1. On the use of indices to study changes in climate extremes
EGS-AGU-EUG Joint Assembly
Nice, France, 10 April 2003
Albert Klein Tank
KNMI, the Netherlands

2. Guide How do we define extreme events?
Extremes in daily precipitation and temperature series from weather stations
Observed trends in Europe, 1946-now
“amplified” response of very wet days
“asymmetric” warming of cold & warm tail

4. !

5. WMO status of global climate in 2002

10. Day-count indices of extremes

11. Day-count indices of extremes

12. Day-count indices of extremes
Relation between relative trend bq/X required for 80% detection probability (5% level) and series length N
for extreme events with average return period Tret 6x 3x
Klein Tank and Können, J. Climate (accepted)

13. “Amplified” response of very wet days
Linear trends in rainy season, last ~50 years
Easterling et al. (BAMS, 2000) in IPCC-TAR

14. Precipitation fraction due to very wet days “R95%tot”
1) Identify very wet days using a site specific threshold = 95th percentile at wet days in the period
2) Determine fraction of total precipitation in each year that is due to these days
3) Trend analysis in series of fractions

16. Frich et al. (Clim. Res., 2002) in IPCC-TAR

17. Trends
APN 2002 (Della-Marta, Haylock et al.)

18. Caribbean workshop, Jamaica 2001 (Peterson et al., 2002, JGR)

19. Trends in 5-day rainfall total 1961-1990
Africa workshop, Casablanca 2001

21. European precipitation trends
Averaged over Europe, all indices of wet extremes saw increases between , although spatial trend coherence is low and many station trends are not significant
The index that represents the fraction of the annual amount due to very wet days gives a signal of disproportionate large changes in the extremes
IPCC-TAR:
“2 to 4% increase in frequency of heavy events in mid- and high latitudes of the NH”
“in regions where total precipitation has increased ... even more pronounced increases in heavy precipitation events”

22. IPCC-TAR, Ch.2, Folland and Karl

24. upper 10-ptile
the year 1996
lower 10-ptile

25. upper 10-ptile
the year 1996
lower 10-ptile
“cold
nights”

26. “warm
nights”
upper 10-ptile
the year 1996
lower 10-ptile
“cold
nights”

31. European temperature trends
Recent trends ( ) in temperature extremes reflect the general warming: fewer cold extremes, more warm extremes
Averaged over all stations, the first decades of slight cooling saw narrowing of temperature distributions, whereas the last decades of strong warming saw widening of temperature distributions
IPCC-TAR:
“lengthening of the freeze-free season in most mid- and high latitude regions”
“reduction in frequency of extreme low monthly and seasonal average temperatures and smaller increase in frequency of extreme high average temperatures”

32. ECA&D website: www.knmi.nl/samenw/eca

33. questions…?