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Using a large acoustical array located midway between Puerto Rico and Bermuda, we have observed enhanced diurnal tidal signals associated with the lowest internal‐wave mode. These signals result from a diurnal internal wave resonantly trapped between the shelf just north of Puerto Rico and the turning latitude, a distance of about 1100 km. The data obtained using the large acoustical array are consistent with the predicted Airy‐function variation with latitude of diurnal internal waves near the turning latitude. The existence of this wave is a striking demonstration of the long spatial and temporal coherence of oceanic internal tides. In general, the energy radiated by such waves is a loss of energy from the earth‐moon system and a source of energy for mixing in the deep ocean.

Broadband acoustic signals were transmitted from a moored 250‐Hz source to a 3‐km‐long vertical line array of hydrophones 1000 km distant in the eastern North Pacific Ocean during July 1989. The sound‐speed field along the great circle path connecting the source and receiver was measured directly by nearly 300 expendable bathythermograph (XBT), conductivity‐temperature‐depth (CTD), and air‐launched expendable bathythermograph (AXBT) casts while the transmissions were in progress. This experiment is unique in combining a vertical receiving array that extends over much of the water column, extensive concurrent environmental measurements, and broadband signals designed to measure acoustic travel times with 1‐ms precision. The time‐mean travel times of the early raylike arrivals, which are evident as wave fronts sweeping across the receiving array, and the time‐mean of the times at which the acoustic reception ends (the final cutoffs) for hydrophones near the sound channel axis, are consistent with ray predictions based on the direct measurements of temperature and salinity, within measurement uncertainty. The comparisons show that subinertial oceanic variability with horizontal wavelengths shorter than 50 km, which is not resolved by the direct measurements, significantly (25 ms peak‐to‐peak) affects the time‐mean ray travel times. The final cutoffs occur significantly later than predicted using ray theory for hydrophones more than 100–200 m off the sound channel axis. Nongeometric effects, such as diffraction at caustics, partially account for this observation.

Long‐range acoustic transmissions made in conjunction with extensive environmental measurements and accurate mooring position determinations have been used to test the accuracy of equations used to calculate sound speed from pressure, temperature, and salinity. The sound‐speed fields computed using the Del Grosso equation [V. A. Del Grosso, J. Acoust. Soc. Am. 56, 1084–1091 (1974)] give predictions of acoustic arrival patterns which agree significantly better with the long‐range measurements than those computed using the Chen and Millero equation [C. Chen and F. J. Millero, J. Acoust. Soc. Am. 62, 1129–1135 (1977)]. The predicted ray travel times and travel time error have been calculated using objectively mapped sound‐speed fields computed from conductivity, temperature, depth (CTD) and expendable bathythermograph (XBT) data. Using the measured and predicted ray travel times, a negligible correction to Del Grosso’s equation of +0.05±0.05 m/s at 4000‐m depth is calculated.

Research in ocean acoustic tomography during the last quadrennium has resulted in substantial progress in understanding the capabilities and limitations of the technique. Theoretical studies and numerical simulations have led to greater understanding of the oceanographic information available in a vertical slice from acoustic transmissions between a single pair of instruments, of the horizontal geometries required to map the ocean mesoscale field with specified precision, and of the properties of tomographic reconstructions of the two‐dimensional vector field of current. Simultaneously, the instrumentation used in tomographic experiments has been significantly improved, increasing the precision of the measurements and making gyre and basin scale experiments feasible between moored instruments. Experimental efforts to test the accuracy with which the ocean temperature and current fields can be measured acoustically have now demonstrated that tomographic techniques provide measurements with oceanographically useful precision up to ranges of about 1000 km. Such demonstrations are difficult due to the incompatibility between point measurements and the spatial averages provided by tomographic techniques. The experiments have also yielded significant information on the character of acoustic propagation at long range in the ocean. Experimental precision is now adequate to distinguish between competing algorithms for sound speed as a function of temperature, salinity, and depth. Finally, and most importantly, emphasis has shifted to use of the technique for studying the ocean, rather than on understanding the capabilities and limitations of the technique. Two major experiments, the Greenland Sea Tomography Experiment and the Gulf Stream Extension Tomography Experiment, both conducted during 1988–89, were devoted to improving our understanding of ocean dynamics, although results are not yet available. There is increased emphasis on exploiting the integrating nature of acoustic transmissions to study gyre and global scale temperature variability, phenomena difficult to study in any other way.

