preliminary data report may 5, 1995 A. Cruise Narrative A.1. Highlights A.1.a WOCE Designation ISS1 A.1.b EXPOCODE 35MF68SUZIL A.1.c Chief Scientist Young-Hyang Park A.1.d Ship M/V Marion Dufresne A.1.e Port of Call Le Port, La Reunion A.1.f Cruise Date April 12 to May 20, 1991 A.2 Cruise Summary A.2.a Geographic boundaries The cruise track followed four near-meridional sections cutting across the ACC core and a zonal section along 38 S, within the area bounded by 37 - 51 S and 50 - 80 E. A.2.b Total number of stations During the cruise, a total of 73 CTD/rosette stations were occupied using a 12 bottle rosette equipped with 12 l Niskin water sampling bottles. A.2.c Floats and drifters deployed A.2.d Moorings deployed or recovered A.3 List of Principal Investigators Name Responsibility Affiliation ---------------------------------------------------------------------------- Y. H. Park CTD, S, 02 Museum National d'Histoire Naturelle (MNHN) Paris, France ----------------------------------------------------------------------------- A.4 Scientific Programme and Methods The cruise was carried out in the Crozet Basin area which has recently been included within the WOCE special study area in the Southwest Indian Ocean (ISS1). Our primary goal was to give a detailed description of the frontal structure and transport associated with the Antarctic Circumpolar Current in the area where the bottom topography of the Crozet and Kerguelen Plateaux together with the eastward extension of the Agulhas Return Current exert a particularly crucial influence on the regional circulation. A secondary, but still very important, goal was to improve knowledge of the water characteristics and circulation at all depths related to the water mass exchanges between the Southern Ocean and the Indian Ocean. Specifically, this includes the formation and modification of the thermocline water masses north of the ACC, the injection of modified waters into the subtropical anticyclonic gyre in the South Indian, and the circulation of deep and bottom waters within the Crozet Basin. Water sampling during the cruise included measurements of temperature, salinity, and oxygen by CTD, and water bottle samples for analysis of salinity and oxygen to calibrate the CTD sensors. The ship departed Le Port, La Reunion on April 12, 1991, and made on April 13 a test station near 26 09'S, 55 20'E to test the CTD and its rosette water sampling equipment. The first station (Sta. 1) was occupied on April 15 at 37 S, 55 E, and the last station (Sta. 73) on May 17 at 38 S, 58 30'E. The CTD used is Neil Brown Mark III B No 01-1116 equipped with a dissolved oxygen sensor. The CTD temperature and pressure sensors were calibrated at the IFREMER calibration facility both before and after the cruise. The conductivity and oxygen sensors were calibrated using data from the on-board analyses of the salinity and oxygen samples which were collected at each station from PVC Niskin bottles mounted on a General Oceanic rosette sampler. Detailed analysis and interpretation of the cruise data set have been given in Park et al. (1992). Selected plots showing property vs property relationships within the cruise area and the property sections at 53 E. A.5 Major Problems and Goals not Achieved During the early stage of the cruise a problem developed with the General Oceanics rosette tripping mechanism which resulted in mis-firings and double-tripping of the water sampling bottles. This problem was resolved simply by replacing the problematic rosette sampler by a spare one. A.6 Other Incidents of Note A.7 List of Cruise Participants Name Responsibility Institution ------------------------------------------------------------------------- Young-Hyang Park Chief Scientist MNHN, Paris Lucien Gamberoni Salts/Oxygens MNHN, Paris Edwige Charriaud CTD Software MNHN, Paris Annie Kartavtseff CTD Hard & Softwares LODYC, Paris Catherine Rouault CTD Soft ware LODYC, Paris Mathilde Nithart Watch Stander Univ. Luminy, Marseille Henri Orengo Watch Stander Univ. Luminy, Marseille Frederic Rigaud Watch Stander Lab. Arago, Banyuls s/mer Jean-Louis Turon Watch