Impact of Upper Ocean–Topographical Coupling and Isopycnal ... · include realistic bottom...

OCTOBER 2000 2535H O G A N A N D H U R L B U R T

Impact of Upper Ocean–Topographical Coupling and Isopycnal Outcropping inJapan/East Sea Models with 1⁄88 to 1⁄648 Resolution*

PATRICK J. HOGAN AND HARLEY E. HURLBURT

Naval Research Laboratory, Stennis Space Center, Mississippi

(Manuscript received 3 November 1998, in final form 23 November 1999)

ABSTRACT

A regional primitive equation ocean model is used to investigate the impact of grid resolution, baroclinicinstability, bottom topography, and isopycnal outcropping on the dynamics of the wind and throughflow-forcedsurface circulation in the Japan/East Sea. The results demonstrate that at least 1⁄328 (3.5 km) horizontal gridresolution is necessary to generate sufficient baroclinic instability to produce eddy-driven cyclonic deep meanflows. These abyssal currents follow the f/h contours of the bottom topography and allow the bottom topographyto strongly influence mean pathways of the upper-ocean currents in the Japan/East Sea. This upper ocean–topographical coupling via baroclinic instability (actually a mixed baroclinic–barotropic instability) requires thatmesoscale variability be very well resolved to obtain sufficient coupling. For example, 1⁄328 resolution is requiredto obtain a realistic separation latitude of the East Korean Warm Current (EKWC) from the Korean coast whenHellerman–Rosenstein monthly climatological wind stress forcing is used. Separation of the EKWC is morerealistic at 1⁄88 resolution when the model is forced with climatological winds formed from the ECMWF 10-mreanalysis due to strong positive wind stress curl north of the separation latitude, but at 1⁄88 the level of baroclinicinstability is insufficient to initiate upper ocean–topographical coupling. Hence, this major topographical effectis largely missed at coarser resolution and leads to erroneous conclusions about the role of bottom topographyand unexplained errors in the pathways of current systems. Results from a 1⁄648 simulation are similar to thoseat 1⁄328, particularly where the EKWC separates from the Korean coast, suggesting statistical simulation con-vergence for mesoscale variability has been nearly achieved at 1⁄328 resolution. Isopycnal outcropping and as-sociated vertical mixing provide an alternate mechanism to topographical control in developing and maintaininga boundary current along the west coast of Japan, but are less important than baroclinic instability in drivingdeep mean flows.

1. Introduction

Located between Japan, Korea, and Russia, the Ja-pan/East Sea (hereafter referred to as the JES) is apart of the chain of adjacent marginal seas in thenorthwest Pacific. With a maximum depth of about3700 m, the JES has been characterized as a micro-cosm of the global ocean (Ichiye 1984). Indeed, large-scale circulation features similar to those found in theglobal ocean include cyclonic and anticyclonic gyresystems, a western boundary current that separatesfrom the coast, and a deep homogeneous water mass.Other interesting phenomena include wind and ther-mohaline driven circulation, deep convection, and a

* Naval Research Laboratory Contribution Number NRL/JA 7323-98-0063.

Corresponding author address: Dr. Patrick J. Hogan, Naval Re-search Laboratory, Stennis Space Center, MS 39529-5004.E-mail: [email protected]

ubiquitous mesoscale eddy field. Because the JEScontains processes similar to those occurring in large-scale ocean basins, but is smaller and may adjust morerapidly, and hence be easier to study, the JES offersocean scientists a unique natural laboratory for in-vestigating oceanic processes. For the same reasons,the JES is an ideal basin for studying dynamics vianumerical simulations. The region is characterized bycomplex circulation patterns and mesoscale variabil-ity, yet is small enough for multiple simulations to beeconomically feasible, including some with very highhorizontal grid resolution. This feasibility is furtherenhanced by using the Naval Research Laboratory’sLayered Ocean Model (NLOM), a modular design thatprovides an effective means to dissect ocean dynamics(Hurlburt et al. 1996) and one that is extremely ef-ficient and portable to several different computer ar-chitectures (Wallcraft and Moore 1997).

Inflow through Tsushima Strait and outflow throughthe Tsugaru and Soya Straits (Fig. 1) couples the JESto the Pacific Ocean. These straits have shallow silldepths; the deepest sill depth (about 200 m) is in the

2536 VOLUME 30J O U R N A L O F P H Y S I C A L O C E A N O G R A P H Y

FIG. 1. THE 1⁄328 bottom topography for the Japan/East Sea. Locations of the inflow and outflow ports are markedas black arrows. Geographical locations used in text are marked. Contour interval is 250 m.

western channel of Tsushima Strait. However, flowthrough these straits has a profound impact on the cir-culation features within the JES. In particular, a fun-damental circulation feature in the JES is the TsushimaWarm Current (TWC), which transports warm salty wa-ter into the basin through Tsushima Strait. The historicalinterpretation of Suda and Hidaka (1932) and Uda(1934) that the TWC splits into three distinct branchesafter entering the basin is still widely accepted, but oth-ers (Moriyasu 1972) have regarded the TWC as a single

meandering current that episodically sheds eddies. Thehighly variable spatial and temporal characteristics ofthe TWC make it difficult to support one theory overthe other. Indeed, hydrographic surveys (Kawabe 1982;Kim and Chung 1984), analyses of satellite infrared data(Kim and Legeckis 1986; Cho and Kim 1996), and anal-ysis of satellite-tracked drifters (Beardsley et al. 1992)demonstrate the complex and chaotic behavior of thiscurrent system.

In contemporary literature, the three branches of the


FIG. 2. Observationally based schematic patterns of surface circulation in the Japan/East Sea (a) in summer by Naganuma (1977)and (b) annual mean by Yarichin (1980). The following currents are labeled: East Korea Warm Current (EKWC), Nearshore Branch(NB), Liman Cold Current (LCC), and North Korea Cold Current (NKCC). Warm (cold) rings are denoted by a ‘‘W’’ (‘‘C’’). FromPreller and Hogan (1998).

TWC (from east to west) are named the NearshoreBranch (NB), the Offshore (or Middle) Branch, and theEast Korea Warm Current (EKWC), respectively. In thethree-branch view, the NB flows eastward along thecoast of Honshu and exits into the Pacific Ocean throughTsugaru Strait (Fig. 2). The Offshore Branch, which ismore variable in space and time, is situated to the west-northwest of the Nearshore Branch and flows alongHonshu farther offshore. The EKWC flows northwardalong the Korean continental slope to about 378–388N,where it meets the southward flowing North KoreanCold Current (NKCC). At the confluence, the currentsseparate from the coast and flow east-northeast towardTsugaru Strait along the polar front. Much of that flowexits through Tsugaru Strait, while the remainder con-tinues northward along the coast of Hokkaido as theTsugaru Warm Current. Some of this flow exits into theSea of Okhotsk through Soya Strait, while the remaindereventually returns southward along the Siberian coastas the Liman (or Primoriye) Cold Current (LCC) andthe NKCC (south of Vladivostok), forming a cyclonicgyre in the northern part of the basin (Fig. 2).

A primary objective of this study is to identify thecritical dynamical processes responsible for the ob-served and modeled circulation patterns in the JES. For

instance, what mechanism determines the separation lat-itude of the EKWC from the coast of Korea? Whatcauses the formation and persistence of the NB of theTWC? What role does the bottom topography play ininfluencing the surface circulation? Of course the abilityto address questions such as these depends on the re-alism of the numerical model being used to investigatethe dynamics. The realism of any given simulation isdependent on many criteria, including choice of modelphysics, numerical implementation of the equations ofmotion, the choice of forcing functions and parameterspace, and model grid resolution. For the simulationsused in this study, the numerical implementation of theequations of motion is identical for all simulations (seeHurlburt and Thompson 1980; Wallcraft 1991; Hurlburtet al. 1996; Moore and Wallcraft 1998). The modelphysics are varied only to the extent that they are usedto explore the parameter space for the simulations. Theprimary forcing functions are inflow and outflowthrough the straits and monthly climatological windstress. Nearly all of the simulations are forced with sea-sonally varying inflow and outflow through the straitsand the Hellerman and Rosenstein (1983, hereafter HR)monthly wind stress climatology, although that windclimatology may not give the best results in the JES.


Indeed, a simulation forced with a different wind set isoffered to accentuate this point. However, because weare interested in the dynamics of the circulation, a lessthan optimal wind set can actually facilitate this taskbecause systematic changes between different solutionscan be easier to identify.

The overall strategy employed in this study is to ex-ploit the modularity of the NLOM to investigate thecirculation dynamics in the JES. In this context we ex-amine the solutions from an ensemble of simulationswith increasing (dynamical) complexity by including orexcluding certain features and dynamical processes. Thesimplest end-member in this ensemble is a linear 1.5-layer reduced-gravity design, while the most complexis a fully nonlinear 4-layer design with realistic bottomtopography. Information on the model design is dis-cussed in section 2 and specific details of the modelimplementation for the JES are presented in section 3.In section 4 the lowest order dynamics of the JES areinvestigated with a series of linear 1⁄88, 1.5-layer reduced-gravity simulations. In section 5 we examine the sen-sitivity to external forcing, as well as the impact ofhigher order dynamics, such as nonlinearity and iso-pycnal outcropping. The impacts of horizontal grid res-olution, mesoscale flow instabilities, and realistic bot-tom topography are discussed in section 6. In these sim-ulations, horizontal grid resolution varies from 1⁄88 (14km) to 1⁄648 (1.7 km), which allows the role of upperocean–topographical coupling to be examined in detail.

Finally, the summary and conclusions are presented insection 7.

2. The NRL Layered Ocean Model (NLOM)

The NLOM is a semi-implicit primitive equationocean model with vertically integrated model equationsfor each layer. In the NLOM, the layered model equa-tions are cast in transport form and (in the version usedhere) the interfaces between the layers are isopycnalsurfaces. The model retains the free surface and caninclude realistic bottom topography as long as it is con-fined to the lowest layer. The fundamental model designis described in detail in Hurlburt and Thompson (1980),although Wallcraft (1991) has significantly enhanced thecurrent version. The mathematical and numerical for-mulation of the equations in spherical coordinates arediscussed in Moore and Wallcraft (1998), with particularattention given to the proper formulation of the isopyc-nal diffusion. The simulations for this study use a por-table, scalable version of the NLOM (Wallcraft andMoore 1997), which runs efficiently and interchange-ably on a wide variety of computer architectures usinga tiled data parallel programming paradigm.

