Observing barotropic currents and tides in the Kuroshio Extension region from long-range reciprocal acoustic transmissions

Abstract

Travel times of reciprocal acoustic transmissions from a 5-transceiver acoustic tomography array over a 1000-km-scale domain in the Kuroshio Extension (KE) region during summer 1997 are used to investigate the barotropic tidal currents and range-averaged large-scale low-frequency currents as well as their relative vorticity. Inverse techniques are employed to estimate the low-frequency range-and-depth averaged (barotropic) currents every 3 or 6 h from the differential acoustic travel times. Eight major tidal constituent amplitudes and phases derived from the high-frequency acoustic travel times agree well with those found from the TOPEX/POSEIDON tidal model TPXO.5 [Egbert et al., 1994]. The results show that the range-averaged barotropic currents along the section in the KE recirculation gyre flowed westward in about 4 cm/s with an uncertainty of 0.8 mm/s; this is consistent with the result of the ADCP survey on WOCE P-14N experiment, and comparable with the surface geostrophic current velocity 5 m/s determined from the SSHA of the TOPEX/POSEIDON altimeter, indicating that barotropic currents are dominant in the Kuroshio Extension recirculation gyre. Estimated daily averaged relative vorticity over a 155<th>000 km2 area in the KE recirculation region has a near-zero mean (1.84×10−7 s−1).