Stander Univ. Bordeaux Jean-Jacques Pichon Watch Stander Univ. Bordeaux Fabienne Maret Watch Stander Univ. Bordeaux -------------------------------------------------------------------------- C.Measurement Techniques and Calibrations C.1 CTD Calibration (L. Gamberoni and E. Charriaud) Temperature The calibrations at the IFREMER calibration facility before and after the cruise showed a temperature change (in a interval of 15 months) of 0.003 C at 0 C and 0.001 C at 25 C. Post-cruise temperature calibration consisted of a fifth order polynomial curve fit to seven temperature points sampled between -2 and 25 C. The polynomial fitting shows a 0.0002 C standard deviation, while the accuracy of the thermometer standard as claimed by the IFREMER calibration facility is 0.002 C. Corrections determined from this polynomial fitting were applied to the CTD data. Pressure Pre- and post-cruise calibrations showed a pressure drift of 10 dbar. During the post-cruise calibration, data were sampled at 400 db intervals with both increasing and decreasing pressure between 0 and 6000 dbar. Corrections were determined from a fifth order polynomial curve fit to sixteen pressure points. The standard deviation from the polynomial fitting is 0.25 dbar for the downcast calibration and 0.14 dbar for the upcast calibration, while the accuracy of the pressure tester as claimed by the IFREMER calibration facility is ± 1.2 dbar at 6000 dbar. The corrections determined from this polynomial fitting were applied to the CTD data from the cruise, with the pressure bias term being set equal to the pre-lowering deck unit pressure reading made during each CTD cast. Conductivity The conductivity sensor was calibrated with shipboard salinity measurements on the maximum 12 water samples collected at each station. The conversion of water sample salinities into the corresponding conductivities was made using the algorithms of Fofonoff and Millard (1983), with C(35, 15, 0) = 42.914 mS/cm. The raw conductivities from the CTD were first corrected for temperature and pressure effects on the conductivity sensor, using the following algorithm proposed by the CTD constructor: Cc = Cr [ 1- a (T-15) + b P ] where Cc and Cr are the corrected and raw conductivities, a is the temperature correction coefficient (7 x 10-6 C-1), b is the pressure correction coefficient (1.48 x 10-8 dbar-1),and T and P are the calibrated temperature and pressure. The calibration consisted in least-square regression fit of the CTD conductivities to their respective water sample data. The data were subdivided into two homogeneous groups (Stas. 1-12 and Stas. 13-73) in which the differences between water sample and CTD conductivities appeared to have homogeneous calibration characteristics. An iterative regression procedure was used, discarding successively the outlying observations in excess of 2.8 times the standard deviation of the conductivity differences in the station group under consideration.The following regression equations were determined from the retained data points (about 90 % of the initial data points for each homogeneous group), which were used to correct the CTD conductivities: Stas. Retained Data Points Regression Eq. St. Dev. (mS/cm) 1-12 72 (91.1 %) C = 0.00494042 + 1.000110 Cc 0.0032 13-73 552 (90.0 %) C = 0.01278070 + 0.999841 Cc 0.0026 Finally, the calibrated CTD conductivities were converted into the corresponding salinities using the algorithms of Fofonoff and Millard (1983). Oxygen The calibration of the CTD oxygen sensor was made against in situ water sample oxygen data. The calibration algorithm used here is that of Millard (1982): OX = Soc*Oc*OXSAT(T, S)*exp{tcor [T + wt (T0 - T)] + pcor*P} where OX is the CTD oxygen in ml/l, Oc is the CTD oxygen current, Soc is the oxygen current slope, OXSAT is the oxygen saturation value after Benson and Krause (1984), T, S, and P are the calibrated CTD temperature, salinity, and pressure, tcor and pcor represent the temperature and pressure corrections, T0 is the CTD oxygen prove internal temperature, and wt is the weighting fraction of oxygen prove internal temperature. The