Coastal trapped waves in a two-layer ocean: Wave properties when the density interface intersects a sloping bottom

Properties of coastal trapped waves when the pycnocline intersects a sloping bottom are studied using a two-layer model which has slopes in both layers. In this system there is an infinite discrete sequence of modes, and four different sorts of waves exist: the barotropic Kelvin wave, the upper shelf wave, the lower shelf wave and the internal Kelvin-type wave. They all propagate with the coast to their right in the Northern Hemisphere. The upper and lower shelf waves are due to the topographic-effect on the upper-layer and lower-layer slopes, respectively. Their motions are dominant in the respective layers being accompanied by significant interface elevations. The properties of the upper (lower) shelf wave are almost unaffected by the existence of a lower-layer (upper-layer) slope. The motion of the internal Kelvin-type wave is confined to the region around the line where the density interface intersects the bottom slope.The modes, except that with the fastest phase speed (the barotropic Kelvin wave), are assigned mode numbers in order of descending frequency. Characteristics of Mode 1 change with wavenumber; the upper shelf wave for small wavenumbers and the internal Kelvin-type wave for large wavenumbers (high frequencies). The higher modes of Mode 2 and above can be classified into the upper and lower shelf waves.     

Numerical experiment on the circulation in the Japan Sea

By using a two-dimensional barotropic model on a-plane, the effect of the bottom topography on the path of the Tsushima Current is investigated. The rectangular model ocean with continental slopes has two openings: one is located at the southern boundary and the other at the eastern boundary. In a steady state, most of the water supplied into the model ocean through the inflow opening, flows along the continental slope with the coast to the right. Continental shelf waves play an important role in the process of adjustment to a steady state. It is suggested that the nearshore branch of the Tsushima Current might be largely topographically controlled.     

Branching of the Tsushima current in the Japan Sea

Although the Tsushima Current exhibits a complicated meander in the interior region of the Japan Sea, its path is more regular in the southwest region near the Tsushima Strait, and three branches have often been recognized there by many investigators. However, the detailed structures and temporal variabilities of these branches have not been clarified, and so they are studied here by analysing temperature, salinity and sea level data. It is shown that the existence of the first branch (the nearshore branch along the Japanese coast) can be detected from salinity distributions at least during the period from March to August. The third branch (the Eastern Korean Current) exists in all seasons. On the other hand, the second branch (the offshore branch) is seasonally variable and can be identified only in summer from June to August. Along the Japanese coast of southwest Japan Sea, the main pycnocline intersects the gentle slope on the shelf at a depth between 150 and 200 m. The first branch is found on the coastal side of the line where the main pycnocline intersects the bottom slope. On the other hand, the second branch is formed just on the seaward side of this line. Sea level differences in the Tsushima Strait, i.e., between Hakata and Izuhara and between Izuhara and Pusan, show that the seasonal variation of the surface velocity (or volume transport) is small in the eastern channel and large in the western channel. The period during which the surface velocity and volume transport in the western channel increase corresponds well to the period during which the second branch exists. These results suggest that the effects of bottom topography and oceanic stratification in the Japan Sea as well as the time variation of inflow through the western channel of the Tsushima Strait play important roles in the formation of the second branch.     

A numerical study on the generation of the small meander of the Kuroshio off southern Kyûshû

The generation of small meanders of the Kuroshio off southern Kyûshû is investigated. Basing on the fact that the small meanders tend to follow an increase in velocity of the Kuroshio in the Satsunan region (Sekine andToba, 1981), the influence of this velocity increase on the quasi-stationary path of the Kuroshio is studied numerically. Simplified bottom and coastal topographies are employed in a two layer model ocean. A quasi-stationary numerical solution with a constant inflow is used for the initial condition, and a temporal increase in the inflow with corresponding leakage is employed as the boundary condition to investigate nonlinear effects due to the increase in current velocity. Experiments for four different physical models are carried out to determine the specific roles of the continental slope, the planetary-effect, and density stratification. Temporal increase in the inflow tends to cause offshore shift of the current path. But the topographic effect of the continental slope is strong enough that no significant shift of the current path occurs in the case of the barotropic ocean. However, in the case of a baroclinic ocean, temporal increase in the inflow does cause generation of small meanders, because density stratification diminishes the topographic effect. A larger density stratification provides more favorable condition for the appearance of the small meander, and a cyclonic eddy is formed on the continental side of the small meander path.     

