[fiber-optic salinity/density sensor: the ocean refractometer] [An inductively charged profiling mooring for ocean observing]

	FIG 1: Fiber-Optic Ocean Refractometer »
	FIG 2: Coupler Used as a Refractometer »
	FIG 3: Summary Spectrum »

Understanding the smallest motions in the ocean, which have scales smaller than a millimeter, is (ironically) important to advance our understanding of the climate, since circulation models cannot resolve turbulence and have to parameterize it. These scales are poorly understood because they are so small, and because salinity is very hard to measure. Using technology modified from AIDS research, I developed a sensor half the diameter of a human hair (figures 1 and 2) to sense these smallest-scale fluctuations in ocean refractive index. These are closely related to salinity fluctuations, which to-date have been measured by conductivity and temperature measurements. Measuring salinity from one small sensor reduces "spiking" resulting from mismatched sensor responses. Testing began summer 2001 in Puget Sound, Washington, and initial results showed that in fact the so-called Batchelor cutoff for salinity could be resolved (figure 3). In addition, however, the sensor responds to the velocity signal of the turbulence, making it an optical "shear probe," but precluding resolution of low-frequency signals. I was able to reduce, but not eliminate, this effect.

Download PDF file: "An Ocean Refractometer: Resolving millimeter-scale turbulent density fluctuations via the refractive index"

		FIG 4: Charged Profiler »

Current moored profiling technology is severely limited by battery power. This means that if we want to observe with a profiling mooring for a year, one must suffice with daily profiles - or worse. Since internal waves, which contain about half the ocean's energy, have higher frequencies than this, we wish to profile at least once an hour or so - resulting in a short (44-day) time series. To improve on this situation, we are developing a next-generation moored profiling system. With Bruce Howe and Time McGinnis (also at APL), we are modifying a buoyancy-driven profiler, the Sea Tramp by Ocean Origo, to be inductively charged from an external power supply. This can be a cabled observatory node, or a large moored battery pack for deployment at non-cabled sites. We are also developing the needed inductive, acoustic and radio communication technologies to allow real-time telemetry back to a nearby ship or shore.

We plan to test the system in fall 2008 in Puget Sound, with the year-long Hawaii deployment early 2009.