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Wind-driven circulation in the Japan Sea and its influence on the branching of the Tsushima Current
Prog O(canog.. Vol. 17. pp. 297-312. 1986.
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W i n d - d r i v e n circulation in the Japan Sea and its influence on the b r a n c h i n g o f the T s u s h i m a C u r r e n t YOSHIHIKO
SEKINE*
(Received 30 November 1985; hi revised form 2 January 1987; accepted 26 February 1987) Abstract--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.
INTRODUCTION
THE JAPAN SEA is a marginal sea surrounded by the Japanese islands and the Asian continent. The major feature of the current system in the Japan Sea is an inflow of the Tsushima Current through the Tsushima Strait and its outflow through the Tsugaru and Soya Straits. During the past five decades, there have been many reports on the flow patterns of the Tsushima Current in the Japan Sea (e.g. SUDA and HIDAKA,1932; KAWAI, 1974; KAWABE, 1982a). The Tsushima Current has been shown to split into three branches in the southern region of the Japan Sea. One branch flows along the Japanese coast, the second branch flows offshore, and the third branch flows along the Korean coast. Based on observed temperature, salinity and sea level data, KAWABE (1982a) showed that the branch along the Japanese coast exists from spring to summer, the second branch only in summer and the third branch throughout the year. The dynamics of branching of the Tsushima Current in the southern Japan Sea was investigated numerically by YOON (1982a,b,c) and KAWABE (1982b). The model characteristics are shown in Table 1. In particular, using a multi-level flat bottom model with the seasonal variation in temperature and salinity of the boundary inflow corresponding to the Tsushima Current and atmospheric conditions at the sea surface, YOON (1982b) * Institute of Geosciences and Astronomy, The National Defense Academy, Yokosuka 239, Japan. 297
298
Y. SEKINE Table 1. Model characteristics of Japan Sea developed so far
Author
Density stratification
Coastal topography
Bottom topography
Wind
Heat*
Inflow
Yoon (1982a)
Yes
Rectangular
Flat
Constant
Constant
Stationary
Yoon (1982b)
Yes
Realistic
Flat
No
Rectangular
Simplified
Seasonal variation No
Stationary
Yoon (1982c)
Seasonal variation No
Kawabc (1982b)
Yes
Rectangular
Simplified
No
No
Variable
Present study
Yes
Realistic
Realistic
Seasonal variation
No
Stationary
Stationary
* Constant, Seasonal Variation and No mean stationary thermal boundary condition, seasonally varing thermal boundary condition and no consideration on the thermal condition, respectively.
demonstrated that the third branch along the Korean coast is a western boundary current governed by the planetary [3-effect. However, his model failed to simulate the first branch along the Japanese coast because of the exclusion of the bootom slope. YOON (1982C) suggested that the first branch is formed by the topographic effect of the continental slope, the topographic guiding effect along the geostrophic contour f/h where f is the Coriolis parameter and h is the depth. While YOON (1982c) assumed a barotropic model, KAWABE (1982b) examined the branching of the Tsushima Current by use of a two-layer model (see Table 1). He showed that the first branch along the Japanese coast is a bottom-controlled steady current subject to the topographic 13-effect. He also pointed out that the second branch of the Tsushima Current, located offshore of the first branch, is caused by propagation of the lowest two modes of the upper shelf wave which in turn is generated by the significant increase in inflow of the Tsushima Current in summer. However, his model is driven by an inflow and an outflow only, and no wind stress effect on the branching of the Tsushima Current is considered. Historical data (JAPAN METEOROLOGICALAGENCY, 1972; KUTSUWADA and SAKURAI, 1982; KUTSUWADA,1982) depict a clear seasonal variation in wind stress on the Japan Sea and the northern Pacific. These data show a strong northwesterly wind in winter (winter monsoon) blowing from the Siberian high atmospheric pressure. In contrast, wind stress in summer is very weak. Consequently, the wind-driven circulation is mush stronger in winter than in summer. Moreover, a seasonal variation in the wind stress might have a significant influence on the branching of the Tushima Current and the current system of the Japan Sea. In this context, the present paper deals with the seasonal variation of wind driven circulation in the Japan Sea with special reference to the branching of the Tsushima Current. DESCRIPTION OF THE MODEL
Boundary and bottom topography of the Japan Sea (Fig. 1) are simplified so that only the main features are modeled. A two-layer model is assumed. Its schematic representation is displayed in Fig. 2. Because the total depth of the model ocean varies spatially, a quasi-geostrophic approximation can not be assumed. The orthogonal coordinates are
Wind-driven circulation in the Japan Sea
299
SoyaStrait ~Japan Sea01 ~,ugaru Strait
Fig. 1. Schematic representation of the model ocean (left). Solid straight lines show the coastal boundaries. Isopleths of the depth (m) (right). Shallow area is stippled.
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taken as in Fig. 1, in which the angle between x axis (y axis) and eastward (northward) is 36 ° . The basic equations describing the model ocean are derived from the equation of motion and the equation of continuity for a two layer model. Assuming hydrostatic balance, the 13-plane, the rigid lid and the Bousinesq approximation, the final forms of the basic equations are as follows:
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Wind-driven circulation in the
JapanSea
301
The notations used in the above equations have the following meanings: t u,v h q
: : : :
D H Z qb f 72 Subscripts 1,2
: : : : : : :
time. horizontal component of velocity in the x- and y-directions. thickness of the layer. deviation of interface positive downward from the average depth of the interface. total depth of the ocean. averaged depth of the upper layer. relative vorticity. volume transport function. Coriolis parameter. Laplacian operator for the horizontal direction. quantities in the upper and the lower layers, respectively.
