Journal of Oceanography, Vol. 55, pp. 247 to 256. 1999

Effects of Eddy Variability on the Circulation of the / East Sea

1 1 2 G. A. JACOBS , P. J. HOGAN AND K. R. WHITMER

1Naval Research Laboratory, Stennis Space Center, Mississippi, U.S.A. 2Sverdrup Technology, Inc., Stennis Space Center, Mississippi, U.S.A.

(Received 5 October 1998; in revised form 17 November 1998; accepted 19 November 1998)

The effect of mesoscale eddy variability on the Japan/East Sea mean circulation is Keywords: examined from satellite altimeter data and results from the Naval Research Laboratory ⋅ Japan Sea, Layered Ocean Model (NLOM). Sea surface height variations from the Geosat-Exact ⋅ eddies, ⋅ altimeter, Repeat Mission and TOPEX/POSEIDON altimeter satellites imply geostrophic velocities. ⋅ At the satellite crossover points, the total velocity and the Reynolds stress due to numerical model- ing, geostrophic mesoscale turbulence are calculated. After spatial interpolation the momentum ⋅ Reynolds stress. flux and effect on geostrophic balance indicates that the eddy variability aids in the transport of the Polar Front and the separation of the East Korean Warm Current (EKWC). The NLOM results elucidate the impact of eddy variability on the EKWC separation from the Korean coast. Eddy variability is suppressed by either increasing the model viscosity or decreasing the model resolution. The simulations with decreased eddy variability indicate a northward overshoot of the EKWC. Only the model simulation with sufficient eddy variability depicts the EKWC separating from the Korean coast at the observed latitude. The NLOM simulations indicate mesoscale influence through upper oceanÐtopographic coupling.

1. Introduction Lie et al., 1995). The eddy variability has been observed The Japan/East Sea (JES) general circulation is forced through analysis of sea surface temperature (SST) data primarily by the inflow through the from the (Toba et al., 1984; Miyao, 1994). The length and time scales East Sea and by the outflow through the Tsugaru and of the eddy field have been well characterized through these Soya Straits. divides the strait into east and studies. The propagation of individual eddies has also been west channels. The total transport through the strait varies documented through the SST studies (Isoda and Saitoh, significantly in time, as does the transport distribution 1993; An et al., 1994; Isoda, 1994). Using SST to perform between the eastern and western branches. The main flow an evaluation of turbulence generated by the eddy field in occurs through the west channel with a smaller portion the JES, Toba et al. (1984) demonstrate that the synoptic through the east channel. The west channel flow enters the JES circulation is dominated by mesoscale eddies. The JES as the East Korean Warm Current (EKWC) and generally mean flow is visible only after averaging over long time separates from the Korean coast between 37° and 39°N. The periods, suggesting a strong influence of the eddy field on east channel flow enters the JES as the Nearshore Branch the mean flow. and generally follows the Japanese coast to the Tsugaru In pursuit of the same goal, we use two tools to attempt Strait. Wind stress also plays a part in the JES circulation to understand eddy variability effects that occur in the JES. with the wind stress curl aiding in generating the cyclonic In this study we use sea surface height (SSH) data from the circulation north of the front. Preller and Hogan (1998) Geosat-Exact Repeat Mission (Geosat-ERM) and TOPEX/ present a review of the historical research in the JES. POSEIDON (T/P) altimeters. Measuring the SSH, the al- Particularly ubiquitous in the JES is the mesoscale eddy timeter provides geostrophic speed estimates in a direction field. perpendicular to the satellite ground track. At points where In situ eddy observations are abundant throughout the each altimeter’s ground track crosses itself, the two cross- JES (Toba et al., 1984; Ichiye and Takano, 1988; Isoda and track estimates of geostrophic velocity may be transformed Saitoh, 1993; An et al., 1994; Isoda, 1994; Lie et al., 1995; into eastward and northward velocity components. From Shin et al., 1995). Eddies have been observed extensively these velocity components, the Reynolds stresses are esti- through temperature measurements off the Korean coast in mated at crossover points (Parke et al., 1987). Gradients of the Ullung basin (Isoda and Saitoh, 1993; An et al., 1994; the Reynolds stresses produce a force on the mean flow, and

