Measuring the Kuroshio Current with ocean acoustic tomography (original) (raw)
Related papers
Geophysical Research Letters, 2005
1] The Kanmon Strait acoustic tomography data acquired at a 5-min interval are assimilated sequentially into a 2D ocean model on the basis of the ensemble Kalman filter scheme to image strong tidal current structures occurring in the strait. When the accurate range-averaged currents obtained for the sound transmission lines connecting eight tomography stations are used as assimilation data, the complicated vortex-imbedded currents are imaged with horizontal resolution and accuracy much better than the result of tomographic inversion. The assimilated currents are well compared to the shipboard ADCP data with a RMS difference of about 24 cm s À1 for both the horizontal velocity components. The assimilated volume transport across the strait also shows a good agreement with the transport estimated from the range-averaged current on a pair of transmission lines crossing the strait, making a RMS difference of 3700 m 3 s À1 . Citation: Lin, J., , Accurate imaging and prediction of Kanmon Strait tidal current structures by the coastal acoustic tomography data, Geophys.
The calculation of Kuroshio current structure in the East China Sea—early summer 1986
Progress in Oceanography, 1988
g)n the basis of hydrographic data and moored current meter records obtained during an early summer cruise (May 20-June 23) of 1986, a three dimensional diagnostic calculation of the circulation is performed in the survey area, which covers the East China Sea continental shelf, Okinawa Trough and an area east of the Ryukyu Island. The Kuroshio Current condition and structure in the East China Sea, its branches and their interrelationship as well as the eddies around the Kuroshio, are discussed. When the Kuroshio entered the area northeast of Taiwan, there were two branches. The main branch flowed northeastward along the continental slope and the other branch was at the eastern part of the Okinawa Trough. The main axis of the Kuroshio followed the continental slope above the 300 m level, but moved gradually eastward to the Okinawa Trough below the 300 m level.
Journal of Physics: Conference Series, 2008
Ocean acoustic tomography (OAT) was proposed in 1979 by Walter Munk and Carl Wunsch as an analogue to x-ray computed axial tomography for the oceans. The oceans are opaque to most electromagnetic radiation, but there is a strong acoustic waveguide, and sound can propagate for 10 Mm and more with distinct multiply-refracted ray paths. Transmitting broadband pulses in the ocean leads to a set of impulsive arrivals at the receiver which characterize the impulse response of the sound channel. The peaks observed at the receiver are assumed to represent the arrival of energy traveling along geometric ray paths. These paths can be distinguished by arrival time, and by arrival angle when a vertical array of receivers is available. Changes in ray arrival time can be used to infer changes in ocean structure. Ray travel time measurements have been a mainstay of long-range acoustic measurements, but the strong sensitivity of ray paths to range-dependent sound speed perturbations makes the ray sampling functions uncertain in real cases. In the ray approximation travel times are sensitive to medium changes only along the corresponding eigenrays. Ray theory is an infinitefrequency approximation, and its eikonal equation has nonlinearities not found in the acoustic wave equation. We build on recent seismology results (kernels for body wave arrivals in the earth) to characterize the kernel for converting sound speed change in the ocean to travel time changes using more complete propagation physics. Wave-theoretic finite frequency kernels may show less sensitivity to small-scale sound speed structure.