Viscous lateral boundaries are set up. The interface displacements at the in- and outflow boundary are given to satisfy tile geostrophic isostacy; an inflow of 2 Sv (1 Sv = 1 × 1012 cm ~ sec -~) corresponding to the Tsushima Current, an outflow of 1.3 Sv of the Tsugaru Warm Current and an outflow of 0.7 Sv of the Soya Current are all assumed to be confined to the upper layer. In the initial state, a flow is confined to the upper layer and it runs along the northern coast of Japan (Fig. 3). Starting from this initial condition, a time variation in the current system is investigated by a numerical time integration of (1), (2) and (3). The grid size is 38 km along x axis and 31 km along y axis. Differencing schemes used are Lilly's scheme for (1) and the Generalized Arakawa scheme for (2) (for detail, see GRAMMELTVZDT, 1969). The leap frog scheme is used fundamentally for the local time change term with a time step of
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8640 sec. To suppress the computational mode, the Euler backward scheme proposed by MATSUNO (1966) is applied every 40 time steps. In winter, a strong northwesterly wind monsoon prevails, while in summer the wind is very weak (Fig. 4). The average of the wind stress curl over the whole model ocean is negative in winter (Fig. 5), which turns out a dominant anticyclonic flow in the basin (SVERDRUP, 1947). However, from April to July, the average of wind stress curl is
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Monthly mean wind stress data, 1958-75 (after JAPANMETEOROLOG,CALAGENCY, 1972; KUTSUWADA and SAKURA1, 1982; KUTSUWADA,1982). No values below 0.05 dyne cm -2 are Fig. 4.
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Wind-driven circulation in the Japan Sea
303
positive and generates a cyclonic flow. Since the time change of the wind stress is remarkable, the large time variation in wind driven circulation is expected. A quasi-stationary state is reached in the first phase under the annual mean wind stress. Next, the model is driven with a seasonally varying wind stress obtained by linear interpolation of monthly averages (Fig. 4) for evaluating to what extent the current system varies from season to season. RESULTS
A time integration with the constant annual mean wind stress was carried out for 4000 days. The main results are shown in Figs 6 and 7. Part of the kinetic energy of initial flow along the Japanese coast shifts to the west, and a western boundary current along the western and northern boundaries is formed gradually. The lower layer velocity field (Fig. 7) has a tendency to flow along the geostrophic contour f/h, and is explained as the
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Fig. 6. (a) Sequential pattern of the volume transport function at days 250, 500, 1000, 2000, 3000 and 4000 in the case of annual mean wind stress. The contour interval of the volume transport function is 0.2 Sv (1 Sv = 1012 cm 3 see-'). (b) Sequential pattern of the upper layer thickness at days 250, 1000 and 4000. The contour interval is 5 m.
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bottom intensified mode in the two-layer system with sloped bottom (RItINES, 1970; SU~INOHARA, 1981)• This tendency is weak in the velocity field of the upper layer, reflecting the surface intensified mode; the western boundary current dominates and the topographic branch along the Japanese coast is weak. A relatively strong flow in the lower layer is found near the Tsushima Strait. Since the lower layer thickness is remarkably reduced in this region by the shallow sloped bottom and descending interface (see Fig. 6B), this flow is restricted to a thin layer• The barotropic response is very fast, and its characteristic time is about 10 days. But time variation in flow pattern is small after about 1,000 days. In the completely stationary state, the geostrophic isostacy should be reached and the topographic effect on the flow pattern should vanish• A result from a flat bottom Japan Sea model examined by YOON (1982a) showed most of the inflow of the Tsushima Current flow as a western boundary current, and the coastal branch along the Japanese coast is not found in the stationary state. However, it takes more than 4000 days to attain such a geostrophic isostacy in the present model and the topographic effect on the current persists for a long period• If a baroclinic response by a baroclinic Rossby wave with a velocity of 1 cm sec-1 is assumed, it takes about 1000-2000 days for the baroclinic wave to propagate over the entire Japan Sea Basin• Therefore, the delay of the baroclinic response shown in the present model originates in the effect of the bottom topography• The solution at day 4000 with a constant wind stress is used as an initial condition for the case with seasonally varying wind stress, and the time integration is carried out for 11
305
Wind-driven circulation in the Japan Sea
years. A periodic annual change in kinetic energy and entrophy with the highest peak in winter is clearly seen (Fig. 8). Because no variation with periods longer than a year is depicted in the energy variation, the flow pattern in the llth year is examined in the following. Distinct seasonal variations in the total transport function are seen (Fig. 9). In winter, most of the inllow bends northward and a strong western boundary current is formed, whereas the coastal branch along the Japanese coast is very weak. Related to the large negative wind stress curl in this period (Fig. 5), the anticyclonic circulation is especially intensified in January. On the other hand, in summer, most of the inflow water flows along the southern boundary and the western boundary current is weak. The inner circulation developed in winter is weak because of a weak wind stress during this season. The disappearance of the western boundary current is related to the positive wind stress curl in late spring to summer (Fig. 5); the positive wind stress curl induces northward Sverdrup transport and the western boundary current flows southward. Therefore the southward western boundary current flow cancels the northward western boundary current formed by the inflow water through the Tsushima Strait. In winter, a large horizontal gradient of the interface depth is found, whereas the gradient is relatively small in summer (Fig. 10). The pronounced tilt of the interface along the northern boundary arises from convergence in the lower layer (upwelling of the interface owing to the strong northwesterly winter wind). >~
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Because the difference in the specific volume between the upper layer 1/p and the lower layer 1/(p + Ap) yields sea level increase (decrease), if the ratio of the upper layer and lower layer thicknesses changes, the increase in the upper layer thickness is proportional to the increase in sea level. The seasonal variation in the calculated upper layer thickness is compared with the observed sea level variation. Data are taken from Tidal Record (1984) published by Geographical Survey Instutute of Japan, and Monthly and Annual Mean Heights of Sea Level (1976) published by PSMSL. The calculation shows similar trends to the observations from winter to spring, but a prominent peak in the observed sea level is seen in summer, while the calculation predicts a minimum. This discrepancy in summer season is most probably due to the surface heating, which is not considered in the calculation. The similar tendency from winter to spring is mainly due to the situation that the deepening of the mixed layer gives good approximation of the two layer model to the actual oceanic condition. A high correlation exists between the calculated upper layer thickness and observed sea level in the south such as Hagi and Ulsan, but the correlation is relatively low in the north (Fig. 11). The latter fact indicates
Wind-driven circulation in the Japan Sea
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JAN.
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.
.
