2520

F.  Große  et  al.:  Looking  beyond  stratification

Biogeosciences,  13,  2511-2535,  2016

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

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Figure  4.  Spatial  distribution  of  observed  and  simulated  average  bottom  Ot  concentrations  in  late  summer  (a)  2001,  (b)  2005  and  (c)  2008,
and  (d)  standard  deviation  in  2005.  Colour  scale  of  panel  (b)  applies  to  panels  (a)—(c).  Circles  indicate  sample  sites,  underlying  colours  show
simulation  results.  Averages  and  standard  deviation  were  calculated  for  the  entire  observation  period  (bottom-left  comer  of  each  panel).
White  circles  in  (d)  mark  model  bottom  grid  cells  with  only  one  corresponding  observed  value  (i.e.,  no  standard  deviation).

due  to  the  decreasing  vertical  resolution  of  the  model  in
greater  depth.
In  2005,  the  observed  minimum  values  are  about
0.3mgO7L _1  higher  than  in  2001.  This  relative  increase
compared  to  2001  is  reproduced  well  by  the  simulation  showing ­
  a  similar  increase  by  about  0.2mgO7L _1 .  The  observations ­
  show  lowest  bottom  O2  a  bit  north  and  south  of
the  simulated  minimum,  but  still  relatively  low  values  less
than  7.2  mgCL  L _1  in  the  centre  of  the  simulated  minimum.
Highest  observed  concentrations  of  9.3mg07L _1  are  located ­
  in  the  very  deep  area  at  the  eastern  end  of  the  Norwegian ­
  Trench,  and  in  the  inner  German  Bight.  In  the  German ­
  Bight,  the  model  indicates  only  slightly  higher  values
compared  to  2001,  while  in  the  northern  North  Sea,  the  simulation ­
  shows  lower  values  compared  to  2001,  which  can  be
seen  in  the  observations  as  well.
In  2008,  the  observations  reveal  significantly  higher  bottom ­
  O2  concentrations  in  the  area  of  the  2001  minimum,
compared  to  the  previous  years.  In  contrast,  observed  concentrations ­
  in  most  other  parts  of  the  North  Sea  are  lower
than  in  2001  and  2005.  The  overall  minimum  concentration

in  2008  of  7.2  mg02  L _1  is  reached  close  to  the  Dutch  coast.
In  the  southern  North  Sea,  the  simulation  yields  lower  bottom ­
  O2  concentrations,  and  significantly  higher  values  in  the
2001  minimum  area.  For  the  western  central  and  northern
North  Sea  the  picture  is  different.  Here,  the  observations  result ­
  in  consistently  lower  values  compared  to  2001  and  2005,
whereas  the  simulated  O2  shows  higher  values  in  most  areas,
except  for  the  eastern  Norwegian  Trench.
The  simulated  SD  in  2005  is  mainly  representative  for
the  years  2001  and  2008  as  well.  It  shows  that  in  most  areas ­
  of  the  North  Sea  the  changes  in  bottom  O2  dining  August/September ­
  are  very  low  throughout  a  period  of  3  to  4
weeks,  indicated  by  a  SD  of  less  than  0.1  mg02L _1 .  The
observations  also  show  only  small  SDs  in  most  areas.
In  general,  the  basin-wide  distributions  of  simulated  bottom ­
  O2  represent  the  observed  spatial  patterns  and  their  yearto-year
  variations  quite  well,  even  though  absolute  values
are  not  always  reflected.  Both  observed  and  simulated  bottom ­
  O2  concentrations  show  that  the  50  m  isobath  (broadly
along  54°  N,  0°  E  to  57°  N,  8°  E;  Thomas  et  al.,  2005)  marks
the  separation  line  between  the  northern  regions  unaffected