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

Transcription

1 Journal of Oceanography, Vol. 55, pp. 247 to Effects of Eddy Variability on the Circulation of the Japan/ East Sea G. A. JACOBS 1, P. J. HOGAN 1 AND K. R. WHITMER 2 1 Naval Research Laboratory, Stennis Space Center, Mississippi, U.S.A. 2 Sverdrup 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 examined from satellite altimeter data and results from the Naval Research Laboratory Layered Ocean Model (NLOM). Sea surface height variations from the Geosat-Exact Repeat Mission and TOPEX/POSEIDON altimeter satellites imply geostrophic velocities. At the satellite crossover points, the total velocity and the Reynolds stress due to geostrophic mesoscale turbulence are calculated. After spatial interpolation the momentum 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 ocean topographic coupling. Keywords: Japan Sea, eddies, altimeter, numerical modeling, Reynolds stress. 1. Introduction The Japan/East Sea (JES) general circulation is forced primarily by the inflow through the Tsushima Strait from the East China Sea and by the outflow through the Tsugaru and Soya Straits. Tsushima Island divides the strait into east and west channels. The total transport through the strait varies significantly in time, as does the transport distribution between the eastern and western branches. The main flow occurs through the west channel with a smaller portion through the east channel. The west channel flow enters the JES as the East Korean Warm Current (EKWC) and generally separates from the Korean coast between 37 and 39 N. The east channel flow enters the JES as the Nearshore Branch and generally follows the Japanese coast to the Tsugaru Strait. Wind stress also plays a part in the JES circulation with the wind stress curl aiding in generating the cyclonic circulation north of the front. Preller and Hogan (1998) present a review of the historical research in the JES. Particularly ubiquitous in the JES is the mesoscale eddy field. In situ eddy observations are abundant throughout the JES (Toba et al., 1984; Ichiye and Takano, 1988; Isoda and Saitoh, 1993; An et al., 1994; Isoda, 1994; Lie et al., 1995; Shin et al., 1995). Eddies have been observed extensively through temperature measurements off the Korean coast in the Ullung basin (Isoda and Saitoh, 1993; An et al., 1994; Lie et al., 1995). The eddy variability has been observed through analysis of sea surface temperature (SST) data (Toba et al., 1984; Miyao, 1994). The length and time scales of the eddy field have been well characterized through these studies. The propagation of individual eddies has also been documented through the SST studies (Isoda and Saitoh, 1993; An et al., 1994; Isoda, 1994). Using SST to perform an evaluation of turbulence generated by the eddy field in the JES, Toba et al. (1984) demonstrate that the synoptic JES circulation is dominated by mesoscale eddies. The mean flow is visible only after averaging over long time periods, suggesting a strong influence of the eddy field on the mean flow. In pursuit of the same goal, we use two tools to attempt to understand eddy variability effects that occur in the JES. In this study we use sea surface height (SSH) data from the Geosat-Exact Repeat Mission (Geosat-ERM) and TOPEX/ POSEIDON (T/P) altimeters. Measuring the SSH, the altimeter provides geostrophic speed estimates in a direction perpendicular to the satellite ground track. At points where each altimeter s ground track crosses itself, the two crosstrack estimates of geostrophic velocity may be transformed into eastward and northward velocity components. From these velocity components, the Reynolds stresses are estimated at crossover points (Parke et al., 1987). Gradients of the Reynolds stresses produce a force on the mean flow, and Copyright The Oceanographic Society of Japan. 247

