== Longitude sign corrected in individual cast headers, was correct in ReadMe file 2014-5-27 === ============================== Originally posted 2013-2-1 ================================== North Pole Environmental Observatory 2012 Aerial CTD Survey NSF Grants OPP-9910305, OPP-0352754, and ARC-0856330 CTD Station Position Latitude _ Longitude Cast Date / Time Cast 1 Borneo_1 89deg 17.250min North 001deg 53.100min East 2012-4-16/0907 UTC Cast 2 85N_90E 84deg 56.970min North 089deg 54.100min East 2012-4-16/1338 UTC Cast 3 86N_90E 85deg 58.500min North 090deg 89.790min East 2012-4-16/1720 UTC Cast 4 85N_170W 84deg 57.313min North 169deg 49.117min West 2012-4-17/1201 UTC Cast 5 86N_175W 86deg 00.287min North 175deg 22.706min West 2012-4-17/1541 UTC Cast 6 88N_180 88deg 00.482min North 179deg 03.631min East 2012-4-18/1036 UTC Cast 7 89N_0 89deg 13.791min North 002deg 15.457min West 2012-4-18/1513 UTC Cast 8 87N_180 87deg 24.040min North 179deg 50.800min East 2012-4-19/1238 UTC Cast 9 89N_180 88deg 56.263min North 179deg 24.674min East 2012-4-19/1703 UTC Cast 10 87N_90E 87deg 14.737min North 091deg 35.883min East 2012-4-20/1046 UTC Cast 11 88N_90E 88deg 0.070min North 089deg 58.788min East 2012-4-20/1403 UTC Cast 12 Borneo_2 89deg 04.907min North 003deg 50.696min West 2012-4-21/1613 UTC Cast 13 90N 89deg 59.656min North 113deg 09.000min West 2012-4-22/0912 UTC Cast 14 89N_90E 89deg 01.075min North 090deg 22.648min East 2012-4-22/1308 UTC Each cast is an ASCII file of eleven numerical columns with a short header- _ Depth (m) _ Pressure (dbar) _ Temperature in situ (deg C) _ Potential Temperature (deg C) _ Conductivity (S/m) _ Salinity (psu) _ Density (sigma-theta) _ Dissolved Oxygen (ml/l) _ Dissolved Oxygen (mg/l) _ Dissolved Oxygen (%sat) _ Dissolved Oxygen (Mmol/Kg) Part of the observational program of the North Pole Environmental Observatory, these CTD-chemistry stations were obtained using a Twin Otter skiplane operating out of the Russian Ice Station Barneo to record ocean sections from the North Pole along 180 degrees and 90 degrees East longitudes. The measurements were made with a Seabird SBE-19plus Seacat (s/n 5076) following a landing at these positions on the Arctic sea ice. Mounted on and plumbed together with the CTD was SBE-43 Dissolved Oxygen Sensor s/n 229. Suspended on the line above the CTD and plumbed to its pump outflow was an ISUS V2 Nitrate Sensor. Using a winch installed in the aft door of the airplane also allowed Niskin Bottles to be mounted and tripped at chosen depths on the line, and water samples from these were drawn inside the heated fuselage. Concentrations of various chemical tracers were obtained, and these and the Nitrate data are archived in a different submission. Ideally the downcasts provide better quality data than the upcasts, since they are freer of instrument wake effects and show the best resolution and detail of small features. In spring Arctic conditions with very cold air temperatures, care must be taken to avoid seawater freezing in the plumbing the instant it enters the water and not dissipating before reaching a substantial depth, which can result in contamination of the top of the downcast. Such precautions as pitching a heated tent over the hole and Twin Otter aft door and waiting a long period with the instrument soaking in the Mixed Layer before beginning the cast have usually been adequate. During the 2012 survey, however, either these steps turned out to be insufficient or another problem developed, and contamination did develop in the downcast oxygen measurements, and on closer examination, even in conductivity. The cause is under investigation. Comparison with shallow profiles from a different Sea-Bird profiler confirmed that the upcasts were a better measure of the actual profile. Issues with the ISUS plumbing and a partial pump malfunction are likely culprits. But as a consequence, the archive reported here contains data from the upcasts at each station of the NPEO 2012 survey. Data processing followed a modified SEASOFT recipe with certain constants determined by empirical trial. Temperature and conductivity were low-pass filtered with a time constant of 0.5 seconds, the dissolved oxygen voltage was filtered with a time constant of 0.25 seconds, and pressure was filtered with a time constant of 1.0 seconds. Alignment is the step that compensates for separate sensors with differing time constants and located at different points in the CTD plumbing by shifting individual data channels in time. Temperature was advanced relative to pressure by 0.5 seconds, a value determined by varying the temperature advance to select the value that did the best job of minimizing salinity spiking. In this case, the temperature advance turned out to agree with the Sea-Bird standard recipe. Further processing was compromised by the lack of trustworthy downcasts. A reasonable cell thermal mass correction could not readily be determined from the naturally smoothed conductivity up-trace, and none was applied. The dissolved oxygen voltage was advanced by 4.0 seconds, selected from a few cases where oxygen minima and maxima from available partial downcasts could be matched with the upcast on profile plots. Sea-Bird expects a range of oxygen advance between 2 and 7 seconds. Finally, the derived variables salinity, density, and dissolved oxygen concentration were calculated. Oxygen is offered in four different units for the user's convenience. Measurements by the Switchyard Project (http://psc.apl.washington.edu/switchyard) taken during May 2012 provide CTD and ocean chemistry coverage of the Lincoln Sea region, and are archived in submissions by Lamont-Doherty Earth Observatory and the University of Washington. Separately archived are other components of the North Pole Environmental Observatory including drifting buoys and bottom-anchored moorings. Deployment history, profile plots and other analysis using these data may be viewed at the NPEO website (http://psc.apl.washington.edu/northpole). Reference: Morison, J., R. Andersen, N. Larson, E. D'Asaro, and T. Boyd, 1994: The Correction for Thermal-Lag Effects in Sea-Bird CTD Data. J. Atmos. Oceanic Technol., 11, 1151-1164. Sea-Bird Electronics, Inc., 2012: "SEASOFT V2: SBE Data Processing UserŐs Manual". For further information, please contact Dr. James Morison morison@apl.washington.edu (206) 543-1394 Dr. Michael Steele mas@apl.washington.edu (206) 543-6586 Dr. Matt Alkire malkire@apl.washington.edu (206) 897-1623 Roger Andersen roger@apl.washington.edu (206) 543-1258 at Polar Science Center, Applied Physics Lab, University of Washington 1013 NE 40th, Seattle, WA 98105-6698 USA FAX (206) 616-3142