Behavior of warm water flowing into a cold ocean

Behavior of warm water flowing into a model ocean with flat bottom filled with cold water is investigated numerically with a three-dimensional level model in a rotating system.The warm water which flows through the southern opening into the ocean is found to progress in two directions. The east front of the warm water progresses along the east coast and reaches the northern opening. And an eastern boundary current associated with this density structure is formed. The west front progresses along the west coast with the speed much less than that of the eastside front. However, the progression of the west front slows down remarkably half way between the southern and northern boundaries due to-effect, and a baroclinic western boundary current system is formed. Unstable waves whose scales are 200–400 km are found in the region of the eastern boundary current and propagate toward the west. These waves seem to be caused by the baroclinic instability.The boundary currents obtained in the present model are considered to represent some characteristics of the Tsushima Current system in the Japan Sea.     

Short-term fluctuations of the Tsushima Current in waters northwest of Yamaguchi Prefecture

We discussed the short-term fluctuations of the Tsushima Current, using ADCP (acoustic Doppler current profiler) data taken by the quadrireciprocal method (Katoh, 1988) for removing tidal currents from observed currents. Transects were set in waters northwest of Yamaguchi Prefecture to capture the first and second branches of the Tsushima Current. Along the transects, ADCP and STD (salinitytemperature-depth recorder) measurements were repeated in May to July 1989. The velocity of the first branch fluctuated considerably in a short period, while the direction and position of its axis were steadly. The stability of its axis position probably results from the topographic effect. The baroclinic transport, which is based on calculations of geostrophic current with assuming zero velocity near the bottom, of the first branch was almost equal to the barotropic one. The range of fluctuation in the barotropic transport was much larger than that in the baroclinic transport. The barotropic fluctuation was correlated with the difference in atmospheric pressure between the east and west sides of the Tsushima Strait. As for the second branch, not only the velocity but also the position of its axis changed noticeably in a short period. There was such a case where the axis was hardly recognized. The changeability of the axis position for the second branch seems to result from that of the cold water front. The baroclinic transport of the second branch was much larger than the barotropic one, but the ranges of their fluctuations were very similar with each other.     

A Box Model of Glacial-Interglacial Variability in the Japan Sea

The Japan Sea has experienced drastic changes in the last 60 ka: the surface water was colder than the present value by five degrees and extremely freshened (24 ppt) in the last glacial maximum (15 ka), and then it contained Oyashio water for a few thousand years. It is an open question whether the inflow-outflow pattern was entirely reversed, opposite to the present exchange with an inflow through Tsushima Strait and an outflow through Tsugaru Strait. A box model is employed with two boxes representing the northern and the southern half domains in the upper (300-m-thick) layer. The model is driven by atmospheric forcing and inflow through Tsushima Strait and/or Tsugaru Strait. Here, the net transport through Tsushima to Tsugaru is given in the model. A baroclinic component is added to the net transport through each strait. It is the baroclinic components that allow the upper and the lower portions to flow to the opposite directions in the straits, and hence a reversal flow becomes possible against the net transport, under the condition of an extremely freshened Japan Sea. The fresh surface layer in 1814 ka is attributable to a near-shutoff of the inflow due to the low sea level. Shortly after the near-shutoff, the baroclinic transport through Tsugaru Strait yields intrusion of the Oyashio water into the Japan Sea. Thus, it is implied that Oyashio water existed in the Japan Sea a few thousand years after the reopening of Tsugaru Strait, even though the net transport was one-way, similar to the present state.     

