The majority of our sampling effort, approximately 60-72 h per standard HOT cruise, is spent at Station ALOHA. High vertical resolution environmental data are collected with a Sea-Bird CTD having external temperature (T), conductivity (C), dissolved oxygen (DO) and fluorescence (F) sensors and an internal pressure (P) sensor. A Sea-Bird 24-place carousel and an aluminum rosette that is capable of supporting 24 12-L PVC bottles are used to obtain water samples from desired depths. The CTD and rosette are deployed on a 3-conductor cable allowing for real-time display of data and for tripping the bottles at specific depths of interest. The CTD system takes 24 samples s-1\u00a0and the raw data are stored both on the computer and, for redundancy, on VHS-format video tapes (prior to HOT-322) and as an audio signal on a laptop PC (HOT-322 - present).
\nIn February 2006, before cruise 178, we replaced our 24 aging 12-L PVC rosette bottles with new 12-L bottles fabricated at the University of Hawaii Engineering Support Facility, using plans and specifications from John Bullister (Pacific Marine Environmental Laboratory).
\nContinuous measurements of temperature, salinity, oxygen, and fluorescence are made with a Sea-Bird SBE-9/11Plus CTD package with dual temperature, salinity, oxygen sensors, and fluorometer described in\u00a0Tupas et al. (1995). In 2023 the CTD underwater unit #91361 was used during cruises HOT-340 through HOT-348. CTD #1487 and a new CTD #1506 acquired in November 2022 were used during the WHOTS-19 cruise.
\nDetails of the CTD processing for each year can be found in HOT Data Reports #1-34 (HOT: Yearly Data Reports).\u00a0CTD data are written to files using formats specified by the WOCE Hydrographic Programme Office. These formats are based on NODC formats, and are detailed in the WHP Office Report WHPO 90-1.\u00a0
\nSince November 2001 CTD fluorescence data have been regularly calibrated against Chlorophyll bottle data and reported in the CTD files as Chloropigments (CHLPIG) in microgram/liter (uG/L). Bottle Fluorometric Chlorophyll-a plus Pheopigments above 175 dbar are matched against the upcast CTD Fluorometry to calibrate the downcast Fluorescence reported in the CTD files. All CTD files for prior cruises were calibrated and updated to reflect this change.
\nStarting HOT-177 (2006), the Transmissometer (XMISS) data that used to be included in the CTD files have been replaced by continuous Nitrate measured using Satlantic's InSitu Underwater Spectrometer (ISUS V2). Satlantic's ISUS V2 is a chemical free sensor that uses UV absorption technology to provide accurate nitrate concentration measurements in real-time.
\nTemperature is reported in the ITS-90 scale. Salinity and all derived units were calculated using the UNESCO (1981) routines; salinity is reported in the practical salinity scale (PSS-78). Oxygen is reported in \u00b5mol kg-1. Chloropigment (Fluorescence) is reported in \u00b5g/l.
CTD data were acquired at a rate of 24 samples per second. Digital data were stored on a laptop personal computer and for redundancy, the CTD signal was recorded using a USB sound card and Audacity\u00ae software on a separate laptop. Backups of CTD data were made onto USB storage cards and compact disks. The raw CTD data were quality controlled and screened for spikes described in\u00a0Winn et al. (1993). Data alignment, averaging, correction and reporting were done as described in\u00a0Tupas et al. (1993). Salinity spike rejection parameters were modified for some cruises in 2023 because of rough sea conditions. Spikes occur when the CTD samples the disturbed water of its wake; therefore, samples from the downcast are rejected when the CTD moves upward or when its acceleration exceeds 0.5 m s-2\u00a0in magnitude.
\nSome cruises were conducted under relatively rough conditions. To relax the data rejection criteria and avoid eliminating excessive points, the CTD acceleration cutoff value had to be increased to between 0.55 and 1 m s-2\u00a0for some casts.\u00a0The World Meteorological Organization (WMO) Sea State codes describe sea surface conditions based on wave height and visual characteristics:
\nThe data were additionally screened by comparing the temperature and conductivity sensor pairs. These differences permitted identification of problems in the sensors. Only the data from one set of T-C sensors and one oxygen sensor, deemed most reliable, are reported here.\u00a0
\nPressure
\nThe pressure calibration strategy employed a high-quality quartz pressure transducer as a transfer standard. Periodic recalibrations of this laboratory standard were performed with a primary pressure standard. The transfer standard was used to check the CTD pressure transducers. The corrections applied to the CTD pressures included a constant offset determined when the CTD first enters the water on each cast, and a pressure-dependent offset, obtained from semi-annual bench tests between the CTD sensor and the transfer standard.
\nThe transfer standard is a Digiquartz portable standard Paroscientific SN 136923 pressure gauge equipped with a 10,000-PSI transducer. This instrument was purchased in May 2016 and was initially calibrated against a primary standard. A subsequent recalibration was performed in May 2020 at Fluke.
\nCTD pressure transducer bench tests were done using an Ametek T-100 pump and a manifold to apply pressure simultaneously to the CTD pressure transducer and the transfer standard. All these tests had points at six pressure levels between 0 and 4500 dbar, increasing and decreasing pressures.