247 Copyright  The Oceanographic Society of Japan. the effects of this force may be examined through a balance Hurlburt (1998) indicate a horizontal resolution of 1/32° is with the Coriolis force. This provides a balance similar to required to generate the interaction between the eddy field the balance between the horizontal pressure gradient and and bottom topography without the parameterization of Coriolis force, and this is one method to understand the topostress. Using this resolution, we examine two mecha- relative importance of the geostrophic turbulence. nisms through which the mesoscale field may influence the Numerical models provide a useful capability to add mean circulation. Both the Reynolds stresses and the upper and remove dynamical processes and then examine the oceanÐtopographic coupling indicate influence on the mean. effects. Hogan and Hurlburt (1998) investigated the dynamics In this examination, we compute geostrophic velocities of the JES in a progressive fashion using the Naval Research from both the Geosat-ERM and T/P satellites at each of their Laboratory’s (NRL) Layered Ocean Model (NLOM). In this respective crossover points (Section 2). The numerical model study we use the NLOM with horizontal grid resolution of experiments of the isopycnal model then demonstrate the 1/32° (3.5 km) to simulate the mesoscale variability. To effects of eddy variability on the mean circulation (Section contrast the circulation with eddy variability to the circulation 3). The results are discussed in Section 4. with reduced variability, we perform simulations with higher eddy viscosity that suppresses the eddy variability. At least 2. Altimeter Data Sets and Reynolds Stresses 1/32° resolution is necessary to resolve the mesoscale fea- This study uses altimeter data from the T/P and Geosat- tures sufficiently to ensure upper oceanÐtopographic coupling ERM satellites. T/P data provided on the merged Geophysical in the JES (Hogan and Hurlburt, 1998). In one 1/32° ex- Data Records (GDRs) from the Physical Oceanography periment, the eddy viscosity is set to a relatively large value Distributed Active Archive Center (PO-DAAC) over the to intentionally suppress the mesoscale eddy field (experi- time period 1993Ð1997 are first broken into arcs composed ment A_High). The other 1/32° experiment uses a more of one satellite revolution. Arcs that fall on the same ground realistic value of eddy viscosity and contains much more track are grouped into a set of repeat (or collinear) passes. energy in the eddy field (experiment A_Low). The mean Computed orbits for T/P are based on the Joint Gravity circulation based on the two simulations represents the Model (JGM)-3 gravity field (Tapley et al., 1996). Atmo- effects of the mesoscale variability on the mean flow. spheric corrections (dry troposphere, wet troposphere, and At low horizontal model grid resolution, the EKWC ionosphere) are applied, as well as solid Earth tides, an typically separates from the Korean coast at a latitude ocean tide estimate from the Grenoble model (Le Provost et further north than the observed separation latitude (an over- al., 1994), tidal loading, an electromagnetic (EM) bias, and shoot). Sueng and Kim (1993) indicate that the EKWC an inverse barometer correction based on the local instanta- overshoot problem is diminished (but not eliminated) by neous barometric pressure. increasing horizontal resolution to 1/5° in the Cox (1984) The National Oceanic and Atmospheric Administration ocean model. One effect of the increased model resolution (NOAA) provides the Geosat-ERM data. The orbit solutions is to better resolve the mesoscale field, and thus the model in the Geosat-ERM altimeter data set are also based on will represent the eddy field more accurately. The interac- JGM-3. Atmospheric corrections (dry troposphere, wet tion of the eddy field with the bottom topography has been troposphere, and ionosphere from the International Refer- indicated in several JES in situ studies (Toba et al., 1984; An ence Ionosphere 1995, IRI95) are applied, as well as solid et al., 1994). and ocean tide estimates (Le Provost et al., 1994), an EM Holloway et al. (1995) use the Modular Ocean Model bias correction of 2.5% of the significant wave height (MOM) at 1/5° resolution with the addition of topostress to (Witter and Chelton, 1991), and a static inverse barometer understand the effects of seasonality, wind stress, buoyancy correction. forcing, latitudinal variations in Coriolis parameter, and SSH from all repeat passes is interpolated along ground bottom interaction in the JES. The topostress parameterizes tracks to points spaced by 1 s (i.e., about 6.5 km along track), the topographic influence on the circulation (Holloway, and the mean SSH at each point along the ground tracks is 1992). Without topostress, the model of Holloway et al. subtracted removing both the geoid signal and the mean (1995) indicates the EKWC separating from the coast at a dynamic topography. The T/P data covers 5 years while the latitude further north than observed (similar to Sueng and Geosat-ERM covers only about 2.5 years. The mean SSH Kim, 1993). The topostress has the effect of increasing the removed from each satellite’s data set is the mean over its strength of the North Korean Cold Current (NKCC), which own time period. There are about 65 measurements at each causes the EKWC to separate closer to the observed latitude. Geosat-ERM ground track point versus about 180 for T/P. It is the interaction between the eddy field and the bottom Because the Reynolds stresses are statistical averages, the topography that the topostress of Holloway (1992) intends Reynolds stresses based on the Geosat-ERM data are ex- to parameterize for numerical models that do not sufficiently pected to contain larger errors. Some differences between resolve the mesoscale variability. the T/P and Geosat-ERM Reynolds stresses are also ex- To accurately model the mesoscale field, Hogan and pected due to the interannual variations in the eddy field.

