Dear Mark,

Thank you for your DQE evaluation of CTD data collected along WOCE section
P18.  We considered each of your suggestions and the following is an
itemized explanation of what we did or didn't change in our data files, as
well as answers to your questions.

Kristy McTaggart and Greg Johnson

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STATION SUMMARY FILE (.sum)

.sum files here were ammended to contain the same maximum pressure values
for stations 25, 27, 32, 46, 61, and 78 as you listed.

The PDR sound speed used for sounder readings was 1500 m/s.  The readings
were not corrected for transducer depth below the waterline.  The depth of
the transducer would've been about 5.5 +/- 0.6 m.  We would prefer to use
the PDR depths as listed and correct them using Carter's tables so that
they serve as independent measurements and can be used as a check on CTD
pressure.


SALINITY 

Regarding suspicious CTD salinity data listed in Table 4:

        station 24      2-6 dbar        flags not changed to 3
        station 51      84 dbar         flag changed to 3
        station 52      74 dbar         flag changed to 3
        station 53      70 dbar         flag changed to 3
        station 55                      flags not changed to 3
        station 67      46 dbar         flag changed to 3


'Scatter of salinity residuals'

There is an incompatibility between the General Oceanics rosette sampler
and the Sea-Bird 911plus CTD system that generates a spike in the data
stream at the moment a bottle is confirmed as tripped.  Because of this,
upcast CTD burst data had to be averaged prior to the bottle confirm bit.
Two-second averages were chosen over a longer interval because the CTD
operators did not always let the package sit at bottle depth for at least
10 seconds before firing the rosette.  Hence no changes were made.


'Biasing of CTD salinity data for individual stations'

Of course one can seemingly make a (very slight) improvement in the
CTD-bottle residual statistics by allowing more degrees of freedom in the
fit as the DQE has suggested (that is, breaking up the fit into small
station groupings).  One could get the best statistics by individually
fitting each station to its bottles, but most experts would argue that
this would be a bad choice, because one would not be taking advantage of
the CTD calibration as a way to average out station-to-station bottle
salinity noise.

We believe that the SBE-9/11 CTD conductivity slope drifts gradually, and
is actually more stable than the day-to-day fluctuations in the autosal-
inometer salinities owing to small temperature drifts in the laboratory
and the fact that severe budgetary constraints on these cruises forced us
to economize even on such things as standard sea water.  We suspect that
the "biasing of the CTD salinity data" mentioned in the DQE evaluations is
actually noise in the bottle data.  Somewhat suspicious is that the station
groupings recommended by the DQE of the correct size (most often 3-5
stations per group) that they could easily be owing to daily drift problems
in the autosalinometer.  For our original calibrations we deliberately
chose to model the conductivity slope adjustments of the entire data sets
for P14S/P15S and P18 using 4th-order polynomial functions of station
number to average out bottle salinity noise.  We did this because we saw
no obvious jumps in the CTD calibration for either cruise, just gradual
drifts.

Statistical support for our philosophy over that of the DQE is given by
the following exercise:  The 2oC potential isotherm is well within the
oldest Pacific Deep Water, and has some of the tightest Theta-S relation-
ships in the Pacific Ocean (and probably the world).  For both P18 and
P14S/P15S, we looked at the absolute values of station-to-station changes
in CTD salinity on Theta=2.0oC (Figure 1) for our original calibration,
creating a histogram of station-to-station differences for each cruise in
0.001 bins.  We then applied the DQE's suggested ad-hoc calibrations for
smaller station groupings to the data and conducted the same analysis.
When the histograms are differenced (Figure 2), one can see that the
Theta-S relations at 2oC after the DQE's corrections are noisier for both
cruises.  For P18, after the DQE's suggested correction there are four 
less station pairs in the 0.000 difference bin and one less in the 0.001
difference bin whereas there are three more in the 0.002 difference bin
and two more in the 0.003 difference bin.  For P15S/P15S there are four
less stations in the 0.000 difference bin after the DQE's suggested
correction, with one more in the 0.001 difference bin and three more in
the 0.002 difference bin.  Since the DQE's "corrections" actually
introduce more noise in the CTD Theta-S relation at 2oC than our original
calibration, we decline application of them.  The small groups do not
improve the calibraiton, they degrade, perhaps by introducing auto-
salinometer drift noise.