Fig. 10. Same as Fig. 6 (b) but for seasonallyvarryingwind stress. The upper layer thickness at day 15 in each month are plotted.
an important role of the thermohaline effect on the oceanic condition in the northern part of the Japan Sea. Although there is neither inflow nor outflow in the lower layer, a current is induced near the inflow region and it flows along the isopleths of the depth (Fig. 12). The motion in the lower layer is most remarkable in January, as shown by the strong peak of the kinetic energy variation (Fig. 8). There is clear coherence of the kinetic energy variation between the upper layer and lower layer (see Fig. 8), so, the barotropic response is more important to the seasonal response of the current system in the Japan Sea. This is mainly due to the rapid change in wind stress in comparison with a baroclinic time scale longer than one year, as discussed in the case of stationary wind stress (Figs 6 and 7). A strong western boundary current is found in winter, and it splits into two paths at the midpoint of the northern boundary (see upper layer velocity field in Fig. 12). One split path leaves the northern boundary and runs zonally toward the Tsugaru Strait. The other split path exists along the northern boundary without separation and its outflow is made at the Soya Strait. In contrast, the different upper layer flow pattern is seen in late spring to summer; most of the inflow water flow along the southern boundary, and the western boundary current is very weak. In late summer, the flow along the southern boundary leaves the coastal region and the main current axis shifts offshore region. In summer, the
Y. SEKINE
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central branch is most remarkable and the western boundary current is weak in comparison with other season (Fig. 13). The appearance of the central branch in the present model coincides with the observational feature examined by KAWABE (1982a). Although KAWABE (1982a) concluded that the central branch is due to the increase in volume transport of inflow of the Tsushima Current during the summer, the present study indicates that the central branch is partly due to the wind-driven circulation in summer; the volume transport of the inflow is assumed to be constant in the present model. In this way, the wind-driven circulation has a significant influence on the branching of the Tsushima Current and the total current system in the Japan Sea.
309
Wind-driven circulation in the Japan Sea
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D I S C U S S I O N AND S U M M A R Y
Wind-driven circulation in the Japan Sea has been investigated numerically by use of observed wind stress data and, in a first phase, a two-layer primitive model with a simplified bottom and coastal topography is driven by the annual mean wind stress. The characteristic time of the response of the barotropic Rossby wave to the stationary wind stress in the Japan Sea is about 10 days, smaller than the time scale of seasonal change in the wind stress. In contrast, the baroclinic response takes a very long time. The topographic effect of the bottom slope still persists in a quasi-stationary state in the last period of time integration of 4000 days. Two predominant branches of the inflow of the
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Tsushima Current exist; one is the topographic branch along the southern boundary and the other branch is associated with the westward intensification along the northern boundary. This result differs from that of a flat bottom model of ¥OON (1982a), which gives a baroclinic response time of about 1000-2000 days. In the second phase, a seasonally varying wind stress obtained by a linear interpolation of monthly mean data allows delineation of the seasonal variation in the wind-driven circulation. A clear annual cycle of the current system is depicted. The main results of the seasonal variation in the wind-driven circulation are as follows: (1) In winter, most of the inflow water of the Tsushima Current bends northward and an intense western boundary current accompanied by an inner circulation is formed, whereas the topographic branch along the southern boundary is very weak. This is due to the strong wind stress and/or negative wind stress curl. (2) In spring to summer, the western boundary current is weak, but the topographic branch along the southern boundary is intensified. This branch is stongest in late spring and moves offshore in summer. A second central branch observed in summer (KAwABE, 1982a) iS successfully simulated.
Wind-drivcn circulation in the Japan Sea
311
(3) The intensification of the western boundary current in winter is brought about by the strong monsoon wind stress in this season. Because the barotropic response is accomplished within half a month, the strong wind stress forms the strong western boundary current in a short time. (4) The intensification of the topographic branch along the southern boundary is mainly attributed to weak wind stress with positive curl. The positive wind stress curl generates cyclonic Sverdrup circulation with a western boundary current flowing southward, opposite to the direction of western boundary current originated by the inflow of the Tsushima Current. Therefore, most of the inflow comes along the southern region of the basin. One shortcoming of the present study is that by dealing with a two-layer model ocean, it ignores the frontal structure of the Tsushima Current. Although recent satellite imagery shows various disturbances in temperature field along the frontal region of the Tsushima Current (e.g., TOBA, HANAWA, KAWAMUKA, YANO and KURASAWA, 1985), the frontal structure cannot be properly handled with the layer model. A level model should be developed. No seasonal variation of the inflow of the Tsushima Current was taken into account in our model. Although the volume transport and the current velocity of the inflow of the Tsushima Current is not yet well-known, historical data show the existence of its seasonal variation (MIYAZAKI,1952; YI, 1966; MHTA,1976). In addition, no effect of the thermohaline circulation in the Japan Sea on the branching of the Tsuhima Current was considered. The thermohaline circulation, which is important for the generation of the Japan Sea bottom water, also might be important. Improving the model in these respects will be important to understand better the total current system of the Japan Sea. Acknowledgments--The author wishes to thank Professor K. Takano for his critical reading of the manuscript and valuable comments. The author also thanks Professors Y. Toba of Tohoku University and N. Suginohara of University of Tokyo and Dr. J. H. Yoon for many fruitful discussions. Thanks are extended to Dr. K. Kutsuwada and Mr. T. Andoh for providing the wind stress data.