The equations for the n-layer finite depth, hydrody-namic model are given below for layers k 5 1, · · · , nwith k 5 1 for the top layer. In places where k is usedto index model interfaces, k 5 0 is the surface and k5 n is the bottom:

]U 1 ](U u ) ](V u cosu)k k k k k1 1 2 V (u sinu 1 aV sin2u)k k[ ]]t a cosu ]f ]u

5 1 max(0, 2v )u 1 max(0, v )u 2 (max(0, 2v ) 1 max(0, v ))uk21 k21 k k11 k k21 k

n ](h 2 H )h j jk1 max(0, 2C v )(u 2 u ) 1 max(0, C v )(u 2 u ) 2 GOM k21 k21 k M k k11 k kja cosu ]fj51

2](h e cosu) ](h e cos u)A k ff k fuk k1 (t 2 t )/r 1 1 (1)f f ok21 k 2 2 [ ]a cos u ]f ]u

]V 1 ](U y ) ](V y cosu)k k k k k1 1 1 U (u sinu 1 aV sin2u)k k[ ]]t a cosu ]f ]u

5 1 max(0, 2v )y 1 max(0, v )y 2 (max(0, 2v ) 1 max(0, v ))yk21 k21 k k11 k k21 k

n ](h 2 H )h j jk1 max(0, 2C v )(y 2 y ) 1 max(0, C v )(y 2 y ) 2 GOM k21 k21 k M k k11 k kja ]uj51

2](h e cosu) ](h e cos u)A k fu k uuk k1 (t 2 t )/r 1 1 (2)u u ok21 k 2 2 [ ]a cos u ]f ]u

]hk 2 2ˆ1 = · V 5 v 2 v 2 K ¹ [¹ (h 2 H )], (3)k k k21 4 k k]t


where1 ]F 1 ](F cosu)f u= · F 5 1

a cosu ]f a cosu ]u

21 ] F 1 ] ]F2¹ F 5 1 cosu

2 2 2 2 1 2a cos u ]f a cosu ]u ]u

h 5 kth layer thicknessk

v 5 kth layer velocity 5 eW u 1 eW yk f k u k

V 5 h v 5 eW U 1 eW Vk k k f k u k

eW 5 angular deformation tensork

] u ] yk ke 5 2 cosu 5 2eff uuk k1 2 1 2]f cosu ]u cosu

] y ] uk ke 5 1 cosu 5 efu ufk k1 2 1 2]f cosu ]u cosu

H 5 kth layer thickness at restk

n21

H 5 D(f, u) 2 HOn jj51

D(f, u) 5 total depth of the ocean at restr 5 constant reference density0

r 5 kth layer density, constant in space and timek

g j $ kG 5kj 5g 2 g(r 2 r )/r j , kk j o

t 5 wind stressw

C 5 coefficient of interfacial frictionk

C 5 coefficient of bottom frictionb

t , for k 5 0wC r |v 2 v |(v 2 v ),k o k k11 k k11t 5k for k 5 1, · · · , n 2 1C r |v |v , for k 5 n b o n n

0 for k 5 0, nv 5k 1 25v 2 v 2 W v for k 5 1, · · · , n 2 1k k k k

1 1 1 2v 5 v [max(0, h 2 h )/h ]k k k k k

2 2 1 2v 5 v [max(0, h 2 h )/h ]k k k k k

1 2v 5 (v 2 v )/ Wk k k k

v 5 kth interface reference diapycnal mixingk

velocity1h 5 kth layer thickness at which entrainmentk

starts2h 5 kth layer thickness at which detrainmentk

starts

W (f, u) 5 kth interface weighting factor for globalk

diapycnal mixing designed to conservemass within a layer in compensation for

1explicit diapycnal mixing due to h , hk k

1 2(i.e., v 2 v ), and net transport throughk k

the lateral boundaries of layer k

C 5 coefficient of additional interfacial frictionM

associated with entrainment

A 5 coefficient of isopycnal eddy viscosity

X(f, u) 5 region wide area average of X

K 5 coefficient of biharmonic horizontal layer4

thickness diffusivity

a 5 radius of the Earth (6371 km)

22g 5 acceleration due to gravity (9.8 m s )

t 5 time

V 5 angular rotation rate of the earth25 21(7.292 205 3 10 s )

u 5 latitude

f 5 longitude.

A hydrodynamic reduced-gravity model with n activelayers has the lowest layer infinitely deep and at rest,that is, vn11 5 0, hn11 5 `, and =hn11 5 0. The modelequations for the active layers are identical to those forn-layer hydrodynamic finite depth model, except that

Hn 5 const

Gkl 5 g(rn11 2 rk)/r0, l # k

Gkl 5 g(rn11 2 rl)/r0, l . k

t k 5 t w, k 5 0

t k 5 Ckr0|vk 2 vk11|vk 2 vk11), k 5 1, · · · , n

vk 5 0, k 5 0

vk 5 max(0, ) 2 max(0, ) 2 hk ,1 2v v vk k k

k 5 1, · · · , n

The model equations were integrated on a C-grid(Mesinger and Arakawa 1976) using second-order, cen-tered finite differences in space. The integrations in timeused an explicit numerical scheme for the reduced-grav-ity simulations and a semi-implicit scheme for the finitedepth simulations. For the finite depth simulations, theexternal and internal gravity waves are treated semi-implicitly in a manner similar to that described by Kwi-zak and Robert (1971). This treatment allows a longertime step than required for primitive equation modelsthat use an explicit free surface or a rigid lid. Overall,the model is up to 100 times more efficient (in termsof central processing time per model year for a givendomain and horizontal resolution) than existing fixed-level primitive equation ocean models (Wallcraft 1991).


An important attribute of the NLOM is the ability toallow isopycnal outcropping via ventilation of modelinterfaces. This process, called hydromixing (Wallcraft1991), prevents nonpositive layer thickness, allowsoverturning cells in the vertical, and permits the exis-tence of relatively thin layers. In practice, when a layerbecomes thinner than a specified thickness , fluid is1hk

entrained into it from the layer below, although thereis no net transfer of fluid between layers. Instead, ifthere is local entrainment or detrainment or net transportthrough the lateral boundaries within a layer, the netmass flux from these processes is instantaneously bal-anced by domainwide vertical mixing, distributed ac-cording to the weight function Wk(f, u). The mixingvelocity, vk, in hydromixing is relatively simple anddepends only on the net mass flux just described; thelayer thickness; a reference vertical mixing velocity,

; a layer thickness at which to start entrainment, ;1v hk k

a layer thickness at which to start detrainment, (typ-2hk

ically deactivated by setting to a large value); and theweight function, Wk(f, u). In the simulations for thisstudy, Wk 5 1, a uniform distribution, although moresophisticated forms have been used in global versionsof the NLOM with considerable success (Shriver andHurlburt 1997).

Hydromixing is an important aspect of the NLOMdesign since it allows layers to outcrop via interfaceventilation. However, this design and restricting the bot-tom topography to the lowest layer also enhances modelefficiency because positive layer thickness is maintainedfor all layers everywhere, a requirement for the modaldecomposition used in the semi-implicitization of ex-ternal and internal gravity waves. From a dynamicalpoint of view, once a layer has reached the thicknessof , it is dynamically inactive over the outcropped1hk

region in the sense that the resulting uniformly thicklayer can no longer contribute to a pressure gradient.The impact of isopycnal outcropping on the circulationin the JES is discussed in more detail in section 5.

The NLOM has been used to investigate ocean dy-namics in both regional and global models. Hurlburt etal. (1996) performed a series of simulations of the Pa-cific Ocean, including the JES, which demonstrated theimportance of baroclinic instability and realistic bottomtopography for simulating the Kuroshio/Oyashio andKuroshio Extension. Most of their simulations includedsix active layers with a relatively thick surface layer(135 m). That layer structure was optimal for the PacificOcean because it allowed the formation of a realisticequatorial undercurrent, but was too thick in the JES topermit the formation of a robust NB via interface ven-tilation (Hurlburt et al. 1996). In addition, the horizontalgrid resolution (1⁄88, or about 14 km) was apparently toocoarse to achieve the upper ocean–topographical cou-pling needed for realistic separation of the EKWC, al-though that resolution was sufficient for the couplingmechanism to occur in the region of the Kuroshio. Hurl-burt and Metzger (1998) reported that 1⁄168 resolution

was required to simulate the bifurcation of the Kuroshioat the Shatsky Rise by way of upper ocean–topograph-ical coupling. They found that this coupling via baro-clinic instability and particular features of the ShatskyRise topographic complex were essential to the bifur-cation and the existence of the northward branch eventhough the simulated Kuroshio never directly impingedon the topography. Of particular interest to this studyis to determine whether the upper ocean–topographicalcoupling described in Hurlburt et al. (1996) and Hurl-burt and Metzger (1998) can affect the separation lat-itude of the EKWC and other features of current systemsin the JES, which is characterized by a significantlydifferent flow regime with volume transports typicallyan order of magnitude less than those observed in theKuroshio.

Others have used layered models in the JES with somesuccess. Seung and Kim (1997) used a version of theMiami Isopycnal Coordinate Ocean Model (Bleck et al.1992) to investigate the renewal time of the upper in-termediate water. Their results indicated particularlystrong mixing and isopycnal outcropping offshore fromthe Siberian coast during winter. These results were inagreement with those of Seung (1997), who applied theventilation theory developed by Luyten et al. (1983) tothe JES. Seung and Kim (1995) used a 2.5-layer, re-duced-gravity model that included throughflow forcingand/or analytic wind stress curl. When buoyancy fluxvia a mixed layer was included, the intermediate layeroutcropped north of the polar front, causing cycloniccirculation in that region and southward displacementof the separation latitude of the EKWC. Vasilev andMakashin (1992) developed a transport streamfunctionmodel to show the impact of deep convection on theventilation of deep water in the northern JES.

3. Design of the model simulations

Implementing the NLOM for the JES region was rel-atively straightforward. The model domain extends from34.258 to 498N. The model domain and bottom topog-raphy are shown in Fig. 1. Unvaried model parametersare given in Table 1. Several simulations were performedto determine the minimum number of layers to includeand to determine the sensitivity of the model solutionsto various combinations of layer thickness. These results(not shown) demonstrated the need for at least four layers,with interface depths of 60 m, 135 m, and 250 m, re-spectively (Table 1). The relatively thin top layer (60 m)allows isopycnal outcropping via interface ventilation tooccur over much of the basin, which, as discussed insection 5, is critical for robust simulation of the NB alongthe coast of Honshu. The depth of the second interface(135 m) is chosen to coincide with the approximate silldepths of the inflow/outflow straits, and also plays a rolein the dynamics associated with isopycnal outcropping.The depth of the third interface (250 m) approximatesthe mean depth of the thermocline which separates the


TABLE 1. Unvaried model parameters.

