North Pacific gyre-scale reciprocal acoustic transmission experiment

Abstract

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.]