15  May  2014

DANGENDORF  ET  AL.

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Januar  2007  [days]

Fig.  1.  Visualization  of  (top)  the  daily  meteorological  situation  during  January  2007  and  (bottom)  the  related  daily  storm  surges
measured  at  the  Cuxhaven  tide  gauge.  In  the  vector  plots  (top),  the  colored  contours  represent  areas  of  similar  pressure,  while  the  vectors
show  the  wind  speed  and  direction.  For  presentation  purposes,  only  the  first  25  days  are  shown.  In  the  graph  (bottom),  daily  surges  (black)
and  daily  NSCI  index  (blue;  see  also  section  3;  based  on  20CRv2  data)  are  shown.  The  red  line  marks  the  long-term  (1843-2012)  95th
percentile  of  daily  (skew)  surges,  while  the  gray  areas  represent  single  events  exceeding  this  threshold.

When  using  hourly  values,  astronomical  tidal  water
levels  are  subtracted  from  contemporaneous  observed
water  levels.  When  using  only  two  tidal  high  and  low
water  levels  per  day,  astronomical  peaks  are  subtracted
from  the  nearest  observed  peaks.  The  two  may  be  up  to
a  few  hours  apart  from  each  other  due  to  the  wind  influence, ­
  and  therefore  the  resulting  surges  are  known  as
“skew  surges."  Hence,  we  first  evaluated  whether  the

skew  surges  (averaged  to  daily  means)  differ  in  the  longterm ­
  from  original  surges  (or  nontidal  residuals)  derived
from  hourly  values  for  the  overlapping  period  from  1918
to  2008  (Fig.  2).  For  that  purpose,  we  have  computed
different  annual  percentiles  (0.1%,  1%,  2%,  5%,  10%,
20%,  30%,  40%,  50%,  60%,  70%,  80%,  90%,  95%,
98%,  99%,  and  99.9%;  Fig.  2a)—the  percentiles  are  computed ­
  for  each  year  separately—and  the  corresponding