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The 1987 Reciprocal Tomography Experiment (RTE87)

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The RTE87 experiment consisted of three moored transceivers separated by 750, 1000, and 1250 km. Reciprocal transmissions were made for about 120 days during summer 1987.
The RTE87 experiment occurred during summer 1987, following the not-so-successful RTE86. This experiment was essentially a test of the ability of the instruments to make reciprocal transmissions, and, in particular, the ability of reciprocal tomography to measure currents. Though this experiment was essentially an engineering test of the tomography instruments, a number of interesting oceanographic properties were learned from the data: (1) The Del Grosso sound speed equation was found to be most consistent with the acoustic data, (2) The formation and heat content of the summer mixed layer at megameter scales was quantified and compared with climatological heat fluxes, (3) Currents (low-frequency and tides) and relative vorticity were measured and compared with forcing by wind-stress curl, and (4) The barotropic tide measurements were compared to state-of-the-art tide models and found to be in good agreement, and (5) Phase-locked, deterministic internal tides were observed and determined to have likely emanated from the Hawaiian Ridge some 2000 km to the south.

The radiation of internal tides from Hawaii was confirmed by Ray and Mitchum a year later using satellite altimetry, though there seems to be lingering controversy over whether the internal tides emanated from Hawaii or the Aleutian island chain. Certain arguments for Hawaii as the source of the radiaton are reiterated from the original publication: A mooring located mid-way along the northern acoustic path obtained temperature and current data consistent with northward internal wave propagation, the "age" of the internal tide as determined by the M2 and S2 phase difference was consistent with a range to Hawaii, not the Aleutians, the stability of the internal tide phase (phase-locked over 120 days) rather rules out a superposition (an interference pattern is quite sensitive) of northward (Hawaiian) and southward (Aleutian) propagating internal waves, and finally the publication made the prediction that coherent internal tides would be emanating from the Hawaiian Ridge, which has been shown to be true.


Publications

Dushaw, B. D., P. F. Worcester, B. D. Cornuelle, B. M. Howe, and D. S. Luther, 1995. Baroclinic and barotropic tides in the central North-Pacific Ocean determined from long-range reciprocal acoustic transmissions. J. Phys. Oceanogr., 25, pp. 631-647.

Dushaw, B. D., P. F. Worcester, B. D. Cornuelle, and B. M. Howe, 1994. Barotropic currents and vorticity in the central North-Pacific during summer 1987 determined from long-range reciprocal acoustic transmissions. J. Geophys. Res., 99, pp. 3263-3272.

Dushaw, B. D., P. F. Worcester, B. D. Cornuelle, and B. M. Howe, 1993. Variability of heat content in the central North Pacific in summer 1987 determined from long-range acoustic transmissions. J. Phys. Oceanogr., 23, pp. 2650-2666.

Dushaw, B. D., P. F. Worcester, B. D. Cornuelle, and B. M. Howe, 1993. On equations for the speed of sound in seawater. J. Acoust. Soc. Am., 93, pp. 255-275.

Dushaw, B. D., 1992. The 1987 Gyre Scale Reciprocal Acoustic Tomography Experiment. Ph.D. thesis, University of California, San Diego.