Parameter Value(s) Definition

Cb 2 3 1023 Bottom drag coefficientCk 0 kth interfacial stress coeffi-

cientCm 1 Coefficient of additional in-

terfacial friction associatedwith entrainment

g 9.8 m s22 Acceleration due to gravityh1

k 50 m (k 5 1)40 m (k 5 2, 3)

Thickness of layer k atwhich entrainment starts

hk2 Deactivated Thickness of layer k at

which detrainment starts(deactivated by makingvery large)

rk11 2 rk 0.8/0.32/0.42 kg m23 Stratification between layersk and k 1 1

k

HO ii51

60/135/250/bottom (m) Mean depth at the bottom oflayer k

vk 0.04 cm s21 kth interface reference verti-cal mixing velocity

warm saline surface waters from the cool homogeneousdeep water. In the context of water masses, the top threelayers approximate the location of the surface and inter-mediate water, while the abyssal layer represents the Ja-pan Sea Proper Water. The maximum depth in the modeldomain is 3750 m.

Communication with the Pacific Ocean occurs viainflow through Tsushima Strait and outflow through theTsugaru and Soya Straits at the model boundaries. Areview of the literature (see Preller and Hogan 1998)indicates significant variability in the distribution andamplitude of flow through the straits (e.g., Miyazaki1952; Yi 1966; Toba et al. 1982). Here, inflow throughTsushima Strait is divided into two separate inflow ports,one each for the western and eastern channels of Tsu-shima Strait. The mean volume transport of the totalinflow is 2.0 Sv (Sv [ 106 m3 s21), but a seasonal signalis superimposed such that the maximum inflow and out-flow of 2.66 Sv occurs in July and the minimum of 1.34Sv occurs in January (33% peak deviation from themean). At any given time the inflow is segmented sothat 75% enters through the western channel and 25%through the eastern channel. Vertically, two-thirds ofthe total inflow (for both channels) enters the basinthrough layer 1, while the remaining one-third entersthrough layer 2. The straits are closed in layers 3 and4. Inflow through Tsushima Strait is instantaneously andexactly balanced by outflow through the Tsugaru andSoya Straits (the Tartar Strait is neglected) using a mod-ified Orlanski (1976) boundary condition. The outflowis distributed such that two-thirds of the flow exitsthrough Tsugaru Strait and the remaining one-thirdthrough Soya Strait. The vertical distribution for theoutflow is the same as for the inflow (⅔ exits throughlayer 1, ⅓ through layer 2). These boundary conditionsare not based on any single observation or dynamical

calculation, but instead represent a compromise for thewide range of limited observations cited in the literature.

A modified version of the 1⁄128 ETOP05 bottom to-pography (National Oceanic and Atmospheric Admin-istration 1986) was used to represent the coastline ge-ometry and bottom topography. The maximum depthwas set to 3750 m and the minimum depth at 200 m(near the shelf break). As such, the 200-m isobath wasused as the model boundary. For the JES, this is a rea-sonable first approximation as much of the basin is char-acterized by narrow continental shelf regions, and shelfdynamics are quite different than those in deep water.The ETOP05 bottom topography was first interpolatedto 1⁄328, then modified for better agreement with detailedtopographic charts (Fig. 1). The 1⁄88 and 1⁄168 versions ofthe model geometries were formed by subsampling the1⁄328 model grid, and the 1⁄648 model geometry was formedby interpolation from the 1⁄328 model grid. To reduce thegeneration of energy at scales that are poorly resolvedby the model, the topographic depths are smoothedtwice using a center-weighted nine-point real smoother.

Most of the simulations used in this study are forcedwith the HR monthly wind stress climatology. The in-dividual wind stress components from this climatologyhave a native grid resolution of 2.08 and were inter-polated to the various model grid resolutions with cubicspline interpolation. To dampen the presence of 48 wave-length noise associated with the wind stress on the orig-inal 28 field, the wind stress fields were smoothed oncewith a center-weighted nine-point real smoother. Twosimulations, (8NEC and 8NEO; Table 2), were forcedwith a monthly mean wind stress climatology formedfrom the European Centre for Medium-Range WeatherForecasts 1979–93 10-m reanalysis winds (Gibson etal. 1997). Overall, the ECMWF climatology has morespatial structure and stronger wind stress and wind stresscurl than the HR climatology. However, both wind setsare generally characterized by positive (negative) windstress curl in the northern (southern) part of the JES,and both show significantly stronger wind stress duringthe winter months (associated with high atmosphericpressure over Siberia) relative to the summer months.

4. Linear simulations

The framework for this study is to use an ensembleof simulations that systematically increases in dynam-ical complexity. The simplest member of this ensembleis a linear 1.5-layer, reduced-gravity (RG) model, whichhas one internal vertical mode and a bottom layer thatis infinitely deep and at rest. As such, it includes onlythe lowest order dynamics. Furthermore, it reveals thedeterministic ocean model response to external forcingand provides a baseline comparison for other simula-tions, which have added features like nonlinearity, bot-tom topography, multiple vertical modes, flow instabil-ities, and isopycnal outcropping. Figure 3 shows themean sea surface height (SSH) from three 1.5-layer,


TABLE 2. List of simulations.

Simulation Resolution Model layers A (m2 s21) Wind forcing Generic description

8LHO8LPO8LHP8NHO8NEO8NPO8NHP8NHP38NEP8NFB8NRG16NHP32NHP32NHPb32NFB32NRG64NHP

1/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/25681/88 3 45/2568

1/168 3 45/51281/328 3 45/102481/328 3 45/102481/328 3 45/102481/328 3 45/102481/648 3 45/20488

1.51.51.54.04.04.04.03.04.04.03.54.04.04.04.03.54.0

505050505050505050505015

55552

HRHRHRHRECMWFHRHRHRECMWFHRHRHRHRHRHRHRHR

Linear RG HR winds onlyLinear RG ports onlyLinear RG ports and HR windsNonlinear HR winds onlyNonlinear ECMWF winds onlyNonlinear ports onlyNonlinear ports and HR windsAs above with three active layersNonlinear ports and ECMWF windsFlat bottom version of 8NHPRG version of 8NHP1/168 analog of 8NHP1/328 analog of 8NHP32NHP with modified topographyFlat bottom version of 32NHPRG version of 32NHP1/648 analog of 8NHP

FIG. 3. Mean SSH from 1⁄88 1.5-layer reduced-gravity linear simulations forced by (a) seasonally varying throughflow (8LPO), (b) monthlyHR winds (8LHO), and (c) both throughflow and HR winds (8LHP). Linearity was achieved by reducing the amplitude of the external forcingby 103. For plotting, the SSH has been scaled upward by the same factor. Contour interval is 1 cm.

reduced-gravity simulations that differ only in the typeof external forcing applied. For these simulations, lin-earity was achieved by reducing the amplitude of thewind and/or throughflow forcing by a factor of 103. Theresult is very small local time derivatives, small non-linear advective terms, and small amplitude variationsin the interface depth. Dynamically, the linear solutionsin Fig. 3 are essentially the same as Munk (1950) witha Sverdrup (1947) interior calculated using realisticwind and/or throughflow forcing and coastline geom-etry, except that horizontal friction is applied every-where and the SSH field includes a component that bal-ances the wind stress but contributes nothing to the ve-locity field. However, this component does allow de-partures from SSH 5 const along the boundary where

there is no flow through the boundary. These are evi-dent, but small in Fig. 3b.

The mean SSH from simulation 8LPO (Table 2),which was driven by seasonally varying throughflowforcing (only) is examined (Fig. 3a). It is uncommon toconsider a ‘‘port-driven’’ solution in the context of Sver-drup dynamics, but this simulation simply shows theimpact of flow through the JES in a linear fashion, anddepicts the TWC as a Munk (b1/3) boundary layer thathugs the western boundary until it is forced to separatefrom the coast of Korea and flow eastward to satisfythe constraints of the outflow ports. Consistent with thedistribution of the outflow ports (described in section3), about two-thirds of the flow separates at the latitudeof the Tsugaru outflow port, and the remaining one-third


separates at the latitude of the Soya outflow port. Thenortheastward flow along the Siberian coast (Fig. 3a) isneeded to satisfy the constraint of the Soya outflow port.This is contrary to the observed cyclonic circulation inthe northern part of the JES and in particular with theobserved southwestward flow along the Siberian coastassociated with the LCC (Fig. 2). This also results in asevere overshoot of the EKWC, which is observed toseparate from the coast at about 378–388N. Indeed,‘‘overshooting’’ of boundary currents has been a prob-lem with previous modeling efforts in the JES, partic-ularly those with relatively coarse horizontal grid res-olution (e.g., Seung and Kim 1993).

The mean SSH from simulation 8LHO, which wasforced with the HR monthly wind stress climatology(only), is shown in Fig. 3b (Table 2). This solution ischaracterized by two large gyre systems, an anticyclonicgyre in the southern half of the basin associated withnegative wind stress curl and a cyclonic gyre in thenorthern half basin associated with positive wind stresscurl. Unlike simulation 8LPO, this solution has realistic(in terms of the flow direction) southwestward flowalong the Siberian coast. Similar to simulation 8LPO,the 8LHO simulation has a northward Munk b1/3 bound-ary current on the western side of the anticyclonic gyrein the southern part of the basin. This Munk boundarylayer also hugs the coast up to about 41.58N, but herethe separation is controlled by the overall pattern of thewind stress curl, and the boundary between the twogyres is approximately the location of the zero windstress curl line. The anticyclonic gyre in the southernpart of the basin results in unrealistic southwestwardflow along the coast of Honshu. Hence, in the linearsense, the HR climatology acts counter to realistic north-eastward flow along the coast of Honshu associated withthe NB of the TWC (Fig. 2).