OXYGEN

Rankings for stations as listed in Table 6 were complied with except for
station 160, which is closer to a rating of 2 than 1 and was flagged as 3
not 4.  A cutoff of 3750 dbar was used to reflag the deep data of stations
21 and 22; 3400 dbar for station 65; 3200 dbar for station 67; and 2200 
dbar for station 85.  Note all flags of 6, 7, or 8 were preserved in the
reflagging.  

Poor oxygen data were owing to poor sensor performance not to the data 
processing or curve fitting.  A few worst case groupings were reexamined
using two sets of fit coefficients blended near the oxygen minimum as was
done for P14S/P15S.  However, there was no significant improvement.
Unfortunately, only one oxygen module was available for this cruise due
to severe budgetary constraints, and it was not a good one.

Suspicious oxygen data listed in Table 5 were examined and near surface
data were reflagged as 3 as suggested.  Note that data files submitted 
before and after the DQE evaluation are 1 dbar averages, not the 2 dbar 
averages referenced.  For suspicious oxygen data deeper in the water 
column, these were interpolated over and flagged as 6 (stations 30, 69, 
70, 71-74, 128, 153, and 180).  The shift in oxygen data between 2084 and 
2384 dbar for station 188 was flagged as 3 and not interpolated over.
Again, all flags of 6, 7, or 8 were preserved in the reflagging.

Stations 26, 89 and 160 were viewed with adjacent profiles and their 
bottles.  Station 26 and 89 oxygen profiles were flagged as 4 as suggested
in Table 6.  Station 160, however, looked to be closer to a rating of 2
than 1 and was flagged as 3 not 4.

CTDOXY flags in the .sea file were changed to 4 for all the station 
samples you listed.  Also, CTDOXY flags were changed to 4 where profiles
were recently interpolated as a result of DQE suggestions:
	
	station  30	sample 121
	  	 70 	       107
	  	 73	       108
	        180	       111


TEMPERATURE

There is a typo in the data report.  The value of the drift for 
temperature sensor T1461 is -0.0006 C.  Temperature calibrations were
applied to the data using Seasoft processing module DATCNV which reads
the sensor's .con file for coefficients.


DESPIKING, INTERPOLATION AND FLAGS

The flag value of 8 used near the surface in the .ctd files represent
data that were continued to the surface from the first assumed good value.
For P14S/P15S we used 7.  For P18, this procedure was done in program 
POSTCAL where temp, cond, oxc and oxt were copied back and flagged as 8,
then salinity was recomputed and flagged as 2 in most cases.  Despiking
done after POSTCAL changed some flags to 6.  Flags of 8 were left in the
data files for this cruise.

As for the large blocks of interpolated data (mostly oxygen) listed in 
Table 2, we maintain that this is the best way to deal with these data
from a poor and failing sensor.  Flags of 6 (as well as 7 and 8) have
been preserved even when reflagging the entire oxygen profile as
suggested in Table 6.


DENSITY INVERSIONS

Original data submitted for P18 were not examined for small density
inversions.  In response to the DQE evaluation, program DELOOP, as applied
to P14S/P15S, with an N^2 criteria of -3x10e-6 was applied to P18 profiles.
Over 82% of the density inversions listed in Table 7 were interpolated
over.  Delooped 1 dbar averaged data files with all the changes noted above
are resubmitted along with this reply to the DQE.


DOCUMENTATION

Again, the PDR sound speed was 1500 m/s, and the readings have not been
corrected for transducer depth (5.5 +/- 0.6 m) below the waterline.

Station groupings used for oxygen calibrations and final values of fit
parameters are given in a separate oxygen calibration table.

Oxygen calibration problems were owing to poor sensor performance.

Temperature pre- and post-cruise calibration difference for sensor T1461
was a typo in the documentation and should read -0.0006 C.

More frequent flagging of surface temperatures compared to surface 
salinities is explained in the previous section, DESPIKING, INTERPOLATION
AND FLAGS.

Data files submitted to the WOCE office were 1 dbar averages, not 2 dbar.