Patrick Willems
Katholieke Universiteit Leuven, Hydraulics Division, Kasteelpark Arenberg 40,
BE-3001 Leuven (Belgium)
Tel. +32 16 321658
E-mail: Patrick.Willems@bwk.kuleuven.be
URL: http://www.kuleuven.be/cv/u0009249e.htm
SI 1

Anomalies in frequency and amplitude of rainfall extremes
Figures 1 and 2 of the article show positive anomalies in extreme precipitation quantiles in the 1910s, 1950-1960s and 1990s and low negative anomalies in the
1930s-1940s and 1970s, and that these are significant at the 5% significance level.
Supplementary Figure 1 shows additional results for the anomalies in extreme
precipitation quantiles, in mean intensity per precipitation event and in the number of extreme precipitation events, based on the 108-years 10-minute precipitation intensities at Uccle, for a 10-years blocksize and the winter period (DJF). For the anomaly in extreme precipitation quantiles and in mean intensity per precipitation event, confidence intervals are plotted around the Uccle-based historical values.
This means they become significant at the 5% significance level when the longterm based “reference” (anomaly 0) is located outside the confidence intervals.
For the number of extreme precipitation events, confidence intervals are analytically derived based on the Poisson distribution and plotted around the mean. The historical numbers now become significant when they are located outside the plotted confidence intervals.
It is apparent from Supplementary Figure 1 that the temporal variability in mean
intensity and in event frequency have similar patterns although the variability in the number of extremes is more similar with the quantile anomaly. Moreover, the event frequency has longer periods with significant deviations, which indicates that oscillations in the event frequency contribute more to the oscillations in the quantile anomalies than oscillations in the mean event intensity. Oscillation
anomalies are in Supplementary Figure 1 more significant for the quantiles, then
for the event frequency and the mean event intensity, as changes in quantiles show the combined effect of changes in event frequency and mean event intensity.
SI 2

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Supplementary Figure 1. Confidence intervals (5% significance level) for the anomalies in extreme quantiles of precipitation (top), the mean intensity per extreme precipitation event (middle), and the number of extreme precipitation events (bottom), for the 108-years (1898-2005) 10-minute precipitation depths > 0.8 mm at Uccle, 10-years blocksize and DJF period
SI 3

Influence of blocksize, aggregation time and season
The procedure for calculation of the anomalies in extreme precipitation quantiles has been repeated for blocksizes of 5, 10 and 15 years, for aggregation times of 10 minutes, 1 day and 1 month, and separately for summer (JJA) and winter (DJF) periods. An overview of the periods with significant anomalies (5% significance
level) is given in Supplementary Figure 2. Conclusions are consistent for all
investigated blocksizes, aggregation times and seasons. Also when the mean recurrence interval, above which the values are considered extreme, is varied,
conclusions remain consistent (Supplementary Figure 3). Oscillation highs are
found in the 1910s, the 1950-1960s and recently during the past 15 years; oscillation lows in the 1930s-1940s, and in the 1970s.
Periods with significantly HIGH extreme precipitation quantiles
DJF
JJA month, 15y day, 15y
10min, 15y month, 10y day, 10y
10min, 10y month, 5y day, 5y
10min, 5y
Periods with significantly LOW extreme precipitation quantiles
DJF
JJA
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Supplementary Figure 2. Overview of periods with significant high and low extreme precipitation quantiles (5% significance level) for the 108-years (1898-2005) precipitation extremes at Uccle, for aggregation times of 10 minutes, 1 day and 1 month, for 5, 10 and 15-years blocksizes, and
DJF and JJA. Coloured dots represent the central time moments of the significant periods (e.g. central time moment for 1898-1902 is 1900)
SI 4

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Supplementary Figure 3. Sensitivity of the threshold for defining extremes (15 or less most extreme events p.a.) on the multidecadal oscillations in the extreme quantiles, for the 108-years
(1880-2005) daily (top figure) or monthly (bottom figure) precipitation depths at Uccle, 15-years blocksize and DJF period
2000
SI 5

Similar multidecadal oscillations in other hydroclimatic series
The multidecadal oscillations identified for the Uccle precipitation series were compared with the results of similar analysis for the extreme quantiles of daily and monthly precipitation, temperature and cloud cover in central-western Europe.
Supplementary Figure 4 shows the results for an exceptionally long record of
monthly precipitation over England and Wales (UK) for 1766-2007. This series was originally constructed by Wigley et al. (1984), and is now routinely updated by the Hadley Centre. It is based on 35 rain gauges across the region, 250 to 750 km north-west of Uccle, and was useful to extend the analysis to the period 1766-
2007. Oscillation highs are detected for the 1990s, 1910s, 1870s, and less strong
for the 1960s, 1830s and 1790s (Supplementary Figure 4). Oak tree ring width
analysis results for Belgium in 1120-1986 indicate that multidecadal oscillations with irregular periodicities in the range 30 - 60 years were present also in previous
centuries (Supplementary Figure 4).
Supplementary Figure 5 shows other results based on monthly and daily
precipitation, temperature and cloud cover series in Belgium and surrounding countries. These series have been obtained from the ECA&D database and the
GHCN (Land Surface Data, Global Historical Climate Network) database
( http://www.ncdc.noaa.gov/oa/climate/ghcn-monthly ). Oscillation highs and lows are found for the same periods, with oscillation highs in the 1870s, 1910s, 1950-
1960s and 1990s (Supplementary Figure 5).
SI 6

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Supplementary Figure 4. Anomalies in extreme quantiles of monthly precipitation at Uccle and
England and Wales, and oak tree ring widths in the Ardennes (relative to 1898-2005 period), for
15-years blocksize and all months in DJF
SI 7

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Supplementary Figure 5. Anomalies in extreme quantiles of daily precipitation, temperature and
2000 cloud cover in central-western Europe (ECA&D and GHCL databases) and monthly precipitation for England and Wales, for 15-years blocksize and a threshold corresponding to an average number of 15 exceedances p.a. (for daily series) and all months (for monthly series) in DJF
SI 8

Significance of spatial correlation in quantile anomalies
The article presents an analysis of the quantile anomalies in precipitation extremes based on all daily precipitation series available in the ECA&D database that have lengths of 40 years or more in common with the Uccle series. A map of these
stations is provided in Supplementary Figure 6. Figure 4 of the article shows the
existence of positive correlations between the Uccle station and stations in central-western Europe, and anti-correlations with stations in southwestern and
northern Europe. In Supplementary Figure 7, the results are filtered based on the
one-sided p-value of the t-test for a zero correlation hypothesis; stations are only shown when the p-value is lower than 10%, 20% or 30%. Most significant correlations are found for stations with positive correlations close to Uccle.
Supplementary Figure 6. Stations considered from the ECA&D database
SI 9

Supplementary Figure 7. Spatial correlations of the anomalies in extreme quantiles of daily precipitation between Uccle and other stations of the ECA&D database with at least 40 years of data in common and with correlation p-value < 10% (top), 20% (middle) and 30% (bottom), 15years blocksize and a threshold corresponding to an average number of 15 exceedances p.a. in DJF
SI 10

Supplementary reference
Wigley TML, Lough JM, Jones PD (1984) Spatial patterns of precipitation in
England and Wales and a revised, homogeneous England and Wales precipitation series. Int. J. Climatol. 4:1-25.
SI 11