\nPressure sensor #75434\u00a0(CTD #91361)
\nThis CTD was serviced and recalibrated at SeaBird in May 2021 due to damages when the winch wire parted during a CTD deployment, and the package fell down from about 2 m height and hit the ship\u2019s deck on the March 2021 HOT-328 cruise. The September 2021 test showed that the sensor\u2019s characteristics changed after this incident.
\n*A correction of 0.137 dbar (from the May 2021 calibration) was applied to the pressure offset at 0 dbar during data collection for casts conducted with sensor #75434 during the HOT-335 through HOT-339 cruises. However, a more accurate offset was later determined when the CTD first enters the water on each cast. On-deck CTD pressures are regularly recorded during cruises at the beginning, and the end of each CTD cast.
\n*A correction of 0.137 dbar (from the May 2021 calibration) was applied to the pressure offset at 0 dbar during data collection for casts conducted with sensor #75434 during the HOT-340 through HOT-348 cruises. However, a more accurate offset was later determined when the CTD first enters the water on each cast. On-deck CTD pressures are regularly recorded during cruises at the beginning, and the end of each CTD cast.
\nThe 0-dbar pressure for sensor #75434 was near constant during 2021 through 2023. These pressures are smaller than the before-cast on-deck pressure because during bench tests the CTD is powered on at least 12 hours before testing to allow the pressure sensor to stabilize, while during cruises, the CTD is powered on only about 15 minutes before each cast. The bench tests show a slow sensor stabilization accounts for the observed differences.
\nThe 0-4500 dbar pressure offset and hysteresis from the bench tests have been near-constant and within expected values, although the first has shown a slight regular increase. No linear pressure-dependent offset was applied during data collection for sensor #75434 to correct the 0-4500 dbar span offset.
\nPressure sensor #53702\u00a0(CTD #1487)
\nSensor #53702 (CTD #1487) was acquired in 2022. It was factory-calibrated on January 5, 2022, and bench tested once in May 2022. The test showed a large 0-dbar offset. A correction of -0.98 dbar (from its original sensor calibration) was applied to the pressure offset at 0 dbar during data collection for casts conducted with this sensor during the WHOTS-18 and WHOTS-19 cruises. However, a more accurate offset was later determined when the CTD first entered the water on each cast. A linear pressure-dependent offset was applied during data collection for this sensor to correct the 0-4500 dbar span offset. The hysteresis from the bench test was within expected values.
\nThe May 2022 bench test indicated a large span offset and hysteresis as compared to the March 2024 test; however, we note that the first test showed pressure fluctuations at each pressure step, indicating that there were leaks in the plumbing during the calibration, and this may have compromised the span offset and hysteresis values.
\nA linear pressure-dependent offset (from its original sensor calibration) was applied during data collection for this sensor to correct the 0-4500 dbar span offset. The hysteresis from the bench test was within expected values.
\nPressure sensor #154451 (CTD #1506)
\nSensor #154451 was acquired in November 22, 2022, it was factory-calibrated on November 9, 2022, and bench tested once in March 2024. The test showed a small 0-dbar offset. No correction was applied to the pressure offset at 0 dbar during data collection for casts conducted with this sensor during the WHOTS-19 cruise. However, an offset was later determined when the CTD first entered the water on each cast. This offset was also very small and no correction was applied. The hysteresis from the bench test was also very small.
\nTemperature
\nSix Sea-Bird SBE-3-Plus temperature transducers, #4448, #5519, #6672, #6631, #6724, and #6676 were used during 2023.\u00a0Four Sea-Bird SBE-3-Plus temperature transducers, #4448, #5519, #6672, and #6631, were used in 2022.
\nThe history of the sensors, as well as the procedures followed to obtain the sensor drift from the Sea-Bird calibrations, are well-documented in previous HOT\u00a0Data Reports: Fujieki et al., 2026, 2024, 2023b, 2023a, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994, 1995, 1997, 1998, 1999, Karl et al. 1996\u00a0(HOT: Yearly Data Reports).
\nCalibration coefficients obtained at Sea-Bird and used in the drift estimates were used in the following formula that gives the temperature (in Deg C) as a function of the frequency signal (f):
\n\n\ntemperature = 1/{a + b[ln(fo/f)] + c[ln2(fo/f)] + d[ln3(fo/f)]} - 273.15\n
For each sensor, the final calibration consists of two parts: first, a single \"baseline\" calibration is chosen from among the ensemble of calibrations during the year; second, for each cruise a temperature-independent offset is applied to remove the temporal trend due to sensor drift. The offset, a linear function of time, is calculated by least-squares fit to the 0-30 Deg C average of each calibration during the year.
\nThe maximum drift correction in 2023 was less than 3.0 x 10-4 Deg C for the data collected with these sensors.\u00a0The maximum drift correction in 2022 was less than 2.0 x 10-4\u00a0Deg C for the data collected with these sensors. The baseline calibration is selected for which the trend-corrected average from 0-5 Deg C is nearest to the ensemble mean of these averages.