Baroclinic circulation and its high frequency variability in Otsuchi Bay on the Sanriku ria coast,Japan

Hydrographic observations were made in Otsuchi Bay on the Sanriku ria coast, Japan, to provide clear images of the baroclinic circulation extending over the bay together with the associated intrusion of lower-layer water (bottom water) from outside the bay. In summer, a prominent baroclinic circulation with flow speeds \({>} 0.1\ \text{ m }\ \text{ s }^{-1} \) extends over the greater part of the bay. A main pycnocline (thermocline), which separates the upper and lower layers, is located at a depth of 15–40 m in and around the bay. The direction of the lower-layer flow (inflow into and outflow from the bay) is opposite to that of the upper-layer flow, which are baroclinically coupled to each other. Moreover, with regard to the lower-layer flow, the inflow tends to occur mainly through the northwestern part of the bay mouth, whereas the outflow tends to occur mainly through the southeastern part. The inflow and outflow alternate on time scales of several to a few tens of hours, and the flow directions are sometimes related to the tidal ones, although the relationship is not applied persistently. In winter, the baroclinic circulation is considerably weaker than in summer, because the stratification breaks down.     

A study on the chemical forms of radionuclides in seawater—II chloride and sulfate complexes of60Co

The apparent stability constants for chloride and sulfate ions with Co (II) at ionic strength of 0.67 were determined by the cation exchange method. The value of the stability constant ₁ for chloride ion with Co (II) ion was 0.79±0.055. The stability constants ₁, ₂ and ₃ for sulfate ion with Co (II) ion were 12.0±0.27, 91.5±11.4 and l,110±250, respectively. The chemical species of Co (II) in seawater was estimated at the pH of 8.0 to be present as Co²⁺ (63%), CoCl⁺ (27%) and CoSO₄ ⁰ (8.6%) using the known value of dissociation constant of Co (II), and under the assumption that only major inorganic anions are responsible for the chemical equilibrium of Co (II).     

The evolution of isolated vortices interacted with steep slope

Numerical solutions are examined for isolated, intense vortices as influenced by western bounding bottom topography through the use of a rigid-lid, two-layer primitive -plane numerical model. Systematic studies are made of the sense of rotation (cyclonic/anticyclonic), the consequence of varying the gradient of bottom slope, and the different vertical shear in a two layer ocean. In the basin with a bottom slope, the nearly barotropic anticyclonic vortex forms a modon-like vortex for S with fixedRo ₂<O(1) (where is the ratio between the variation of the Coriolis parameter across the eddy to the Coriolis parameter in the center, S the topographic effect and,Ro ₂ the Rossby number in the lower layer) and its generation is due to a compound effect of the planetary beta, topographic beta, avvection, and mirror image. The formation of the modon-like vortex and the propagation of the original vortex onto the bottom slope depends on the strength of slope gradient and the baroclinicity of the vortex. The nearly barotropic anticyclonic vortex evolves into the stronger upper ocean one with increasing S: the gradient of the bottom slope becomes steeper. Then the original vortex lives longer because the barotropic component of the energy is converted to the baroclinic one and it moves toward southeast in forming a modon-like vortex in the lower layer. The evolution of a vortex in the model results are compared to observational results of a Kuroshio warm core ring (KWCR) obtained from hydrographic data (June, 1985) and from NOAA satellite infrared images (April, 1985 to July, 1985). It is shown that a KWCR (June, 1985) is influenced by the western continental slope/shelf of the East Japan.     

Methane in the East China Sea water

Methane in the East China Sea water was determined four times at a fixed vertical section along PN line consisting of 11–14 stations, in February 1993, October 1993, June 1994 and August 1994. The mean concentration of methane in the surface water was not significantly higher than that in the open ocean. The methane concentration below the pycnocline increased during the stratified period in summer to autumn and reached to 15 nmoles/l at most in October. The concentration of methane was fairly well correlated with AOU in the layer below the pycnocline in the stratified season. This means that methane in the bottom water has only a single source, which is expected to be anoxic sediments near the coast, and that the oxidation rate of methane in the water is extremely slow in the oxic water. The high methane observed in October completely disappeared in February, indicating that the methane was escaped to the atmosphere or transported to the pelagic ocean by the Kuroshio current. The East China Sea, therefore, is not a large direct and stationary source for the atmospheric methane, but may have some role as a source by supplying it sporadically to the atmosphere in early winter or indirectly from the surface of the pelagic ocean.     