When the linear solution is forced with both windsand ports, as in simulation 8LHP (Fig. 3c), the result isa linear superposition of simulations 8LPO and 8LHO,and in any given region the solution is altered dependingon which of the individually forced solutions dominates.For instance, in the area off the Siberian coast, simu-lation 8LHP shows southwestward flow because thesouthwestward flow from simulation 8LHO is strongerthan the northeastward flow that resulted from simu-lation 8LPO. The net result is realistic flow to the south-west, which is the model’s representation of the LCC.This result suggests that the LCC is a predominantlywind-driven current. Although simulation 8LHP showsan unrealistic latitude for the EKWC separation and asouthwestward boundary current along the coast ofHonshu, it is noteworthy that the rudiments of most ofthe basic current systems are present in a solution that(only) contains the lowest order dynamics. In particular,simulation 8LHP shows a cyclonic (anticyclonic) gyrein the northern (southern) part of the basin, a polar frontseparating the two gyres, the southward flowing LCC,and the EKWC. However, the NB of the TWC, a major

current in the JES, is totally absent and the large over-shoot of the EKWC beyond its observed separation lat-itude means the NKCC is absent as well.

In the following sections, the dynamical complexityof the simulations is systematically increased by in-cluding some combination of nonlinearity, multiple in-ternal modes, realistic bottom topography, isopycnaloutcropping, and flow instabilities. This modular ap-proach to investigating the dynamics provides a roadmap for identifying the crucial processes needed to sim-ulate the JES circulation in a robust and realistic fashionand the major changes needed to correct the deficienciesseen in the linear solutions.

5. Impact of nonlinearity, external forcing, andisopycnal outcropping

The linear simulations discussed in the previous sec-tion demonstrated that most of the large-scale circula-tion features in the JES can be reproduced with minimaldynamical ingredients when the model includesthroughflow and wind forcing. A major exception isunrealistic southwestward flow along the coast of Hon-shu due to the wind stress curl in the HR climatology.In the following sections, the model complexity is pro-gressively increased by adding nonlinearity, bottom to-pography, multiple internal modes (i.e., more layers),interface ventilation, and diapycnal mixing. The modelsensitivity to external forcing is also examined. Thedynamical consequences of these changes, which canbe examined in a systematic fashion, include the pos-sibility of barotropic and baroclinic flow instabilities,the formation of inertial jets and boundary layers, andisopycnal outcropping.

As in the previous section, we begin by investigatingthe impact of the external forcings, individually andcollectively, but here using a series of nonlinear sim-ulations. The mean SSH and surface layer currents froma sequence of 4-layer, 1⁄88 simulations forced with sea-sonal throughflow and/or HR monthly winds are ex-amined (Fig. 4). In addition to nonlinearity, these sim-ulations include multiple internal baroclinic modes andrealistic bottom topography. The addition of these dy-namical ingredients results in O(1) changes from thelinear solutions, providing much more realistic depic-tions of the current systems. Even when driven by sea-sonal throughflow forcing only (simulation 8NPO),most of the major surface layer currents, nonlinear re-circulation gyres, and realistic mesoscale circulationfeatures occur throughout the JES (Fig. 4a). In contrastto the linear simulations, the currents are inertial in char-acter and the fronts meander, forming rings and eddies.For instance, in simulation 8NPO, the EKWC flowsnorthward along the Korean coast until it separates fromthe coast near 418N, which is farther north than theobserved separation latitude. However, unlike simula-tion 8LPO, after separation from the coast the EKWCforms a large meander before flowing southward and


FIG. 4. Mean SSH from 4-layer 1⁄88 nonlinear simulations forced by (a) seasonally varying throughflow (8NPO), (b) HR monthly climato-logical winds (8NHO), and (c) both throughflow and HR winds (8NHP). Contour interval is 1 cm.

bifurcating into two distinct branches (Fig. 4a). Thenorthern branch joins the eastward flow along 408N con-tributing to the polar front, while the other branch joinsthe NB, which flows along the coast of Honshu. Thelatter current is totally absent in linear simulation 8LPO.In the northern part of the JES the currents are char-acterized by overall cyclonic circulation, even in theabsence of wind forcing. Some of the flow from thisgyre exits to the Pacific Ocean via Soya Strait, whilethe remainder forms a nonlinear recirculation gyre, in-cluding the LCC along the Siberian coast even thoughwind forcing is absent.

Simulation 8NHO is identical to simulation 8NPOexcept that it is forced by the HR monthly wind stressclimatology (only). Again, the impact of nonlinearity isevident. The mean SSH and surface layer currents (Fig.4b) are characterized by cyclonic circulation in thenorthern part of the basin and anticyclonic circulationin the southern part, although flow along the coast ofHonshu is quite weak and unrealistically southwestwardwest of the Noto Peninsula. The polar front, which isalso weak, marks the boundary between the two gyres.The cyclonic gyre in the northern part of the basin isassociated with positive wind stress curl, which pro-duces isopycnal outcropping and locally intense openocean upwelling due to Ekman suction offshore fromthe Siberian coast.

When both seasonal throughflow and wind forcingare included, as in simulation 8NHP (Fig. 4c), the im-pact of each forcing mechanism is visible in Fig. 4. Asevidenced by the similarity of Figs. 4a and 4c, the sea-sonal throughflow forcing clearly dominates the mod-eled circulation, particularly the meandering and bifur-cation associated with the EKWC. However, the wind-driven component clearly strengthens the northern gyre

and the flow along the polar front and it affects theshape of the eddy that forms where the EKWC separatesfrom the Korean coast. Except for the separation latitudeof the EKWC, which is farther to the north than ob-served, simulation 8NHP reasonably replicates all of themajor current systems depicted in Fig. 2. Perhaps thesingle largest improvement in simulations 8NPO and8NHP is the formation of the NB of the TWC alongthe coast of Honshu, which was either missing (simu-lation 8LPO) or flowing in the wrong direction (simu-lations 8LHO and 8LHP) in the linear solutions. Usinga simplified model with port and wind forcing, Yoon(1982) attributed the formation and persistence of theNB to topographical control. This hypothesis was sup-ported in numerical studies by Kawabe (1982) and Ichi-ye (1984). In the simulations described here, a differentmechanism is offered as an alternative explanation forthe formation of this boundary current.

In these simulations isopycnal outcropping is the keydynamical mechanism responsible for the formation ofthe NB of the TWC. In regions of outcropping, themodel interfaces ventilate via strong diapycnal mixing,a process described in section 2. Isopycnal outcroppingand ventilation can occur in response to mechanismssuch as (i) Ekman suction, (ii) coastal upwelling in re-sponse to longshore winds or Kelvin wave propagation,(iii) geostrophic response of an interface to a current(the mechanism for isopycnal outcropping north of thepolar front in simulation 8NPO, a simulation with nowind forcing), or (iv) formation of a cyclonic eddy. Ingeneral, interface ventilation in the NLOM is a conse-quence of having O(1) variations in layer thickness andis enhanced by including a relatively thin top layer inthe model (see Table 1). These conditions permit thethin top layer (60-m mean depth in these simulations)


FIG. 5. Mean SSH from 1⁄88 nonlinear simulations forced by HR monthly climatological winds and seasonally varying throughflow for (a)4-layer case (H1 5 60 m) and (b) 3-layer case where the top two layers from (a) have been combined (H1 5 135 m). The removal of thethin top layer in (b) inhibits isopycnal outcropping and hence the formation of the Nearshore Branch along the coast of Honshu. Contourinterval is 1 cm.

to outcrop over much of the basin for most of the time.In simulations without a thin top layer, such as simu-lation 8NHP3 (Table 2), the NB is nonexistent, and flowsouth of the polar front is weakly anticyclonic (Fig. 5).This anticyclonic flow is similar to the flow depicted inthe linear solutions that included wind forcing (e.g.,Figs. 3b,c). In simulation 8NHP (Figs. 4c and 5a), how-ever, the NB is a well-defined boundary current alongthe entire coast of Honshu. The meridional segments ofthis current remain as eastern boundary currents in themean (rather than propagating westward as nondisper-sive Rossby waves) because the vertical mixing term islarger in magnitude than the mass divergence term inthe continuity equation [Eq. (3)]. Restated, this meansthat the rate of upward vertical mixing in the region ofoutcropping is faster than the rate of interface deepeningproduced by westward Rossby wave propagation. Fun-damentally, this is an extreme example of the geo-strophic current mechanism for outcropping (‘‘iii’’above), which occurs because there is a surface layerthat has insufficient volume to carry the portion of theTWC transport within that layer, unless it is confinednear the southeastern boundary between the Tsushimaand Tsugaru Straits. This is discussed further shortly.

Consistent with this explanation and the wind forcing,the flow immediately below in layer 2 (which does notoutcrop in this region) is in the opposite direction. Incontrast, Kim and Yoon (1994) used a multilevel modelwith 1⁄68 resolution to describe the seasonal variabilityof the NB, and reported that it drifted offshore due tothe b effect and developed into a meandering phase fromsummer to autumn.

From section 2, the outcropped layers do not attainzero-layer thickness, but rather outcrop to a specifiedlayer thickness that controls activation of vertical mix-ing. In the JES, this depth (50 m) is chosen to be anominal mixed layer depth for layer 1. The result, how-ever, is that layers with an outcropped bottom interfaceare baroclinically inactive (i.e., do not contribute to thepressure gradient) because they are isopycnal and haveminimal variation in layer thickness in outcropped re-gions. The interface ventilation for simulations 8NPO,8NHO, and 8NHP is examined using maps of the frac-tion of time that each interface ventilates (Fig. 6). FromFig. 6a, the top interface from simulation 8NPO ven-tilates over 90% of the time north of the polar front.Simulation 8NHO (Fig. 6b) on the other hand, producesmuch less interface ventilation except in a small region


FIG. 6. Maps of the percent of time that an interface ventilates from 4-layer 1⁄88 nonlinear simulations. (a–c) show interfaces(1–3) for throughflow forced simulation (8NPO); (d–f ) show interfaces (1–3) for HR wind forced simulation (8NHO), and(g–i) show interfaces (1–3) for simulation forced by both throughflow and winds (8NHP). Units are percent of time; contourinterval is 10%. Note 0% is a separate color.


FIG. 7. Meridional cross section of mean interface depth along1388E from 4-layer, 1⁄88 simulation forced by both winds and ports(8NHP). The horizontal lines at depths 50 m, 90 m, and 130 m indicatethe cumulative depths of the (Table 1). For the thickness of a1hi

given layer, vertical mixing is turned on when hi # .1hi

in the central subpolar gyre. For a given simulation, theeffect of outcropping due to the external forcing is cu-mulative over all layers, and that is reflected in the in-terface ventilation maps for simulation 8NHP (Figs.6g–i).

