(Swiss Re, 2012)

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

2. Flood overview • 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 km2 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).

3. Historical ranking

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

5. Data sources 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).

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

7. Climate causes:Monsoon & tropical storms Time series (1992-2011) for Thailand (north of 12N) for: • MJJASO MSLP anomaly (mb) • MJJASO precipitation anomaly (mm) • Storm precipitation anomaly (mm) 2011 = +246.1 mm 2011 = +89.3 mm 2011 = –0.58 mb

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

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

10. Distribution fitted return periods: TMD data Chiang Mai Bangkok Annual rainfall return level (mm) Annual rainfall return level (mm) 2500 1600 1400 2000 1200 1000 1500 800 2011 RP = 9.4 years 1 1 2011 RP = 18.7 years 1000 1000 0.1 0.1 10 10 100 100 1000 Return Period (years) Return Period (years) 600

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. Summary • 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

13. Future work

14. Precipitation sensitivity to dataset Gridded Station

15. Return period sensitivity to record length Chiang Mai Bangkok 2000 Annual rainfall return level (mm) Annual rainfall return level (mm) 2200 1800 1500 1400 1000 2011 = 3.6 years 2011 = 76.6 years 1 1 1000 1000 0.1 0.1 10 10 100 100 1000 Return Period (years) Return Period (years) 500