(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, 24th 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 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).

Flood overview

Top 10 Thailand flood events (1985-2012) (Brakenridge, 2012)

Year Dates Magnitude Duration (days) Area affected (km2)

1995 1 Aug – 9 Nov 7.9 101 444 000

2002 18 Aug – 26 Nov 7.9 101 372 000

2006 20 Aug – 13 Dec 7.7 116 213 000

2004 6 Aug – 3 Oct 7.6 159 378 000

2011/12 5 Aug – 9 Jan 7.5 158 197 000

2007 5 Sep – 10 Nov 7.3 167 300 000

1994 3 Sep – 10 Nov 7.1 107 165 000

2005 13 Aug – 26 Sep 7.1 145 134 000

2003 12 Sep – 12 Oct 7.0 131 315 000

1996 18 Jul – 21 Aug 7.0 135 314 000

Flood magnitude = log(Duration × Severity × Area Affected).Severity depends on the estimated recurrence interval of floods in the region affected and is defined on a scale between 1 and 2.

Historical ranking

Data sources: Precipitation

Thailand Meteorological Department (TMD) • 99 stations• 20 years of daily

data (1992-2011)• 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

+ 273 mm

+ 515 mm

Climate causes:Monsoon & tropical stormsTime series (1992-2011) for Thailand (north of 12N) 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

• 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

160

014

00

120

010

00

800

600 A

nn

ual

rai

nfa

ll r

etu

rn l

evel

(m

m)

0.1 1 10 100 1000

Return Period (years)

100

015

00

200

025

00

An

nu

al r

ain

fall

ret

urn

lev

el

(mm

)

Chiang Mai Bangkok

2011 RP = 9.4 years 2011 RP = 18.7 years

0.1 1 10 100 1000

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 2002-2012. (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 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

FUTURE WORK

Precipitation sensitivity to dataset

0

500

1000

1500

2000

2500

3000Bangkok (1992-2007)

TMD - stationCRU - stationAPHRODITE - gridded (0.25; Yatagai et al., 2009)APHRODITE - gridded (0.5)CRU - gridded (0.5)GPCC - gridded (0.5; Schneider et al., 2011)

Aver

age

annu

al p

reci

pita

tion

(mm

/yea

r)

Station Gridded

Return period sensitivity to record length

220

018

00

140

010

00

An

nu

al r

ain

fall

ret

urn

lev

el

(mm

)

0.1 1 10 100 1000

Return Period (years)

0.1 1 10 100 1000

Return Period (years)

500

100

015

00

200

0

An

nu

al r

ain

fall

ret

urn

lev

el

(mm

)

Chiang Mai Bangkok

2011 = 3.6 years 2011 = 76.6 years