REFERENCES GEOGRAPHICALSURVEYINSTITUTEOFJAPAN (1984) Tidal Record B2 No. 40. GRAMMELTVEDT,A. (1968) A survey of finite difference scheme for the primitive equations for a barotropic fluid. Monthly Weather Review, 97,384-404. JAPAN METEOROLOGICALAGENCY(1972) Marine meteorological study of the Japan Sea. Technical Report of the Japan Meteorological Agency, 80, 1-161. KAWABE,M. (1982a) Branching of the Tsushima Current in the Japan Sea. Part I Data analysis. Journal of Oceanographical Society of Japan, 38, 95-107. KAWABE,M. (1982b) Branching of the Tsushima Current in the Japan Sea. Part lI. Numerical experiment. Journal of Oceanographical Society of Japan, 38, 183-192. KAWAI, H. (1974) Transition of current images in thc Japan Sea, In: The Tsushima warm current--ocean structures and fishery, Edited by Fishery Society of Japan, Koseisha-koseikaku, pp. 7-26 (in Japancsc). KUTSUWADA,K. (1982) New computation of wind stress over the North Pacific Ocean. Journal Oceanographical Society of Japan, 38, 159-171. KUTSUWADA,K. and K. SAKURAI(1982) Climatological maps of wind stress field over the north Pacific Ocean. Oceanographical Magazine, 32, 25-46. MATSUNO,T. (1966) Numerical integrations of the primitive equations by a simulated backward difference method. Journal of Meteorological Society of Japan, 44, 76-84. MIITA, T. (1976) Current characteristics measured with current meters at fixed stations. Bulletin of Japan Society Fish. Oceanography, 28, 33-58 (in Japanese). MIYAZAKI, M. (1952) The heat budget in the Japan Sea. Report of Hokkaido Regional Fishery Research Laboratory, 4, 1-45 (in Japanese).
3 12
Y. S[~KJNE
RmNI~S, P. B. (1970) Edge-, bottom- and Rossby waves in a rotating stratiticd fluid. Geophysical Fluid Dynamics, 1,273--302. PI-RMANI-NTSERVICE for MEAN SEA LEVEl. (1976) Monthly and Annual Mean Height of Sea Level. SuDA, K. and K. HIOAKA (1932) The results of the oceanographical observations on board R.M.S. Syumpu Maru in the southern part of the Japan Sea in the summer of 1929, Part i. Journal Oceanographical Imperial Marine Observation, 3, 291-375 (in Japanese). SI.I(ilN()IIARA, N. (198(]) Effects of bottom topography and density stratilic~ltion on the form~ltion of western boundary currents. Journal of Oceanographical Society of Japan, 35,224-232. SUGINOIIARA, N. (1981) Quasi-geostrophic waves in a stratified ocean with bottom topography. Journal of Physical Oceanography, 11, 107-115. SVERDRUP, H. U. (1947) Wind-driven currcnts in a baroclinic ocean: with application to the equatorial currents of thc castcrn Pacific. Proceedings of the National Academic of Science, U.S.A., 33,318-326. TOI~A, Y., K. HANAWA,H. KAWAMURA,Y. YANO and Y. KURASAWA(1985) Horizontal processes involved in the formation of sea surface temperature near a western boundary current. In: The ocean surface. Y. TOgA and H. MITSUYASU,editors, D. Reidcl Publishing Company, 571-576. YI, S. (1966) Seasonal and secular variations of lhe ,.V~llcrvolume tral'tsport across the Korea SIr~lil. JOlll'll{tl()/ Oceanographical Society of Korea. I, 7-13. YooN, J. H. (1981a) Numerical expcrimcnt on the circulation in the Japan Sea Part I. Formation of thc cast Korc~m warn] current. Jottrnal ()1"Oce,nogrtq~hical Society of Jtq)an, 38, 43-51. Yoo~r, J. H. (1981b) Numerical experiment on the circultaion in the Japan Sea Part II. Influence of seasonal variations in atmospheric conditions on the Tsushima Current. Journal of Oceanographical Society of Japan, 38, 81-94. YOON, J. H. (1981c) Numerical experiment on thc circulation in the Japan Sea Part III. Mechanism of nearshore branch of the Tsushima Current. Journal of Oceanographical Society of Japan, 38, 125-130.
0 0 7 9 - ~ d I/~.t~ $1~ rio + 51~ ( tlp}righl © 1'1,~7 ])crganlorl Jourrl~ll ~, I.td
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W i n d - d r i v e n circulation in the Japan Sea and its influence on the b r a n c h i n g o f the T s u s h i m a C u r r e n t YOSHIHIKO
SEKINE*
(Received 30 November 1985; hi revised form 2 January 1987; accepted 26 February 1987) Abstract--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.
INTRODUCTION
THE JAPAN SEA is a marginal sea surrounded by the Japanese islands and the Asian continent. The major feature of the current system in the Japan Sea is an inflow of the Tsushima Current through the Tsushima Strait and its outflow through the Tsugaru and Soya Straits. During the past five decades, there have been many reports on the flow patterns of the Tsushima Current in the Japan Sea (e.g. SUDA and HIDAKA,1932; KAWAI, 1974; KAWABE, 1982a). The Tsushima Current has been shown to split into three branches in the southern region of the Japan Sea. One branch flows along the Japanese coast, the second branch flows offshore, and the third branch flows along the Korean coast. Based on observed temperature, salinity and sea level data, KAWABE (1982a) showed that the branch along the Japanese coast exists from spring to summer, the second branch only in summer and the third branch throughout the year. The dynamics of branching of the Tsushima Current in the southern Japan Sea was investigated numerically by YOON (1982a,b,c) and KAWABE (1982b). The model characteristics are shown in Table 1. In particular, using a multi-level flat bottom model with the seasonal variation in temperature and salinity of the boundary inflow corresponding to the Tsushima Current and atmospheric conditions at the sea surface, YOON (1982b) * Institute of Geosciences and Astronomy, The National Defense Academy, Yokosuka 239, Japan. 297
298
Y. SEKINE Table 1. Model characteristics of Japan Sea developed so far
Author
Density stratification
Coastal topography
Bottom topography
Wind
Heat*
Inflow
Yoon (1982a)
Yes
Rectangular
Flat
Constant
Constant
Stationary
Yoon (1982b)
Yes
Realistic
Flat
No
Rectangular
Simplified
Seasonal variation No
Stationary
Yoon (1982c)
Seasonal variation No
Kawabc (1982b)
Yes
Rectangular
Simplified
No
No
Variable
Present study
Yes
Realistic
Realistic
Seasonal variation
No
Stationary
Stationary
* Constant, Seasonal Variation and No mean stationary thermal boundary condition, seasonally varing thermal boundary condition and no consideration on the thermal condition, respectively.
