Observation of barotropic-tide relative vorticity in the northwest Atlantic

Abstract

Time series of reciprocal ray travel times were obtained at 350-, 410-, and 670-km ranges in the western North Atlantic during the 1991–1992 Acoustic Mid-Ocean Dynamics Experiment (AMODE). Transmissions were recorded for approximately 300 days between six transceivers in a pentagonal array. Barotropic current along each of the 15 propagation paths is derived from the difference of reciprocal ray travel times, while ten independent estimates of areal-averaged relative vorticity are found by integrating current around triangles in the pentagonal array. The estimated tidal currents are highly accurate, and tidal relative vorticity at the M2 frequency is detected. This vorticity is induced primarily by the stretching of vortex lines by tidal elevation. Harmonic constants (amplitude, phase) of M2 tidal vorticity are about (4–8±2×10−9 s−1, 270°−320° ±20°), while harmonic constants of about (2–3×10−9 s−1, 300°−340°) are predicted using the shallow-water equations. The measured tidal harmonic constants are compared with those derived from a global barotropic tidal model.