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2.0 WOCE/HOT cruise organization

Each of the (approximately monthly) HOT cruises follows the same basic pattern with some flexibility for ancillary projects to be done after the core sampling has been completed. During transit from Honolulu to the time-series station ALOHA (A Long-term Oligotrophic Habitat Assessment) one weight test is done to between 700 and 1000 m at station 1 off Kahe Point (16 km offshore from the western tip of Oahu, 21° 20.6' N, 158° 16.4' W, 1500 m water depth). During this test the WOCE electronics technician checks the status of the CTD wire and its correct level winding on the drum. Following the successful winch test, a CTD/rosette cast to 1000 m is conducted. This cast serves as a "shakedown" for the remainder of the cruise, and the functioning of the components of the CTD/rosette system as well as coordination between winch, deck and console operators can be tested. The training of new personnel in activities such as taking meteorological observations, and sampling salinities is also done in this station. The data taken at Kahe Point (station 1) represent an additional time-series of water properties at a near-shore site.

Upon arrival at ALOHA (station 2²), operations commence with a deep cast (maximum depth approximately 4750 m), 36-hour burst sampling³ of the upper 1000 m at the same location, plus CTD casts to support ancillary BEACH work. Time permitting, the last CTD cast at station ALOHA will be a deep cast. After work at station ALOHA is completed one more cast is done at station 8 (HALE-ALOHA). The data from this cast are used to calibrate some of the instruments on a mooring located 30 km southwest of ALOHA (22° 20' N, 158° 10.6' W). This cast should be taken at a location cross-wind, cross-current, and about 1 nm from the mooring. One near-bottom CTD cast is also conducted at the Kaena Pt. Station (station 6: 21° 50.8'N, 158° 21.8' W), usually at the end of the cruise. Care should be taken when the CTD is near the bottom (~2500 dbar) during this cast, given that this station is located on a steep slope.

During the entire cruise the shipboard ADCP (Acoustic Doppler Current Profiler) is used to measure the velocity profile in the upper 300 m. The exact penetration depth depends on the ship-specific acoustic noise level and the amount of entrained bubbles (see Winn et al., 1991, p. 32). From data taken during the transit to and from station ALOHA at the beginning and end of each cruise, one obtains a blurred snapshot of the spatial variability of the absolute horizontal current components. Temporal variability is seen when the ship remains at one location for more than half a day, but we must be mindful of spatial gradients that may exist within the allowed radius around station ALOHA as well, which contaminate the "time-series" ADCP record somewhat. The example given by Chiswell et al. (1990, p. 25) illustrates the difficulty of separating spatial from temporal variability in ADCP measurements. Although the CTD watch leader is not directly responsible for the actual setup of the ADCP operations on the ship - this is done by the ship's technician under the guidance of Dr. E. Firing (UH) before the cruise - some basic guidelines for ADCP data logging and monitoring are given in Section 4.3.

As outlined in the WOCE Implementation Plan (1988), measurements at the HOT site contribute to the global description of heat, fresh water and chemical fluxes at a location in the central subtropical gyre of the North Pacific. In particular, the documentation and understanding of the temporal variability and its role in the earth's climate is an important scientific objective. As the abyssal circulation is an important factor on longer (decadal to climatic) time scales, the waters between 1000 m and the bottom should be sampled at 200 m depth intervals or closer to meet WOCE standards (ALOHA station 2, cast 1, see Appendix A).

