Coastal Acoustic Tomography of the Neko-Seto Channel with a Focus on the Generation of Nonlinear Tidal Currents—Revisiting the First Experiment (original) (raw)
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Mapping Tidal Currents and Residual Currents by Use of Coastal Acoustic Tomography
Proceedings of International Conference "Managinag risks to coastal regions and communities in a changinag world" (EMECS'11 - SeaCoasts XXVI), 2016
A coastal acoustic tomography (CAT) experiment for mapping the tidal currents in the Zhitouyang Bay was successfully carried out with seven acoustic stations during July 12 to 13, 2009. The horizontal distributions of tidal current in the tomography domain are calculated by the inverse analysis in which the travel time differences for sound traveling reciprocally are used as data. Spatial mean amplitude ratios M2 : M4 : M6 are 1.00 : 0.15 : 0.11. The shallow-water equations are used to analyze the generation mechanisms of M4 and M6. In the deep area, velocity amplitudes of M4 measured by CAT agree well with those of M4 predicted by the advection terms in the shallow water equations, indicating that M4 in the deep area where water depths are larger than 60 m is predominantly generated by the advection terms. M6 measured by CAT and M6 predicted by the nonlinear quadratic bottom friction terms agree well in the area where water depths are less than 20 m, indicating that friction mechanisms are predominant for generating M6 in the shallow area. Dynamic analysis of the residual currents using the tidally averaged momentum equation shows that spatial mean values of the horizontal pressure gradient due to residual sea level and of the advection of residual currents together contribute about 75% of the spatial mean values of the advection by the tidal currents, indicating that residual currents in this bay are induced mainly by the nonlinear effects of tidal currents.
Coastal tomographic mapping of nonlinear tidal currents and residual currents
Continental Shelf Research, 2017
Depth-averaged current data, which were obtained by coastal acoustic tomography (CAT) July 12-13, 2009 in Zhitouyang Bay on the western side of the East China Sea, are used to estimate the semidiurnal tidal current (M 2) as well as its first two overtide currents (M 4 and M 6). Spatial mean amplitude ratios M 2 :M 4 :M 6 in the bay are 1.00:0.15:0.11. The shallow-water equations are used to analyze the generation mechanisms of M 4 and M 6. In the deep area, where water depths are larger than 60 m, M 4 velocity amplitudes measured by CAT agree well with those predicted by the advection terms in the shallow water equations, indicating that M 4 in the deep area is predominantly generated by the advection terms. M 6 measured by CAT and M 6 predicted by the nonlinear quadratic bottom friction terms agree well in the area where water depths are less than 20 m, indicating that friction mechanisms are predominant for generating M 6 in the shallow area. In addition, dynamic analysis of the residual currents using the tidally averaged momentum equation shows that spatial mean values of the horizontal pressure gradient due to residual sea level and of the advection of residual currents together contribute about 75% of the spatial mean values of the advection by the tidal currents, indicating that residual currents in this bay are induced mainly by the nonlinear effects of tidal currents. This is the first ever nonlinear tidal current study by CAT.
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.
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.
Tomographic measurement of tidal current and associated 3-h oscillation in Bali Strait
Estuarine, Coastal and Shelf Science, 2020
A coastal acoustic tomography experiment involving four acoustic stations was conducted from June 1-3, 2016 in the northern part of the Bali Strait. Range-average currents obtained along four transmission paths for a 30-h observation period were investigated in two-period bands: (a) > 6 h and (b) 10 min-6 h. The dominant phenomena for period bands (a) and (b) were semidiurnal tides with periods of approximately 12 h and internal modes with periods of approximately 3 h, respectively. The phase of the semidiurnal tidal currents was significantly deviated from a sea surface height-current relation expected for the progressive waves in the observation site. The 3-h oscillation had an amplitude comparable to that of the semidiurnal tidal current (with amplitudes of 1.5 ms 1) and was embedded in a semidiurnal envelope oscillation. In relation to the semidiurnal sea surface variations, the 3-h oscillation reached a peak at low water and diminished at high water. The velocity error of 0.07 ms 1 was estimated from the differential travel-time variations in the high-frequency band (10 min-1 h). The error was significantly smaller than the amplitudes of the semidiurnal tidal currents and the 3-h oscillation. Finally, it was suggested that the 3-h oscillation is an internal seiche synchronized with the semidiurnal tides.
