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Monday, January 24, 2011

Jeff Masters: 2011 -- Year of the Flood

2011 -- Year of the Flood

by Jeff Masters, wunderblog, January 21, 2011

The year 2010 was one the worst years in world history for high-impact floods. But just three weeks into the new year, 2011 has already had an entire year's worth of mega-floods. I'll recap here six remarkable floods that have already occurred this year.

Brazil suffered its deadliest natural disaster in history last week, when torrential rains inundated a heavily populated, steep-sloped area about 40 miles north of Rio de Janeiro. Flash floods and mudslides from the heavy rains have claimed at least 772 lives, including 357 in Nova Friburgo and 323 in Teresópolis. The storm left 126 people missing, the Brazilian Health and Civil Defense Ministry said Thursday. Rainfall amounts of approximately 300 mm (12 inches) fell in just a few hours in the hardest-hit regions. Damage estimates are currently $1.2 billion, and 13,000 are homeless. Latest rainfall forecasts from the GFS model show the heaviest rains during the coming week staying well south of the Rio de Janeiro area, which will give the flood region time to dry out and recover.

Figure 1. Flooded stream in Teresópolis. Image credit: Wikipedia.

Australia Queensland
Australia's most expensive natural disaster in history is now the Queensland flood of 2010-2011, with a price tag now as high as $30 billion. At least 31 have been killed since December in the floods, and another 40 are missing. According to the Australian Bureau of Meteorology, in 2010 Australia had its wettest spring (September-November) since records began 111 years ago, with some sections of coastal Queensland receiving over 4 feet (1200 mm) of rain. Rainfall in Queensland and all of eastern Australia in December was the greatest on record, and the year 2010 was the rainiest year on record for Queensland. 

Queensland typically has its rainiest years when La Niña events occur, due to the much warmer than average ocean temperatures that occur along the coast. The BOM noted,"Previous strong La Niña events, such as those of 1974 and 1955, have also been associated with widespread and severe flooding in eastern Australia. Sea surface temperatures off the Queensland coast in recent months have also been at or near record levels." The BOM's annual summary also reported, "Sea surface temperatures in the Australian region during 2010 were the warmest value on record for the Australian region. Individual high monthly sea surface temperature records were also set during 2010 in March, April, June, September, October, November and December. Along with favourable hemispheric circulation associated with the 2010 La Niña, very warm sea surface temperatures contributed to the record rainfall and very high humidity across eastern Australia during winter and spring."

Queensland has an area the size of Germany and France combined, and 3/4 of the region has been declared a disaster zone. The latest GFS precipitation forecast for the coming week shows new heavy rains of 3-5 inches can be expected over the extreme northern portion of Queensland, but the majority of the state will receive lesser rains that should not further aggravate the flooding situation.

Figure 2. The airport, the Bruce Highway, and large swaths of Rockhampton, Australia, went under water due to flooding from the Fitzroy River on January 9, 2011. The town of 75,000 was completely cut off by road and rail, and food, water and medicine had to be brought in by boat and helicopter. Image credit: NASA.

Australia Victoria
From January 12-14, extremely heavy rains over the southern Australian state of Victoria caused major flooding that killed one person and caused hundreds of millions in damage. Kevin Parkyn, a senior forecaster with the Bureau of Meteorology said "Victoria is experiencing one of its worst flood events in its history" after "a week in which rainfall totals have been smashed in parts of Victoria." Bureau of Meteorology senior forecaster Terry Ryan said "It's the worst flood in western Victoria in their history as far as our records go in terms of the depth of water and the number of places affected." According to atmospheric moisture expert Dr. Kevin Trenberth of the National Center for Atmospheric Research, extratropical storm systems like the one that affected Victoria get 70% of their moisture from the surrounding atmosphere, and 30% due to evaporation from the surface. Since the airmass that supplied Victoria with its flooding rains traveled over the already-flooded portions of Queensland to the north before reaching Victoria, the moisture from the Queensland floods contributed significantly to the Victoria floods. Little rain is predicted over Victoria during the coming week, fortunately.

100-year flood in Sri Lanka
As I reported in my previous post, at least 43 are dead and damage estimates are at $500 million in Sri Lanka, which suffered a 1-in-100 year flood this month.

South Africa
Heavy rains of up 345 mm (13.6") have fallen in South Africa so far this month, resulting in deadly floods that have killed 40 people. Seven of the country's nine provinces have been declared disaster zones. Agricultural damage alone from the floods is estimated at $145 million. Heavy rains and severe flooding have also affected neighboring Mozambique, where 13 people are dead and 13,000 homeless or suffering damaged homes. Neighboring Zimbabwe has seen its heaviest rains in 30 years in recent weeks, according to the nation's Civil Protection Unit, but severe flooding has not yet hit that nation. La Niña events commonly cause heavy rains in southern Africa. Sea surface temperatures off the east coast of South Africa were 0.2-0.4 °C above average during December 2010 -- nowhere near record levels, but warm enough to contribute to enhanced rainfall.

Very heavy rains since late December have triggered a major flooding disaster in the Philippines, where 40 are dead, 453,000 people displaced, and 1.2 million people affected. The heavy rains were caused when a cold front moved over the eastern Philippines and lingered for many days. Heavy rains are common in the Philippines during La Niña events, as unusually warm waters accumulate by the islands. This winter, the waters in the central Philippines (10°-15° N, 120°-130° E) were at the warmest levels in history -- 1.0 °C above average during December. The exceptionally warm waters allowed more moisture than usual to evaporate into the air, enhancing rainfall.

The year 2011 has begun with a remarkable number of high-impact floods world-wide, and much of the blame for this can be placed on the current La Niña event occurring in the Eastern Pacific. NOAA's Climate Prediction Centercurrently puts the La Niña event in the "strong" category, and whenever a La Niña or El Niño event reaches the strong category, major perturbations to global weather patterns occur. This typically results in record or near-record flooding in one or more regions of the globe. When one combines the impact of La Niña with the increase of global ocean temperatures of 0.5 °C (0.9 °F) over the past 50 years, which has put 4% more water vapor into the atmosphere since 1970, the result is a much increased chance of unprecedented floods. A 4% increase in atmospheric moisture may not sound like much, but it turns out that precipitation will increase by about 8% with that 4% moisture increase. Critically, it is the extreme rainfall events that tend to supply the increased rainfall. For example, Groisman et al. (2004) found a 20% increase in very heavy (top 1%) precipitation events over the U.S. in the past century, and a 36% rise in cold season (October-April) "extreme" precipitation events (those in the 99.9% percentile -- 1 in 1000 events). These extreme rainfall events are the ones most likely to cause floods.


Groisman, P.Y., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004, "Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from In Situ Observations," J. Hydrometeor., 5, 64.85.

Milly, P.C.D., R.T. Wetherald, K.A. Dunne, and T.L. Delworth, 2002, Increasing risk of great floods in a changing climate," Nature 415, 514-517 (31 January 2002);  doi:10.1038/415514a.

Santer, B.D., C. Mears, F. J. Wentz, K. E. Taylor, P. J. Gleckler, T. M. L. Wigley, T. P. Barnett, J. S. Boyle, W. Brüggemann, N. P. Gillett, S. A. Klein, G. A. Meehl, T. Nozawa, D. W. Pierce, P. A. Stott, W. M. Washington, and M. F. Wehner, 2007, "Identification of human-induced changes in atmospheric moisture content," PNAS, 104, 15248-15253.

Trenberth, K.E., J. Fasullo, and L. Smith, 2005: "Trends and variability in column-integrated atmospheric water vapor",Climate Dynamics 24, 741-758.


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