2 the effects of this force may be examined through a balance with the Coriolis force. This provides a balance similar to the balance between the horizontal pressure gradient and Coriolis force, and this is one method to understand the relative importance of the geostrophic turbulence. Numerical models provide a useful capability to add and remove dynamical processes and then examine the effects. Hogan and Hurlburt (1998) investigated the dynamics of the JES in a progressive fashion using the Naval Research Laboratory s (NRL) Layered Ocean Model (NLOM). In this study we use the NLOM with horizontal grid resolution of 1/32 (3.5 km) to simulate the mesoscale variability. To contrast the circulation with eddy variability to the circulation with reduced variability, we perform simulations with higher eddy viscosity that suppresses the eddy variability. At least 1/32 resolution is necessary to resolve the mesoscale features sufficiently to ensure upper ocean topographic coupling in the JES (Hogan and Hurlburt, 1998). In one 1/32 experiment, the eddy viscosity is set to a relatively large value to intentionally suppress the mesoscale eddy field (experiment A_High). The other 1/32 experiment uses a more realistic value of eddy viscosity and contains much more energy in the eddy field (experiment A_Low). The mean circulation based on the two simulations represents the effects of the mesoscale variability on the mean flow. At low horizontal model grid resolution, the EKWC typically separates from the Korean coast at a latitude further north than the observed separation latitude (an overshoot). Sueng and Kim (1993) indicate that the EKWC overshoot problem is diminished (but not eliminated) by increasing horizontal resolution to 1/5 in the Cox (1984) ocean model. One effect of the increased model resolution is to better resolve the mesoscale field, and thus the model will represent the eddy field more accurately. The interaction of the eddy field with the bottom topography has been indicated in several JES in situ studies (Toba et al., 1984; An et al., 1994). Holloway et al. (1995) use the Modular Ocean Model (MOM) at 1/5 resolution with the addition of topostress to understand the effects of seasonality, wind stress, buoyancy forcing, latitudinal variations in Coriolis parameter, and bottom interaction in the JES. The topostress parameterizes the topographic influence on the circulation (Holloway, 1992). Without topostress, the model of Holloway et al. (1995) indicates the EKWC separating from the coast at a latitude further north than observed (similar to Sueng and Kim, 1993). The topostress has the effect of increasing the strength of the North Korean Cold Current (NKCC), which causes the EKWC to separate closer to the observed latitude. It is the interaction between the eddy field and the bottom topography that the topostress of Holloway (1992) intends to parameterize for numerical models that do not sufficiently resolve the mesoscale variability. To accurately model the mesoscale field, Hogan and Hurlburt (1998) indicate a horizontal resolution of 1/32 is required to generate the interaction between the eddy field and bottom topography without the parameterization of topostress. Using this resolution, we examine two mechanisms through which the mesoscale field may influence the mean circulation. Both the Reynolds stresses and the upper ocean topographic coupling indicate influence on the mean. In this examination, we compute geostrophic velocities from both the Geosat-ERM and T/P satellites at each of their respective crossover points (Section 2). The numerical model experiments of the isopycnal model then demonstrate the effects of eddy variability on the mean circulation (Section 3). The results are discussed in Section Altimeter Data Sets and Reynolds Stresses This study uses altimeter data from the T/P and Geosat- ERM satellites. T/P data provided on the merged Geophysical Data Records (GDRs) from the Physical Oceanography Distributed Active Archive Center (PO-DAAC) over the time period are first broken into arcs composed of one satellite revolution. Arcs that fall on the same ground track are grouped into a set of repeat (or collinear) passes. Computed orbits for T/P are based on the Joint Gravity Model (JGM)-3 gravity field (Tapley et al., 1996). Atmospheric corrections (dry troposphere, wet troposphere, and ionosphere) are applied, as well as solid Earth tides, an ocean tide estimate from the Grenoble model (Le Provost et al., 1994), tidal loading, an electromagnetic (EM) bias, and an inverse barometer correction based on the local instantaneous barometric pressure. The National Oceanic and Atmospheric Administration (NOAA) provides the Geosat-ERM data. The orbit solutions in the Geosat-ERM altimeter data set are also based on JGM-3. Atmospheric corrections (dry troposphere, wet troposphere, and ionosphere from the International Reference Ionosphere 1995, IRI95) are applied, as well as solid and ocean tide estimates (Le Provost et al., 1994), an EM bias correction of 2.5% of the significant wave height (Witter and Chelton, 1991), and a static inverse barometer correction. SSH from all repeat passes is interpolated along ground tracks to points spaced by 1 s (i.e., about 6.5 km along track), and the mean SSH at each point along the ground tracks is subtracted removing both the geoid signal and the mean dynamic topography. The T/P data covers 5 years while the Geosat-ERM covers only about 2.5 years. The mean SSH removed from each satellite s data set is the mean over its own time period. There are about 65 measurements at each Geosat-ERM ground track point versus about 180 for T/P. Because the Reynolds stresses are statistical averages, the Reynolds stresses based on the Geosat-ERM data are expected to contain larger errors. Some differences between the T/P and Geosat-ERM Reynolds stresses are also expected due to the interannual variations in the eddy field. 248 G. A. Jacobs et al.

