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JOURNAL  OF  CLIMATE

Volume  27

1880  1900  1920  1940  1960  1980  2000

Time  [yr]

Fig.  7.  (a)  Box  plot  of  residual  percentile  trends  (20CRv2  generated ­
  surges  minus  observation)  from  all  ensemble  members  over
the  period  from  1871  to  2011.  The  gray  shaded  areas  represent  the
maximal  range  of  SEs  (95  %  confidence  level):  that  is,  residual  trends
are  significant  for  all  ensemble  members  and  percentiles,  (b)  30-yr
moving  trends  for  the  99th  percentile  residuals  (20CRv2  generated
surges  minus  obseivation).  All  56  ensemble  members  are  shown  in
orange.  The  ensemble  mean  together  with  the  related  SE's  is  given
by  the  red  line  and  the  red  shaded  area,  respectively,  (c)  As  in  (b),
but  for  the  99.9th  percentile  residuals.

observations  and  20CRv2  found  by  Krueger  et  al.
(2013a,b)  over  the  North  Atlantic  region  prior  the  1940s.
In  contrast  to  this  earlier  study,  the  inconsistencies  with
surge  levels  at  Cuxhaven  are  less  pronounced  and  become ­
  clearly  visible  only  for  the  period  before  the  1910s.
It  should  be  noted  that  the  surges  itself  differ  to  the
conventional  storminess  proxies  insofar  that  they  measure ­
  both  changes  in  wind  speed  and  direction.  However, ­
  there  are  three  reasons  why  this  does  not  affect  our
main  conclusion  that  the  surge  record  is  representative
for  storminess  in  the  region:
1)  The  rank  correlation  (Spearman)  between  wind
speeds  and  absolute  surges  has  a  value  of  0.53,  which

is  significantly  different  statistically  from  zero  (see
Fig.  S2  of  the  supplementary  material).  Since  during
periods  of  low  wind  speeds  the  influence  of  bathymetric ­
  effects  on  the  surge  generation  increases
relative  to  the  winds,  the  correlation  between  the
extreme  events,  which  are  analyzed  here,  increases
noticeably.  For  example,  the  correlation  between  the
annual  95th  percentile  time  series  of  both  factors
increases  to  a  value  of  0.64  (not  shown).
2)  Dangendorf  et  al.  (2013c)  investigated  changes  in
the  frequency  of  different  wind  directions  between
January  and  March  from  1871  to  2008  and  could  not
find  any  evidence  for  significant  changes  during  the
period  of  interest.
3)  The  fact  that  Krueger  et  al.  (2013b)  previously
detected  similar  differences  in  conventional  proxies
(although  for  a  slightly  different  region  and  with
larger  differences  than  detected  here)  and  Donat
et  al.  (2011b)  pointed  to  significant  trends  in  the  wind
speed  further  supports  the  reliability  of  the  surge
record  as  a  measure  of  storminess.
Additionally,  one  could  argue  that  the  use  of  a  single
grid  point  time  series  rather  than  the  entire  wind  field
over  the  North  Sea  as  input  for  the  empirical  wind  surge
model  could  bias  the  results.  When  looking  at  Fig.  3  in
Donat  et  al.  (2011b),  it  can  clearly  be  seen  that  the  linear
trends  are  evident  over  the  entire  North  Sea  area.  In
fact,  strongest  trends  in  wind  speeds  have  been  found  for
the  grid  points  not  used  as  input  data  in  the  present
study.  Therefore,  using  other  additional  grid  points  as
input  parameters  for  the  wind  surge  formulas  would  lead
to  even  larger  inconsistencies  between  20CRv2  and
surge  observations.
On  this  basis,  we  conclude  that  the  significant  trend
detected  by  Donat  et  al.  (2011b)  was  less  a  result  of  the
large  decadal  trends  in  storminess  in  the  last  decades  but
rather  reflects  lower  occurrence  of  extreme  values  in  the
early  decades  of  the  reanalysis.  The  latter  are  supported
by  neither  pressure-based  storm  indices  (Krueger  et  al.
2013a,b)  nor  the  surge  observations  at  Cuxhaven  since
1843.

4.  Discussion  and  conclusions
We  have  established  a  new  storm  surge  record  for  the
tide  gauge  of  Cuxhaven,  extending  conventional  records
back  to  1843.  This  could  be  achieved  by  using  a  generally
known  but  in  the  last  decades  less  considered  method
(Horn  1948,  1960)  that  enables  us  to  decompose  measurements ­
  of  tidal  high  and  low  water  levels  into  tides,
MSL,  and  (skew)  surges.  The  method  can  open  a  worldwide ­
  available  but  in  terms  of  surges  unappreciated  data