CTD profiles were made using a SBE 9/11 CTD system with dual temperature and salinity sensors, a Paroscientific pressure gauge and a Seabird oxygen sensor. The CTD worked flawlessly although there were a few correctable human errors on the early stations. The dual sensors were particularly useful in tracking down several minor coefficient errors early in the cruise and tracking the slow drift of the conductivity sensors with time.

Problems

Data from station 30 was partially lost through a computer crash. Data from the up and down casts were combined to form a composite profile which spans nearly all of the water column.

Temperature

No temperature standards were carried onboard, since SeaBird temperature sensors are generally highly accurate and stable. Temperatures computed using precruise and postcruise calibrations were interpolated in time to obtain a calibration for each cast. The secondary temperature sensor (sn 2107) was consistently about 0.5 mK warmer than the primary sensor (sn 2034) (Figure A1). On the last cast, the secondary temperature was replaced by temperature sensor sn 1050 . It agreed with the primary within about 0.2 mK suggesting that sn 2107 is in error. Accordingly, the primary temperature sensor is considered to be more accurate and used in the released data.

 
Figure A1. Difference between primary and secondary temperatures (upper) and salinities (lower) after final calibrations.

Salinity

Salinity samples were taken on all casts using 24 4l bottles on a Seabird rosette and their salinity measured onboard using a Guidline autosal in a temperature controlled van. Two salinities were computed, one from the primary T-C pair and one from the secondary T-C pair. These drifted apart during the cruise with the primary indicating about 0.0025 psu saltier than the secondary by the end of the cruise.The primary drifts high relative to the bottles with time and the secondary drifts low (Figure A2, dots and circles). The drift was modeled by a quadratic function of cast number (Figure A2, lines). The CTD salinities from both primary and secondary sensors, corrected by this model, generally fall within 0.001 psu of the bottle samples. This is approximately the accuracy of the salinometer. The primary salinity is used in the released data.

  Figure A2 Difference between salinity from bottles and CTD. Value from each bottle indicate by ('+'), average for entire cast, excluding outliers, by ('o'). Heavy red line is quadratic fit; light red lines are plus and minus 0.001 psu. Fit coefficients are listed on figure.

Dynamic Response

Figure A3 illustrates the accuracy of the CTD in following rapid temperature and salinity fluctuations. A temperature jump of 0.15 C is almost completely compensated by salinity. There is no evidence of salinity spiking and the density fluctuations associated with the temperature jump are less than 0.01 kg /m³. This is sufficiently good that no corrections to improve dynamic response have been made.

Oxygen

Oxygen concentration was computed from the CTD oxygen sensor using coefficients supplied by a previous user and computed using the Seabird software. Oxygen samples were taken on most casts, with a full water column of bottles taken on all AR7W stations. Oxygen was measured using an automated Winkler tritration system . Oxygen computed from the oxygen sensor is nearly useless without calibration by the bottles. There is an approximately 0.2 ml/l hysterisis between the up and down casts; a trend with depth of nearly 1 ml/l, and a small decrease in the sensor oxygen when the CTD stops for each bottle. Transient sensors responses, several hundred meters long are seen at the top of each cast and at the start of each upcast. The offset between the sensor oxygen and the bottles changes by about 0.1 ml/l with time, apparently depending on how often and how long the CTD is turned on.

 A corrected oxygen profile has been brought into reasonable agreement with the bottle values as follows:
The oxygen sensor values on the up and down casts were brought into approximate agreement by 1) deleting the bottom 700 m of the upcast, all sections of the upcast with a vertical velocity less than 0.2 m/s, and the top 350 m of the downcast , 2) adding 0.23 ml/l to the upcast, 3) combining the up and down casts to form a combined oxygen profile 4) fitting the difference between the bottles and the combined oxygen profile with a quadratic polynomial, and 5) adding this polynomial to the combined oxygen profile. Figure A4 shows the up (red) and down (blue) components of the combined profile, the corrected profile (black), and the bottle oxygen values ('o'). The average rms deviation of the corrected profile from the bottles is about 0.03 ml/l, only slightly larger than the error in the Winkler titrations. The corrected profiles are least accurate for shallow casts as there are fewer bottles and the sensor has less time to equilibriate.

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Other Errors

The temperature and salinity differences (Figure A1) show subtle differences between the up and down casts. Similarly, the oxygen profiles(Figure A3) show a slight decrease at many of the bottles. Both suggest small dependence of the the measured values on the flow past the instrument. Since the SBE CTD is pumped, this is possible only if the plumbing is slighty restricted or improperly installed. Suspecting this, we carefully inspected the plumbing before station 24 and reattached a loose piece of tubing. This may explain the anomalous temperature in station 24.