By measuring the net mass flux across each interface,the strength of the mean meridional overturning cell insimulation 8NHP is determined to be about 1.7 Sv overthe entire basin. Of that, 0.36 Sv of exchange occurswith the abyssal layer. This exchange rate gives a res-idence time for renewal of the deep water of about 130years, close to that estimated by Kim and Kim (1997)and Seung and Kim (1997). As Fig. 6i indicates, nearlyall of the abyssal layer outcropping occurs in the north-western part of the basin, where the third interface isventilated over 90% of the time. Previous studies (Se-njyu and Sudo 1994; Seung and Yoon 1995a) indicatethat this is a likely area for deep convection, and thatthis convection produces cyclonic circulation of the in-termediate and deep water north of the polar front, anindication which is also supported by numerical studies(Seung and Kim 1993; Seung and Yoon 1995b).

To rephrase an earlier statement, the uniformly thinoutcropped layers contribute very little to the pressuregradient and are dynamically inactive in that sense. Assuch, a robust current (the NB) persists in the regionwhere the thin top layer is not outcropping, and althoughthe pressure gradient in layer 1 is due to the gradientsof layer thickness anomaly in all four layers, the gra-dients in layer 1 are essential for the formation of theNB, and the flow is reversed in layers below the surfacelayer as in the wind-forced contribution. The mean layerstructure can be depicted in meridional cross sectionsof interface depth. One cross section (Fig. 7), for sim-ulation 8NHP along 1388E, demonstrates that the topinterface in simulation 8NHP does not ventilate southof 398N, the location of the NB. Likewise, the secondinterface ventilates along the polar front at about 40.58N.

Finally, where all three interfaces ventilate, betweenabout 42.58–458N along 1388E (Figs. 6i and 7), thereis no contribution to the pressure gradient except fromthe barotropic mode. Hence, the mean SSH shows littlevariation there (Fig. 4c), and the current velocities areweak (although current transports can be large becausethe flow is barotropic).

Analysis of the simulations in Fig. 4 revealed theimpact of the wind-driven circulation in the JES whenHR monthly winds are used. However, different windforcing can produce greatly different results, particularlywith respect to separation of the EKWC. For example,Kim and Yoon (1996) achieved realistic separation ofthe EKWC in a 1.5-layer, reduced-gravity model. In thatstudy the overshoot problem was diminished by usingthe Na et al. (1992) wind stress, which is characterizedby strong positive wind stress curl over the subpolargyre. The strong positive wind stress curl resulted instrengthening of the subpolar gyre, thereby moving theseparation latitude of the EKWC to the south. To assessthe impact that a different wind climatology has on theNLOM, two simulations (8NEO and 8NEP) were forcedby a monthly wind stress climatology formed from the1979–1993 ECMWF 10-m reanalysis winds (see section3). Except for the wind forcing, these simulations areidentical to simulations 8NHO and 8NHP (Table 2).

The mean SSH from simulation 8NEO (Fig. 8b) issignificantly different than the mean SSH from simu-lation 8NHO (Fig. 4b). Most striking is the large anti-cyclonic gyre in the center of the basin. Another sig-nificant difference is the continuation of the southwardflow associated with the subpolar gyre south of 428N.In essence, this simulation contains a well-definedNKCC as well as the LCC. The NKCC flows southwardto about 37.58, where it separates from the coast, andflows eastward forming part of a large-scale cycloniccirculation that encompasses most of the basin, includ-ing northeastward flow along much of the coast of Hon-shu. That the separation of the NKCC occurs near thesame latitude as the observed separation latitude of theEKWC is significant because it demonstrates that re-alistic separation of the EKWC at 1⁄88 resolution may besubstantially influenced by the wind-driven circulation.

When the model is forced with seasonal throughflowand the ECMWF monthly wind stress climatology (sim-ulation 8NEP), the separation latitude of the EKWC ismuch more realistic (Fig. 8c) than its HR counterpart(simulation 8NHP). In simulation 8NEP, the EKWC sep-arates from the coast near 378N, but the core of thecurrent flows northward to about 398N due to the influ-ence of the throughflow forcing. After separation fromthe coast, the EKWC flows eastward along the polarfront until it bifurcates into two branches in responseto the large anticyclonic gyre that was observed in sim-ulation 8NEO. The northern branch of this bifurcationflows eastward forming part of the southern boundaryof the subpolar gyre. Some snapshots of the SSH (notshown) show the southern branch of the bifurcation far-


FIG. 8. Mean SSH from 4-layer 1⁄88 nonlinear simulations forced by (a) seasonally varying ports only (8NPO), (b) ECMWF climatologicalwinds only (see text) (8NEO), and (c) both ports and ECMWF winds (8NEP). Contour interval is 1 cm.

ther offshore than the NB, suggesting that this branchmight represent the highly variable Offshore Branch ofthe TWC. In these cases, the NB is fed more directlyby inflow through the eastern channel of TsushimaStrait, although it tends to merge with the OffshoreBranch east of 1368E where the continental shelf be-comes very narrow and deepens considerably.

The significance of simulation 8NEP is that it clearlyillustrates the model sensitivity to the atmospheric forc-ing, particularly with respect to separation and branch-ing of the EKWC. It is reasonable, and indeed likely,that other wind stress climatologies give significantlydifferent solutions than the two discussed here. How-ever, the sensitivity of the circulation to atmosphericforcing is not the focus of this paper. Hence, in thefollowing section, we limit our discussion to simulationsforced with the HR monthly wind stress climatology,and investigate the role of horizontal grid resolution andthe impact of bottom topography. Given the comparisonof simulations 8NHP and 8NEP, this choice may seemcounterintuitive. However, as will be shown, by pur-posefully choosing a wind stress climatology that isknown a priori to give unrealistic results (e.g., overshootof the EKWC), the impact of certain dynamical pro-cesses can be evaluated independent of any contributionfrom the atmospheric forcing, and hence, in a more clearand concise fashion.

6. Impact of horizontal grid resolution and bottomtopography

Studies of the JES eddy field from observations havesuggested a relationship between the upper ocean cur-rents and the bottom topography (Toba et al., 1982; Anet al. 1994; Lie et al. 1995). Numerical studies (Hurlburt

and Metzger 1998) show the importance of numericalmodel resolution on the mesoscale eddy field by dem-onstrating the effect of eddy variability in the KuroshioExtension on the Kuroshio bifurcation at the ShatskyRise. The mechanism through which the eddy field af-fects the mean circulation is complex, but shows thatthe mean flow is related to the bottom topography. Hol-loway (1992) introduced the concept of topostress as ameans of parameterizing the topographic influence oncirculation. Holloway et al. (1995) included this topos-tress parameterization in a relatively coarse resolutionmodel of the JES, and showed that it had the effect ofincreasing the strength of the NKCC and of separatingthe EKWC close to the observed latitude. Because ofNLOM efficiency, it was possible to progressively in-crease the horizontal grid resolution, thereby enhancingthe mesoscale eddy field, to determine to what extentthe mean flow, mesoscale eddy field, and bottom to-pography are related.

Simulations 8NHP and 64NHP form the 1⁄88 (14 km)and 1⁄648 (1.7 km) end members of a sequence of sim-ulations that are identical except for the horizontal gridresolution and the corresponding decrease in eddy vis-cosity afforded by the increase in grid resolution (Table2). Snapshots of SSH for each grid resolution (Fig. 9)and maps of mean SSH (Fig. 10) are examined. Asdescribed in section 5, the separation latitude of theEKWC in simulation 8NHP occurs farther to the norththan observed. When the resolution is increased to1⁄168 (simulation 16NHP, Figs. 9b and 10b), there is mod-est improvement in the separation latitude of the EKWCin that the current no longer hugs the Korean coastline,but there is still overshoot of the EKWC beyond 408N.Also, there is a marginal increase in the number of ed-dies when the resolution is increased to 1⁄168, particularly


FIG. 9. Snapshots of SSH (color) and currents (vectors) on 15 Jan for 4-layer nonlinear simulations with realistic bottomtopography forced by throughflow and HR winds. Resolution is (a) 1⁄88 (8NHP), (b) 1⁄168 (16NHP), (c) 1⁄328 (32NHP), and (d)1⁄648 (64NHP). Realistic separation of the EKWC is not achieved until 1⁄328 resolution. Contour interval of SSH is 1 cm.


FIG. 10. Mean SSH from the same suite of simulations shown in Fig. 9. Resolution is (a) 1⁄88 (8NHP), (b) 1⁄168 (16NHP), (c) 1⁄328(32NHP), and (d) 1⁄648 (64NHP). At least 1⁄328 resolution is required for realistic separation of the EKWC.


south of the polar front. It is not until the resolution isincreased to 1⁄328 (simulation 32NHP, Figs. 9c and 10c)that the EKWC realistically separates at about 388N.Also, the eddy field at this resolution is ubiquitous,forming predominantly warm (cold) rings on the north(south) side of the Polar Front (Fig. 9c), although thesimulations at all four resolutions are eddy-resolvingand demonstrate eddies in Fig. 9.

Separation of the EKWC also occurs near 378–388Nin simulation 64NHP (Figs. 9d and 10d). Indeed, thechange in the solution when the resolution is increasedfrom 1⁄328 to 1⁄648 is qualitatively much smaller than thechange in solution that accompanies any of the otherchanges in grid resolution (and eddy viscosity). Thelargest differences appear in the region of the YamotoBasin. In that region, simulation 32NHP is characterizedby a strong cyclonic gyre but simulation 64NHP is char-acterized by a strong cyclonic/anticyclonic gyre pair.However, comparison of several 10-yr means (notshown) from either simulation 32NHP or 64NHP (i.e.,from a single simulation) indicate that this is an area ofsubstantial interdecadal variability, even though thesesimulations used climatological monthly mean forcing.This is one possible reason for the differences in thisregion. However, in all other regions of the JES, thefact that the mean SSH in simulations 32NHP and64NHP is quite similar suggests that the simulation isnearly converged for mesoscale variability at 1⁄328 res-olution.