parameters Soc, tcor, wt, and pcor were determined from water sample oxygen values by performing a linear least-square regression. The calibration procedure was analogue to that used for the conductivity; stations were subdivided into four homogeneous groups (Stas. 1-12, Stas. 13-43, Stas. 44-58, and Stas. 59-73), and the outlying observations in excess of 2.0 times the standard deviation of the oxygen differences in the station group under consideration were discarded successively from the iterative least-square fits. The resulting parameter values and the standard deviation of oxygen differences for each group are shown below. Stas. Retained Data Points Soc tcor wt pcor St. Dev. (ml/l) ------------------------------------------------------------------------------- 1-12 55 (83.3 %) 2.6354 - 0.0354 0.818 0.000150 0.046 13-43 202 (70.4 %) 2.6632 - 0.0351 0.680 0.000151 0.038 44-58 101 (78.3 %) 2.6663 - 0.0363 0.844 0.000144 0.046 59-73 78 (63.4 %) 2.6421 - 0.0336 0.817 0.000150 0.034 ------------------------------------------------------------------------------- The conversion of oxygen content in ml/l into that in micromol/kg (as reported in Data Files) was made using the following equation from Culberson (1991): O2 (micromol/kg) = 44.660 x O2(ml/l) / rsw where rsw is the density of seawater at the temperature at which the oxygen sample was pickled. C.2.2 Salinity and Oxygen Measurements during the MD 68-SUZIL Cruise by: L. Gamberoni Salinity The water sample salinities were measured with a Guildline Portasal Model 8410 salinometer that was standardized daily with IAPSO Standard Sea Water Batch P-110. The salinity measurements during this cruise were made within a temperature controlled (1 C) portable laboratory, generally maintained about 1-2 C below that of the salinometer water bath. Oxygen The dissolved oxygen samples measured during this cruise were analyzed with the Winkler titration method described by Carritt and Carpenter (1966), with two exceptions. The thiosulfate normality was reduced from 0.14 to 0.01, and the microburette was replaced by a 20 ml Metrohm Dosimat digital piston buret. Potassium hydrogen iodate standard solutions (KH(IO3)2) were previously prepared in the laboratory. D. Acknowledgments E. References Benson, B., and D. Krause Jr. (1984) The concentration and isotopic fractionation of oxygen dissolved in fresh and seawater in equilibrium with the atmosphere. Limnol. Oceanogr., 29, (3), 620-632. Carritt, D. E., and J. H. Carpenter. (1966) Comparison and evaluation of currently employed modifications of the Winkler method for determining dissolved oxygen in seawater; a NASCD report. J. Mar. Res., 24, 286-318. Culberson, C. H. (1991) Dissolved oxygen. WHP Operations and Methods, WHPO 91-1, WHP Office, WHOI, Woods Hole. Fofonoff, N. P., and R. C., Millard. (1983) Algorithms for computation of fundamental properties of seawater. Unesco technical papers in marine science No. 44, 53 pp. Millard, R. C. (1982) CTD calibration and data processing techniques at WHOI using the 1978 practical salinity scale. Proc. Int. STD Conference and Workshop, La Jolla, Mar. Tech. Soc., 19 pp. Park, Y. H., L. Gamberoni, and E. Charriaud. Frontal structure, water masses and circulation in the Crozet Basin. J. Geophys. Res. (accepted). Unesco, 1983. International Oceanographic tables. Unesco Technical Papers in Marine Science, No. 44. Unesco, 1991. Processing of Oceanographic Station Data, 1991. By JPOTS editorial panel. F. WHPO Summary Several data files are associated with this report. They are the mf68.sum, mf68.hyd, mf68.csl and *.wct files. The mf68.sum file contains a summary of the location, time, type of parameters sampled, and other pertient information regarding each hydrographic station. The mf68.hyd file contains the bottle data. The *.wct files are the ctd data for each station. The *.wct files are zipped into one file called mf68wct.zip. The mf68.csl file is a listing of ctd and calculated values at standard levels. The *.csl files are not aviliable at this time due to the CTD data files being given in non-uniform levels. G. Data Quality Evalutation