Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i at Manoa
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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 and the raw data are stored both on the computer and, for redundancy, on VHS-format video tapes.
In 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 (PMEL).
Continuous measurements of temperature, salinity, oxygen and fluorescence are made with a Sea-Bird SBE-9/11Plus CTD package with dual temperature, salinity and oxygen sensors and fluorometer described in Tupas et al. (1995). In 2019, the CTD underwater unit #91361 was used during cruises HOT-309 through HOT-317. In addition, an ISUS nitrate sensor (SN 97) had been installed in one of the CTD channels since cruise HOT-253, but it was no longer used after HOT-304.
CTD casts were made at Stations Kahe and ALOHA during each cruise in 2019. At Station ALOHA, a burst of consecutive CTD casts to 1000 m was made over 36 hours to span the local inertial period and three semi-diurnal tidal cycles. The full 36-hour burst sampling period was completed during all HOT cruises in 2019, except for HOT-310 and -317, when operations were canceled early due to rough weather. Two near-bottom CTD casts within about 10 m of the bottom were made during each 2019 cruise, except for HOT-317 which only had one near-bottom cast.
CTD casts to depths ranging between 2400 and 2500 m were conducted at Station Kaena during HOT-311, -312, -313, -315, and -316.
CTD casts have been conducted during cruises near the WHOTS mooring since August 2004 to calibrate the moorings’ sensors. Five yo-yo cycles to 200 m depth were performed near the WHOTS-15 mooring (Station 50: 22° 46.045'N, 157° 53.888' W) near the eastern edge of the ALOHA circle during 2019 for cruises HOT-309 through -315, except for HOT-314, which consisted of four cycles conducted to 200 dbar and one cycle to 1020 dbar. After cruise HOT-315, the WHOTS-15 mooring was recovered during the WHOTS-16 cruise, and the WHOTS-16 mooring was deployed near the southeastern edge of the ALOHA circle (Station 52: 40° 0.10'N, 157° 56.961' W). Station 52 was occupied during HOT-316. Five yo-yo cycles were conducted to 200 dbar during this cruise. CTD yo-yo casts were also conducted near the mooring during the WHOTS-16 cruise before recovering the WHOTS-15 mooring and after the WHOTS-16 mooring’s deployment.
Data Acquisition and Processing
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 on VHS videotapes. Backups of CTD data were made onto USB storage cards and later onto compact disks. The raw CTD data were quality controlled and screened for spikes as described in Winn et al. (1993). Data alignment, averaging, correction and reporting were done as described in Tupas et al. (1993). Salinity spike rejection parameters were modified for some cruises in 2019 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 is moving upward or when its acceleration exceeds 0.5 m s-2 in magnitude.
Some cruises were conducted under relatively rough conditions. The CTD acceleration cutoff value had to be increased to between 0.55 and 0.70 ms-2 for some casts to relax the data rejection criteria and avoid eliminating excessive points.
The 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, whichever was deemed most reliable, are reported here.
Temperature 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 µmol kg-1. Chloropigment (Fluorescence) is reported in µg/l.