Clearly, a profound change in the circulation dynam-ics has occurred between 1⁄168 and 1⁄328 resolution, par-ticularly with respect to the separation latitude of theEKWC. What are the dynamics responsible for this fun-damental change? Examination of the abyssal circula-tion (Fig. 11) suggests that the strength and directionof the abyssal circulation can strongly influence the sur-face circulation. In simulation 8NHP (Fig. 11a), the deepcurrents are relatively weak and are largely anticyclonic.In simulation 16NHP (Fig. 11b), localized areas ofeddy-driven deep mean flows develop and are mainlyassociated with the topographic highs and lows. How-ever, in simulation 32NHP (Fig. 11c), there is a profoundchange in the abyssal circulation compared to the coars-er resolution simulations. In particular, the eddy-drivendeep mean flows are much stronger and occur over mostof the basin. Furthermore, there is a reversal in the cur-rent directions from anticyclonic at 1⁄88 resolution to cy-clonic at 1⁄328 and 1⁄648 resolution. Indeed, in simulation32NHP (Fig. 11c) there is southward abyssal flow alongthe coast of Korea to about 388N, the observed sepa-ration latitude of the EKWC. Near 1298E, 37.58N thestrength of the southward flowing current is 2.4 cm s21,which is in close agreement with current meter mea-surements (Lie et al. 1989) that indicate a mean currentspeed of about 3 cm s21 in nearly the same location.With the exception of the gyres in the Yamato Basin,the strength and pathways of the mean abyssal layercurrents in simulation 64NHP (Fig. 11d) are close to

those from simulation 32NHP, which is another indi-cation that the mesoscale variability in simulation64NHP has nearly converged with respect to that insimulation 32NHP.

In Fig. 11 we have seen the increase in the kineticenergy of the mean flow and the accompanying reversalin the direction of the abyssal circulation as the modelresolution is increased from 1⁄88 to 1⁄328. This is the resultof strengthened baroclinic instability (actually a mixedbaroclinic–barotropic instability) in the higher resolu-tion simulation. Baroclinic instability is very efficientat transferring energy from the upper layers into theabyssal layer. Maps of surface and abyssal eddy kineticenergy (EKE) (Figs. 12 and 13, respectively) demon-strate how dramatically the surface and abyssal eddyenergy levels increase with increasing grid resolutionup to 1⁄328. Those maps also show that the EKE maximain the surface and abyssal layers are geographically col-located, a classic signature of baroclinic instability (Hol-land and Lin 1975).

There is a clear correlation between the unrealisticseparation of the EKWC (which is accompanied byweak anticyclonic deep circulation) at 1⁄88 and realisticseparation of the EKWC (and the reversal in the deepcirculation) at resolution of 1⁄328 and higher. This sug-gests that baroclinic instability and the inclusion of re-alistic bottom topography are the critical factors influ-encing the surface circulation in the JES. Hence, to in-vestigate the role of baroclinic instability involving thebarotropic mode, it is useful to compare simulationswith flat or realistic bottom topography, and reducedgravity simulations that exclude baroclinic instabilityinvolving the barotropic mode. To examine the impactof the bottom topography (or lack thereof ) on the sur-face layer circulation, a sequence of simulations thatinclude realistic bottom topography, a flat bottom, anda lowest layer that is infinitely deep and at rest (e.g.,reduced-gravity simulations) were performed at both1⁄88 and 1⁄328 resolution (Fig. 14; Table 2).

The circulation features and dynamics of simulation8NHP (which included realistic bottom topography)were discussed in sections 5 and 6, but the mean SSHfrom that simulation is repeated (Fig. 14a) for ease ofcomparison. The mean SSH from reduced gravity sim-ulation 8NRG (Fig. 14c) is strikingly similar to that fromsimulation 8NHP. Both show overshoot of the EKWC,a robust NB of the TWC and realistic meandering ofthe polar front. The presence of a robust NB in simu-lation 8NRG, which does not include bottom topogra-phy, is a further indication that bottom topography isnot required for the formation of this current. Both alsoshow a robust LCC off the Siberian coast. The similarityof the mean SSH from simulations 8NHP and 8NRGindicates that the bottom topography and baroclinic in-stability (involving the baroclinic and barotropicmodes), in general, have little if any influence on thesurface circulation since simulation 8NRG has a lowestlayer that is infinitely deep and at rest. However, sim-


FIG. 11. Kinetic energy per unit mass of the mean abyssal flow (color) and mean abyssal currents (vectors) for thesame simulations shown in Fig. 9: (a) 1⁄88 resolution (8NHP), (b) 1⁄168 resolution (16HP), (c) 1⁄328 resolution (32HP), and(d) 1⁄648 resolution (64NHP). Contour interval of KEM is 0.25 log10 m2 s22, and is also expressed in cm s21 (mean speed)and cm2 s22 (KEM per unit mass).


FIG. 12. Surface layer EKE per unit mass (color) and bottom topography (black contours) for the same simulations shown in Fig.9: (a) 1⁄88 resolution (8NHP), (b) 1⁄168 resolution (16NHP), (c) 1⁄328 resolution (32NHP), and (d)1⁄648 resolution (64NHP). Contour intervalof EKE is 0.125 log10 m2 s22, and is also expressed in cm s21 (mean speed) and cm2 s22 (EKE per unit mass). Contour interval ofbottom topography is 400 m.


FIG. 13. Abyssal layer EKE per unit mass (color) and bottom topography (black contours) for the same simulations shown in Fig.9: (a) 1⁄88 resolution (8NHP), (b) 1⁄168 resolution (16NHP), (c) 1⁄328 resolution (32NHP), and (d) 1⁄648 resolution (64NHP). Contour intervalof EKE is 0.125 log10 m2 s22, and is also expressed in s21 (mean speed) and cm2 s22 (EKE per unit mass). Contour interval of bottomtopography is 400 m.


FIG. 14. Mean SSH from 1⁄88 nonlinear simulations with (a) realistic bottom topography (8NHP), (b) a flat bottom (8NFB), and (c) areduced gravity version (8NRG). Panels (d)–(f ) are the same except for 1⁄328 resolution. Realistic separation of the EKWC is only achievedin simulation 32NHP. Contour interval is 1 cm.

ulation 8NFB (Fig. 14b) shows notable deviations fromsimulations 8NRG and 8NHP and therefore a substantialimpact of the barotropic mode. The relatively high abys-sal EKE in the southern half of the basin in simulation8NFB (not shown) indicates that baroclinic instabilityis mainly responsible. Baroclinic instability is an effi-cient process for surface to abyssal KE transfer and itis the only mechanism that can explain the larger EKEand stronger abyssal currents in 8NFB, because it is theonly possibility that was either absent or inhibited inthe other two simulations.

In baroclinic instability involving the barotropicmode, the abyssal circulation plays an essential role.Rapid vortex stretching and compression associatedwith rapidly moving upper ocean eddies (compared to

nondispersive Rossby wave propagation) efficientlytransfers energy to the abyssal layer, generating closedabyssal circulation eddies with a quarter wavelength off-set from the surface layer. When strongly sloping bottomtopography is present, the f/h contours of the topog-raphy tend to inhibit baroclinic instability by inhibitingthe formation of the associated abyssal circulation ed-dies [because the conservation of potential vorticity lim-its the crossing of f/h contours depending on the relativeamplitudes of the relative and planetary vorticity; e.g.,see section 5.5 of Hurlburt (1986)]. In addition, Hurlburtet al. (1990) contains a clear illustration of a baroclin-ically unstable event and a comparison to one that isbarotropically unstable (see their section 3 and Figs. 4and 5). Thus, the upper ocean circulation of simulation


8NHP with realistic bottom topography is more like thatof reduced gravity simulation 8NRG than flat bottomsimulation 8NFB.

The mean SSH fields for simulations 32NHP, 32NFB,and 32NRG are examined (Figs. 14d–f, respectively).At 1⁄328 resolution all three simulations show strikingdifferences from each other as well as changes fromtheir 1⁄88 counterparts. For example, simulation 32NHPshows separation of the EKWC from the Korean coastnear 378N, unlike its 1⁄88 counterpart (8NHP) whichshows EKWC separation at 408–418N, as do both of thereduced-gravity simulations (8NRG and 32NRG). Sim-ulation 32NFB also shows EKWC overshoot of the ob-served separation latitude and at least weak northwardwestern boundary current flow as far north as 428N,while 32NHP shows southward flow along this coast asfar as 378N. A similar situation is found in the abyssallayer (not shown). The upper ocean circulation of sim-ulation 32NHP exhibits many other departures fromsimulations 32NFB and 32NRG, which are clearly re-lated to the bottom topography as discussed next.

The upper ocean–topographical coupling that allowsthe EKWC to separate near the observed latitude re-quires stronger baroclinic instability and larger ampli-tude abyssal layer relative vorticity (generated by vortexcompression and stretching) than occurred at 1⁄88 or1⁄168 resolution. This is possible only when mesoscaleflow instabilities are very well resolved. At least 1⁄328resolution was needed in the JES simulations. This res-olution is substantially finer than the first internal Ross-by radius of deformation, which is resolved even in the1⁄88 simulations. The coupling mechanism relies on thefact that baroclinic instability is very efficient at trans-ferring energy down to the abyssal layer and that en-ergized deep flows tend to follow the f/h contours inthe abyssal layer as required by the conservation ofpotential vorticity. Using the continuity equation, Hurl-burt and Thompson (1980) show how abyssal currentscan steer upper ocean currents in a two-layer modelsince

v1g · =h1 5 v2g · =h1, (4)

where vkg is the geostrophic velocity in layer k and theleft-hand side of (4) is the geostrophic advective con-tribution to the mass divergence term in the continuityequation (3) for layer 1. The geostrophic balance forthe internal mode is given by

k 3 f (v1g 2 v2g) 5 g9=h1, (5)

where g9 5 (r2 2 r1)/ro. In essence, if v1g k v2g, then(4) and (5) state that if =h1 is a good measure of y 1g,then in a two-layer model the currents in the lower layercan advect gradients of upper-layer thickness and thussteer upper-layer currents, especially where the currentsintersect at large angles. This argument formally breaksdown in multilayer models, but the steering effect re-mains in situations where the barotropic and first bar-oclinic modes are dominant (Hurlburt et al. 1996; Hurl-

burt and Metzger 1998), a situation which exists in theJES.

