5

Tidal  and  Residual  Currents  in  the  Northern  North  Sea
Observations

Holger  Klein,  Wolfgang  Lange  and  Ekkehard  Mittelstaedt

UDC  551.466.78:551.466.75

Summary
One-month  current  and  water-level  time  series  from  the  northern  North  Sea  are  analyzed  with  regard ­
  to  residual  flows,  volume  transports,  and  tidal  constituents.  The  data  originate  from  the  NORA  and
MO  YENS  field  programs  (along  59°  N,  July/August  1990  and  roughly  62°  N,  September/October  1986  respectively). ­
  With  regard  to  the  residual  flows,  the  results  generally  confirm  the  known  circulation  pattern  of
the  northern  North  Sea.  Differences  between  these  and  other  investigations  are  due  to  the  great  short  and
long-term  variability  of  the  current  systems.  The  main  objective  of  this  paper,  however,  is  to  describe  tides
and  tidal  streams.  Firstly,  we  relate  the  tidal  streams  along  59°  N  to  the  spring  and  neap  high  waters  at  Aberdeen; ­
  secondly,  up  to  17  tidal  constituents  are  determined  by  means  of  harmonic  analysis.

Gezeiten  und  Restströme  in  der  nördlichen  Nordsee:  Beobachtungen  (Zusammenfassung)
Einmonatige  Strömungs-  und  Wasserstandszeitserien  aus  der  nördlichen  Nordsee  werden  hinsichtlich ­
  der  Restströme,  Volumentransporte  und  Gezeiten  analysiert.  Die  Daten  stammen  aus  den  beiden  Meßprogrammen ­
  NORA  entlang  59°  N  (Juli/August  1990)  und  MO  YENS  entlang  ca,  62°  N  (September/Oktober ­
  1986).  Die  berechneten  Restströme  entsprechen  dem  bekannten  generellen  Zirkulationsmuster  in  der
nördlichen  Nordsee.  Im  einzelnen  auftretende  Abweichungen  zu  anderen  Untersuchungen  haben  ihre  Ursache ­
  in  der  starken  Variabilität  der  Restströme  in  diesem  Seegebiet.  Hauptanliegen  dieser  Arbeit  ist  die
Darstellung  der  Gezeiten  und  Gezeitenströme,  Im  ersten  Schritt  werden  die  Gezeitenströme  entlang  59°  N
zu  den  Spring-  und  Nipp-Gezeiten  in  Aberdeen  in  Beziehung  gesetzt.  In  einem  zweiten  Schritt  werden  mit
Hilfe  der  harmonischen  Analyse  die  Koeffizienten  von  bis  zu  17  Partialtiden  berechnet.

1  Introduction
The  water  exchange  across  the  northern  boundary  of  the  North  Sea  is  dominated  by  the  in-  and  outflow
through  the  Norwegian  Trench.  The  trench  is  the  main  link  between  the  shallow  North  Sea  and  the  deep  Atlantic ­
  Ocean.  The  transition  zone  between  the  shelf  and  the  deep  sea  is  also  the  northern  boundary  of  several
numerical  North  Sea  circulation  models.  A  main  problem  in  modelling  is  posed  by  the  great  length  of  the
open  northern  boundary  of  the  North  Sea,  together  with  the  great  variability  of  volume  transports  in  several
time  scales.  Most  of  these  models  use  only  the  M 2  tidal  elevation.  Higher  tidal  harmonics,  however,  are  also
important  for  North  Sea  dynamics  and  are  generated  on  the  shelf  itself  (cf.  D  e  1  h  e  z  and  Martin  [1992]).
Besides  discussing  tidal  and  residual  currents,  we  therefore  also  determine  tidal  constituents  which  could  be
used  to  improve  the  boundary  information  for  circulation  models.  The  results  of  two  field  experiments  are
discussed  below.

Methods:
During  the  NORA  experiment  (Nord  Rand,  northern  boundary),  8  current  meter  moorings,  each
equipped  with  3-5  Aanderaa  RCM4/5  current  meters,  were  deployed  along  59°  N  between  2°  W  and
4°  45’  E  (N1-N8  in  Fig.  1).  The  moorings  were  maintained  for  about  one  month  from  15-17  July  until
18-21  August,  1990.
The  MO  YENS  moorings  (Model  Verification  North  Sea)  were  deployed  between  6  and  8  September
1986,  and  recovered  between  15  and  19  October  1986.  The  MOVENS  moorings  (M7-M10  in  Fig.  1)  were