Gyre-Scale Reciprocal Acoustic Transmissions

Abstract

Three acoustic transceivers moored in a triangle approximately 1000 km on a side simultaneously transmitted broadband signals to one another for four months during summer 1987. The triangle extended in latitude from the Subtropical Front to the Subarctic Front in the North Central Pacific Ocean. Individual ray arrivals were resolved and their arrival times measured with a precision of a few milliseconds using phase-coded transmissions centered at 250 Hz. Using CTD data taken on the three legs, mean sound speed profiles were calculated using the Chen and Millero and Del Grosso sound speed algorithms. The measured arrival patterns agree best with arrival patterns predicted using the Del Grosso algorithm. Predicted and measured absolute travel times also agree best using the Del Grosso algorithm. All resolvable ray paths are surface-reflected, and the travel times therefore represent both range and depth averages. The sum of the travel times of oppositely traveling signals depends on the sound speed field. The sum travel times decreased as the summer thermocline formed during the experiment. The travel times suggest that the thermocline deepened episodically on the northern leg of the triangle. Historical data gives a low mesoscale energy level for the experimental area, with small temperature fluctuations in the main thermocline. The sum travel times correspondingly show little low-frequency variability except for that caused by the formation of the summer thermocline. The transmissions along the east and west legs of the triangle propagated through a frontal region, which was located at 34°-38° N. CTD and XBT data collected on the deployment and recovery cruises suggests that the frontal region was stable throughout the experiment. The difference in the travel times of oppositely traveling signals depends on the current field. Since the experimental area has both a low mesoscale energy level and low mean currents, the differential travel times are dominated by tidal currents. Tidal currents determined acoustically agree well with Schwiderski’s tidal model and with barotropic tidal currents determined from a current meter mooring located approximately in the center of the northern leg of the triangle. At frequencies below one cycle per day the differential travel times give depth and range-averaged currents of a few mm/s, with an eastward current along the northern leg. The circulation around the entire triangle corresponds to relative vorticities of about 10-8 s -1, of the same magnitude as expected from Sverdrup dynamics.