demonstrated that the third branch along the Korean coast is a western boundary current governed by the planetary [3-effect. However, his model failed to simulate the first branch along the Japanese coast because of the exclusion of the bootom slope. YOON (1982C) suggested that the first branch is formed by the topographic effect of the continental slope, the topographic guiding effect along the geostrophic contour f/h where f is the Coriolis parameter and h is the depth. While YOON (1982c) assumed a barotropic model, KAWABE (1982b) examined the branching of the Tsushima Current by use of a two-layer model (see Table 1). He showed that the first branch along the Japanese coast is a bottom-controlled steady current subject to the topographic 13-effect. He also pointed out that the second branch of the Tsushima Current, located offshore of the first branch, is caused by propagation of the lowest two modes of the upper shelf wave which in turn is generated by the significant increase in inflow of the Tsushima Current in summer. However, his model is driven by an inflow and an outflow only, and no wind stress effect on the branching of the Tsushima Current is considered. Historical data (JAPAN METEOROLOGICALAGENCY, 1972; KUTSUWADA and SAKURAI, 1982; KUTSUWADA,1982) depict a clear seasonal variation in wind stress on the Japan Sea and the northern Pacific. These data show a strong northwesterly wind in winter (winter monsoon) blowing from the Siberian high atmospheric pressure. In contrast, wind stress in summer is very weak. Consequently, the wind-driven circulation is mush stronger in winter than in summer. Moreover, a seasonal variation in the wind stress might have a significant influence on the branching of the Tushima Current and the current system of the Japan Sea. In this context, the present paper deals with the seasonal variation of wind driven circulation in the Japan Sea with special reference to the branching of the Tsushima Current. DESCRIPTION OF THE MODEL
Boundary and bottom topography of the Japan Sea (Fig. 1) are simplified so that only the main features are modeled. A two-layer model is assumed. Its schematic representation is displayed in Fig. 2. Because the total depth of the model ocean varies spatially, a quasi-geostrophic approximation can not be assumed. The orthogonal coordinates are
Wind-driven circulation in the Japan Sea
299
SoyaStrait ~Japan Sea01 ~,ugaru Strait
Fig. 1. Schematic representation of the model ocean (left). Solid straight lines show the coastal boundaries. Isopleths of the depth (m) (right). Shallow area is stippled.
•
~
hI
Z
P
UPPERLAYER
LOWER LAYER
Fig. 2. Schematic representation of the two-layer model, p and p + Ap denote the density of upper and lower layer, respectively.
taken as in Fig. 1, in which the angle between x axis (y axis) and eastward (northward) is 36 ° . The basic equations describing the model ocean are derived from the equation of motion and the equation of continuity for a two layer model. Assuming hydrostatic balance, the 13-plane, the rigid lid and the Bousinesq approximation, the final forms of the basic equations are as follows:
300
Y. SEKINF
(i) Vorticity equation for the barotropic mode
at
Ox O
+
{1(
~ u|v,h, + Uy 0
O Oy D Ox
+ ~
po
g
)} o{ (o
O v~h2
+
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Txx
+-~y
Ty
~x
)}
/1~h2 +
u2v2h2
-~x (1)
+ AhVeZ + ~ curl
po
where f=
7.9 x 10-5 + 1 . 0 x 10- ~ s x + 1.74 x 10-13y = f o +
Z=os
~
13~ + 13yy
+~y a4,
ulhl + u2h2 --
vlhl + v2h2 -
Oy
Ox
h~ = H + q , ~ ~ q , H hz = D - H - r l (ii) Shear equation for the baroclinic mode OSu Ot OSv Ot
Ap 811 Po g ~x + f S v + AhV2Su Ap 011 Po g ~ Y - f S u
+ Aj, VZSv
OU 2 Ou~ Oulvl - - + ~ + Ox Oy Ox
OU2V 2
Oy
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+
Tx
Poh 1
~y pohl
(2)
(iii) Continuity equation Oq Ot
--
O h a u z Ohzvz
(3)
.9I- _ _
Ox
Oy
and where S/1 ~ Sv
121
--/12
-~ V l -
V2
Ah = 107 cm 2 s-~, Ap = 2.9
x
10-3 g cm -3
Wind-driven circulation in the
JapanSea
301
The notations used in the above equations have the following meanings: t u,v h q
: : : :
D H Z qb f 72 Subscripts 1,2
: : : : : : :
time. horizontal component of velocity in the x- and y-directions. thickness of the layer. deviation of interface positive downward from the average depth of the interface. total depth of the ocean. averaged depth of the upper layer. relative vorticity. volume transport function. Coriolis parameter. Laplacian operator for the horizontal direction. quantities in the upper and the lower layers, respectively.
Viscous lateral boundaries are set up. The interface displacements at the in- and outflow boundary are given to satisfy tile geostrophic isostacy; an inflow of 2 Sv (1 Sv = 1 × 1012 cm ~ sec -~) corresponding to the Tsushima Current, an outflow of 1.3 Sv of the Tsugaru Warm Current and an outflow of 0.7 Sv of the Soya Current are all assumed to be confined to the upper layer. In the initial state, a flow is confined to the upper layer and it runs along the northern coast of Japan (Fig. 3). Starting from this initial condition, a time variation in the current system is investigated by a numerical time integration of (1), (2) and (3). The grid size is 38 km along x axis and 31 km along y axis. Differencing schemes used are Lilly's scheme for (1) and the Generalized Arakawa scheme for (2) (for detail, see GRAMMELTVZDT, 1969). The leap frog scheme is used fundamentally for the local time change term with a time step of
//
~
//o~now \
/
_r~/zj
..
,o,,o=
2CM_~/5
.
D
~
;ugaru Strait
Japan S e : ~ J
%
'(UI . v I )
Soya Strait
/
t (u2, v2
)
2CM_Z/$
Tsushi~a ~'%~/~/ Strait :~
Pacific Ocean
~-/0.
I*Y$
Fig. 3. Specified volume transport of the inflow corresponding to the Tsushima Current and two outflows corresponding to the Tsugaru Warm Current and the Soya Current (left). Initial velocity fields for the upper layer (top, right) and the lower layer (bottom, right).