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

4 where θ is the angle between the eastward direction and the ascending ground track, and the direction of positive crosstrack 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 (v a and v d ) 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 u u = 1 N N j =1 u j u j ( 2a) u v = 1 N v v = 1 N N j =1 N j =1 u j v j v j v j ( 2b) ( 2c) Fig. 2. The gradient of the Reynolds stresses after the data from the Geosat-ERM and TOPEX/POSEIDON have been interpolated spatially may be viewed as either a momentum flux or an additional forcing to the dynamical equations. The major flux areas are associated with the East Korean Warm Current, the 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 stress ellipses based on the Reynolds stress estimates are generated for both T/P and for the Geosat-ERM (Fig. 1). The distance from the center to the edge of the ellipse in the given direction is the variability of the currents in the prescribed direction. Thus the ellipse principal axis points in the direction of the major flow variability. The horizontal Reynolds stresses enter into the linearized unforced dynamical equations for the horizontal flow (ignoring molecular viscosity and boundary stresses) by u t = 1 p ρ x + fv u u x u v y ( 3a) v t = 1 p v v u v fu ρ y y x. ( 3b) The overbars indicate a long period mean, the primes indicate the deviation from the mean, f is the Coriolis parameter, and p is the pressure. The horizontal derivatives of the Reynolds stresses act as forcing on the mean circulation. The Reynolds stresses from the Geosat-ERM and T/P data are interpolated to a regular 1/4 grid by a weighted averaging as described by Zlotnicki et al. (1989). The weighting is a given by a Gaussian function with a 150 km e-folding length scale. The e-folding scale is larger than the typical eddy size (between 50 to 150 km). The scales of the Reynolds stresses (Figs. 1(a) and (b)) are generally larger than 150 km except near the Polar Front. In this region some smoothing of the Reynolds stresses occurs in the interpolation procedure. The net result is a reduction in the subsequent effective fluxes, forcing, and effect on the geostrophic flow. The effective forcing induced (Fig. 2) is the vector defined by the derivatives of the Reynolds stress on the right 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 mean flow (pressure gradient, Coriolis force, and Reynolds stress force) balance. If the Reynolds stresses were zero, a geostrophic balance would give the mean flow. There are several ways to interpret the Reynolds stress effects. One interpretation is that the Reynolds stress alters the pressure gradient that would balance the Coriolis force. This first interpretation approaches the problem from the point of 250 G. A. Jacobs et al.

