2514

F.  Große  et  al.:  Looking  beyond  stratification

Biogeosciences,  13,  2511-2535,  2016

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

A  further  sink  of  O2  is  nitrification,  the  bacterial  transformation ­
  of  ammonium  to  nitrate.  Within  ECOHAM,  this  process ­
  is  light-dependent  and  links  the  O2  cycle  to  the  N  cycle.
Nitrification  only  occurs  under  aerobic  conditions  (i.e.,  concentrations ­
  >  OmgCE  L _1 ),  which  is  a  realistic  constraint  for
the  pelagic  North  Sea  environment.  It  is  light-dependent,  being ­
  stronger  under  low  light  conditions.  Pelagic  denitrification ­
  is  implemented,  but  is  negligible  as  it  only  occurs  under ­
  anaerobic  conditions.  Pelagic  anaerobic  ammonium  oxidation ­
  (anammox)  is  not  implemented,  however,  it  can  be
neglected  for  the  same  reason.  Except  for  primary  production, ­
  the  biological  processes  involved  in  the  Ot  cycle  are
not  temperature-dependent  in  the  present  model  setup.
For  the  representation  of  the  benthic  remineralisation  processes ­
  a  simple  sediment  module  is  used.  A  layer  of  zero  extent ­
  is  defined  below  the  deepest  pelagic  layer  of  each  water
column.  There  the  deposited  organic  matter  is  collected  and
remineralised  (Patsch  and  Ktihn,  2008).  The  benthic  remineralisation ­
  of  the  organic  matter  is  defined  as  a  first-order  process ­
  with  relatively  high  remineralisation  (C,  N,  P)  and  dissolution ­
  rates  (Si;  opal)  preventing  year-to-year  accumulation
of  deposited  matter.  The  released  dissolved  inorganic  matter
is  returned  directly  into  the  pelagic  bottom  layer.  Different
rates  are  applied  to  organic  C,  N,  P  and  Si  resulting  in  different ­
  timescales  for  the  release  into  the  pelagic.  In  ECOHAM,
the  O2  cycle  is  affected  by  the  benthic  remineralisation  in
a  direct  and  indirect  way.  First,  the  remineralisation  in  the
sediment  is  accompanied  by  the  direct  reduction  of  the  O2
concentrations  in  the  pelagic  bottom  layer  above.  Second,  inorganic ­
  nitrogen  is  released  from  the  sediment  in  the  form  of
ammonium,  which  can  be  nitrified  within  the  water  column
under  O2  consumption.  According  to  Seitzinger  and  Giblin
(1996),  who  suggested  a  tight  coupling  between  benthic  nitrification ­
  and  denitrification,  benthic  denitrification  depends
on  the  benthic  O2  consumption  in  our  model.  Direct  benthic
nitrification  and  benthic  anammox  are  neglected  as  the  sediment ­
  has  zero  vertical  extent  (Patsch  and  Ktihn,  2008).
For  a  more  detailed  description  of  the  ECOHAM  model,
including  the  full  set  of  the  differential  equations  and  parameter ­
  settings  of  ECOHAM,  the  reader  is  referred  to
Lorkowski  et  al.  (2012).  A  detailed  description  and  analysis
of  the  O2  module  can  be  found  in  Müller  (2008).
2.1.1  Model  setup  and  forcing  data
The  model  domain  extends  from  15.250°  W  to  14.083°  E  and
from  47.583  to  63.983°  N  and  comprises  the  entire  North
Sea,  large  parts  of  the  northwestern  European  continental
shelf  and  parts  of  the  adjacent  northeastern  Atlantic.  The  horizontal ­
  resolution  is  1  /5°  with  82  grid  points  in  latitudinal  direction ­
  and  1  /3°  with  88  grid  points  in  longitudinal  direction.
The  horizontal  grid  of  the  model  domain  is  shown  in  Fig.  2.
The  vertical  dimension  with  a  maximum  depth  of  4000  m  is
resolved  by  31  z-layers  with  a  surface  layer  of  10  m.  The
vertical  has  a  resolution  of  5  m  between  10  and  50  m  depth.

Figure  2.  Horizontal  grid  and  bottom  topography  of  the
HAMSOM-ECOHAM  model  domain.  White  numbers  indicate
depth  levels.  Yellow  boxes  A-D  mark  the  4x4  regions  used  for  the
characterisation  of  key  features  presented  in  Sect.  3.3.  Black-filled
boxes  (1,  2)  mark  the  validation  sites  discussed  in  Sect.  3.1.1.  Redframed ­
  boxes  (2-5  )  indicate  regions  used  for  the  O2  mass  balance
calculations  in  Sects.  3.4-3.7.

which  is  relevant  for  the  calculation  of  the  MLD  (Sect.  2.2).
Below  50  m,  the  layer  thicknesses  successively  increase  with
depth.
The  model  system  was  run  over  the  period  1977  to  2012.
HAMSOM  was  initialised  with  a  monthly-averaged  climatology ­
  based  on  the  World  Ocean  Atlas  (WOA;  Conkright
et  al.,  2002).  The  meteorological  forcing  was  derived  from
NCEP/NCAR  reanalysis  data  (Kalnay  et  al.,  1996;  Kistler
et  al.,  2001)  and  provides  6  hourly  information  for  air  temperature, ­
  cloud  coverage,  relative  humidity,  wind  speed  and
direction.  Short  wave  radiation  was  calculated  from  astronomic ­
  insulation  and  cloud  coverage  applying  a  correction
factor  of  0.9  (Lorkowski  et  al.,  2012).  The  data  were  interpolated ­
  to  the  model  grid  and  time  step  according  to  O’Driscoll
et  al.  (2013)  and  Chen  et  al.  (2014).  Daily  freshwater  run-off
data  for  249  rivers  were  provided  by  Cefas  and  represent  an
updated  data  set  of  that  used  by  Lenhart  et  al.  (2010)  covering
the  entire  simulation  period.  The  same  data  set  encompassed
nutrient  loads  used  for  the  ECOHAM.
At  open  boundaries,  surface  elevation  was  prescribed  as
a  fixed  (Dirichlet)  open  boundary  condition  (OBC)  according ­
  to  the  M2  tide,  while  for  horizontal  transport  velocities
radiation  OBCs  were  applied.  For  tracers  (T  and  S)  radiation
and  radiative-nudging  OBCs  were  used  in  the  case  of  inflow
and  outflow,  respectively.  A  detailed  description  of  the  OBCs