12

Depending  on  the  spatial  distribution  of  the  data  the  size  of  the  geographical  box  within
which  a  mean  density-parameter  relation  is  assumed  to  be  spatially  invariant  varied  between
111  km  by  222  km  and  333km  by  666  km.

Eight  parameters  were  subjected  to  quality  control:  potential  temperature,  salinity,  oxygen,
silicate,  nitrate,  phosphate,  ratio  [NO]/[PO].  Since  every  density  value  (determined  by  T  and
S)  is  characterised  also  by  the  depth  of  the  respective  isopycnal,  the  depth  of  the  T-S  pair
falling  within  a  density  bin  was  also  checked  as  an  independent  parameter.  The  following
explains  the  inclusion  of  the  statistical  check  for  the  sample  depth.

Before  the  introduction  of  CTD-type  probes,  which  measure  pressure  (depth)  with  an
accuracy  of  about  0.1%,  a  common  oceanographic  practice  was  to  estimate  depth  by  means
of  wire  length  and  angle  between  the  bottles.  The  bottle  trip  depth  was  (sometimes)
determined  by  means  of  pairs  of  protected/unprotected  thermometers  attached  to  some  deep
bottles.  For  a  considerable  number  of  historical  casts  unprotected  thermometers  were

Nunberol  observations  f"10
o  40  so  133  lee  zoo

lAjrrter  ol  observations  I"  10^j
o  33  40  eo  so  ioo

-  -1000

J=-

  40»

Nunberol  observations  f10- 2 j

0  33  40  60  30  100

Ibiuber  ol  observations  (‘10- 3 )
0  33  40  60  30  100

-  -1000

-3300

-  <4000

I
I

Fig.  7:  Percentage  of  rejected  observations  (red)  and  a  total  number
of  observations  (blue)  versus  depth.  Depth  bin  width  is  1  meter.
not  used  or  their  number  was  insufficient  to  determine  the  depth  of  each  sample.  The
frequency  distribution  of  sample  depth  is  depicted  in  Fig.  5  and  demonstrates  that