README NAtl-93 CTD files NOAA AOML designation: North_Atlantic-93, NAtl-93 WOCE/WHP and NODC designation: WOCE AR21, Aug. 22- Oct., 1993. Contact person: Rik Wanninkhof, Wanninkhof@aoml.noaa.gov CTD data reduction: Libby Johns, Johns@aoml.noaa.gov Address NOAA/AOML 4301 Rickenbacker Causeway Miami FL 33149 FILE STRUCTURE AND HEADER INFORMATION: Cast_0XX.cal files The cast_0XX.cal files, where XX is the cast number, are space delimited text files with the following column headings Header: line 1-3: Cast: CTD cast #, 0XX lat: latitude (N), decimal XX.XXX long: longitude (W), decimal XX.XXX jday: year day, DDD gmt: time (GMT), HHMM CTD information: line 4-end Pre: pressure (dB) at 2-dB intervals Tem: temperature (_C) Sal: salinity (practical salinity scale) The location&time.csv file is a comma-delimited file that contains the time and geographic information for each cast: Station: station # Cast: Cast #, note every over the side operation has a consecutive cast number CTD Cast: # of CTD cast (consecutive) Date: M/DD/YR Time_GMT: HH:MM Year_day: fractional day DDD.DD Ą Latitude: degree minute (north or south) DD MM.MMM Ą Longitude: degree minute (W) DD MM.MMM Remark: comments about CTD DESCRIPTION: During the July-August 1993 North Atlantic cruise a total of 94 CTD stations were occupied, between Fortaleza, Brazil and Reykjavik, Iceland. The CTD instrumentation consisted of three Neil Brown Mark III systems, including pressure, temperature and conductivity sensors and a 24-bottle sampling rosette for collecting calibration bottle salinities and chemical tracers. The instruments scanned at a rate of 30 scans/s. The descent rate was typically 30 m/min in the upper 200 m, increasing to 60 m/min for the remainder of each cast. CTD casts were taken to within 25 m of the bottom in most cases where instrument problems did not preclude this. CTD data were acquired and processed at sea using the software package of Millard (ref.). Pre-cruise laboratory calibrations were used for the pressure and temperature sensors. Typical laboratory accuracies are +/- 6.5 dB for pressure and +/- .005 deg C for temperature. The conductivity sensor was also calibrated in the laboratory, but due to the nature of the conductivity cell there is the possibility of at-sea calibration drift, so bottle salinities collected during each CTD upcast were used for the final calibration of the CTD salinities. Bottle salinities were analyzed at sea using a Guildline Autosal, having typical accuracies of better than +/- .002 (referenced to the practical salinity scale, pss). General calibration methodology is as follows. Departures from standard methodology due to instrumental failures, etc., experienced at sea during this particular cruise will be discussed in the following section. Standard methodology: Bottle salinities obtained from the rosette and analyzed at sea using the Guildline Autosal are first edited for outliers using graphical P-S and T-S and historical data comparisons, and by examination of the residual differences between the bottle and CTD upcast salinities (BOTS-CTDS). Problems due to malfunctions of the rosette (i.e. missed bottle trips, bottle trips at the wrong pressures, etc.) are also determined in this way. Next, the uncalibrated CTD salinity profiles are checked for drift or other changes in the conductivity sensor by examination of the time history of the BOTS-CTDS offset throughout the cruise, and divided into subgroups of stations sharing common BOTS-CTDS characteristics. An iterative least-squares regression is run on the BOTS-CTDS residual versus pressure for each subgroup, and linear or polynomial corrections are obtained over appropriate portions of the water column. Cast-by-cast regressions are used when the conductivity drift is large enough to warrant it. The upcast CTD values at the bottle trip locations are calibrated using the results of the regression analysis, and a final bottle data set is produced to be used with the chemical tracer data. Additionally, the downcast CTD profiles are calibrated using the regressions, the data are despiked if necessary, and a final set of CTD profiles subsampled to a 2-dB increment is produced. These files are appended with designation cast_XXX.cal where XXX is the cast #. All data are quality-controlled by rechecking the residual differences between the (now calibrated) upcast and downcast salinities and the bottle salinities, matching pressure and/or density. Statistics are generated to describe the accuracy of the final calibrated data set. Post-cruise laboratory calibrations are typically obtained, and used to double-check the accuracy of the salinity calibration and also used as the final confirmation of the temperature and pressure sensor performance. Observed drift in the temperature or pressure sensors from pre-cruise to post-cruise laboratory calibrations is interpolated over the duration of the cruise if no other information (such as a from a second temperature sensor, or the knowledge that the CTD unit hit the side of the ship at a certain cast causing a temperature offset, for example) is available. Performance during the NAtl-93 cruise: The July-August NAtl-93 CTD performance was not up to standard due to a number of factors. The Neil Brown CTDs were coming to the end of their "useful lifetime" after heavy use during the previous decade (and, in fact, the summer 1993 work was the last time they were ever used before AOML changed to Seabird CTD systems). Although three Neil Brown units were available during the cruise (serial numbers 1148, 2156 and 2769), each of the units experienced a number of independent problems which began during the previous cruise (AOML/PHOD, Miami to Fortaleza). Symptoms ranging from a noisy conductivity sensor to sensor drift and rosette problems were experienced during this previous cruise. In Fortaleza, during preparation for the NAtl-93 leg of the cruise, all three CTD units were swapped into the rosette in an attempt to determine the best CTD, but the problems continued joined by signs of noisiness and drift of the temperature sensor in one of the units as well. Dave Bitterman, AOML's