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For over fourteen years, the Hawaii Ocean Time-series (HOT) project has
been successfully building and maintaining a database for observing and
interpreting physical and biogeochemical variability in oligotrophic waters
around the Hawaiian Islands. The HOT project has established an extensive
compilation of data, collected at nearly monthly intervals, which are readily
available to the public and to the scientific community (http://www.soest.hawaii.edu/HOT_WOCE/dataftp.html).
The Physical Oceanography (PO) component of the HOT project has been studying
the relation of water mass variations to gyre fluctuations, determining
the needs and methods for monitoring currents at Station ALOHA (A Long-term
Oligotrophic Habitat Assessment) and developing a climatology of short-term
physical variability (http://www.soest.hawaii.edu/HOT_WOCE/).
Thus, it is extremely important to maintain standardized methods for effective
sampling and analysis in order to provide reliable data to the scientific
community.
Salinity sample measurements, one of the components for which the PO group is responsible, are extremely important in that 1) they provide primary calibration for CTD profiles and thermosalinograph data, 2) they comprise a database of water salinity characteristics at various depths in the water column for an extended period of time at Station ALOHA, and 3) they provide quality control on other Niskin bottle water samples for other measurements. Standardized methods in sampling, analysis, documentation, and calibration are crucial in setting a context for extracting meaningful signals from a time-series that has been in effect for as long as the HOT Project. In this report, we have reviewed the history of the HOT salinity database in order to quality-control sections of data with confidence parameters. We have reviewed past and present methods and will recommend guidelines and standards for optimal procedures. As a comprehensive history analysis, this report is organized into four parts: Instrumentation (Section 1), Methodology (Section 2), Standardization (Section 3), and Quality Control (Section 4).
The Instrumentation section (Section 1) presents the history of salinity sample measurement equipment and documentation of Autosal maintenance over the years. We present changes in salinity measurement instruments and materials as well as changes in instrument operators. We also stress the importance of documentation in the Autosal Run Log by the Autosal operator, including any physical, electrical, or chemical repairs done on the Autosal instrument, which should be entered into the Maintenance Database by the Electronics Technician.
In the Methodology section (Section 2), we evaluate the overall methodology of the salinity bottle sampling and data analysis process. Here we discuss which of the methods employed in the past have remained, been updated, or been discontinued, and ways in which we can improve organization and eliminate confusion in the future. Some of the important procedures that need to be standardized include methods in Autosal operation, documentation, and data processing.
The Standardization section (Section 3) is a subset of the Methodology section in that they both relate to specific methods in the salinity measurement and analysis process. The Standardization section, however, is specific towards standardization and calibration methods using IAPSO and substandard seawater (collected in a 50 liter carboy from 1000 m at Station ALOHA). We also examine the significance of Autosal electronics drift throughout the years. A close look at standardization methodology and Autosal drift may give clues to whether adjustments are needed for specific groups of data.
The Quality Control section (Section 4) includes the methods used to determine the quality of the salinity samples by flagging them as good, suspect or bad. We also include plots of salinity values for samples below 3000 dbar, to give an indication of the salinity variability in the deep and bottom water.
A final section with Conclusions and Suggestions (Section 5) contains a summary of this report, including recommendations for improving the salinity data quality.