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 511 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.
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. 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."
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
November 2010 Colombia floods: 3rd warmest SSTs on record, +0.65°C
December 2010 Australian floods: 3rd warmest SSTs on record, +1.05°C
July 2010 Pakistani floods: 2nd warmest SSTs on record, +0.95°C