248 G. A. Jacobs et al. That is, the two satellites have observed different events that Because the mean SSH has been removed at each will lead to different statistical characteristics. ground track point, the calculated cross-track geostrophic Significant time-varying orbit errors exist in the Geosat- velocities are actually the deviations from the mean veloc- ERM data (Jacobs and Mitchell, 1997). These errors are ity. At a given ground track point, the cross-track geostro- removed by subtracting a least squares fit sinusoid with a phic velocity anomalies of the ascending (va′) and de- frequency of once cycle per satellite revolution through an scending (vd′) tracks are calculated. The SSH values used in iterative procedure (Jacobs et al., 1992). This procedure computing the cross-track geostrophic velocities are from adjusts each repeat pass to the mean SSH along a given the 1-second SSH samples. Before the computation, the data ground track. In addition, a climatological seasonally vary- are smoothed using a Gaussian filter with a 25 km e-folding ing dynamic height is used to retain a majority of the scale. The filtering reduces noise that may bias the Reynolds seasonal steric signal. Typical amplitudes removed are 15 stress estimates. The eastward (u′) and northward (v′) ve- cm RMS. At these large wavelengths (40,000 km) the locity anomaly components are calculated by removed amplitudes have very little effect on the geostro- phic velocities computed at a given point. Typical noise ′− ′ va vd amplitudes observed here are of the order of 1 cm over u′= ()1a 2sin θ distances of 10 to 100 km. Thus, any residual orbit error, or removal of ocean signal will not have a significant influ- v ′+v ′ ence. For consistency the T/P data are treated in an identical v′= a d ()1b manner. 2 cosθ

(a) (b)

Fig. 1. The Reynolds stress ellipses based on (a) TOPEX/POSEIDON data and (b) Geosat-ERM data. The higher eddy variability south of the Polar Front is apparent as larger ellipses. At a given point, the main direction of velocity fluctuations is in the direction of the ellipse major axis. Though the Geosat-ERM data contains a higher spatial resolution, the noise level is higher than TOPEX/ POSEIDON due to the shorter time period covered. The Geosat-ERM data covers about 2.5 years (or 60 samples) while TOPEX/ POSEIDON covers about 5 years (or 185 samples).

