High-frequency (≳1 cpd) travel time variability of long-range reciprocal acoustic transmissions in the western North Atlantic

Abstract

Time series of ray travel times were obtained at 350-, 410-, and 670-km ranges in the western North Atlantic during the 1991–2 Acoustic Mid-Ocean Dynamics Experiment (AMODE). Transmissions were recorded for approximately 300 days between six transceivers in a pentagonal array. Sound-speed (current) variability is observed by calculating the sum (difference) of reciprocal travel times. The sum and difference of reciprocal travel times are high-pass filtered by removing a daily average. The barotropic-tide current is measured by the differential travel times. Both phase-locked and narrow-band internal-tide sound-speed variability, caused by the internal-tide isotach displacement, are observed by the sum travel times. The acoustic array acts as a high-directivity antenna for the incident internal tide. The observed internal tide is likely generated at the continental shelf surrounding the North American Basin, or perhaps the Mid-Atlantic Ridge. The nontidal, high-frequency variability (≳1 cpd) is due to internal-wave sound-speed and current variability.