5 view that we prescribe (or measure) the velocity field, and the pressure gradient is the dependent variable. Another interpretation is that the Reynolds stress effects are linearly additive to the geostrophic flow. This second approach assumes the pressure gradient is prescribed and that the mean velocity is the dependent variable. Thus the Reynolds stress effects on the mean flow may be examined by the velocity induced by balancing the Reynolds stress force with the Coriolis force v = 1 f u u x + u = 1 v v f y u v y + u v x. ( 4a) ( 4b) 3. Numerical Model Results Numerical model experiments are performed to examine the influence of the eddy variability. The simulations are conducted with the same numerical model using different eddy viscosities to determine the effects on the mesoscale variability. For the experiments, the horizontal friction prescribed in the numerical model dynamics is Laplacian given by the form A[ (h )] v, where A is the eddy viscosity, h is the layer thickness, and v is the velocity vector (Hogan and Hurlburt, 1998). The two eddy viscosities used are 50 m 2 /s (A_High) and 5 m 2 /s (A_Low), and the grid resolution is 1/32 latitude and 45/1024 longitude (distance between like variables). Two model tests are thus made, and we refer to these tests as A_High and A_Low. The model is a primitive equation formulation with the vertical structure described by Lagrangian layers. The layers are capable of representing the barotropic and baroclinic ocean structure with a few layers. Each of the 4 layers in the model represents the vertically integrated momentum equations, and the interfaces between layers represent isopycnals. The model has a free surface and contains realistic bottom topography that is restricted to the bottom layer. Hurlburt and Thompson (1980) describe the basic model formulation and Cartesian numerics in detail, and Wallcraft (1991) introduces substantial enhancements. Moore and Wallcraft (1998) discuss the mathematical and numerical formulation in spherical coordinates. The JES model mean interface depths are 60 m, 135 m, and 250 m. The top three layers represent the warm saline inflow from the Tsushima Strait and the JES Intermediate Water, while the lowest layer represents the JES Proper Water. The Tsushima Strait mean transport is 2.0 Sv with a seasonal variation that has 2.66 Sv peak transport in July and 1.34 Sv minimum transport in January. At all times, 75% of the transport passes through the western channel. The top layer contains two thirds of the transport with the remaining third in the second layer. The lower layers are closed with a no-slip boundary condition. The JES outflow vertical distribution is identical to the inflow with two thirds of the outflow through the Tsugaru Strait and one third through the Soya Strait. The Hellerman-Rosenstein (1983) monthly wind stress climatology provides wind forcing. The model simulations are each integrated to statistical equilibrium, and an additional 10 year s of integration are used to form the averages. Hogan and Hurlburt (1998) discuss the JES model in detail. Dynamics of the model simulations from Hogan and Hurlburt (1998) are summarized as follows. Realistic separation of the EKWC from the coast of Korea is only achieved at 1/32 resolution or higher when forced with the Hellerman-Rosenstein wind stress climatology. Some atmospheric data sets can produce more realistic EKWC separation at coarser model resolution due to strong positive wind stress curl north of the separation latitude of the EKWC. However, simulations with insufficient horizontal grid resolution forced by alternate wind stresses do not adequately resolve mesoscale flow instabilities, which are needed to properly simulate the upper ocean topographic coupling. The upper ocean topographic coupling mechanism relies on the fact that baroclincally unstable surface layer currents are very efficient at transferring energy to the abyssal layer. This generates eddy-driven deep mean flows that are constrained to follow the f/h contours of the bottom topography. The deep flows in turn influence the surface circulation via a conservation of mass process described by Hurlburt and Thompson (1980) (also see Hurlburt et al. (1996) and Hurlburt and Metzger (1998)). This topographical effect is missed at coarser resolution, which can lead to erroneous conclusions about the role of the bottom topography and unexplained errors in the pathways of the current systems. In the A_High simulation, baroclinic instability is suppressed, upper ocean topographical coupling is greatly weakened, and the EKWC flows further to the north than the observed separation latitude (Fig. 4). 4. Discussion The mean flows of the model experiments contain dramatic differences. The A_High mean flow indicates that the EKWC overshoots to a latitude between 40 and 42 N (Fig. 3). Only in the A_Low experiment does the EKWC separate from the Korean coast between 37 and 39 N, which is much closer to the observed separation latitude (Fig. 3). The currents within the Polar Front are also stronger in the low viscosity experiment as demonstrated by the higher kinetic energy of the mean flow. The change in eddy variability in the model experiments causes the changes in the mean currents. The distribution and intensity of eddy kinetic energy changes when the model viscosity is altered (Fig. 4). The A_Low experiment contains an area of high EKE around the Ullung Basin and Effects of Eddy Variability on the Circulation of the Japan/East Sea 251

6 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 m 2 /s, and (b) 1/32 resolution and an eddy viscosity of 5 m 2 /s. The interval between color levels is.25 cm 2 /s 2. 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 m 2 /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 m 2 /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 m 2 /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 m 2 /s 2. 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.

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

8 This is due to the wind-driven setup and setdown on the continental shelf. The calculation of geostrophic currents from heights on the shelf is not accurate, and this leads to excessive EKE along the coastlines. The Geosat-ERM EKE contains generally higher EKE south of the Polar Front and very low EKE north of the Polar Front. The peak EKE occurs in the Ullung Basin and southeast of the Yamato Rise. The spatial distribution of the A_Low EKE (Fig. 4(a)) is in much better agreement with the Geosat-ERM EKE than is the A_High EKE. Within both the Geosat-ERM observations and the A_Low experiment, the high EKE extends from the Korean coast to the Tsugaru Strait. In the A_High experiment, the high EKE does not extend past the Yamato Rise. Higher eddy activity south of the Polar Front has been observed in prior studies (Ichiye and Takano, 1988), and the altimeter Reynolds stress results indicate the higher eddy variability by the larger ellipses (Fig. 1). The model also produces higher EKE south of the Polar Front (Fig. 4). The variability north of 40 N is generally low except near the Tsugaru Strait outflow. The direction in which the velocity variations are highest at a given point is in the direction of the ellipse principle axis (Fig. 1). A majority of the velocity variability throughout the JES is in the north-south direction. This is indicative that the mesoscale field is composed mainly of meanders of the Polar Front and Nearshore Branch 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 v 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 variations. 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 variability 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 northward 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 is weakly northward from 42 N to 44 N. From 44 N to 48 N, the momentum flux is southward, and the flux magnitude increases at 44 N. The effect of the Reynolds stresses on the mean flow (Fig. 6) is determined from (4) and indicates a strengthening of the general JES circulation. The induced flow along the Korean peninsula is in contrast to this, indicating a flow away from the coast. The general cyclonic circulation north of the Polar Front is also intensified. The effect of the spatial interpolation of the Reynolds stress is to reduce the magnitude of the peak Reynolds stress and spread the stress over a larger area. This affects the flux and current estimates in a similar manner. Thus it is possible that the effects of the Reynolds stress are stronger and more localized along the Polar Front and Nearshore Branch than is indicated by the results presented here. The effect of the geostrophic eddy Reynolds stress on the mean flow (Fig. 6) indicates an eastward drift along the Korean coast with the strongest eastward velocities near the Tsushima Strait. The suggestion is that the eddy variability is a factor in separating the EKWC from the Korean coast or that the eddy variability broadens the mean current. The EKWC in the A_High experiment indicates an overshoot to a point between 40 and 42 N. In the A_Low experiment, the EKWC separates from the coast near the observed latitude. Thus the model also indicates the importance of the mesoscale eddy field for the EKWC separation. Fig. 6. The effect of the Reynolds stresses on the mean flow indicates a divergence from the Korean peninsula as well as an intensification of the Polar Front and the cyclonic circulation north of the Polar Front. 254 G. A. Jacobs et al.

