WHOTS (19-20) Cruise Shipboard Observations
Contents
4. WHOTS (19-20) Cruise Shipboard Observations¶
The hydrographic profile observations made during the WHOTS cruises were obtained with a Sea-Bird CTD package with dual temperature, salinity, and oxygen sensors. This CTD was installed on a rosette sampler with 5 L Niskin sampling bottles for calibration water samples. Furthermore, the ship Oscar Sette came equipped with a thermosalinograph system that provided a continuous depiction of the near-surface layer’s temperature and salinity. However near-surface temperatures were not available during the WHOTS-20 cruise, because the thermosalinograph remote temperature sensor was not functional. Horizontal currents over the depth range of 30-700 m were measured from the shipboard 75 kHz Ocean Surveyor (OS75) ADCP (narrowband) with a vertical resolution of 16m for the WHOTS-19 and WHOTS-20 cruises. Broadband mode for the OS75 provided additional current data over the range upper 200 m with a vertical resolution of 8m. Unfortunately, the broadband mode was non-functional during WHOTS-20.
Data gaps occurred when the system was shut down temporarily during communications with the acoustic releases used for the moorings during both cruises. Periods of missing data between 300 and 450 m in the broadband ADCP were apparent due to the lack of scattering material in the water.
4.1. Conductivity, Temperature, and Depth (CTD) Profiling¶
Continuous measurements of temperature, conductivity, dissolved oxygen, and pressure were made with the UH Sea-Bird SBE-9/11Plus CTD underwater units #1506 and #1487 during WHOTS-19 cruise and #0895 during WHOTS-20 cruise. The CTD was equipped with an internal Digiquartz pressure sensor and pairs of external temperature, conductivity, and oxygen sensors.
Each temperature-conductivity sensor pair used a Sea-Bird TC duct, which circulated seawater through independent pump and plumbing installations. The CTD configuration also included two oxygen sensors, installed in the plumbing for each sensor set. In both cruises, the CTD was mounted in a vertical position in the lower part of a rosette sampler, with the sensors’ water intakes located at the bottom of the rosette.
The package was deployed on a conducting cable, which allowed for real-time data acquisition and display. The deployment procedure consisted of lowering the package to approximately 10 dbar and waiting until the CTD pumps started operating. The CTD was then raised until the sensors were close to the surface to begin the CTD cast. The time and position of each cast were obtained via a GPS connection to the CTD deck box. Four salinity samples were taken on each cast for calibration of the conductivity sensors.
4.1.1. Data Acquisition and Processing¶
CTD data were acquired at the instrument’s highest sampling rate of 24 samples per second. Digital data were stored on a laptop computer, and, for redundancy, the analog signal was recorded on a separate computer using a sound card and Audacity (TM) software. Backups of CTD data were made onto USB storage cards.
The raw CTD data were quality controlled and screened for spikes described in the WHOTS Data Report 1 [Santiago-Mandujano et al., 2007]. Data alignment, averaging, correction, and reporting were done as described in [Tupas et al., 1993]. Spikes in the data occur when the CTD samples the disturbed water of its wake. Therefore, the downcast samples were rejected when the CTD was moving upward or when its acceleration exceeded 0.5 m s-2 in magnitude. The data were subsequently averaged into 2-dbar pressure bins after calibrating the CTD conductivity with the bottle salinities.
The data were additionally screened by comparing the T-C sensor pairs. These differences permitted the identification of problems with the sensors. The data from only one T-C pair, whichever was deemed most reliable, is reported here. Only data from the downcast are reported, as wake effects from the rosette commonly contaminate upcast data.
Temperature is reported on the ITS-90 scale. Salinity and all derived units were calculated using the UNESCO (1981) routines; salinity is reported in the Practical Salinity(SA) scale (PSS-78). Oxygen is reported in umol kg-1.