Internal flow structure of short wind waves

The minimum value of wind stress under which the flow velocity in short wind waves exceeds the phase speed is estimated by calculating the laminar boundary layer flow induced by the surface tangential stress with a dominant peak at the wave crest as observed in previous experiments. The minimum value of the wind stress is found to depend strongly on, the ratio of the flow velocity just below the boundary layer and the phase speed, but weakly onL, the wavelength. For wind waves previously studied (=0.5,L=10 cm), the excess flow appears when the air friction velocityu _* is larger than about 30 cm sec^–1. The present results confirm that the excess flow found in my previous experiments is associated with the local growth of a laminar boundary layer flow near the wave crest.     

Two layer model of the steady ocean response to large-scale heating and cooling

Steady response of a linear, two-layer baroclinic ocean to a steady thermal forcing was investigated on a mid-latitude -plane. Surface heat flux and its relaxation processes were parametrized as a form of heating function and Newtonian cooling term of cross interfacial velocityw in the continuity equation. Ageostrophic linear drag terms were employed to represent the western boundary layer for accomplishment of the steady circulation in the closed domain. In the interior, the dynamics belongs to thef-plane regime. The flow in the upper layer is zonal and eastward, which is the same as that expected in the channel without meridional boundary. The eastern boundary region is the so called Sverdrup region, in which the vortex stretching balances the term, thoughw contains a damping term. It directs the zonal flow southward (northward) in the south (north) followed by upwelling (downwelling) and forms the two gyres. Western boundary region is of the Stommel type except the upwelling (downwelling) in the south (north). The meridional circulation is similar to the Hadley Cell except southern and northern boundary layers. However, its strength is much greater than the latters.     

Does the water retained in a filter paper affect the chlorophylla measurement?

The present study investigated the effects of water seeped in a filter paper on the variations of chlorophylla measurement. As little as 184 l water retained in a 25 mm diameter GF/F filter paper after filtration resulted in the settlement of 60% chlorophylla at the bottom 20% of the extractant (90% acetone) in the extraction tube. More than one order difference in concentration of chlorophylla was found between the uppermost 20% and the bottom 20% of the extractant. Chlorophylla is trapped at the bottom by a pycnocline created by the water seeped into the filter paper and by chlorophylla itself, which is heavier than the extractant. Once the extractant was well mixed, good reproducibility of measurement was obtained. White precipitation occurred during the mixing of seawater sample and the extractant and was not found to affect the measurement of chlorophylla. The dilution of the extractant by the seawater seeped in the filter paper during extraction probably did not affect the extraction efficiency.     

Wind-driven circulation in the Japan Sea and its influence on the branching of the Tsushima Current

Seasonal variation in the wind-driven circulation in the Japan Sea is studied with reference to the branching of the Tsushima Current using a two-layer model with simplified bottom and coastal topography. The system is driven by wind stress, an inflow corresponding to the Tsushima Current and by the two outflows corresponding to the Tsugaru and Soya Currents.In the first phase, an annual mean wind stress is imposed and a quasi-stationary state is obtained. In the next phase, a seasonally varying wind stress is imposed. Seasonal variation in the wind stress plays an important role in the branching system of the Tsushima Current. In winter, an intensified western boundary current with a prominent inner circulation is formed as a result of a strong wind stress of winter monsoon with negative wind stress curl. In spring to summer, the western boundary current is weak, but the topographic branch along the Japanese coast is intensified. The weak western boundary current is caused by weak wind stress with positive wind stress curl, which induces cyclonic Sverdrup flow in the Japan Sea and causes its western boundary current to flow in the opposite direction to the prescribed northward boundary inflow current. The topographic branch is strongest in late spring and moves offshore in summer, in agreement with the central branch denoted by Kawabe (1982b). Some of the observational features of the Tsushima Current are successfully simulated.     