Moving ship tomography seeks to synthesize a high‐resolution three‐dimensional snapshot of the ocean sound speed field from a few tomographic instruments by moving them during the survey. A moored array of six acoustic transceivers was deployed in the Greenland Sea from September 1988 to August 1989 as part of the Greenland Sea Tomography Experiment. A prototype ship‐deployed array of ten hydrophones was used to receive about 30 transmissions after mooring deployment and about 40 prior to recovery, distributed at locations encircling the moored sources. The positions of the sources and receivers are imperfectly known at this time, but it is possible to compare the observed arrivals with ray theory and identify many of the observed arrivals with numerical ray paths. The second survey yielded enough identified rays to produce estimates of the ocean sound‐speed field in spite of the need to solve for the unknown instrument positions. The resolution of the sound‐speed maps will improve as the ship and mooring positions are refined using NAVSTAR Global Positioning System data. The tests in the Greenland Sea are preparatory to a trial of moving ship tomography in the western North Atlantic in 1991. The experimental design for that experiment includes simulations examining the effects of ocean dynamics on the reconstruction process. Cases including both linear planetary wave propagation and advection by uniform barotropic currents show that the mapping can proceed in the presence of realistic dynamical evolution. [Work supported by ONR and ONT.]

Three acoustic transceivers moored in a triangle approximately 1000 km on a side in the North Pacific simultaneously transmitted broadband signals to one another for 4 months during summer 1987. All resolvable ray paths were surface reflected; the travel times represent both range and depth averages. Sound‐speed profiles were calculated from CTD data taken on the three legs using both the Chen and Millero and Del Grosso sound‐speed algorithms. The absolute travel times and measured arrival patterns agree best with predictions made using the Del Grosso algorithm. Sum travel times decreased as the summer thermocline formed during the experiment. Inverting the sum travel times yields range‐ and depth‐averaged sound speeds consistent with those computed from CTD and XBT data obtained during transceiver deployment and recovery. Differential travel items are dominated by tidal curents since the experimental area has both a low‐mesoscale energy level and low mean currents. Tidal currents determined acoustically agree well with Schwiderski's tidal model and with barotropic tidal currents determined from a current meter mooring located on the northern leg of the triangle. Inverting the differential travel times gives low‐frequency depth‐ and range‐averaged currents of a few mm/s, with an eastward current along the northern leg in agreement with the barotropic current determined from bottom‐mounted electric field measurements. The circulation around the triangle corresponds to relative vorticities of about 10−8 s−1, of the same magnitude as expected from Sverdrup dynamics. [Work supported by ONR and NSF.]

Large‐scale depth‐integrated currents, relative vorticity, and tides were measured in the northcentral Pacific Ocean during summer 1987 using long‐range reciprocal acoustic transmissions between transceivers in a triangle approximately 1000 km on a side. Tidal harmonic constants found from the acoustically determined currents agree with those found from current meters and with the tidal models of Schwiderski [E. W. Schwiderski, Mar. Geod. 3, 161–255 (1980)] and Cartwright etal. [D. E. Cartwright etal., NASA Tech. Mem. 104578 (1992)]. Sum travel times were used to calculate the baroclinic tide isotach displacement. A significant part of the derived internal tide is deterministic. Currents are calculated using the topographic Sverdrup balance with the Fleet Numerical Oceanography Center wind field. The measured time derivative of the areal‐averaged relative vorticity is insignificant to the Sverdrup balance. Currents and vorticity calculated using the Sverdrup balance are an order of magnitude smaller than the observations. The magnitude and variability of the large‐scale currents and vorticity determined from the Semtner–Chervin eddy‐resolving model of ocean circulation [A. J. Semtner and R. M. Chervin, J. Geophys. Res. 93, 15 502–15 522 (1988)] are similar to the direct measurements. [Work supported by NSF and ONR.]