F.  Große  et  al.:  Looking  beyond  stratification

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www.biogeosciences.net/13/2511/2016/

Biogeosciences,  13,  2511-2535,  2016

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Figure  3.  Annual  time  series  of  observed  and  simulated  bottom
Cb  concentrations  at  Cefas  station  North  Dogger  in  (a)  2007  and
(b)  2008,  and  at  MARNET  station  Ems  in  (c)  2010  and  (d)  2011.
Same  legend  for  all  panels.  Grey  shaded  areas  indicate  the  stratification ­
  periods  derived  from  simulated  T  according  to  Eq.  (1).

also  affect  the  difference  between  simulated  and  observed  0 2
concentrations.
At  MARNET  station  Ems  the  observed  bottom  0 2  concentrations ­
  show  significantly  larger  intra-seasonal  fluctuations
than  at  North  Dogger.  This  applies  to  both  years  2010  and
2011,  and  mainly  results  from  the  shallower  station  depth,
i.e.,  sampling  depth  (sensor  in  30  m).  As  at  North  Dogger,
differences  may  also  relate  to  different  depths  of  the  time  series ­
  (32.5  m  for  observation)  and  the  vertical  resolution  with
only  six  layers.
In  2010,  the  onset  of  0 2  decline  in  the  observations  is  in
good  agreement  with  that  in  the  simulations.  Stratification
lasts  shorter  and  is  less  persistent  than  at  North  Dogger.  As  at
North  Dogger,  intra-seasonal  fluctuations  in  the  0 2  evolution
are  not  fully  reproduced.  The  model  tends  to  overestimate
bottom  0 2  in  2010,  revealing  a  maximum  difference  of  about
1  mg0 2  L -1 .
In  2011,  persistent  stratified  periods  derived  from  the
simulation  do  not  exceed  2  months  at  station  Ems.  Consequently, ­
  the  temporal  evolution  of  bottom  0 2  represents

mainly  the  temporal  evolution  of  the  0 2  saturation  concentrations. ­
  Again  large  fluctuations  of  up  to  ±2  mg0 2  L -1  can
be  seen  in  the  observations  which  are  not  fully  reproduced
by  the  model.  Besides  these  short-term  changes,  the  difference ­
  between  simulated  and  observed  bottom  concentrations
is  less  than  0.8  mg0 2  L -1  with  higher  summer  values  in  the
simulation.
The  validation  of  bottom  0 2  at  the  stations  North  Dogger ­
  and  Ems  shows  that  the  HAMSOM-ECOHAM  model
is  capable  of  reproducing  the  main  features  of  the  bottom
0 2  dynamics  at  these  two  stations.  The  minor  differences
in  the  concentrations  (<  ±0.4mgO 2 L _1 )  at  the  beginning
and  end  of  the  year,  representing  mainly  the  saturation  concentrations, ­
  show  that  the  general  physical  setting  provided
by  the  model  is  reasonable.  The  slightly  slower  0 2  reduction
in  the  simulation  may  indicate  an  underestimation  of  the  biological ­
  consumption,  e.g.,  due  to  benthic  remineralisation.
Intra-seasonal  fluctuations  at  both  stations  are  not  fully  reproduced, ­
  due  to  the  limited  spatial  resolution  of  the  model
grid.  Additionally,  the  tides  may  have  an  effect  at  station  Ems
on  the  short-term.  However,  they  are  not  resolved  due  to  the
daily  time  step  of  the  simulated  current  fields.
The  generally  good  agreement  between  simulation  and  observation ­
  is  also  shown  by  the  Taylor  diagram  (Taylor,  2001,
see  Fig.  5,  markers  “a”  for  North  Dogger  and  “b”  for  Ems),
which  presents  the  correlation  coefficients  (COR),  standard
deviations  (SDs)  and  centred  root-mean-square  differences
(RMSD)  of  the  simulation  relative  to  the  observations.  SDs
and  RMSDs  are  normalised  by  the  SD  of  the  corresponding ­
  observations.  For  analysis,  the  data  of  each  data  set  was
merged  into  a  continuous  series  of  data.  For  both  stations,
COR  is  high  with  values  of  about  0.95  and  the  normalised
RMSD  is  less  than  0.38.  The  RMSD  values  are  mainly  due
to  the  larger  range  and  higher  (seasonal  and  intra-seasonal)
variability  in  the  observed  bottom  0 2 ,  which  is  also  indicated ­
  by  the  normalised  SDs  of  about  0.73  and  0.82  for  Cefas
North  Dogger  and  MARNET  Ems,  respectively.
3.1.2  Spatial  distribution  of  late  summer  bottom  0 2
Figure  4  shows  the  spatial  distribution  of  the  average  simulated ­
  and  observed  0 2  concentrations  in  the  model  bottom
layer  for  the  years  2001  (a),  2005  (b)  and  2008  (c),  and  the
SD  related  to  the  averages  in  2005  (d).  The  averaging  period
for  the  simulations  corresponds  to  the  complete  observation
period  for  each  year,  listed  in  the  bottom  right  comer  of  each
panel.
In  2001,  the  observations  show  the  lowest  concentrations
of  all  years  with  minimum  values  of  5.9mg0 2 L _1  in  the
area  54-57°  N,  4.5-7°  E.  This  minimum  is  similarly  present
in  the  model  yielding  6.96mg0 2 L _1 .  Maximum  observed
concentrations  were  found  off  the  southern  tip  of  Norway
(9.3mg0 2 L _1 )  and  in  the  deepest  parts  of  the  Norwegian
Trench  (8.7  to  8.8  mg0 2  L -1 ).  The  very  high  observed  value
at  the  northwesternmost  sampling  site  represents  an  outlier