(Swiss Re, 2012) Emma Gale & Mark Saunders Department of Space & Climate Physics, University College London, UK The 2011 Thailand flood: climate causes and return periods Bangkok, 24 th October 2011 (Cpl. Robert J. Maurer, U.S. Marine Corps)

The floods in 2011 caused estimated losses of US $30 billion (economic) and US $12 billion (insured) (Swiss Re, 2012). Insured loss ranks as the highest ever from a freshwater flood disaster worldwide. An area of 30,000 km 2 was inundated – mainly in the Chao Phraya River basin, home to 20 million people (30% of the Thai population) (DHI, 2012). 65 out of 77 provinces were affected (Aon Benfield, 2012). Flood overview

Historical ranking

Data sources: Precipitation Thailand Meteorological Department (TMD) 99 stations 20 years of daily data ( ) Data coverage = 100%

Monsoon MJJASO MSLP data – NCEP/NCAR (Kalnay et al., 1996). Southern Oscillation Index (SOI) data – Australian Bureau of Meteorology (2012). Tropical Storms Best-track data for storms affecting Thailand – JTWC (Chu et al., 2002). River discharge Satellite-derived river flows in the Chao Phraya River – Dartmouth Flood Observatory (Brakenridge et al., 2012). Data sources

Climate causes: Precipitation in 2011 Annual precipitation (mm) Annual precipitation anomaly (mm) 1345 mm 2043 mm mm mm

Climate causes: Monsoon & tropical storms Time series ( ) for Thailand (north of 12  N) for: (a)MJJASO MSLP anomaly (mb) (b)MJJASO precipitation anomaly (mm) (c)Storm precipitation anomaly (mm) 2011 = –0.58 mb 2011 = mm 2011 = mm

In 2011, La Niña enhanced the summer monsoon (moderately high SOI of +7 during ASO 2011). Climate causes: ENSO Composite difference of precipitation anomalies between La Niña and El Niño events Single year Multi year %

Return periods: Precipitation 2011 annual precipitation return period (years) 2011 storm precipitation return period (years) RP = 12.7 years RP = 9 years RP = 19.7 years RP = 5.6 years

Distribution fitted return periods: TMD data Annual rainfall return level (mm) Return Period (years) Annual rainfall return level (mm) Chiang MaiBangkok 2011 RP = 9.4 years 2011 RP = 18.7 years Return Period (years)

Return periods: River discharge Satellite-derived river discharge data for two locations on the Chao Phraya River basin ((a) and (b) marked on the return period maps) for (Adapted from Dartmouth Flood Observatory data). (a) (b)

The 2011 Thailand flood ranks as the country’s most damaging to date. Unusually high rainfall caused by a strong monsoon and four tropical storms. The 2011 La Niña enhanced the summer monsoon anomalies. The Chao Phraya River could not cope with the volume of water runoff and burst, inundating an area home to 30% of the Thai population. A consensus of various different estimates suggests a return period for the flood of years. Further information: Gale, E. L. and Saunders, M. A. (2013), The 2011 Thailand flood: climate causes and return periods. Weather, 68: 233–237. doi: /wea.2133 Summary

FUTURE WORK

Precipitation sensitivity to dataset Station Gridded

Return period sensitivity to record length Annual rainfall return level (mm) Return Period (years) Return Period (years) Annual rainfall return level (mm) Chiang MaiBangkok 2011 = 3.6 years 2011 = 76.6 years