Effects of Eddy Variability on the Circulation of the Japan/East Sea 249 where θ is the angle between the eastward direction and the ascending ground track, and the direction of positive cross- track velocity is 90° to the left of the ground track direction. For the purposes of computing the velocity components u′ and v′, the ascending and descending track velocities (va and vd) are chosen by finding the closest measurements in time. Thus, for T/P the values are measured no more than 5 days apart, and for the Geosat-ERM the values are measured no more than 8.5 days apart. The effects of the temporal offset in the ascending and descending tracks has been examined in detail by Morrow et al. (1994). The barotropic variations in the JES are of a time period much shorter than the 5 days of T/P. Thus the Reynolds stresses due to barotropic turbulence will not be properly represented from the altimeter data. The Reynolds stresses are calculated at each crossover point for both T/P and the Geosat-ERM from the velocity components

N ′ ′ = 1 ′ ′ () u u ∑ uj uj 2a N j =1

N ′ ′ = 1 ′ ′ () u v ∑ uj v j 2b Fig. 2. The gradient of the Reynolds stresses after the data from N = j 1 the Geosat-ERM and TOPEX/POSEIDON have been interpo- lated spatially may be viewed as either a momentum flux or an N additional forcing to the dynamical equations. The major flux ′ ′ = 1 ′ ′ () v v ∑ v j v j 2c areas are associated with the East Korean Warm Current, the N j =1 Polar Front, and the Nearshore Branch. where N is the total number of samples at the crossover point and the subscript j refers to the sample in time. The Reynolds stresses from the Geosat-ERM and T/P data The Reynolds stress ellipses based on the Reynolds are interpolated to a regular 1/4° grid by a weighted averaging stress estimates are generated for both T/P and for the as described by Zlotnicki et al. (1989). The weighting is a Geosat-ERM (Fig. 1). The distance from the center to the given by a Gaussian function with a 150 km e-folding length edge of the ellipse in the given direction is the variability of scale. The e-folding scale is larger than the typical eddy size the currents in the prescribed direction. Thus the ellipse (between 50 to 150 km). The scales of the Reynolds stresses principal axis points in the direction of the major flow (Figs. 1(a) and (b)) are generally larger than 150 km except variability. The horizontal Reynolds stresses enter into the near the Polar Front. In this region some smoothing of the linearized unforced dynamical equations for the horizontal Reynolds stresses occurs in the interpolation procedure. The flow (ignoring molecular viscosity and boundary stresses) net result is a reduction in the subsequent effective fluxes, by forcing, and effect on the geostrophic flow. The effective forcing induced (Fig. 2) is the vector ∂u 1 ∂p ∂u′ u′ ∂u′ v′ defined by the derivatives of the Reynolds stress on the right =− + fv − − ()3a ∂t ρ ∂x ∂x ∂y hand side of (3), and these are also referred to as the eddy momentum flux. If we are interested in the mean flow then the time derivatives in (3) are zero. The three terms for the ∂v 1 ∂p ∂ v′ v′ ∂u′ v′ =− − fu − − .3b() mean flow (pressure gradient, Coriolis force, and Reynolds ∂t ρ ∂y ∂y ∂x stress force) balance. If the Reynolds stresses were zero, a geostrophic balance would give the mean flow. There are The overbars indicate a long period mean, the primes several ways to interpret the Reynolds stress effects. One indicate the deviation from the mean, f is the Coriolis pa- interpretation is that the Reynolds stress alters the pressure rameter, and p is the pressure. The horizontal derivatives of gradient that would balance the Coriolis force. This first the Reynolds stresses act as forcing on the mean circulation. interpretation approaches the problem from the point of