4.1.2. CTD Sensor Calibration and Corrections¶
4.1.2.1. Pressure¶
The pressure calibration strategy for CTD pressure transducers #154451 and #53702 used during WHOTS-19 and #101430 used during WHOTS-20 cruise employed a high-quality quartz pressure transducer as a transfer standard. Periodic recalibrations of this lab standard were performed with a primary pressure standard. The only corrections applied to the CTD pressures were a constant offset determined when the CTD first enters the water on each cast. Also, a span correction determined from bench tests on the sensor against the transfer standard was applied. These procedures and corrections are thoroughly documented in the HOT-2022 data report [Fujieki et al., 2024] and HOT-2023 data report [Fujieki et al., 2025].
4.1.2.2. Temperature/Conductivity¶
Sea-Bird SBE-3-Plus temperature and SBE 4C conductivity transducers were used during WHOTS-19 and -20 cruises. These sensors’ history and performance have been monitored during HOT cruises, and calibrations and drift corrections applied during WHOTS cruises are thoroughly documented in the HOT-2022 data report [Fujieki et al., 2024] and HOT-2023 data report [Fujieki et al., 2025]
4.1.2.3. Dissolved Oxygen¶
Sea-Bird SBE-43 oxygen sensors were used during the WHOTS-19 and -20 cruises. The WHOTS-19 oxygen data were calibrated using calibration coefficients obtained during the HOT-342 cruise conducted on 24-30 May 2023, before the WHOTS-19 cruise, which used the same primary oxygen sensor. The CTD empirical calibration was performed using oxygen water samples and the procedure from [Owens and Millard, 1985]. See [Tupas et al., 1996] for details on these calibrations procedures. The oxygen data from WHOTS-20 were calibrated using calibration coefficients obtained during the HOT-350 cruise conducted on April 27 to May 1, 2024 before the WHOTS-20 cruise, which used the same oxygen sensors.
4.2. Water Sampling and Analysis¶
4.2.1. Salinity¶
Salinity samples were collected by a rosette sampler during CTD casts at selected depths during WHOTS-19 and -20, and then sub-sampled in 250 ml glass bottles. The top of each bottle and thimble were thoroughly dried before being tightly capped to prevent water from being trapped between the cap or thimble and the bottle’s mouth. It has been observed that residual water trapped in this way increases its salinity due to evaporation, and it can leak into the sample when the bottle is opened for measuring. Samples from each cruise were measured after the cruise in the UH laboratory using a Guildline Autosal 8400B SN 73647 for WHOTS-19 and WHOTS-20. International Association for Physical Sciences of the Ocean (IAPSO) standard seawater samples were measured to standardize the Autosal, and samples from a large batch of “secondary standard” (substandard) seawater were measured after every 24-48 samples to detect drift in the Autosal. Standard deviations of the secondary standard measurements were less than ± 0.001 for WHOTS-19 and -20 cruises Table 4.1.
The substandard water was collected by a rosette sampler from 1020 m at station ALOHA during HOT cruises and drained into a 50-liter Nalgene plastic carboy. In the laboratory, the water was then thoroughly mixed in a glass carboy for 20 minutes by manually shaking, rolling, and tilting the carboy vigorously, after which a 2-inch protective layer of white oil was added on top to deter evaporation. The substandard water was allowed to stand for approximately three days before it was used and was stored in the same temperature-controlled room as the Autosal, protecting it from the light with black plastic bags to inhibit biological growth. Substandard seawater batch #74 was prepared on July 19, 2023, and it was used for WHOTS-19 (See WHOTS-19 Salinity Measurements. The batch #76 was prepared on April 11, 2024, and it was used for WHOTS-20 (See WHOTS-20 Salinity Measurements.
Samples from the WHOTS-19 were measured on October 28, 2019 and samples from WHOTS-20 were measured on September 13, 2021. Table 4.1 shows the precision measurements of the secondary sub-standards.
