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Residual Current System in Kii Channel in August 1996

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Residual Current System in Kii Channel in August 1996 農 業 土 木 学 会 論 文 集 研究 論 文 Trans. of J S I D R E No.194, pp.87～96 (1998. 4) Residual Current System in Kii Channel in August 1996 Masayuki FUJIHARA*, Tateki FUJIWARA**and Gyozo OHASHI* *C ollegeof Agriculture, Ehime University (3-5-7 Tarumi, Matsuyama, Ehime 790, JAPAN) **G raduateSchool of Agriculture, Kyoto University (Sakyou-ku, Kyoto 606-01, JAPAN) Abstract Residual current in Kii Channel in August 1996 was simulated by using a robust diagnostic numerical model. The computed results were verified by an Acoustic Doppler Current Profiler (ADCP) data obtained along the two observational lines. The computed residual current is as follows. From the water surface to the depth of 30m, the water coming from Osaka Bay flows southward to the line of 34•‹ N and veers to the west because of the existence of the counterclockwise eddy in the central part of Kii Channel. After the changing of its direction, the water flows southward along the Shikoku island. The water again changes its direction to the east off Man City and approaches the Kii Peninsula, then flows out to the Pacific Ocean along the Kii Peninsula. As a whole, the southward current is apparent in this layer. Below the depth of 50m, the northward current is dominant as a whole . The forcing balances among the terms in momentum equation were investigated. The tidal stress balances with the pressure gradient around the strait. In other regions, the tidal stress is relatively small. Main balance is established between the pressure gradient force and the Coriolis force in Kii channel. Therefore the residual current in Kii Channel is supposed to be geostrophic . Key words: Residual current, Diagnostic model, Kii Channel, Geostrophic current, Density current, Tidal stress 1. INTRODUCTION Therefore a diagnostic model, which gives the flow field from the observed density field, is suitable to compute the Kii Channel surrounded by Kii Peninsula, Shikoku residual current in Kii Channel. Island and Awaji Island opens to the Pacific Ocean and In view of verification, the water temperature and connects with Osaka Bay through Kitan Strait and with salinity data recorded simultaneously with the current the Harima-Nada sea through Naruto Strait. Because of data are preferable. Fujiwara, one of the authors, difficulty of setting moored current meters, characters of measured current velocities, water temperature, salinity residual currents in Kii Channel have not been known and nutrients along two cross-sections on 6-8 August yet. 1996(not published). The current was measured by an On the other hand, it was reported that nutrients are Acoustic Doppler Current Profiler (ADCP), thus the transported from the inner shelf to Kii Channel during distribution of residual currents in two vertical planes the summer (Fujiwara et al., 1997). This could be one were obtained in detail. The three Prefectural Fisheries reason why the water quality in Osaka Bay, especially Experimental Stations also measured water temperature offshore region has not been restored, though the and salinity in and around Kii Channel when the above terrestrial nutrients loading has been reduced. Long-term field observation was carried out. Therefore we obtained transport of substance such as nutrients is mainly affected synoptic data set, which is suitable for a verification of by Residual Current(RC) in coastal waters. In this way, computational results. the residual current in Kii Channel also has an important This paper aims to know the RC distribution in Kii meaning from the point of water quality in Osaka Bay. Channel in the August by developing a multi-level robust To the authors' knowledge, a three-dimensional diagnostic numerical model and resorting it. numerical simulation about the residual current system in Kii Channel has not been conducted. 2. MODEL DESCRIPTION In Kii Channel, water temperature and salinity have been measured at many fixed stations once a month by 2.1 Basic equations three prefectural fisheries experimental stations. A multi-level robust diagnostic numerical model 農 土 論 集 194 (66-2) 283 88 農 業 土 木 学 会 論 文 集 第194号(第66巻 第2号) presently employed is represented by the momentum the prescribed cell-density calculated from observed equation of the RC including the tidal stress on the values, u(u, v) the horizontally two-dimensional M2 tidal rotating earth accepting f-plane approximation, the current vector, w the vertical M2 tidal current velocity, t continuity equation and the density transport equation the time, z the vertical coordinate positive upward, and (Fujihara and Kawachi, 1995b); the over-bar in Eq.(6) denotes time average over one tidal cycle. The last term on the right-hand side of Eq.