1. (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)

2. 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

3. Historical ranking

4. Data sources: Precipitation Thailand Meteorological Department (TMD) 99 stations 20 years of daily data (1992-2011) Data coverage = 100%

5. 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

6. Climate causes: Precipitation in 2011 Annual precipitation (mm) Annual precipitation anomaly (mm) 1345 mm 2043 mm + 273 mm + 515 mm

7. Climate causes: Monsoon & tropical storms Time series (1992-2011) 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 = +246.1 mm 2011 = +89.3 mm

8. 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 %

9. 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

10. Distribution fitted return periods: TMD data 1600 1400 1200 1000 800 600 Annual rainfall return level (mm) 0.1 1 10100 1000 Return Period (years) 1000 1500 2000 2500 Annual rainfall return level (mm) Chiang MaiBangkok 2011 RP = 9.4 years 2011 RP = 18.7 years 0.1 1 10100 1000 Return Period (years)

11. 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 2002-2012. (Adapted from Dartmouth Flood Observatory data). (a) (b)

12. 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 10-20 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: 10.1002/wea.2133 Summary

13. FUTURE WORK

14. Precipitation sensitivity to dataset Station Gridded

15. Return period sensitivity to record length 2200 1800 1400 1000 Annual rainfall return level (mm) 0.1 1 10100 1000 Return Period (years) 0.1 1 10100 1000 Return Period (years) 500 1000 1500 2000 Annual rainfall return level (mm) Chiang MaiBangkok 2011 = 3.6 years 2011 = 76.6 years