an analysis I did of the UK Met Office Hadley Centre sea surface temperature data set, December 2010 sea surface temperatures in the 5x5 degree region of Earth's surface along the Brazilian shore nearest the disaster area, 20S to 25S and 45W to 40W, were the second warmest on record since 1900. Temperatures were 1.05°C (1.9°F) above average in this region last month. Only 2007, with a 1.21°C departure from average, had warmer December ocean temperatures.
Meteorologist Eugenio Hackbart, with the Brazilian private weather forecasting company Metsul, wrote in his blog today, "Heavy rains early this year coincide with the strong warming of the Atlantic along the coasts of southern and southeastern Brazil. With waters up to 2°C warmer than average in some places, there is a major release of moisture in the atmosphere essential for the formation of storms."
Figure 2. Newspaper front page story in Brazil after the March 18, 1967 flooding disaster, Brazil's previous deadliest single-day natural disaster. Image credit: Metsul.
Brazil's previous worst natural disaster: the March 18, 1967 flood
The previous worst natural disaster in Brazilian history occurred on March 18, 1967 when a tsunami-like flood of water, mud and rocks swept down a hillside in the coastal city of Caraguatatuba, near Sao Paulo, killing 300 - 500 people. According to meteorologist Eugenio Hackbart with the private Brazilian weather company Metsul, a rainguage at nearby Sao Sebastao measured 115 mm (4.5") on March 17, and 420 mm (17") on March 18. Hackbart puts the death toll from the 1967 disaster at 300 - 500, and refers to it as Brazil's deadliest single-day natural disaster in history. Heavy rains at other locations in Brazil that month caused additional mudslides and flooding deaths, and Wikipedia lists the total death toll for the Brazil March 1967 floods at 785.
I looked at the sea surface temperatures for March 1967 to see if unusually warm ocean waters may have contributed to that year's flooding disaster. Sea surface temperatures in the 5x5 degree region of Earth's surface nearest the disaster site (20S to 25S, 50W to 45W) were 0.24°C (0.4°F) above average, which is not significantly different from normal. So, we can get record rains and flooding when sea surface temperatures are near normal, and it is possible that this week's catastrophe was not significantly impacted by the exceptionally warm water near the coast. However, heating up the oceans loads the dice in favor of extreme rainfall events, and makes it more likely we will have an unprecedented flood. If we look at the departure of temperature from average for the moisture source regions of the globe's four most extreme flooding disasters over the past 12 months, we find that these ocean temperatures ranked 2nd or 3rd warmest, going back through 111 years of history:
January 2011 Brazilian floods: 2nd warmest SSTs on record, +1.05°C (20S to 25S, 45W to 40W)
November 2010 Colombia floods: 3rd warmest SSTs on record, +0.65°C (10N to 0N, 80W to 75W)
December 2010 Australian floods: 3rd warmest SSTs on record, +1.05°C (10S to 25S, 145E to 155E)
July 2010 Pakistani floods: 2nd warmest SSTs on record, +0.95°C (Bay of Bengal, 10N to 20N, 80E to 95E)
The size of the ocean source region appropriate to use for these calculations is uncertain, and these rankings will move up or down by averaging in a larger or smaller region of ocean. For example, if one includes an adjacent 5x5 degree area of ocean next to Brazil's coast that may have also contributed moisture to this week's floods, the SSTs rank as 7th warmest in the past 111 years, instead of 2nd warmest. It would take detailed modeling studies to determine just how much impact these near-record sea surface temperatures had on the heavy rains that occurred, and what portion of the ocean served as the moisture source region.
Figure 3. Predicted total precipitation amounts in South America for the 7-day period ending at 7am EST January 21, 2011, as forecast by the 06Z run of the GFS Ensemble model made January 14, 2011. Image credit: Florida State University.
More rain in the forecast
The coast of Brazil is embedded in a warm, moist tropical airmass that is expected to continue to bring heavy rains over he Rio de Janeiro area for at least the next week. Heavy rains in excess of five inches in the next seven days (Figure 3) are predicted by the GFS Ensemble computer model for the disaster region, just north of Rio de Janeiro. The additional heavy rains are likely to cause more life-threatening mudslides and floods.
2010 tied for warmest year in Earth's history
Earth's warmest year in history occurred in 2010, NASA reported this week. The globe's temperature beat the previous record set in 2005 by just .01°C, so we should consider 2010 and 2005 tied for the warmest year on reecord. Reliable global temperature records go back to 1880. The National Oceanic and Atmospheric Administration (NOAA) also announced this week that 2010 was tied with 2005 as the warmest year on record, with temperatures during 2010 1.12°F (0.62°C) above the 20th century average. When the planet stops producing record weather catastrophes to blog about, I'll discuss the 2010 global temperature record in more detail.
Anniversary of the Haitian earthquake
Yesterday was the 1-year anniversary of the great Haitian earthquake of 2010. I want to thank all of you who offered donations to such great charities as the Lambi Fund of Haiti and Portlight.org. The people of Haiti need our continued financial support and prayers in the difficult rebuilding years to come.
I'll have a new post on Monday.
Comments will take a few seconds to appear.