250 G. A. Jacobs et al. view that we prescribe (or measure) the velocity field, and third in the second layer. The lower layers are closed with a the pressure gradient is the dependent variable. Another no-slip boundary condition. The JES outflow vertical distri- interpretation is that the Reynolds stress effects are linearly bution is identical to the inflow with two thirds of the additive to the geostrophic flow. This second approach outflow through the and one third through the assumes the pressure gradient is prescribed and that the Soya Strait. The Hellerman-Rosenstein (1983) monthly mean velocity is the dependent variable. Thus the Reynolds wind stress climatology provides wind forcing. The model stress effects on the mean flow may be examined by the simulations are each integrated to statistical equilibrium, velocity induced by balancing the Reynolds stress force and an additional 10 year’s of integration are used to form with the Coriolis force the averages. Hogan and Hurlburt (1998) discuss the JES model in detail. Dynamics of the model simulations from Hogan and 1  ∂u′ u′ ∂u′ v′ v = + ()4a Hurlburt (1998) are summarized as follows. Realistic  ∂ ∂  f  x y  separation of the EKWC from the coast of is only achieved at 1/32° resolution or higher when forced with the Hellerman-Rosenstein wind stress climatology. Some at- 1  ∂ v′ v′ ∂u′ v′ u =− + .4b() mospheric data sets can produce more realistic EKWC  ∂ ∂  f  y x  separation at coarser model resolution due to strong positive wind stress curl north of the separation latitude of the 3. Numerical Model Results EKWC. However, simulations with insufficient horizontal Numerical model experiments are performed to exam- grid resolution forced by alternate wind stresses do not ine the influence of the eddy variability. The simulations are adequately resolve mesoscale flow instabilities, which are conducted with the same numerical model using different needed to properly simulate the upper oceanÐtopographic eddy viscosities to determine the effects on the mesoscale coupling. The upper oceanÐtopographic coupling mechanism variability. For the experiments, the horizontal friction relies on the fact that baroclincally unstable surface layer currents are very efficient at transferring energy to the prescribed in the numerical modelv dynamics is Laplacian ∇ ∇ abyssal layer. This generates eddy-driven deep mean flows given by the form A[ ¥(h )] v v, where A is the eddy viscos- ity, h is the layer thickness, and v is the velocity vector (Hogan that are constrained to follow the f/H contours of the bottom and Hurlburt, 1998). The two eddy viscosities used are 50 topography. The deep flows in turn influence the surface m2/s (A_High) and 5 m2/s (A_Low), and the grid resolution circulation via a conservation of mass process described by is 1/32° latitude and 45/1024° longitude (distance between Hurlburt and Thompson (1980) (also see Hurlburt et al. like variables). Two model tests are thus made, and we refer (1996) and Hurlburt and Metzger (1998)). This topographi- to these tests as A_High and A_Low. The model is a cal effect is missed at coarser resolution, which can lead to primitive equation formulation with the vertical structure erroneous conclusions about the role of the bottom topogra- described by Lagrangian layers. The layers are capable of phy and unexplained errors in the pathways of the current representing the barotropic and baroclinic ocean structure systems. In the A_High simulation, baroclinic instability is with a few layers. Each of the 4 layers in the model repre- suppressed, upper oceanÐtopographical coupling is greatly sents the vertically integrated momentum equations, and the weakened, and the EKWC flows further to the north than the interfaces between layers represent isopycnals. The model observed separation latitude (Fig. 4). has a free surface and contains realistic bottom topography that is restricted to the bottom layer. Hurlburt and Thompson 4. Discussion (1980) describe the basic model formulation and Cartesian The mean flows of the model experiments contain numerics in detail, and Wallcraft (1991) introduces sub- dramatic differences. The A_High mean flow indicates that stantial enhancements. Moore and Wallcraft (1998) discuss the EKWC overshoots to a latitude between 40° and 42°N the mathematical and numerical formulation in spherical (Fig. 3). Only in the A_Low experiment does the EKWC coordinates. separate from the Korean coast between 37° and 39°N, The JES model mean interface depths are 60 m, 135 m, which is much closer to the observed separation latitude and 250 m. The top three layers represent the warm saline (Fig. 3). The currents within the Polar Front are also stronger inflow from the Tsushima Strait and the JES Intermediate in the low viscosity experiment as demonstrated by the Water, while the lowest layer represents the JES Proper higher kinetic energy of the mean flow. Water. The Tsushima Strait mean transport is 2.0 Sv with a The change in eddy variability in the model experiments seasonal variation that has 2.66 Sv peak transport in July and causes the changes in the mean currents. The distribution 1.34 Sv minimum transport in January. At all times, 75% of and intensity of eddy kinetic energy changes when the the transport passes through the western channel. The top model viscosity is altered (Fig. 4). The A_Low experiment layer contains two thirds of the transport with the remaining contains an area of high EKE around the Ullung Basin and

Effects of Eddy Variability on the Circulation of the Japan/East Sea 251 Fig. 3. The mean currents and kinetic energy of the mean flow from the numerical model using (a) 1/32° resolution and an eddy viscosity of 50 m2/s, and (b) 1/32° resolution and an eddy viscosity of 5 m2/s. The interval between color levels is .25 cm2/s2. The high eddy viscosity simulation indicates an overshoot of the East Korean Warm Current to 40° or 42°N which is far beyond the observed separation latitude. Using a lower eddy viscosity of 5 m2/s (b), the EKWC separates from the Korean coast between 37° and 38°N. Associated with the low viscosity experiment is an increase in EKE (Fig. 4).