The cycle of upper ocean–topographical coupling issummarized in Fig. 15. Baroclinically unstable surfacecurrents efficiently transfer energy to the abyssal layer.The resulting abyssal flow follows the f/h contours ofthe bottom topography to conserve potential vorticity(Fig. 15b). The mean surface layer currents are stronglyinfluenced by the bottom topography even though theydo not impinge on the bottom topography (Fig. 15a).The EKWC separates from the coast of Korea wherethe southward abyssal flow near the coast is interruptedby anticyclonic flow around the ridge near 129.58E,38.48N in the bottom topography. Offshore, near 1308Ethe latitude of the eastward flow associated with thisdeep anticyclonic eddy coincides with the southernboundary of another strong southward abyssal current.While the topographic steering is strongest where upperocean and abyssal currents intersect at large angles, theend result is upper ocean and abyssal currents that arenearly parallel in many locations as seen in Fig. 15.

Figure 16 demonstrates unequivocally that the bottomtopography in this region is critical for separation of theEKWC to occur near the observed latitude of 378–388N.Figures 16a,b are simply enlargements of Figs. 15a,bin the region of the EKWC. Figures 16c,d are the sameexcept from simulation 32NHPb, which is identical tosimulation 32NHP except that all of the bottom topog-raphy shallower than 2000 m (excluding the continentalslope) in the region where the EKWC separates fromthe coast, particularly the north–south oriented ridge,has been set to a constant depth of 2000 m. This mod-ification removes the topographic features required toconstrain the deep mean flows in this region (Fig. 16d),thereby removing their influence on the surface circu-lation. As a result, in simulation 32NHPb the EKWCovershoots too far to the north, similar to simulation8NHP (although simulation 32NHPb is clearly morerealistic than 8NHP where the bottom topography wasnot modified).

The similarity of 1⁄88 simulations 8NHP (realistic to-pography) and 8NRG (reduced gravity) suggests thatrealistic bottom topography plays no role in realisticseparation of the EKWC nor in the surface layer JEScirculation in general. However, that is an incorrect con-clusion. Only when the results from the 1⁄328 simulationsare examined does the importance of the bottom to-pography become clear since only the 1⁄328 simulation(32NHP) with realistic bottom topography shows anEKWC that separates at a realistic latitude (when HRwind plus straits forcing is used). This result is signif-icant because, if the 1⁄328 simulations had not been per-formed, an incorrect conclusion concerning the role ofthe bottom topography would have been reached. Un-fortunately, this scenario may be more common thannot, as few ocean models are economical enough atpresent to run with such high resolution on a basin-scaledomain.


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FIG. 16. Bottom topography (color) and mean currents from simulation 32NHP in the region where the EKWCseparates from the coast of Korea for (a) the surface layer, and (b) the abyssal layer. The same is shown in (c) and(d) for simulation 32NHPb, which has the topographic high located near 398N, 1308E removed.


7. Summary and conclusions

An ensemble of layered ocean simulations with in-creasing dynamical complexity has been used to inves-tigate the circulation dynamics in the JES. The simu-lations used horizontal resolution that ranged from 1⁄88to 1⁄648 for each variable and vertical resolution thatranged from 1.5-layer reduced-gravity to four-layer fi-nite depth with realistic bottom topography. All includea free surface and realistic coastline geometry. Mostwere forced by prescribed seasonally varying transportsthrough the Tsushima, Tsugaru, and Soya Straits and bythe Hellerman and Rosenstein (1983; HR) monthly windstress climatology, but two were forced with a monthlyclimatology formed from the 1979–1993 ECMWF 10-mreanalysis winds. The modularity of the NLOM allowedcertain dynamical processes to be included, excluded,or varied, revealing the impact of specific dynamicalprocesses on circulation features in the JES.

Linear versions of the model revealed the impact ofthe wind and throughflow forcing, separately and col-lectively. The linear versions of the model depicted theTWC as a Munk (1950) western boundary layer with aSverdrup (1947) interior. In these simulations, the large-scale circulation was dominated by the throughflowforcing, but the LCC was only reproduced when windforcing was included. Most of the basic current systemscan be reproduced with a linear version of the modelforced with (both) throughflow forcing and the HR windstress climatology. A major exception is the absence ofthe NB, which is observed to flow northeastward alongthe Honshu coast. Instead, the linear simulations showunrealistic southwestward flow along the Honshu coastwith HR wind stress forcing present and no flow alongthis coast with the throughflow forcing (only). TheNKCC is also missing because the separation latitudeof the EKWC is too far north.

Nonlinear versions of the model include higher orderdynamical processes, such as flow instabilities, inertialjets and boundary layers, and isopycnal outcropping viainterface ventilation when some combination of realisticbottom topography, the barotropic mode, and multipleinternal modes are included. Nonlinear simulations weremuch more realistic than their linear counterparts andhave inertial currents and fronts that form meanders andrings at all resolutions used in this study. The O(1)variations in layer thickness allow for the possibility ofisopycnal outcropping via ventilation of model inter-faces. For the NLOM, isopycnal outcropping is crucialfor the formation of the NB and has substantial impacton other aspects of the JES circulation, including theLCC and water mass formation. In a 1⁄88 simulationforced with seasonal throughflow and the HR monthlywind stress climatology, the residence time for Japan/East Sea Proper Water, the abyssal layer in the model,is about 130 years. This is consistent with observation-ally based estimates of about 80 years by Kim and Kim(1997). Some authors (e.g., Yoon 1982; Kawabe, 1982)

have suggested topographic control as a mechanism forthe NB, but the nonlinear simulations performed heresuggest isopycnal outcropping as an alternate expla-nation.

All major current systems in the JES are realisticallysimulated at 1⁄88 except that the EKWC separates fromthe Korean coast farther to the north than the observedlatitude when the model is forced with the HR monthlywind stress climatology. More realistic separation of theEKWC can be achieved when the model is forced witha different wind climatology, such as the climatologyformed from the 1979–1993 ECMWF 10-m reanalysiswinds used in this study. The more realistic separationlatitude of the EKWC is due to the stronger positivewind stress curl north of the separation latitude in thiswind product. This result suggests that great care mustbe exercised in the interpretation of model results, sincethe external forcing might provide a realistic depictionof the current systems without including all the relevantdynamics. Since the HR climatology resulted in over-shoot of the EKWC at 1⁄88 resolution, increased realismat higher resolution is due to changes in the dynamics,not the external forcing.

When forced with HR winds, overshoot of the EKWCpersists at 1⁄168 resolution. However, when 1⁄328 resolutionis used, the EKWC separates from the coast near theobserved latitude between 378 and 388N. This changein the behavior of the EKWC as a function of gridresolution is accompanied by much higher levels of up-per ocean and abyssal EKE at higher resolution anddistinct changes in the abyssal circulation. Specifically,the 1⁄88 simulation shows a weak anticyclonic abyssalcirculation, whereas the 1⁄328 simulation shows stronger,cyclonic deep flows. Maps of EKE in the surface andabyssal layers show geographical colocation of highEKE in these layers, indicating the presence of baro-clinic instability. This instability (actually mixed bar-oclinic–barotropic) is very efficient at transferring en-ergy downward. Once the energy is transferred into thelowest layer, it energizes the abyssal flow, which is con-strained to follow the f/h contours of the bottom to-pography. The abyssal circulation can then influence thesurface circulation via a process described in Hurlburtand Thompson (1980) and Hurlburt et al. (1996), evenwhen the upper-ocean flow does not impinge on thebottom topography. This cycle of upper ocean–topo-graphical coupling requires sufficient levels of baro-clinic instability, which were only attained at 1⁄328 res-olution and higher. Hence, this effect is missed at coarserresolution.

The roles of high horizontal grid resolution and re-alistic bottom topography are strongly interrelated. The1⁄88 simulations that have a bottom layer that is infinitelydeep and at rest (i.e., reduced-gravity) and include HRand throughflow forcing are very similar to those withrealistic bottom topography, including almost identicalovershoot of the EKWC. Corresponding 1⁄88 flat-bottomsimulations show significant differences from these, but


again show the EKWC overshoot. This result suggeststhat realistic bottom topography is unimportant for re-alistic separation of the EKWC. However, when a cor-responding trio of three model simulations is performedat 1⁄328 resolution, only the case with realistic bottomtopography depicts realistic separation of the EKWC.Moreover, realistic separation is dependent on a specific,relatively shallow topographic feature near the Koreancoast. When this topographic feature is removed, theEKWC separates too far to the north. Hence, simplyusing high horizontal resolution to generate mesoscaleflow instabilities is not sufficient, in and of itself, forattaining upper ocean–topographical coupling; realisticbottom topography is also required, particularly to attainseparation of the EKWC at the observed latitude.

The mean pathways of the current systems from the1⁄328 and 1⁄648 simulations are quite similar, as are thesurface and abyssal EKE from these simulations, withsome notable exceptions in the southern JES. In con-trast, when the resolution was increased from 1⁄88 to1⁄168 or from 1⁄168 to 1⁄328, the changes in the solutions werequite dramatic. This comparison suggests that statisticalequilibrium for mesoscale variability has been nearlyachieved at 1⁄328 resolution. This convergence is signif-icant since a doubling of the horizontal grid resolutionusually results in significant changes in the model so-lution. In short, the results presented in this paper de-scribe the process by which the surface circulation canbe influenced by the abyssal circulation through me-soscale flow instabilities. The mechanism by which thisoccurs, known as upper ocean–topographical coupling,requires that mesoscale eddies be very well resolved aswell as the presence of realistic bottom topography.Well-resolved eddies, certainly those smaller than theinternal Rossby deformation radius, are needed to gen-erate sufficient flow instabilities and the presence of thebottom topography serves to steer upper ocean currentsand to regulate the strength and location of the flowinstabilities. In numerical ocean models, the couplingrequires high horizontal grid resolution, at least 1⁄328 inthe JES. Thus, the role of upper ocean–topographicalcoupling is missed in coarser resolution models, whichcan lead to unexplained errors in the mean pathways ofthe current systems.

Acknowledgments. This is a contribution to the Japan/East Sea Departmental Research Initiative (DRI) spon-sored by the Office of Naval Research (ONR) underProgram Element 601153N. The 6.1 Low Latitude West-ern Boundary Currents (LLWBC) project under thesame program element provided matching funds inFY98. The 6.2 Global Ocean Prediction System projectcontributed in earlier years under Program Element602435N. The simulations were performed on Sunworkstations, the Cray T3E and YMP at the NavalOceanographic Office, the Cray T3E at the Army HighPerformance Computer Resource Center, and the CrayT3D and T3E at the Arctic Region Supercomputing

Center. The nonworkstation computations were per-formed under grants of computer time from the DefenseDepartment High Performance Computing Initiative.Dr. Alan Wallcraft is recognized for substantial contri-bution to this effort through his work on model devel-opment and his computer expertise.