Cruise |
Mean Salinity +/- SD |
# Samples |
Substandard Batch |
IAPSO Batch |
|---|---|---|---|---|
WHOTS-19 |
34.4954 ± 0.0004 |
5 |
74 |
P166 |
WHOTS-20 |
34.4900 ± 0.0001 |
2 |
76 |
P167 |
4.3. Thermosalinograph Data Acquisition and Processing¶
4.3.1. WHOTS-19 Cruise¶
Near-surface temperature and salinity data during the WHOTS-19 cruise were acquired from the thermosalinograph (TSG) system installed on the NOAA Ship Oscar Sette. The sensors were sampling water from the continuous seawater system running through the ship, and were comprised of one thermosalinograph model SBE-21 (SN 3168) and a micro-thermosalinograph model SBE-45 (SN 0290), both with (internal) temperature and conductivity sensors located in the ship’s chemistry lab, about 70 m from the hull intake; and an SBE-38 (SN 212) external temperature sensor located at the entrance of the water intake. All instruments recorded data every second. The water intake is located at the ship’s bow, forward from the starboard side bow thruster at a depth of 3 m. The system has a flow meter in the chemistry lab, showing a flow rate of about 1.1 liter/minute during the cruise. Only the SBE-45 has a debubbler. Salinity water samples were taken every 8 hours from the exhaust in the Chemistry lab using 0.25 litter glass bottles to be measured in the UH lab to correct for any drift in the thermosalinograph conductivities.
4.3.1.1. Temperature Calibration¶
External temperature data from the SBE-38 sensor (last calibrated at Sea-Bird on December 08, 2022) were used to measure the seawater temperature. These data were compared to the data collected during CTD casts.
4.3.1.2. Nominal Conductivity Calibration¶
Data from the SBE-45 conductivity and temperature sensors were used to calculate the intake seawater salinity. The SBE-45 was last calibrated on November 11, 2022, and the SBE21 sensor on December 29, 2022. All conductivity data from the thermosalinograph were nominally calibrated with coefficients from this calibration. However, all the final salinity data reported here were calibrated against bottle data, as explained below.
4.3.1.3. Data Processing¶
Daily files containing navigation data recorded every second were concatenated with the thermosalinograph data. The thermosalinograph data were then screened for gross errors, with upper and lower bounds of 18°C and 35°C for temperature and 3 and 6 \(\text{S m}^{-1}\) for conductivity. There were no points outside the valid temperature range and no points outside the valid conductivity range.
A 5-point running median filter was used to detect one- or two-point temperature and conductivity glitches in the thermosalinograph data. Glitches in temperature and conductivity detected by the 5-point median filter were immediately replaced by the median. Threshold values of 0.3°C for temperature and 0.1 \(\text{S m}^{-1}\) for conductivity were used for the median filter. After running the filter, there were 5 internal temperature, 0 external temperature, and 5 conductivity points replaced with the median.
A 3-point triangular running mean filter was used to smooth the temperature and conductivity data after passing the glitch detection.
The thermosalinograph aboard the Ship Oscar Sette was set to record data every second. Data were visually scanned to flag spikes likely caused by contamination due to the introduction of bubbles to the flow-through system during transits or rough conditions. Of 434647 data points, 11014 conductivity data points were flagged as bad.
4.3.1.4. Bottle salinity and CTD Salinity Comparisons¶
The thermosalinograph salinity was calibrated by comparing it to bottle salinity samples drawn from a water intake next to the thermosalinograph every 8 hours throughout the cruise. Of the twenty thermosalinograph bottles sampled, no casts were identified as a conductivity outlier. Samples were analyzed as described in Water Sampling and Analysis. The comparison was made in conductivity to eliminate the effects of temperature. The conductivity of each bottle sample was computed using the salinity of the bottle, thermosalinograph temperature, and a pressure of 10 dbar, which includes the pressure of the flow-through system’s pump.
Salinity samples were drawn from the flow-through system, located less than 0.5 m from the SBE-45. Consequently, there should be virtually no delay between when the water passes through the thermosalinograph and sampled. A 90-second average centered on the sample draw time was chosen for processing purposes.
In order to make the comparison in conductivity units, the CTD conductivity was calculated using the 4 dbar downcast CTD salinity, the internal thermosalinograph temperature, and a pump pressure of 10 dbar. There were five CTD casts conducted during WHOTS-19 while the thermosalinograph was running. No casts were removed from the analysis as temperature and conductivity outliers.