Fig. 4. The model surface layer eddy kinetic energy (EKE) using an eddy viscosity of 50 m2/s (a) indicates low EKE across the Yamato Rise and high EKE north of 41°N which is beyond the observed separation latitude. Using a lower eddy viscosity of 5 m2/s (b), the EKE extends from the Korean peninsula at 38°N, across the northern side of the Yamato Rise, and to the Tsugaru Strait. The interval between color levels is .125 m2/s2. The model EKE from the experiment using lower eddy viscosity is in much better agreement with the observed EKE from the Geosat-ERM (Fig. 5).

252 G. A. Jacobs et al. two branches of high EKE meandering eastward (Fig. 4(b)). along the Korean coast to 42°N (Fig. 4(a)). The eastward The northern branch extends from the northern Ullung extension of the high EKE region from the Korean coast is Basin, north of the Yamato Rise, and to the Tsugaru Strait. relatively short compared to the low viscosity model experi- The southern high EKE branch extends from the southern ment. side of the Ullung Basin, along the Japanese coast, and To evaluate the A_Low experiment EKE realism, we toward the Tsugaru Strait. The A_High experiment indicates compute the cross-track EKE of the Geosat-ERM data (Fig. the overshoot of the EKWC by the high EKE extending 5). Along the coastlines, the EKE indicates local maxima.

Fig. 5. The cross-track eddy kinetic energy (EKE) computed from the Geosat-ERM data indicates a band of high EKE extending from the Korean peninsula at 38°N, across the northern side of the Yamato Rise, and to the Tsugaru Strait. High EKE also occurs across the Nearshore Branch just off the Japanese coast. The Geosat-ERM EKE is in agreement with the model EKE using the lower eddy viscosity (Fig. 4(b)).