To monitor seasonal and interannual changes, detailed measurements of temperature, salinity, nutrients and other geochemical tracers are required in the surface layer and thermocline which are influenced more directly by atmospheric forcing. The second and following casts at station ALOHA are sampled to at least 1000 m depth. One of these casts (usually cast 2) is called a "density cast" because water samples are taken at a number of specified density values ranging from sigma-theta = 27.37 to the surface, with the intent to resolve the profiles of salinity, dissolved oxygen, and nutrients in potential density coordinates (see Appendix A for sample log sheet). Because the deep cast together with the density cast yield a detailed sampling of the water column, it is important to conduct these two casts back to back. Therefore the density cast should be conducted before or after the deep cast if for some reason this has to be moved to the end of the 36-hour period. Depths sampled during the following casts within the 36-hour burst sampling period are chosen both by the BEACH group and the WOCE team, who have to ensure that at least one water sample each is taken within the mixed layer, the shallow salinity maximum, the intermediate salinity minimum and the deepest position of the rosette for calibration of the CTD conductivity sensor. If oxygen bottles will be taken from the cast, then the sampling should include at least the mixed layer, oxygen maximum, oxygen minimum and the deepest rosette position for calibration of the CTD oxygen sensor. The second deep cast of the cruise (if there is one) should include sampling of oxygen bottles in at least seven levels appropriate for calibration of the CTD oxygen sensor, i.e. in the oxycline and two more levels below the oxygen minimum, in addition to the four levels mentioned before. Examples of continuous vertical profiles of temperature, salinity, depth, dissolved oxygen and fluorescence obtained during one cast are shown in Appendix A.

Because we sample over the 36 hours, we can average out the effects of short-term changes of the depth of density surfaces and the magnitude of hydrographic and nutrient variables (inertial, tidal, and shorter periods). The resulting mean property values of each HOT cruise can be contoured in depth-time plots which show the time variability on time scales greater than one month without aliasing by these high frequency signals (see Figures 6.3.1-14 in Tupas et al., 1995 and sample figure). Further instructive details concerning sampling procedures and analytical methods are to be found in the ten HOT Data Reports published so far (Chiswell et al., 1990; Winn et al., 1992, 1993; Tupas et al., 1993, 1994, 1995, 1997, 1998; Karl et al., 1996; Santiago-Mandujan et al., 1999).

Graphical displays of the data are of great diagnostic value. However, one must always keep in mind that only well-calibrated data are useful for inter-cruise and spatial comparisons. Excellent data quality is the backbone of oceanographic applications, analyses and model verification. Good sampling habits, precise and detailed record keeping and diligence in computer console operations are prerequisites for a successful build-up of the WOCE/BEACH database. It is the purpose of this detailed description of shipboard operational procedures to highlight the skills required for successful data collection, to assist in the achievement of the high standards set forth by data quality experts of the WOCE Hydrographic Program.

There are several components to a typical WOCE cast at station ALOHA:

Preparation of the CTD underwater unit and rosette,
Launch and recovery of the rosette package,
Console operations in the electronics laboratory,
Sea surface (bucket) temperature measurement,
Water sampling from Niskin bottles after the upcast,
Data management and plotting of downcast,
On-board processing of cast,

And most importantly,
Detailed logging of all proceedings before deployment, during down- and upcast of the CTD, and at the time of water sampling.

The details of these operations are described in the following sections.

The University of Hawaii HOT group uses Sea-Bird SBE-9/11 Plus CTD systems, of which there should be two on board during each cruise. The electronic technician tests the electronic equipment, e.g. VCR and Bernoulli disk drives, check the Sea-Bird CTD, and provide sensor calibrations. He/she also inspects and repairs (replaces) Niskin bottles, surface temperature buckets and thermometers, and psychrometers.
Station 2 is a deep-water station 100 km north of Kahuku Point, Oahu, Hawaii, to be occupied within a circle of radius 10 km (6 nm), centered on 22° 45' N, 158° W. This station is identified as the WOCE/WHP Time-series station PRS2/CP1 (see WOCE Implementation Plan, Vol. I, 1988).
A 36-hour burst of consecutive CTD casts at 3 hours intervals is carried out at station ALOHA each month, with the intent to average out the local near-inertial and tidal periods to prevent aliasing longer-term fluctuations whose detection is one of the main objectives of the WOCE Hydrographic Time-series program.

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