REFERENCES

An, H., K. Kim, and H. R. Shin, 1994: On the warm eddies in thesouthwestern part of the East Sea (the Japan Sea). J. KoreanSoc. Oceanogr., 29, 152–163.

Beardsley, R. C., R. Limeburner, K. Kim, and J. Candela, 1992:Lagrangian flow observations in the East China, Yellow, andJapan Seas. La Mer, 30, 297–314.

Bleck, R., C. Rooth, D. Hu, and L. Smith, 1992: Salinity-driventhermocline transients in a wind- and thermohaline-forced iso-pycnic coordinate model of the North Atlantic. J. Phys. Ocean-ogr., 22, 1486–1505.

Cho, Y. K., and K. Kim, 1996: Seasonal variation of the East KoreaWarm Current and its relation with the cold water. La Mer, 34,172–182.

Gibson, J. K., P. Kallberg, S. Uppala, A. Hernandez, A. Nomura, andE. Serrano, 1997: ERA description. ECMWF Re-Analysis Pro-ject Report Series 1, ECMWF, Reading, Berkshire, United King-dom, 71 pp.

Hellerman, S., and M. Rosenstein, 1983: Normal monthly wind stressover the world ocean with error estimates. J. Phys. Oceanogr.,13, 1093–1104.

Holland, W. R., and L. B. Lin, 1975: On the generation of mesoscaleeddies and their contribution to the oceanic general circulation.I. A preliminary numerical experiment. J. Phys. Oceanogr., 5,642–657.

Holloway, G., 1992: Representing topographic stress for large-scaleocean models. J. Phys. Oceanogr., 22, 1033–1046., T. Sou, and M. Eby, 1995: Dynamics of circulation of the JapanSea. J. Mar. Res., 53, 539–569.

Hurlburt, H. E., 1986: Dynamic transfer of simulated altimeter datainto subsurface information by a numerical ocean model. J. Geo-phys. Res., 91, 2372–2400., and J. D. Thompson, 1980: A numerical study of Loop Currentintrusions and eddy shedding. J. Phys. Oceanogr., 10, 1611–1651., and E. J. Metzger, 1998: Bifurcation of the Kuroshio Extensionat the Shatsky Rise. J. Geophys. Res., 103, 7549–7566., D. N. Fox, and E. J. Metzger, 1990: Statistical inference ofweakly correlated subthermocline fields from satellite altimeterdata. J. Geophys. Res., 95, 11 375–11 409., A. J. Wallcraft, W. J. Schmitz Jr., P. J. Hogan, and E. J. Metzger,1996: Dynamics of the Kuroshio/Oyashio current system usingeddy-resolving models of the North Pacific Ocean. J. Geophys.Res., 101, 941–976.

Ichiye, T., 1984: Some problems of circulation and hydrography ofthe Japan Sea and the Tsushima Current. Ocean Hydrodynamicsof the Japan and East China Seas, T. Ichiye, Ed., Elsevier, 15–54.

Kawabe, M., 1982: Branching of the Tsushima Current in the JapanSea. II. Numerical experiment. J. Oceanogr. Soc. Japan, 38,183–192.

Kim, C. H., and J. Y. Yoon, 1994: A numerical study on the seasonalvariation of the Tsushima Warm Current along the coast of Japan.Proc. Third CREAMS Workshop, Seoul, Korea, Research Insti-tute of Oceanography, Seoul National University, 73–79.

Kim, K., and J. Y. Chung, 1984: On the salinity minimum and dis-solved oxygen maximum layer in the East Sea (Sea of Japan).Ocean Hydrodynamics of the Japan and East China Seas, T.Ichiye, Ed., Elsevier, 55–65., and R. Legeckis, 1986: Branching of the Tsushima Current in


1981–83. Progress in Oceanography, Vol. 17, Pergamon, 265–276., and J. Y. Yoon, 1996: Modeling of the wind-driven circulationin the Japan Sea using a reduced gravity model. J. Oceanogr.,52, 359–373., and K. Kim, 1997: What is happening in the East Sea (JapanSea)? Recent chemical observations during CREAMS 93–96. J.Korean Soc. Oceanogr., 31, 164–172.

Kwizak, M., and A. J. Robert, 1971: A semi-implicit scheme for gridpoint atmospheric models of the primitive equations. Mon. Wea.Rev., 99, 32–36.

Levitus, S., 1982: Climatological Atlas of the World Ocean. NOAAProf. Paper No. 13, U.S. Govt. Printing Office, 173 pp.

Lie, H.-J., M.-S. Suk, and C. Kim, 1989: Observations of south-eastward deep currents off the west coast of Korea. J. Oceanol.Soc. Korea, 24, 63–68., S. K. Byun, I. Band, and C. H. Cho, 1995: Physical structureof eddies in the southwestern East Sea. J. Korean Soc.Oceanogr., 30, 170–183.

Luyten, J. R., J. Pedlosky, and H. Stommel, 1983: The ventilatedthermocline. J. Phys. Oceanogr., 13, 292–309.

Mesinger, F., and A. Arakawa, 1976: Numerical methods used inatmospheric models. GARP Publ. Ser., No. 17, WMO, 64 pp.

Miyazaki, M., 1952: The heat budget in the Japan Sea (in Japanese).Bull. Hokkaido Reg. Fish. Res. Lab., 4, 1–54.

Moore, D. R., and A. J. Wallcraft, 1998: Formulation of the NRLLayered Ocean Model in spherical coordinates. Naval ResearchLaboratory Rep. NRL CR 7323-96-0005, Naval Research Lab-oratory, Stennis Space Center, 24 pp. [Available from NavalResearch Laboratory, Stennis Space Center, MS 39529.]

Moriyasu, S., 1972: The Tsushima Current. Kuroshio: Its PhysicalAspects, H. Stommel and K. Yoshida, Eds., University of TokyoPress, 353–369.

Munk, W. H., 1950: On the wind-driven ocean circulation. J. Meteor.,7, 79–93.

Na, J.-Y., J. W. Seo, and S. K. Han, 1992: Monthly mean sea surfacewinds over the adjacent seas of the Korean Peninsula (in Ko-rean). J. Oceanol. Soc. Korea, 27, 1–10.

Naganuma, K., 1977: The oceanographic fluctuations in the JapanSea. Kaiyo Kayaku, 9, 137–141.

National Oceanic and Atmospheric Administration, 1988: Digital re-lief of the surface of the earth. Data Ann. 88-MGG-02. [Availableonline at http://www.ngdc.noaa.gov:80/mgg/global/seltopo.html.]

Orlanski, I., 1976: A simple boundary condition for unbounded hy-perbolic flows. J. Comput. Phys., 21, 251–269.

Preller, R. H., and P. J. Hogan, 1998: Oceanography of the Sea ofOkhotsk and the Japan/East Sea. The Sea, Vol. 11: The GlobalCoastal Ocean, Regional Studies and Synthesis, A. R. Robinsonand K. H. Brink, Eds., John Wiley and Sons, 429–481.

Senjyu, T., and H. Sudo, 1994: The upper portion of the Japan SeaProper Water: Its source and circulation as deduced from iso-pycnal analysis. J. Oceanogr., 50, 663–690.

Seung, Y.-H., 1997: Application of ventilation theory to the East Sea.J. Korean Soc. Oceanogr., 32, 8–16., and K.-J. Kim, 1993: A numerical modeling of the East Seacirculation. J. Oceanol. Soc. Korea, 28, 292–304., and , 1995: A multilayer model for dynamics of upperand intermediate layer circulation of the East Sea. J. KoreanSoc. Oceanogr., 30, 227–236., and J.-H. Yoon, 1995a: Some features of winter convection inthe Japan Sea. J. Oceanogr., 51, 61–73., and , 1995b: Robust diagnostic modeling of the Japan Seacirculation. J. Oceanogr., 51, 421–440., and K.-J. Kim, 1997: Estimation of the residence time forrenewal of the East Sea Intermediate Water using MICOM. J.Korean Soc. Oceanogr., 32, 17–27.

Shriver, J. F., and H. E. Hurlburt, 1997: The contribution of the globalthermohaline circulation to the Pacific to Indian Ocean through-flow via Indonesia. J. Geophys. Res., 102, 5491–5511.

Suda, K., and K. Hidaka, 1932: The results of the oceanographicalobservations on board R.M.S. Syunpu Maru in the southern partof the Sea of Japan in the summer of 1929, part 1 (in Japanese).J. Oceanogr. Imp. Mar. Obs., 3, 291–375.

Sverdrup, H. U., 1947: Wind-driven currents in a baroclinic ocean:With application to the equatorial currents of the eastern Pacific.Proc. Natl. Acad. Sci. U.S.A., 33, 318–326.

Toba, Y., K. Tomizawa, Y. Kurasawa, and K. Hanawa, 1982: Seasonaland year to year variability of the Tsushima–Tsugaru Warm Cur-rent system with its possible cause. La Mer, 20, 41–51.

Uda, M., 1934: The results of simultaneous oceanographical inves-tigations in the Japan Sea and its adjacent waters in May andJune, 1932 (in Japanese) J. Imp. Fish. Exp. Stn., 5, 57–190.

Vasilev, A. S., and V. P. Makashin, 1992: Ventilation of the JapanSea waters in winter. La Mer, 30, 169–177.

Wallcraft, A. J., 1991: The Navy Layered Ocean Model users guide.NOARL Rep. 35, Naval Research Laboratory, Stennis SpaceCenter, MS, 21 pp. [Available from Naval Research Laboratory,Stennis Space Center, MS 39529.], and D. R. Moore, 1997: A scalable implementation of the NRLLayered Ocean Model. Parallel Comput., 23, 2227–2242.

Yarichin, V. G., 1980: Steady state of the Japan Sea circulation (inRussian). Problems of Oceanography, V. V. Pokudov, Ed., Hy-drometeoizdat, Leningrad, 46–61.

Yi, S. U., 1966: Seasonal and secular variations of the water volumetransport across the Korea Strait. J. Oceanol. Soc. Korea, 1,7–13.

Yoon, J. H., 1982: Numerical experiment on the circulation in theJapan Sea. Part III, Mechanism of the Nearshore Branch of theTsushima Current. J. Oceanogr. Soc. Japan., 38, 125–130.

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