A cubic spline was fit to the time series of the differences between the bottle and TSG conductivity, and a correction was obtained for the TSG conductivities. Salinity was calculated using these corrected conductivities, the thermosalinograph temperatures, and 10 dbar pressure. After applying corrections, the mean difference between the bottle and thermosalinograph salinities was 0.0001 psu with a standard deviation of 0.0044 psu. The mean CTD - thermosalinograph difference was 0.0077 psu with a standard deviation of 0.0150 psu.
4.3.1.5. CTD Temperature Comparisons¶
There were five CTD casts conducted during WHOTS-19, one of which was a test cast offshore Honolulu (Station 20) and four at Station 52, respectively. The 3 dbar downcast CTD temperature data from those casts were used to compare with the thermosalinograph data at the time of the casts. This comparison gives an estimate of the quality of the thermosalinograph measurements. Of the 5 casts, none was identified as temperature outliers after comparing it against the thermosalinograph data. The mean difference between the CTD and the internal temperature sensor was -0.2114°C, with a standard deviation of ± 0.0183°C.
4.3.2. WHOTS-20 Cruise¶
Near-surfacesalinity data during the WHOTS-20 cruise were acquired from the thermosalinograph (TSG) system installed on the NOAA Ship Oscar Sette. The instrument was sampling water from the continuous seawater system running through the ship, and was comprised of one micro-thermosalinograph model SBE-45 (SN 0290), with (internal) temperature and conductivity sensors located in the ship’s chemistry lab,about 70 m from the hull intake. Another thermosalinograph model SBE-21 also installed on the system was not working during the cruise. Also not working was the SBE-38 remote temperature sensor located at the entrance of the water intake, and consequently we were not able to collect sea-water temperatures during the cruise. The SBE-45 thermosalinograph was recording data every second. The water intake is located at the ship’s bow, forward from the starboard side bow thruster at a depth of 3 m. The system has a flow meter in the chemistry lab, showing a flow rate of about 1.5 liter/minute during the cruise, and the SBE-45 has a debubbler. Salinity water samples were taken every 8 hours from the exhaust in the Chemistry lab using 0.25 litter glass bottles to be measured in the UH lab to correct for any drift in the thermosalinograph conductivities.
4.3.2.1. Temperature Calibration¶
External temperature data from the SBE-38 sensor was not collected during this cruise.
4.3.2.2. Nominal Conductivity Calibration¶
Data from the SBE-45 conductivity and temperature sensors were used to calculate the intake seawater salinity. These sensors were last calibrated at Sea-Bird on November 11, 2022. All conductivity data from the thermosalinograph were nominally calibrated with coefficients from this calibration. However, all the final salinity data reported here were calibrated against bottle data, as explained below.
4.3.2.3. Data Processing¶
Daily files containing navigation data recorded every second were concatenated with the thermosalinograph data. The thermosalinograph data were then screened for gross errors, with upper and lower bounds of 18°C and 35°C for temperature and 3 and 6 \(\text{S m}^{-1}\) for conductivity. There were 488 points outside the valid temperature range and no points outside the valid conductivity range.
A 5-point running median filter was used to detect one- or two-point temperature and conductivity glitches in the thermosalinograph data. Glitches in temperature and conductivity detected by the 5-point median filter were immediately replaced by the median. Threshold values of 0.3°C for temperature and 0.1 \(\text{S m}^{-1}\) for conductivity were used for the median filter. After running the filter, there was no internal temperature, no external temperature. There were 25 conductivity points replaced by the median. A 3-point triangular running mean filter was used to smooth the temperature and conductivity data after passing the glitch detection.
The thermosalinograph aboard the Ship Oscar Sette was set to record data every second. Both thermosalinographs exhibited a number of conductivity and temperature glitches due to air going into the plumbing.
Data set was visually scanned to flag spikes likely caused by contamination due to the introduction of bubbles to the flow-through system during transits or rough conditions. Of 705386 data points, 137989 conductivity data points were flagged as bad.