High-precision measurement of tidal current structures using coastal acoustic tomography
Estuarine, Coastal and Shelf Science, 2017
A high-precision coastal acoustic tomography (CAT) experiment for reconstructing the current variation in Dalian Bay (DLB) was successfully conducted by 11 coastal acoustic tomography systems during March 7e8, 2015. The horizontal distributions of tidal currents and residual currents were mapped well by the inverse method, which used reciprocal travel time data along 51 successful sound transmission rays. The semi-diurnal tide is dominant in DLB, with a maximum speed of 0.69 m s À1 at the eastern and southwestern parts near the bay mouth that gradually decreases toward the inner bay with an average velocity of 0.31 m s À1. The residual current enters the observational domain from the two flanks of the bay mouth and flows out in the inner bay. One anticyclone and one cyclone were noted inside DLB as was one cyclone at the bay mouth. The maximum residual current in the observational domain reached 0.11 m s À1 , with a mean residual current of 0.03 m s À1. The upper 15-m depth-averaged inverse velocities were in excellent agreement with the moored Acoustic Doppler Current Profiler (ADCP) at the center of the bay, with a rootemeanesquare difference (RMSD) of 0.04 m s À1 for the eastward and northward components. The precision of the present tomography measurements was the highest thus far owing to the largest number of transmission rays ever recorded. Sensitivity experiments showed that the RMSD between CAT and moored-ADCP increased from 0.04 m s À1 to 0.08 m s À1 for both the eastward and northward velocities when reducing the number of transmission rays from 51 to 11. The observational accuracy was determined by the spatial resolution of acoustic ray in the CAT measurements. The costoptimal scheme consisted of 29 transmission rays with a spatial resolution of acoustic ray of 2.03 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi km 2 =ray numbers p. Moreover, a dynamic analysis of the residual currents showed that the horizontal pressure gradient of residual sea level and Coriolis force contribute 38.3% and 36.0%, respectively. This indicates that the two terms are the dominant factors of the residual currents in DLB.
Travel-time correction and preliminary results for ocean acoustic tomography in South China Sea
MATEC Web of Conferences, 2019
An acoustic tomography trial experiment was conducted in South China Sea during May to August in 2016. Two moorings are installed apart from about 56.94 km, while each consists of one low frequency source, 20 hydrophones deployed from the depth of about 400 m to 1600 m, total 32 depth sensors and 3 compass and tilt sensors. Due to internal waves and currents in this area, as a typical value, horizontal drift of a mooring can reach 300 m, thus moorings drift need to be considered to correct ray travel-time. In this paper, the shape of a mooring is estimated firstly and locations of all hydrophone array elements are then calculated and finally used to determine travel-time perturbation of acoustic arrivals. The mooring is modelled as 2 curves, while the end of the mooring is fixed at the cement anchor on the sea floor. Optimization is used to acquire hydrophone location inferential solution. The inferred shape of hydrophone array and element locations are used to correct the travel-ti...
Monitoring of ocean current perturbations using acoustic phase variations
2009
In this paper, we review an acoustic method for measuring both average ocean current speed and sound speed along an acoustic path. The method requires two or more stations on the sea floor, synchronized through a connecting underwater electric cable, that can transmit and receive an acoustic signal and The method is based on reciprocal acoustic transmissions to determine time of flight in both the forward and reverse directions. A specific implementation of this method is proposed. We also consider a novel method for monitoring ocean current perturbations through acoustic phase measurements. In simplest form, a continuous sinusoidal signal is transmitted from one station to a second station. Any variations in ocean current speed will introduce a phase shift in the received signal proportional to the time derivative of the current speed. This effect has been demonstrated through ultrasonic in-air experiments. Finally, the two methods are combined into a single system that continuously measures small-scale ocean current velocity changes.
Variability of Northeastward Current Southeast of Northern Ryukyu Islands
Journal of Oceanography, 2004
To better understand the mechanism underlying the variation of the Kuroshio south of central Japan, we have examined the variability of current structure in its upstream region, southeast of Amami-Ohshima Island in the northern Ryukyu Islands. By combined use of ship-mounted Acoustic Doppler Current Profiler (ADCP) and the TOPEX/POSEIDON satellite altimeter data on Path 214, the sea surface absolute geostrophic currents were estimated every ten days from January 1998 to July 2002. The 4.5-year mean surface current was found to flow northeastward north of 26.8°N with a maximum speed of 14 cm s−1 over the shelf slope at 3000 m depth. The moored current-meter observations at three or four mooring stations from Dec. 1998 to Oct. 2002 suggested the existence of a northeastward undercurrent with a maximum core velocity of 23 cm s−1 at 600 m depth over the shelf slope at 1600 m depth. The mean volume transport in the top 1500 m between 27.9°N and 26.7°N is estimated to be 16 × 106 m3s−1 northeastward, including the subsurface core current related component of 4 × 106 m3s−1.