Effects of Eddy Variability on the Circulation of the Japan/East Sea 253 This is due to the wind-driven setup and setdown on the nitude increases at 44°N. continental shelf. The calculation of geostrophic currents The effect of the Reynolds stresses on the mean flow from heights on the shelf is not accurate, and this leads to (Fig. 6) is determined from (4) and indicates a strengthening excessive EKE along the coastlines. The Geosat-ERM EKE of the general JES circulation. The induced flow along the contains generally higher EKE south of the Polar Front and Korean peninsula is in contrast to this, indicating a flow very low EKE north of the Polar Front. The peak EKE occurs away from the coast. The general cyclonic circulation north in the Ullung Basin and southeast of the Yamato Rise. The of the Polar Front is also intensified. The effect of the spatial spatial distribution of the A_Low EKE (Fig. 4(a)) is in much interpolation of the Reynolds stress is to reduce the magnitude better agreement with the Geosat-ERM EKE than is the of the peak Reynolds stress and spread the stress over a A_High EKE. Within both the Geosat-ERM observations larger area. This affects the flux and current estimates in a and the A_Low experiment, the high EKE extends from the similar manner. Thus it is possible that the effects of the Korean coast to the Tsugaru Strait. In the A_High experiment, Reynolds stress are stronger and more localized along the the high EKE does not extend past the Yamato Rise. Polar Front and Nearshore Branch than is indicated by the Higher eddy activity south of the Polar Front has been results presented here. observed in prior studies (Ichiye and Takano, 1988), and the The effect of the geostrophic eddy Reynolds stress on altimeter Reynolds stress results indicate the higher eddy the mean flow (Fig. 6) indicates an eastward drift along the variability by the larger ellipses (Fig. 1). The model also Korean coast with the strongest eastward velocities near the produces higher EKE south of the Polar Front (Fig. 4). The Tsushima Strait. The suggestion is that the eddy variability variability north of 40°N is generally low except near the is a factor in separating the EKWC from the Korean coast or Tsugaru Strait outflow. The direction in which the velocity that the eddy variability broadens the mean current. The variations are highest at a given point is in the direction of EKWC in the A_High experiment indicates an overshoot to the ellipse principle axis (Fig. 1). A majority of the velocity a point between 40° and 42°N. In the A_Low experiment, variability throughout the JES is in the north-south direction. the EKWC separates from the coast near the observed This is indicative that the mesoscale field is composed latitude. Thus the model also indicates the importance of the mainly of meanders of the Polar Front and Nearshore Branch mesoscale eddy field for the EKWC separation. rather than rings separating from these currents. The meanders create larger north-south velocity variations than east-west velocity variations. For example, assume the velocity along the Polar Front is a constant v, and a southward meander passes by a point south of the front. The east-west velocity varies between zero and v, while the north-south velocity varies from Ðv to +v. Thus the variance of the north-south velocity is larger than the east-west velocity variance. Rings, on the other hand, generate more isotropic velocity varia- tions. The altimeter sampling and noise also generate biases that produce anisotropic errors in the Reynolds stresses. A portion of the north-south orientation may also be due to the altimeter noise. The Reynolds stress generated by the mesoscale vari- ability may be viewed as an eddy momentum flux or forcing on the flow. A strong northward flux exists at the Tsushima Strait entrance to the JES (Fig. 2). There is also a northward eddy momentum flux along the Korean coast from the Tsushima Strait to 41°N. The northward flux extends from the Korean coast at 41°N to the Tsugaru Strait. The north- ward flux along the Korean coast and along the Polar Front to the Tsugaru Strait is associated with the EKWC and Polar Front, respectively. Along the Japan coast, the momentum flux from the Tsushima Strait to the Tsugaru Strait is generally perpendicular away from the coast. The flux away from the coast continues north of the Tsugaru Strait to 43°N. North of the Polar Front, the flux is much reduced. The flux Fig. 6. The effect of the Reynolds stresses on the mean flow indicates a divergence from the Korean peninsula as well as an is weakly northward from 42°N to 44°N. From 44°N to ° intensification of the Polar Front and the cyclonic circulation 48 N, the momentum flux is southward, and the flux mag- north of the Polar Front.