4.3.2.4. Bottle salinity and CTD Salinity Comparisons¶
The thermosalinograph salinity was calibrated by comparing it to bottle salinity samples drawn from a water intake next to the thermosalinograph every 8 hours throughout the cruise. Of the twenty-four thermosalinograph bottles sampled, no bottles were identified as a conductivity outlier. Samples were analyzed as described in Water Sampling and Analysis. The comparison was made in conductivity to eliminate the effects of temperature. The conductivity of each bottle sample was computed using the salinity of the bottle, thermosalinograph temperature, and a pressure of 10 dbar, which includes the pressure of the flow-through system’s pump.
Salinity samples were drawn from the flow-through system, located less than 0.5 m from the SBE-45. Consequently, there should be virtually no delay between when the water passes through the thermosalinograph and sampled. A 90-second average centered on the sample draw time was chosen for processing purposes.
In order to make the comparison in conductivity units, the CTD conductivity was calculated using the 3 dbar downcast CTD salinity, the internal thermosalinograph temperature, and a pump pressure of 10 dbar. There were nine CTD casts conducted during WHOTS-20 while the thermosalinograph was running. Cast two was removed from the analysis as temperature and conductivity outlier.
A cubic spline was fit to the time series of the differences between the bottle and TSG conductivity, and a correction was obtained for the TSG conductivities. Salinity was calculated using these corrected conductivities, the thermosalinograph temperatures, and ten dbar pressure. After applying corrections, the mean difference between the bottle and thermosalinograph salinities was less than -1 mpsu with a standard deviation of 0.0119 psu. The mean CTD - thermosalinograph difference was 0.0087 psu with a standard deviation of 0.0461 psu.
4.3.2.5. CTD Temperature Comparisons¶
There were nine CTD casts conducted during WHOTS-20, one of which was a test cast offshore Honolulu (Station 20), five at Station 50 (near the WHOTS-19 buoy), and 3 at Station 52 (near the WHOTS-20 buoy). The 3 dbar downcast CTD temperature data from those casts were used to compare with the thermosalinograph data at the time of the casts. This comparison gives an estimate of the quality of the thermosalinograph measurements. Of the nine casts, cast two was identified as temperature outlier after comparing it against the thermosalinograph data and removed from the analysis. The mean difference between the CTD and the internal temperature sensor was -0.1399°C, with a standard deviation of ± 0.0527°C.
4.4. Shipboard ADCP¶
4.4.1. WHOTS-19 Deployment Cruise¶
Currents were measured for the cruise duration over the depth range of 30-700 m with a 75 kHz RDI Ocean Surveyor (OS75) ADCP working in narrowband mode with a vertical resolution of 16 m and broadband mode with a vertical resolution of 8 m. The system yielded good data [Santiago-Mandujano et al., 2024] during operations near the WHOTS-18 and WHOTS-19 moorings. The broadband system only recorded good data in the upper 200 m. The times of the datasets from the OS75 kHz are shown in Table 4.2.
WHOTS-19 |
OS75nb |
OS75bb |
|---|---|---|
File starting time |
06/16/23 02:12:16 |
06/16/23 02:12:16 |
File ending time |
06/22/23 18:17:13 |
06/22/23 18:17:13 |
4.4.2. WHOTS-20 Deployment Cruise¶
Currents were measured for the duration of the cruise over the depth range of 30-700 m with a 75 kHz RDI Ocean Surveyor (OS75) ADCP working in narrowband mode with a vertical resolution of 16 m. Unfortunately the broadband mode was non-functional during WHOTS-20. The narrowband system yielded good data (see [Santiago-Mandujano et al., 2024]) during operations near the WHOTS-19 and WHOTS-20 moorings. The times of the datasets from the OS75 kHz are shown in Table 4.3.
WHOTS-20 |
OS75nb |
OS75bb |
|---|---|---|
File starting time |
05/31/24 22:21:54 |
N/A |
File ending time |
06/08/24 18:08:47 |
N/A |