RAFOS Floats: Defining and Targeting Surfaces of Neutral Buoyancy

Dana D. Swift School of Oceanography, University of Washington, Seattle, Washington

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Stephen C. Riser School of Oceanography, University of Washington, Seattle, Washington

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Abstract

For timescales much greater than the local buoyancy period, the buoyant response of a RAFOS float is virtually dictated by its compressibility. As the compressibility of a thermally inert RAFOS float increases from zero, its oceanic equilibrium surface undergoes a smooth continuous deformation starting from an in situ density surface, eventually merging with an isopycnal surface and finally with a neutral surface. Thus there is a continuum of operational modes available to RAFOS floats; each mode is associated with a critical compressibility. Hypothetically, the compressee that transforms an isobaric RAFOS float into an isopycnal float can be modified to make a neutral-surface drifter by altering the critical value of its compressibility. The ballast procedure used to target a float to a prescribed equilibrium surface can be viewed as an accurate (±3%) laboratory measurement of the float's compressibility. For current “isobaric” RAFOS floats, the mean measured compressibility was approximately 2.71×10−6 db−1 (i.e., about 60% that of seawater), which can induce pressure deviations from an isobar as large as 125–175 db in the main thermoclines of the North Pacific and North Atlantic. Errors in ballasting a float yield targeting errors that depend on the float's compressibility and the local density stratification of the ocean. For isobaric floats deployed to 1000 db in the North Pacific Ocean, the targeting errors are approximately 23 db per gram of ballast error. By optimizing the ballast procedure, the ballast errors (and hence the targeting errors) can be minimized. For 59 shallow (1000 db) floats to which the optimized procedure was applied, preliminary estimates of the mean and maximum targeting errors are 25 and 50 db.

Abstract

For timescales much greater than the local buoyancy period, the buoyant response of a RAFOS float is virtually dictated by its compressibility. As the compressibility of a thermally inert RAFOS float increases from zero, its oceanic equilibrium surface undergoes a smooth continuous deformation starting from an in situ density surface, eventually merging with an isopycnal surface and finally with a neutral surface. Thus there is a continuum of operational modes available to RAFOS floats; each mode is associated with a critical compressibility. Hypothetically, the compressee that transforms an isobaric RAFOS float into an isopycnal float can be modified to make a neutral-surface drifter by altering the critical value of its compressibility. The ballast procedure used to target a float to a prescribed equilibrium surface can be viewed as an accurate (±3%) laboratory measurement of the float's compressibility. For current “isobaric” RAFOS floats, the mean measured compressibility was approximately 2.71×10−6 db−1 (i.e., about 60% that of seawater), which can induce pressure deviations from an isobar as large as 125–175 db in the main thermoclines of the North Pacific and North Atlantic. Errors in ballasting a float yield targeting errors that depend on the float's compressibility and the local density stratification of the ocean. For isobaric floats deployed to 1000 db in the North Pacific Ocean, the targeting errors are approximately 23 db per gram of ballast error. By optimizing the ballast procedure, the ballast errors (and hence the targeting errors) can be minimized. For 59 shallow (1000 db) floats to which the optimized procedure was applied, preliminary estimates of the mean and maximum targeting errors are 25 and 50 db.

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