JPO
17-]/4
- J
302
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8640 sec. To suppress the computational mode, the Euler backward scheme proposed by MATSUNO (1966) is applied every 40 time steps. In winter, a strong northwesterly wind monsoon prevails, while in summer the wind is very weak (Fig. 4). The average of the wind stress curl over the whole model ocean is negative in winter (Fig. 5), which turns out a dominant anticyclonic flow in the basin (SVERDRUP, 1947). However, from April to July, the average of wind stress curl is
STRE5
\\3"
~
!
N
D$TR[~*S
"
Monthly mean wind stress data, 1958-75 (after JAPANMETEOROLOG,CALAGENCY, 1972; KUTSUWADA and SAKURA1, 1982; KUTSUWADA,1982). No values below 0.05 dyne cm -2 are Fig. 4.
plotted.
-- I0,_~ C'O
2
'E U E
f ~
.
P
"',~"~..
o
v L.
Iy
x b "J -I
U
-~0
Fig. 5.
Mean x- and y-components (zx, Zy) of monthly averaged wind stress and wind stress curl over the whole model domain.
Wind-driven circulation in the Japan Sea
303
positive and generates a cyclonic flow. Since the time change of the wind stress is remarkable, the large time variation in wind driven circulation is expected. A quasi-stationary state is reached in the first phase under the annual mean wind stress. Next, the model is driven with a seasonally varying wind stress obtained by linear interpolation of monthly averages (Fig. 4) for evaluating to what extent the current system varies from season to season. RESULTS
A time integration with the constant annual mean wind stress was carried out for 4000 days. The main results are shown in Figs 6 and 7. Part of the kinetic energy of initial flow along the Japanese coast shifts to the west, and a western boundary current along the western and northern boundaries is formed gradually. The lower layer velocity field (Fig. 7) has a tendency to flow along the geostrophic contour f/h, and is explained as the
5U. D^~'5
)
I ooo.D^~S
? ) 000.[jA15
A
~
4000.[jAT5
B
Fig. 6. (a) Sequential pattern of the volume transport function at days 250, 500, 1000, 2000, 3000 and 4000 in the case of annual mean wind stress. The contour interval of the volume transport function is 0.2 Sv (1 Sv = 1012 cm 3 see-'). (b) Sequential pattern of the upper layer thickness at days 250, 1000 and 4000. The contour interval is 5 m.
31)4
Y.
S,;~,n,~
P
l•Vl )
•
.
•
' •
•
•
•
t
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. • ,
• •
p
l /
(
•
•
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~/~1000, D/,T$
r
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•
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•
l. Vl )
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I
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'
* .
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P
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,
i
/
°" •
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p
i" ~
I
hATS
Velocity fields of the u p p e r (U1, V1) and lower (U2, V2) layers in the case of annual m e a n wind stress. No values bclow 0.05 cm sec ' are plotted.
bottom intensified mode in the two-layer system with sloped bottom (RItINES, 1970; SU~INOHARA, 1981)• This tendency is weak in the velocity field of the upper layer, reflecting the surface intensified mode; the western boundary current dominates and the topographic branch along the Japanese coast is weak. A relatively strong flow in the lower layer is found near the Tsushima Strait. Since the lower layer thickness is remarkably reduced in this region by the shallow sloped bottom and descending interface (see Fig. 6B), this flow is restricted to a thin layer• The barotropic response is very fast, and its characteristic time is about 10 days. But time variation in flow pattern is small after about 1,000 days. In the completely stationary state, the geostrophic isostacy should be reached and the topographic effect on the flow pattern should vanish• A result from a flat bottom Japan Sea model examined by YOON (1982a) showed most of the inflow of the Tsushima Current flow as a western boundary current, and the coastal branch along the Japanese coast is not found in the stationary state. However, it takes more than 4000 days to attain such a geostrophic isostacy in the present model and the topographic effect on the current persists for a long period• If a baroclinic response by a baroclinic Rossby wave with a velocity of 1 cm sec-1 is assumed, it takes about 1000-2000 days for the baroclinic wave to propagate over the entire Japan Sea Basin• Therefore, the delay of the baroclinic response shown in the present model originates in the effect of the bottom topography• The solution at day 4000 with a constant wind stress is used as an initial condition for the case with seasonally varying wind stress, and the time integration is carried out for 11
305
Wind-driven circulation in the Japan Sea
years. A periodic annual change in kinetic energy and entrophy with the highest peak in winter is clearly seen (Fig. 8). Because no variation with periods longer than a year is depicted in the energy variation, the flow pattern in the llth year is examined in the following. Distinct seasonal variations in the total transport function are seen (Fig. 9). In winter, most of the inllow bends northward and a strong western boundary current is formed, whereas the coastal branch along the Japanese coast is very weak. Related to the large negative wind stress curl in this period (Fig. 5), the anticyclonic circulation is especially intensified in January. On the other hand, in summer, most of the inflow water flows along the southern boundary and the western boundary current is weak. The inner circulation developed in winter is weak because of a weak wind stress during this season. The disappearance of the western boundary current is related to the positive wind stress curl in late spring to summer (Fig. 5); the positive wind stress curl induces northward Sverdrup transport and the western boundary current flows southward. Therefore the southward western boundary current flow cancels the northward western boundary current formed by the inflow water through the Tsushima Strait. In winter, a large horizontal gradient of the interface depth is found, whereas the gradient is relatively small in summer (Fig. 10). The pronounced tilt of the interface along the northern boundary arises from convergence in the lower layer (upwelling of the interface owing to the strong northwesterly winter wind). >~
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.
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~b.
HUNDREd]5 )
'
sb
.i -0
0
• ab. Days
' 2b. ' ~b. " (IN MUNOREnS
,b. )
"
Fig. 8. Time change of the total kinetic energy defined by ff~(ht(u] + v~) + h-,(u~ + v2)ds/2ff, ds where s is the surface area of the model (upper left), the enstrophy defined by ff~DZ2ds/ffsds (bottom left), the total k net c energy of the upper layer defined by ff~h~(u'~ + v~)ds/2ff~ds (upper right) and the total kinetic energy of the lower layer defined by • fYsh2(u2 + v2)ds/2ff~ds (bottom right).
sb.