(5), called ƒÁ-term, is that introduced by Sarmiento and Bryan (1982) to prevent computed density from deviating (1) greatly from the corresponding prescribed cell-density ρOB・This term plays a role of artificial source/sink of density to converge computed densities to prescribed ones. Special treatments for boundary conditions are not (2) necessary in solving the density transport equation Eq.(5), because all the density values along the exterior boundaries are also given in advance. Boundary (3) conditions for the momentum equation Eq.(1) are expressed as ; at the free surface, (4) (8) (5) where, pa is the atmospheric density (=1.23kg/m3), γ2a the drag coefficient of free surface (=0.0013), and Ww the horizontally two-dimensional wind vector, at the sea bottom, (6) (9) (7) where, ƒÁ2b is the frictional coefficient of bottom (=0.0026), where, U(U, V) is the horizontally two-dimensional and at the land boundary, velocity vector of residual current, W the vertical component of residual current, V h the horizontal (10) differential operator, f Coriolis parameter, k the locally vertical unit vector, p the pressure, vh,vv the kinematic which is the no-slip condition. eddy viscosity coefficients in the horizontal and vertical Prior to operation of the diagnostic model, the tidal respectively, L the tidal stress, g the gravitational stress Ts is computed through tidal current simulation acceleration, ƒÅ the sea surface elevation above the mean under the barotropic flow condition in the area which sea level, H the water depth, p the water density, Kh, Kv encompasses Kii Channel. the turbulent diffusion coefficients in the horizontal and The value of vh is changed proportionally to the vertical respectively, y the inverse time constant, ƒÏob square of the amplitude of tidal current variations, in the 284 Trans. JSIDRE Apr. 1998 Residual Current System in Kii Channel in August 1996 89 same manner ( vh = 0.016 •E Tp •E u2am, ; Tp = tidal period, uam latitude 33•‹ 42'N and 34•‹ 25'N which encompass the = amplitude of tidal current) as in the previous work by southern part of Osaka Bay and that of the Harima-Nada sea and Kii Channel, as shown in Fig.1. The whole body Fujihara and Kawachi (1995a). Thus the resulting vh - of water is discretized into 1km •~ 1km square meshes values are 800m2/s in Naruto Strait and 300m2/s in Kitan horizontally and ten levels vertically ( 1 st level : 0-2m, Strait, while these values are about 1m2/s in zones where 2nd level : 2-5m, 3rd level : 5-10m, 4th level : 10-20m , tidal current is weak. The value of v, generally depends 5th level 20-30m, 6th level : 30-40m, 7th level : 40-50m, on the strength of stratification that is closely related to 8th level : 50-75m, 9th level : 75-100m, 10th level : the tidal current intensity. Therefore the vv -value is set 100m-bottom) in order to employ the finite difference to a hundred-thousandth of vh -value. Thereby it takes numerical method. 80cm2/s at the Naruto Strait and lcm2/s as a minimum. 2.3 Distribution of density The values of K,, and IC, are set to 1m2/s and 1 cm2/s , The model used in this study requires the data of respectively. density, wind stress and tidal stress as the input data at Equation (1) is integrated in time by using the same each computational cell. At first, we describe the method numerical scheme as in the previous work (Fujihara and to estimate the density distribution. The observation Kawachi, 1995b), until steady state solutions of the RC stations of Wakayama, Osaka and Tokushima Prefectural are obtained. For successful correction of cell-densities, Fisheries Experimental Stations are also shown in Fig.1. the coefficient y is now set to 0.5/ At (ƒ¢t = 30sec ) At individual locations, water temperature, salinity that intermediately underrelaxes the correction , etc. have been measured at several depths once a month. (ƒÏob-ƒÏ). Moreover the intensive observation mentioned before was carried out from 6th to 8th in August 1996. 2.2 Study area In this study, the water temperature and salinity data The area under consideration is that lying between obtained from 1st to 9th in August 1996 by three Fig.1 Bottom topography of study area and observation stations of density (•œ) and wind (•›). Numerals show the depth in meter. 農 土 論集194(66-2) 285 90 農 業 土 木 学 会 論 文 集 第194号(第66巻 第2号) prefectural fisheries experimental stations and those obtained from the intensive observation were used to estimate the density distribution in Kii Channel. For estimating cell-densities from the scattered densities obtained, interpolation and smoothing techniques are employed. Firstly, level-densities (ƒÏot) at an observation station (i) are determined by the linear interpolation from the measured densities. Secondly, interpolated cell-densities (pc,b) are estimated by using the expression; (11) where L is the distance to the observation station (i) from Fig.2(a) Interpolated density distribution(ƒÐt) in Kii Channel from the cell under consideration and n the number of surface to the deth of 2m.
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