head CTD engineer, was flown to Madeira to be of assistance. After examining the units, the decision was made to build a "fourth" CTD out of what appeared to be the best parts of the three original CTDs. This CTD performed better but other problems remained as described below. Additionally, the building of a fourth CTD negated the pre-cruise pressure and temperature laboratory calibrations for this hybrid unit. Following NAtl-93 this CTD was used for a short cruise in September 1993 off Abaco, the Bahamas, where it finally developed even worse problems. Post-cruise laboratory calibration was not possible as the conductivity sensor was found to have cracked and was non-functional. Various other problems plagued the cruise, ranging from a recurrent problem with the rosette mis-tripping bottles, to deck unit troubles necessitating switching deck units several times, to problems with the (new) software data acquisition package which had not been worked out prior to the cruise. These problems were dealt with post-cruise on a cast by cast basis by various methods. For example, incorrect bottle depths were adjusted using a careful comparison of the bottle salinities and the CTD salinity profiles, using knowledge of the history and trend of the BOTS-CTDS residuals. On several casts where the upcast bottle trip CTD values failed to be logged due to software problems, downcast values were matched to the nominal bottle trip depths and the BOTS-CTDS residuals used to confirm the match. Finally, on a few isolated casts (cast #'s) the temperature developed an unrealistically large offset based on comparisons with surrounding casts as well as comparison with historical data (probably software related? electrical?) and a correction was made to the temperature based on interpolation over adjacent casts. Subsequently the temperature sensor behavior returned to normal. Final data quality: Despite the many and varied problems listed above, a reasonably high quality CTD data set was obtained from this cruise which will be useful for most scientific purposes. Studies which by their nature push the limit of CTD technology and accuracy (for example fine structure studies, or comparative studies of long term temporal changes in temperature and/or salinity based on detailed comparisons with the results of other cruises, etc.) will probably not be possible with this data set. As explained above, it is not possible to quantitatively assess the accuracy of the temperature and pressure sensors as there was no post-cruise laboratory calibration available. However, comparisons with historical data and checks for internal consistency such as examination of the computed density profiles for each CTD cast did not raise any particular doubts about the pre-cruise calibration values which were used. The accuracy of the conductivity sensor was determined by comparison with the bottle salinities, which were within +/- .002. Table 1: Range of salinity correction (results of polynomial): casts ds (0 m) ds (deep) comment 1-16 -.001 .006 17 .009 .004 18 -.004 .002 19-22 .000 .007 23 -.083 -.050 t=t-.173; computer restart 28 .006 .007 29 .001 .013 30 .013 .015 casts 28-32: changed deck units 31 .005 .008 nearly every cast 32 .006 .013 33-34 .000 .003 35 -.343 -.291 changed to CTD_1, 35 and 36 36 -.300 -.399 37 (no cast 37; same location as 38) back to CTD_2 for cast 38 38-42 .001 .007 no cast 41; at-sea memory loss 43 .000 .009 44-45 .001 .008 46 .021 .024 switched to CTD_2..., 46-53 47 .019 .024 48 -.013 .019 49 -.045 .038 50 -.201 .003 t=t-.109 51 -.083 .032 t=t-.109 52 -.357 .008 t=t-.109 53 -.200 .091 t=t-.109 54-57 .002 .007 switched to CTD_4 for duration 58 -.041 .009 59-61 .001 .007 62-65 .003 .009 66 -.010 -.009 67-70 .004 .010 71 .005 .009 72 (no 72; same location as 71) 73-79 (no 74) .005 .009 80-81 .010 .012 82-84 .005 .010 85-86 .005 .008 87 .005 .010 88 .015 .013 89 .007 .009 90-94 (no 93) .008 .010 ************************************************************************ ds: delta salinity (residual best fit polynomial- observed) After calibrations were applied, a statistical comparison of the final upcast and downcast CTD data was done. For the calibrated downcasts, matching the pressure of each bottle salinity with the same pressure in the calibrated downcast, the results were as follows: Over all of the casts (1-94), over the entire water column (0-6000 m), for all of the bottles, the BOTS-CTD salinity difference was .000 +/- .007; iteratively tossing out BOTS-CTD values that exceeded 2 standard deviations improved the comparison to .000 +/- .004 using 86% of the bottles. If just the deep values are used (1000 m) the difference is again .000, and the standard deviation is less than +/- .002 psu. For the calibrated upcasts, again matching pressure, the results were as follows: Over all 94 casts, over the entire water column, for all of the bottles, the BOTS-CTDS salinity difference was .001 +/- .010; the standard deviation reduced to .000 +/- .004 for 95% of the bottles, and .000 +/- .003 for 89% of the bottles when the iterative process was performed. For just the deep (1000 m) values, the difference is .000 +/- .002 using 97% of the bottles. The higher standard deviations when the comparisons are done over the whole water column is to be expected due to the large T and S gradients in the upper water column. For the downcast comparison, this effect is combined with the temporal variability possible between the downcast and upcast (a deep CTD cast takes approximately 4 hours). This analysis suggests that the salinity data on average are good to +/- .002 for the deep-water column ( 1000 m) with no apparent bias, and only a little worse (+/- .003 to .004) for the entire water column. The temperature and pressure accuracies are as stated previously from laboratory calibrations, +/- .005 deg C and +/- 6.5 dB. Of course, without a post-cruise laboratory calibration this is impossible to check. Millard, R.C., and K. Yang, CTD calibration and processing methods used at Woods Hole Oceanographic Institution. Technical report WHOI-93-44. Woods Hole Oceanographic Institution, Woods Hole, MA1993.