306
Y. SEKINE
A~.
.
MONIM
MJJ~.
e~ON1 t l
rO~H
•
.
MOI~| N
/¢OV.
MONTN
~f'e.
AUG.
DEC .
Fig. 9. Sameas Fig. 6 (a) but for seasonallyvaryingwindstress. The volumetransport function at day 15 in each month is plotted.
Because the difference in the specific volume between the upper layer 1/p and the lower layer 1/(p + Ap) yields sea level increase (decrease), if the ratio of the upper layer and lower layer thicknesses changes, the increase in the upper layer thickness is proportional to the increase in sea level. The seasonal variation in the calculated upper layer thickness is compared with the observed sea level variation. Data are taken from Tidal Record (1984) published by Geographical Survey Instutute of Japan, and Monthly and Annual Mean Heights of Sea Level (1976) published by PSMSL. The calculation shows similar trends to the observations from winter to spring, but a prominent peak in the observed sea level is seen in summer, while the calculation predicts a minimum. This discrepancy in summer season is most probably due to the surface heating, which is not considered in the calculation. The similar tendency from winter to spring is mainly due to the situation that the deepening of the mixed layer gives good approximation of the two layer model to the actual oceanic condition. A high correlation exists between the calculated upper layer thickness and observed sea level in the south such as Hagi and Ulsan, but the correlation is relatively low in the north (Fig. 11). The latter fact indicates
Wind-driven circulation in the Japan Sea
~0NIM
307
JAN.
•
°
.
.
Fig. 10. Same as Fig. 6 (b) but for seasonallyvarryingwind stress. The upper layer thickness at day 15 in each month are plotted.
an important role of the thermohaline effect on the oceanic condition in the northern part of the Japan Sea. Although there is neither inflow nor outflow in the lower layer, a current is induced near the inflow region and it flows along the isopleths of the depth (Fig. 12). The motion in the lower layer is most remarkable in January, as shown by the strong peak of the kinetic energy variation (Fig. 8). There is clear coherence of the kinetic energy variation between the upper layer and lower layer (see Fig. 8), so, the barotropic response is more important to the seasonal response of the current system in the Japan Sea. This is mainly due to the rapid change in wind stress in comparison with a baroclinic time scale longer than one year, as discussed in the case of stationary wind stress (Figs 6 and 7). A strong western boundary current is found in winter, and it splits into two paths at the midpoint of the northern boundary (see upper layer velocity field in Fig. 12). One split path leaves the northern boundary and runs zonally toward the Tsugaru Strait. The other split path exists along the northern boundary without separation and its outflow is made at the Soya Strait. In contrast, the different upper layer flow pattern is seen in late spring to summer; most of the inflow water flow along the southern boundary, and the western boundary current is very weak. In late summer, the flow along the southern boundary leaves the coastal region and the main current axis shifts offshore region. In summer, the
Y. SEKINE
308
HAGI IK
OBS.
~z °
~-,c ,:-x
~ i C l
flc Ocean
r FMAMJ J A$ONO
=l ULSAN
. I
CAL.
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TAJIRI
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d
t 9
5
..........
I
I
;
O
-,
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.,'~
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FMAMJ JA$OND
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o -
o 2
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~- -
CAL.
-¢a~l
I FMAMJ JA$ON
s
x
.,,~
J A$OND
)
l5
CAL.
,I, FMAMJ
I
ASOfl 0
J FMAMJ J ASONC)
JFMAMJJA$OND
Fig. 11. Comparison of the observed sea level variation at seven observational points shown in left top and seasonal variation in calculated upper layer thickness. The variation around the annual mean value is plotted.
central branch is most remarkable and the western boundary current is weak in comparison with other season (Fig. 13). The appearance of the central branch in the present model coincides with the observational feature examined by KAWABE (1982a). Although KAWABE (1982a) concluded that the central branch is due to the increase in volume transport of inflow of the Tsushima Current during the summer, the present study indicates that the central branch is partly due to the wind-driven circulation in summer; the volume transport of the inflow is assumed to be constant in the present model. In this way, the wind-driven circulation has a significant influence on the branching of the Tsushima Current and the total current system in the Japan Sea.
309
Wind-driven circulation in the Japan Sea
cUl. Vl)
2¢,~/S
.'J/.''"
°" "
lUl'Vl
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.
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"
;i:i .. ..-
,~,
5 ,°2.,2,
" ,,,2" ~~ili
" "°'Jl
2:1 ......
Same as Fig. 7 but for seasonally varying wind stress. The velocity vectors at day 15 in odd numbered months are plotted.
D I S C U S S I O N AND S U M M A R Y
Wind-driven circulation in the Japan Sea has been investigated numerically by use of observed wind stress data and, in a first phase, a two-layer primitive model with a simplified bottom and coastal topography is driven by the annual mean wind stress. The characteristic time of the response of the barotropic Rossby wave to the stationary wind stress in the Japan Sea is about 10 days, smaller than the time scale of seasonal change in the wind stress. In contrast, the baroclinic response takes a very long time. The topographic effect of the bottom slope still persists in a quasi-stationary state in the last period of time integration of 4000 days. Two predominant branches of the inflow of the
310
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Tsushima Current exist; one is the topographic branch along the southern boundary and the other branch is associated with the westward intensification along the northern boundary. This result differs from that of a flat bottom model of ¥OON (1982a), which gives a baroclinic response time of about 1000-2000 days. In the second phase, a seasonally varying wind stress obtained by a linear interpolation of monthly mean data allows delineation of the seasonal variation in the wind-driven circulation. A clear annual cycle of the current system is depicted. The main results of the seasonal variation in the wind-driven circulation are as follows: (1) In winter, most of the inflow water of the Tsushima Current bends northward and an intense western boundary current accompanied by an inner circulation is formed, whereas the topographic branch along the southern boundary is very weak. This is due to the strong wind stress and/or negative wind stress curl. (2) In spring to summer, the western boundary current is weak, but the topographic branch along the southern boundary is intensified. This branch is stongest in late spring and moves offshore in summer. A second central branch observed in summer (KAwABE, 1982a) iS successfully simulated.