254 G. A. Jacobs et al. Hurlburt and Metzger (1998) show the importance of and “Dynamical Linkage of the Asian Marginal Seas”. The numerical model resolution on the mesoscale field by 1/32° simulations were performed on the Cray T3E at the demonstrating the effect of eddy variability in the Kuroshio Naval Oceanographic Office and the Cray T3E at the Army Extension on the Kuroshio bifurcation at the Shatsky Rise. High Performance Computer Resource Center under grants The mechanism through which the eddy field affects the of computer time for the Department of Defense High mean circulation is complex, but involves the upper oceanÐ Performance Computing Initiative. This work is a contribu- topographical coupling discussed in Section 3. The net tion of the Naval Research Laboratory, paper number JA/ result is that including realistic eddy variability in numerical 7323-98-0059. models causes the baroclinic flow to be more sensitive to the bottom topography. Thus, the mean flow is related to the References bottom topography, and because the eddy field derives An, H., K. Shim and H. R. Shin (1994): On the warm eddies in the energy from the mean flow the eddy field is related to the southwestern part of the East Sea (the Japan Sea). J. Korean bottom topography. The experiments by Holloway et al. Soc. Oceanogr., 29, No. 2, 152Ð163. (1995) include the topostress to parameterize the coupling Cox, M. D. (1984): A primitive equation, 3-dimensional model of the ocean. GFDL Ocean Group Technical Report No. 1, between the mean circulation and the topography in numerical GFDL/NOAA, Princeton Univ. models that are not eddy resolving. The experiments includ- Eby, M. and G. Holloway (1994): Sensitivity of a large scale ing topostress also underscore the importance of the me- ocean model to a parameterization of topographic stress. J. soscale circulation on the mean flow. Phys. Oceanogr., 24, 2577Ð2588. Studies of the JES eddy field from observations have Hellerman, S. and M. Rosenstein (1983): Normal monthly wind suggested a relation of both the mean and eddy circulation stress over the world ocean with error estimates. J. Phys. to the bottom topography (Toba et al., 1984; An et al., 1994; Oceanogr., 13, 1093Ð1104. Lie et al., 1995). The results here provide evidence sup- Hogan, P. J. and H. E. Hurlburt (1998): Impact of upper oceanÐ portive of these studies. The EKE in simulation A_Low topographic coupling and isopycnal outcropping in Japan/ ° ° (Fig. 4) is characterized by a band of high EKE extending East Sea models with 1/8 to 1/64 resolution. J. Phys. from the Korean coast between 37° and 38°N across the Oceanogr. (submitted). Holloway, G. (1992): Representing topographic stress for large northern Ullung Basin and the northern Yamato Rise to the scale ocean models. J. Phys. Oceanogr., 22, 1033Ð1046. Tsugaru Strait. This particular band lies between the 2000 Holloway, G., T. Sou and M. Eby (1995): Dynamics of circulation and 3000 m isobaths. A second high EKE band extends from in the Japan Seas. J. Marine Res., 53, 539Ð569. the Tsushima Strait along the Japan coast toward the Tsugaru Hurlburt, H. E. and E. J. Metzger (1998): Bifurcation of the Strait. Both bands show the influence of the bottom topog- Kuroshio Extension at the Shatsky Rise. J. Geophys. Res., raphy on the distribution of the surface layer EKE in the JES. 103(C4), 7549Ð7566. Hurlburt, H. E. and J. D. Thompson (1980): A numerical study of 5. Conclusions Loop Current intrusions and eddy-shedding. J. Phys. From the altimeter data results, the mesoscale eddy Oceanogr., 10, 1611Ð1651. field increases the cyclonic circulation north of the Polar Hurlburt, H. E., A. J. Wallcraft, W. J. Schmitz, Jr., P. J. Hogan and Front and indicates a possible influence on the separation of E. J. Metzger (1996): Dynamics of the Kuroshio/Oyashio current system using eddy-resolving models of the North the EKWC from the Korean peninsula. In addition, the Pacific Ocean. J. Geophys. Res., 101, 941Ð976. model simulations indicate that the mesoscale field is im- Ichiye, T. and K. Takano (1988): Mesoscale eddies in the Japan portant in the proper separation of the EKWC at the ob- Sea. La Mer, 26, 69Ð75. served latitude. The model experiments with the eddy field Isoda, Y. (1994): Warm eddy movements in the eastern Japan Sea. suppressed show the EKWC overshooting the observed J. Oceanogr., 50, 1Ð15. separation latitude, and the EKE does not extend far eastward. Isoda, Y. and S. I. Saitoh (1993): The northward intruding eddy Only in the high resolution, low viscosity experiment do the along the East Coast of Korea. J. Oceanogr., 49, 443Ð458. EKE and mean currents appear to be strongly related to the Jacobs, G. A. and J. L. Mitchell (1997): Combining multiple bottom topography. altimeter missions. J. Geophys. Res., 102(C10), 23,187Ð23,206. Jacobs, G. A., G. H. Born, M. E. Parke and P. C. Allen (1992): The Acknowledgements global structure of the annual and semiannual sea surface height variability from Geosat altimeter data. J. Geophys. Res., We thank two anonymous reviewers and Professor 97(C11), 17,813Ð17,828. Akira Masuda for valuable insight and suggestions that have Le Provost, C., M. L. Genco, F. Lyard, P. Vincent and P. Canceil improved this paper. This work was sponsored by the Office (1994): Spectroscopy of the world ocean tides form a finite of Naval Research (program element PE0601153N) as part element hydrodynamic model. J. Geophys. Res., 99(C12), of the projects “Japan (East) Sea Dynamics Using Numerical 24,777Ð24,798. Models with 1/8 degree to 1/64 degree Resolution”, “Yel- Lie, H. J., S. K. Byun, I. Band and C. H. Cho (1995): Physical low and East China Seas Response to Winds and Currents”, structure of eddies in the southwestern East Sea. J. Korean Soc.

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