\nA small residual pressure effect on the temperature sensors documented in\u00a0Tupas et al. (1997)\u00a0has been removed from measurements obtained with our sensors. Another correction to our temperature measurements was for the viscous heating of the sensor tip due to the water flow. This cOscillationsorrection is thoroughly documented in\u00a0Tupas et al. (1997).
\nDual sensors were used during each of the 2022 and 2023 cruises. The temperature differences between sensor pairs were calculated for each cast to evaluate the quality of the data, and to identify possible problems with the sensors. Means and standard deviations of the differences in 2-dbar bins were calculated from the ensemble of all casts at Station ALOHA for each cruise. Both sensors performed correctly during the 2022 and 2023 cruises, showing temperature differences within expected values. The mean temperature difference as a function of pressure was typically less than 1 x 10-3\u00a0Deg C, with a standard deviation of less than 0.5 x 10-3\u00a0Deg C below 500 dbar. The largest variability was observed in the thermocline, with standard deviation values up to 5 x 10-3\u00a0Deg C.
\nConductivity
\nSix conductivity sensors were used during the 2023 cruises, #3162, #3984, #4939, #2959, #6092, and #6079.\u00a0The history of our sensors is well documented in previous HOT\u00a0Data Reports: Fujieki et al., 2026, 2024, 2023b, 2023a, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994, 1995, 1997, 1998, 1999, Karl et al. 1996 (HOT: Yearly Data Reports). Sensors #6052, and #6056 are new sensors acquired in February 2022 and were used during the WHOTS-18 cruise. The water pump for sensor #6052 had problems during this cruise and calibrations against salinity samples could not be conducted.\u00a0Sensors #6092, and #6079 are new sensors acquired in November 2022 and were used during the WHOTS-19 cruise.
\nFor each sensor, the nominal calibrations were used for data acquisition, and a final calibration was determined empirically from the salinities of discrete water samples acquired during each cast. Before empirical calibration, conductivity was corrected for the thermal inertia of the glass conductivity cell as described in\u00a0Chiswell et. al. (1990).
\nProcedures for preliminary screening of bottle samples and empirical calibration of the conductivity cell are described in Tupas et al. (1993, 1994a). For cruises HOT-340 through -348, the standard deviation cutoff values for screening of bottle salinity samples were: 0.0034 (0-150 dbar), 0.0049 (151-500 dbar), 0.0018 (501- 1050 dbar), and 0.0009 (1051-5000 dbar).
\nA least squares fit (\u0394C = b0\u00a0+ b1C + b2C2) to the CTD-bottle conductivity differences was used. None of the cruises required a quadratic calibration. The calibrations were best below 500 dbar because the weaker vertical salinity gradients at depth lead to less error when the bottle and CTD pressures are slightly mismatched.
\nThe final step of conductivity calibration was a cast-dependent bias correction described in\u00a0Tupas et. al. (1993)\u00a0to allow for drift during each cruise or sudden offsets due to fouling. Note that a change of 1 x 10-4\u00a0Siemens m-1\u00a0in conductivity is approximately equivalent to 0.001 in salinity.
\nConductivity differences between sensor pairs were calculated the same way for the temperature sensors. The range of variability as a function of pressure was about \u00b1 1 x 10-4\u00a0Siemens m-1, with a standard deviation of less than 0.5 x 10-4\u00a0Siemens m-1\u00a0below 500 dbar, from the ensemble of all the cruise casts. The largest variability was in the halocline, with standard deviations reaching up to 5 x 10-4\u00a0Siemens m-1\u00a0between 50 and 300 dbar.
\nOxygen
\nDuring the 2023 cruises, six Sea-Bird SBE-43 oxygen sensors were used: #433761, #43918, #434232, and #434246. New sensors #434321 and #433791 acquired on November 22, 2022 were used during the WHOTS-19 cruise. Sensor #43918 showed glitches during various cruises in 2023 and was sent to Sea-Bird for inspection in November 2024. The sensor had significant repairs (replaced the anode sub assembly, membrane, electrolyte and electronic board). The history of our sensors is documented in previous HOT\u00a0Data Reports: Fujieki et al., 2026, 2024, 2023b, 2023a, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994, 1995, 1997, 1998, 1999, Karl et al. 1996 (HOT: Yearly Data Reports). All these sensors have been calibrated annually at Sea-Bird.
\nWater bottle oxygen data were screened and the oxygen sensors were empirically calibrated following procedures described previously (Winn et. al. 1992;\u00a0Tupas et. al., 1993). The calibration procedure follows Owens and Millard (1985) and fits a non-linear equation to the CTD oxygen current and oxygen temperature. The bottle values of dissolved oxygen and the downcast CTD observations at the potential density of each bottle trip were grouped for each cruise to find the best set of parameters with a non-linear least squares algorithm. Two sets of parameters were usually obtained per HOT cruise, corresponding to the casts at Station 1 and 2 (calibration coefficients from cast 2 are also used to calibrate the cast at station 6, 50 and 52). The calibration procedure for the Sea-Bird SBE-43 sensors is documented in\u00a0Santiago-Mandujano et. al. (2001).
\nDual sensors were used during cruises, but only the sensor whose data were deemed more reliable is reported.