Wind-drivcn circulation in the Japan Sea
311
(3) The intensification of the western boundary current in winter is brought about by the strong monsoon wind stress in this season. Because the barotropic response is accomplished within half a month, the strong wind stress forms the strong western boundary current in a short time. (4) The intensification of the topographic branch along the southern boundary is mainly attributed to weak wind stress with positive curl. The positive wind stress curl generates cyclonic Sverdrup circulation with a western boundary current flowing southward, opposite to the direction of western boundary current originated by the inflow of the Tsushima Current. Therefore, most of the inflow comes along the southern region of the basin. One shortcoming of the present study is that by dealing with a two-layer model ocean, it ignores the frontal structure of the Tsushima Current. Although recent satellite imagery shows various disturbances in temperature field along the frontal region of the Tsushima Current (e.g., TOBA, HANAWA, KAWAMUKA, YANO and KURASAWA, 1985), the frontal structure cannot be properly handled with the layer model. A level model should be developed. No seasonal variation of the inflow of the Tsushima Current was taken into account in our model. Although the volume transport and the current velocity of the inflow of the Tsushima Current is not yet well-known, historical data show the existence of its seasonal variation (MIYAZAKI,1952; YI, 1966; MHTA,1976). In addition, no effect of the thermohaline circulation in the Japan Sea on the branching of the Tsuhima Current was considered. The thermohaline circulation, which is important for the generation of the Japan Sea bottom water, also might be important. Improving the model in these respects will be important to understand better the total current system of the Japan Sea. Acknowledgments--The author wishes to thank Professor K. Takano for his critical reading of the manuscript and valuable comments. The author also thanks Professors Y. Toba of Tohoku University and N. Suginohara of University of Tokyo and Dr. J. H. Yoon for many fruitful discussions. Thanks are extended to Dr. K. Kutsuwada and Mr. T. Andoh for providing the wind stress data.
REFERENCES GEOGRAPHICALSURVEYINSTITUTEOFJAPAN (1984) Tidal Record B2 No. 40. GRAMMELTVEDT,A. (1968) A survey of finite difference scheme for the primitive equations for a barotropic fluid. Monthly Weather Review, 97,384-404. JAPAN METEOROLOGICALAGENCY(1972) Marine meteorological study of the Japan Sea. Technical Report of the Japan Meteorological Agency, 80, 1-161. KAWABE,M. (1982a) Branching of the Tsushima Current in the Japan Sea. Part I Data analysis. Journal of Oceanographical Society of Japan, 38, 95-107. KAWABE,M. (1982b) Branching of the Tsushima Current in the Japan Sea. Part lI. Numerical experiment. Journal of Oceanographical Society of Japan, 38, 183-192. KAWAI, H. (1974) Transition of current images in thc Japan Sea, In: The Tsushima warm current--ocean structures and fishery, Edited by Fishery Society of Japan, Koseisha-koseikaku, pp. 7-26 (in Japancsc). KUTSUWADA,K. (1982) New computation of wind stress over the North Pacific Ocean. Journal Oceanographical Society of Japan, 38, 159-171. KUTSUWADA,K. and K. SAKURAI(1982) Climatological maps of wind stress field over the north Pacific Ocean. Oceanographical Magazine, 32, 25-46. MATSUNO,T. (1966) Numerical integrations of the primitive equations by a simulated backward difference method. Journal of Meteorological Society of Japan, 44, 76-84. MIITA, T. (1976) Current characteristics measured with current meters at fixed stations. Bulletin of Japan Society Fish. Oceanography, 28, 33-58 (in Japanese). MIYAZAKI, M. (1952) The heat budget in the Japan Sea. Report of Hokkaido Regional Fishery Research Laboratory, 4, 1-45 (in Japanese).
3 12
Y. S[~KJNE
RmNI~S, P. B. (1970) Edge-, bottom- and Rossby waves in a rotating stratiticd fluid. Geophysical Fluid Dynamics, 1,273--302. PI-RMANI-NTSERVICE for MEAN SEA LEVEl. (1976) Monthly and Annual Mean Height of Sea Level. SuDA, K. and K. HIOAKA (1932) The results of the oceanographical observations on board R.M.S. Syumpu Maru in the southern part of the Japan Sea in the summer of 1929, Part i. Journal Oceanographical Imperial Marine Observation, 3, 291-375 (in Japanese). SI.I(ilN()IIARA, N. (198(]) Effects of bottom topography and density stratilic~ltion on the form~ltion of western boundary currents. Journal of Oceanographical Society of Japan, 35,224-232. SUGINOIIARA, N. (1981) Quasi-geostrophic waves in a stratified ocean with bottom topography. Journal of Physical Oceanography, 11, 107-115. SVERDRUP, H. U. (1947) Wind-driven currcnts in a baroclinic ocean: with application to the equatorial currents of thc castcrn Pacific. Proceedings of the National Academic of Science, U.S.A., 33,318-326. TOI~A, Y., K. HANAWA,H. KAWAMURA,Y. YANO and Y. KURASAWA(1985) Horizontal processes involved in the formation of sea surface temperature near a western boundary current. In: The ocean surface. Y. TOgA and H. MITSUYASU,editors, D. Reidcl Publishing Company, 571-576. YI, S. (1966) Seasonal and secular variations of lhe ,.V~llcrvolume tral'tsport across the Korea SIr~lil. JOlll'll{tl()/ Oceanographical Society of Korea. I, 7-13. YooN, J. H. (1981a) Numerical expcrimcnt on the circulation in the Japan Sea Part I. Formation of thc cast Korc~m warn] current. Jottrnal ()1"Oce,nogrtq~hical Society of Jtq)an, 38, 43-51. Yoo~r, J. H. (1981b) Numerical experiment on the circultaion in the Japan Sea Part II. Influence of seasonal variations in atmospheric conditions on the Tsushima Current. Journal of Oceanographical Society of Japan, 38, 81-94. YOON, J. H. (1981c) Numerical experiment on thc circulation in the Japan Sea Part III. Mechanism of nearshore branch of the Tsushima Current. Journal of Oceanographical Society of Japan, 38, 125-130.