U.  Callies  et  al.:  Surface  drifters  in  the  inner  German  Bight

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www.ocean-sci.net/13/799/2017/

Ocean  Sci.,  13,  799-827,  2017

tidal  cycles,  relative  contributions  from  wind  forcing  increase
(Fig.  6).  However,  even  small  systematic  errors  in  the  simulation ­
  of  oscillating  tides  might  possibly  give  rise  to  erroneous
residual  current  components  similar  in  size  to  the  contributions ­
  from  windage.  A  finding  that  needs  further  analysis  is
whether  near-shore  residual  currents  underestimated  in  simulations ­
  indicate  such  inaccuracies  in  regions  where  tides  increase ­
  with  decreasing  water  depth.
This  study  did  not  substantiate  benefits  from  including
Stokes  drift  simulated  offline.  Directions  of  winds  and  waves
coincided  most  of  the  time  and  effects  of  Stokes  drift  on  surface ­
  currents  could  successfully  be  mimicked  in  terms  of  additional ­
  windage.  In  TRIM,  such  effects  seemed  already  sufficiently ­
  parametrized  as  part  of  momentum  transfer  from  the
atmosphere  to  marine  currents.  When  winds  quickly  abate,
increase  or  turn,  waves  adjust  with  a  time  lag,  needing  a  certain ­
  fetch  to  fully  develop.  Although  in  these  cases  the  different ­
  roles  of  winds  and  waves  could  be  more  marked,  in
the  present  study  errors  in  atmospheric  or  marine  circulation
modelling  seemed  predominant.  Nevertheless,  fully  coupled
modelling  of  currents  and  waves  (Staneva  et  ah,  2017)  could
probably  improve  simulated  surface  currents,  given  that  the
vertical  resolution  is  fine  enough.  It  must  also  be  kept  in  mind
that  the  present  study  did  not  include  any  extreme  events.
The  incident  of  two  drifters  converging  quickly  and  separating ­
  about  10  days  later  provided  evidence  that  at  least  in
some  situations  an  unavoidable  increase  in  prediction  uncertainty ­
  would  be  of  the  order  of  3-5  km  day -1 ,  regardless  of
however  sophisticated  a  model  used  might  be.  Further  studies ­
  would  be  needed  to  substantiate  this  finding  in  terms  of
its  representativity  and  possible  dependence  on  specific  locations ­
  or  atmospheric  conditions.  The  observed  separation
rate  happened  to  roughly  agree  with  the  average  magnitude
of  simulation  errors  we  identified.  More  experiments  would
help  identify  the  way  to  go  for  further  model  improvements.

Data  availability.  The  raw  data  of  observed  drifter  locations  are
freely  available  from  Carrasco  and  Horstmann  (2017).  Results  of
the  PCA  of  residual  currents  are  freely  accessible  at  Callies  (2016).