Continental Shelf Research, 2015
Quantifying the tidal current and volume transport through the Qiongzhou Strait (QS) is vital to understanding the circulation in the northern South China Sea. To measure the tidal current in the strait, a 15-day coastal acoustic tomography (CAT) experiment was carried out at four acoustic stations in March 2013. The horizontal distributions of the tidal currents were calculated by inverse analysis of CAT data. The diurnal tidal current constituents were found to dominate: the ratio of the amplitudes O 1 , K 1 , M 2 , S 2 , and MSF was 1.00:0.60:0.47:0.21:0.11. The residual currents were found to flow westward in the northern QS and turn southward in the southern QS. The residual current velocities were larger in the northern area than in the southern area, with a maximum westward velocity of 12.4 cm s À 1 in the northern QS. Volume transport estimated using the CAT data varied between À 0.710 Sv and 0.859 Sv, with residual current transport of À 0.044 Sv, where negative values indicate westward. We conducted a dynamic analysis of the observations made during the study, which suggested that tidal rectification and sea level difference between the two entrances of the QS are important in maintaining the residual current through the strait. This is the first estimation, from synchronous measurements, of major tidal current constituents, residual currents, and volume transport in this strait.
Profiling measurement of internal tides in Bali Strait by reciprocal sound transmission
Acoustical Science and Technology
A reciprocal sound transmission experiment was carried out during June 10-13, 2015 along a cross-strait line in the Bali Strait with strong tidal currents to measure the vertical section structures of the range-averaged current and temperature at a 3 min interval. The five-layer structures of the range-averaged current and temperature in the vertical section were reconstructed by regularized inversion of the travel time data for two rays. The hourly-mean current showed the generation of nonlinear internal tides with amplitudes of 1.0-1.5 ms À1 and periods of 6 h superimposed on semidiurnal internal tides with amplitudes decreasing from the upper to lower layer. The hourly-mean temperature was characterized by variations with amplitudes of 1.0-1.5 C and periods of 6 and 8 h. The current variations showed an out-of-phase relation between the upper and lower layers while the temperature data varied in-phase for all five layers. The two-day-mean current and temperature showed a stratified structure, varying from À0:6 to À0:1 ms À1 and from 23.8 to 28.0 C, respectively. The five-layer current and temperature were significantly above the inversion errors.
1998
Using hydrodynamic and thermodynamic equations appropriate for modelling internal tides, one can predict the current and temperature distributions associated with the ocean's dynamic modes. Comparing such predictions with observations from the INTIMATE'96 experiment, we find a high degree of correlation between the first 3 theoretically calculated dynamic modes and corresponding empirical orthogonal functions (EOF's) derived from an ensemble of temperature and current profiles. The implications are twofold. First, this implies that the dominant variability in the INTIMATE'96 experiment is indeed associated with internal tides. Secondly, it suggests that in future tidal experiments a theoretically generated basis may be used as effectively as an EOF basis (which requires more extensive oceanographic measurements). We have also used the set of dynamic modes to simulate the effect of the tides on acoustic propagation to understand the relative importance of the usual surface tide (barotropic) and the internal (baroclinic) tides.
Journal of Geophysical Research: Oceans, 2018
Understanding residual (i.e., total minus tidal) currents in coastal seas is important because the residual currents affect long‐term material transports. In the Seto Inland Sea, Japan (SIS), a Bungo Channel to Kii Channel sea level difference causes a horizontal pressure gradient in the SIS and thus affects the residual current in the SIS. This study applies a linear regression method to examine how the residual current responds to the Bungo‐Kii sea level difference. The residual current is obtained using the reciprocal acoustic transmission data collected in the eastern portion of the Aki‐Nada sea area in 2012. The residual currents are estimated in the following three periods: from 12 April to 9 June, from 15 June to 21 July, and from 20 September to 27 October. In the regression analysis, an additional term is included to account for the fortnightly variation of the tide‐induced residual current. More than 75% of the observed residual currents can be explained by the sea level d...