Volume transport of the western boundary current penetrating into a marginal sea

The amount of penetration of a western boundary current into a marginal sea which is connected to an open ocean by two narrow straits is estimated from a linear, steady and barotropic theoretical model. In this model the western boundary current in the open ocean is driven by a wind stress imposed at the sea surface. The inflow of the water of the open ocean into the marginal sea is caused by the pressure difference between two straits produced by the wind-driven circulation in the open ocean.Main external parameters are combined into two non-dimensional parameters; and (the ratio of the depth of the marginal sea to that of the open ocean), whereb is the distance between north and south boundaries of the ocean,D ₀ is the depth of the open ocean, is the latitudinal variation of the Coriolis parameter andR is the coefficient of friction. The friction is assumed to be proportional to the flow velocity.In the limit of infinite the volume transport into the marginal sea is not affected by the width of two straits and . It is mainly controlled by the wind stress and the positions of two straits. For finite values of , however, the volume transport depends considerably on and the width of the straits.Guided by both this model and physical considerations, we obtained a relation between the volume transport into the marginal sea and the external parameters. This relation predicts that about 2 % of the volume transport of the Kuroshio penetrates into the Japan Sea.     

Topographic effect of a marine ridge on the spin-down of a cyclonic eddy

The topographic effect of a meridional marine ridge on the spin-down of a cyclonic eddy is discussed by use of a two layer model. It is shown that cyclonic eddies on the eastern side of a marine ridge decay in a shorter time in comparison with those on the western side. The fast decay on the eastern side is mainly due to the planetary effect, which carries the eddy westward to a shallower region on the ridge, where remarkable frictional spin-down is carried out. Conversely, westward shift of the similar cyclonic eddy yields a weak bottom effect in the western side. It is inferred that the resulting remarkable spin-down on the eastern side of a marine ridge is one of the causes of the observed fast decay of the cyclonic cold water mass accompanied by the large meander of the Kuroshio on the eastern side of the Izu Ridge south of Japan.     

Occurrence of coprostanol, 24-ethylcoprostanol and 5α-stanols in the marine sediments

The present study deals with the elucidation of sterol composition of the marine sediments in Kagoshima Bay. The identification of each sterol was performed by gas-liquid chromatography, Ag⁺ impregnated column chromatography, mass spectrometry, and nuclear magnetic resonance spectrometry. The sediment obtained near the estuary of River Koutsuki contained large portions of 5-stanols such as coprostanol and 24-ethylcoprostanol basides 5-stanols such as cholestanol, 24-methylcholestanol, and 24-ethylcholestanol in the sterol fraction. These 5-stanols in the marine sediment may be derived from the fecal contamination by domestic sewages.     

Some characteristics of the development process of the wind-wave spectrum

The development process of wind-waves of which spectral peak distributes from 0.6 cps to 9.3 cps will be discussed on the basis of the wind tunnel experiments and of the field observations performed at Lake Biwa. The characteristics of power and slope spectra are here presented. The development process of these wind-waves is characterized by three stages;i.e. initial-wavelets, transition stage and sea-waves. In the wind tunnel experiments, the transition from the stage of the initial-wavelets to the transition stage occurs when the wave spectral peak arrives at the line 6.40×10^–4 k ^–2cm²·sec (wherek is wave number) or when the slope spectral density at the frequencyf _max becomes larger than 6.40×10^–4 sec. In the stage of sea-waves, the component wave of a wave-spectral peak is steepest in the component waves. And the wave spectral peak develops along the line 1.02×10² f ^–6 cm²·sec (wheref is the frequency corresponding to the wave numberk) untill it reaches the line 33.3f ^–4cm²·sec, and thereafter develops along the latter line, which indicates the constant density of slope spectrum. It is suggested that the nonlinearity of wind-waves must become stronger as wind-waves develop. The effective momentum flux _ws from the air flow to wind-waves in this stage is evaluated to be about 49% of the total stress ₀.     

A proof and applications of the expansion theorem for the Rossby normal modes in a closed rectangular basin

An expansion theorem is derived for Rossby normal modes in a closed rectangular basin and the set of Rossby normal modes is proved to be complete. This theorem provides a general linear solution to the initial value problem as well as to the response problem. In particular, the Green's function is obtained for the instantaneous localized torque anywhere in the basin. Weakly nonlinear versions are solved also by the combination of the general linear solution with the asymptotic expansion in terms of small amplitude. Further, an application is suggested to the spectral method of numerical simulation based on Rossby normal modes relevant to the more nonlinear evolution equation on a-plane, instead ofsin functions or Chebyshev polynomials, which have been employed conventionally for this purpose.