Deutsche  Hydrographische  Zeitschrift  -  German  Journal  of  Hydrography

f

158

U a  (cm/s]
Fig.7:  Thickness  of  the  Ekman  layer  /i L .  estimated  according ­
  to  [4]  and  percentage  frequency  distribution ­
  of  i/ f  (1  cm/s  intervals)  as  observed  at  MK1
and  MK2  at  504  mab  (52  days),  and  at  D1  at  200
mab  (861  days).  i/ f  is  averaged  over  the  respective ­
  4  preceding  days  which  is  the  time  scale  for
the  maintenance  of  the  Ekman  layer.
extrapolations  from  a  few  measurement  positions
may  be  unreliable  for  a  larger  area.
Because  of  the  manganese  nodule  coverage,
there  will  be  no  viscous  sublayer,  i.  e.  the  flow  will
be  turbulent  directly  above  the  sea  bed.  The  logarithmic ­
  layer  is  of  the  order  of  1  metre.  The  height  of
the  Ekman  layer  varies  according  to  [4].  The  time
scale  of  the  EL  is  of  the  order  of  half  a  pendulum
day  (Bowden  [1978]),  i.e.  106  hours  at  the  SEDI-PERU
  site  and  98  hours  at  the  DEA  site.  Therefore,

not  any  increase  in  the  current  speed  will  cause  an
expansion  of  the  EL,  but  the  speed  level  must  be
maintained  for  about  4  to  5  days.
Figure  7  shows  the  percentage  frequency  distribution ­
  of  the  mean  (geostrophic)  flow  U e  for  MK1
and  MK2  at  503  mab  -  which  is  clearly  outside  the
BBL  -  and  for  D1  at  200  mab.  The  mean  velocity
magnitude  used  for  this  diagram  is  a  mean  over  the
respective  4  preceding  days,  i.  e.  the  velocity  which
is  available  for  maintaining  the  Ekman  layer.  During
periods  of  slow  bottom  currents  (MK1,  MK2)  we  can
expect  an  Ekman  layer  height  of  between  10  and  30
m.  The  D1  time  series  which  covers  a  much  broader
time  interval  (861  days)  with  several  events  of  relatively ­
  strong  currents  shows  that  h c  can  reach
values  up  to  60  m  during  such  events.  Figure  8  presents ­
  a  time  series  of  U.,  (4  day  average)  and  h c  at
D1.  On  average  /¡ c  amounts  to  20  m,  but  there  are
also  periods  of  1  to  2-month  duration  with  values  of
about  40  m  and  even  higher  values.
Armi  and  Millard  [1976]  found  a  correlation
between  BML  height  and  the  daily  mean  value  of
the  ‘outer’  flow.  The  recordings  at  MK1  between  50
and  503  mab  ended  a  few  days  before  the  CTD  profiles ­
  shown  in  Figure  6  were  taken,  but  the  daily
mean  value  at  0.6,  6,  and  13  mab  gives  a  current
speed  of  less  than  1  cm/s.  Therefore,  the  weak
BML’s  we  observe  in  the  profiles  can  be  regarded
as  a  relict  of  an  old  BML  which  has  been  established ­
  during  an  earlier  period  of  stronger  flow.

Fig.8:  Daily  mean  values  of  Ekman  layer  height  h c  (top)  und  mean  geostrophic  velocity  U e  at  D1  200  mab  (bottom).
U  is  averaged  over  the  respective  4  preceding  days  which  is  the  time  scale  for  the  maintenance  of  the  Ekman
layer.