9 Hurlburt and Metzger (1998) show the importance of numerical model resolution on the mesoscale field by demonstrating the effect of eddy variability in the Kuroshio Extension on the Kuroshio bifurcation at the Shatsky Rise. The mechanism through which the eddy field affects the mean circulation is complex, but involves the upper ocean topographical coupling discussed in Section 3. The net result is that including realistic eddy variability in numerical models causes the baroclinic flow to be more sensitive to the bottom topography. Thus, the mean flow is related to the bottom topography, and because the eddy field derives energy from the mean flow the eddy field is related to the bottom topography. The experiments by Holloway et al. (1995) include the topostress to parameterize the coupling between the mean circulation and the topography in numerical models that are not eddy resolving. The experiments including topostress also underscore the importance of the mesoscale circulation on the mean flow. Studies of the JES eddy field from observations have suggested a relation of both the mean and eddy circulation to the bottom topography (Toba et al., 1984; An et al., 1994; Lie et al., 1995). The results here provide evidence supportive of these studies. The EKE in simulation A_Low (Fig. 4) is characterized by a band of high EKE extending from the Korean coast between 37 and 38 N across the northern Ullung Basin and the northern Yamato Rise to the Tsugaru Strait. This particular band lies between the 2000 and 3000 m isobaths. A second high EKE band extends from the Tsushima Strait along the Japan coast toward the Tsugaru Strait. Both bands show the influence of the bottom topography on the distribution of the surface layer EKE in the JES. 5. Conclusions From the altimeter data results, the mesoscale eddy field increases the cyclonic circulation north of the Polar Front and indicates a possible influence on the separation of the EKWC from the Korean peninsula. In addition, the model simulations indicate that the mesoscale field is important in the proper separation of the EKWC at the observed latitude. The model experiments with the eddy field suppressed show the EKWC overshooting the observed separation latitude, and the EKE does not extend far eastward. Only in the high resolution, low viscosity experiment do the EKE and mean currents appear to be strongly related to the bottom topography. Acknowledgements We thank two anonymous reviewers and Professor Akira Masuda for valuable insight and suggestions that have improved this paper. This work was sponsored by the Office of Naval Research (program element PE N) as part of the projects Japan (East) Sea Dynamics Using Numerical Models with 1/8 degree to 1/64 degree Resolution, Yellow and East China Seas Response to Winds and Currents, and Dynamical Linkage of the Asian Marginal Seas. The 1/32 simulations were performed on the Cray T3E at the Naval Oceanographic Office and the Cray T3E at the Army High Performance Computer Resource Center under grants of computer time for the Department of Defense High Performance Computing Initiative. This work is a contribution of the Naval Research Laboratory, paper number JA/ References An, H., K. Shim and H. R. Shin (1994): On the warm eddies in the southwestern part of the East Sea (the Japan Sea). J. Korean Soc. Oceanogr., 29, No. 2, Cox, M. D. (1984): A primitive equation, 3-dimensional model of the ocean. GFDL Ocean Group Technical Report No. 1, GFDL/NOAA, Princeton Univ. Eby, M. and G. 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