Advancing Refractometry for the Measurement of Absolute Salinity

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Accurate measurements of ocean salinity and density are critical for almost all areas of ocean science. Some of these include, ocean circulation and mixing, biological productivity, nutrient transport, pollutant dispersal, and the ocean’s capacity to store heat and carbon dioxide. For decades, much of the field work to measure salinity and density has relied upon traditional sensors for detecting conductivity and temperature within ocean water masses and at their interfaces, and from millimeter to global scales. However, conductivity sensors for measuring salinity still have shortcomings due to their temperature cross-sensitivity and because they detect the electrical charge of seawater instead of its true mass. Accordingly, measuring refractive index has been suggested as a complimentary and in many cases superior alternative to measuring conductivity. For example, refractive index’s low sensitivity to temperature makes it particularly valuable for studying the arctic pumps which drive many of the world’s currents and influence global climate. The difficulty with measuring refractive index, however, has traditionally been the extraordinary precision with which the measurement must be made. Nevertheless, modern optics technologies have begun to increase its feasibility. Accordingly, this project seeks to develop and utilize two technologies that together will, for the first time, enable accurate measurements of refractive index and Absolute Salinity in the ocean. The first technology is an in-situ refractometer that improves upon an existing experimental seagoing refractometer. The second technology is the benchtop equipment necessary for developing an accurate equation-of-state for seawater refractive index. To date, no sufficient experimental reference data exists for which to compare seawater index measurements. Thus, developing equipment capable of producing this data is a critical step for refractive index’s utility. Overall, the technologies developed in this work will provide the means and the experimental reference data for an additional measurement, the refractive index, to be widely used in observational oceanography.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.