Gyre Mode Research Articles

On the anomalous structure of the Southeast Vietnam Offshore Current during 1994 to 2015

The summer circulation in the South China Sea (SCS) often has a dipole structure associated with the eastward Southeast Vietnam Offshore Current (SVOC). The dipole has an anti-cyclone to the south and a cyclone to the north. A 22-year (1994–2015) HYbrid Coordinate Ocean Model (HYCOM) global reanalysis is used to study why the SVOC in the SCS is anomalous in summer of 1995, 1998, 2007, 2010, and 2015. The normal SVOC Sea Surface Height (SSH) pattern is defined based on previous observations of the dipole associated with it. An Empirical Orthogonal Function (EOF) analysis is performed on the monthly mean SSH and wind stress curl to seek the answer. SSH EOF1 and EOF2 explain 43.3% and 18.9% of the total SSH variance, respectively. Wind stress curl EOF1 and EOF2 explain 63.4% and 14.9% of the total variance, respectively. SSH EOF1 is a basin-scale gyre mode that represents the barotropic response to the basin-scale monsoon wind stress curl EOF1. SSH EOF2 is a dipole mode that represents a baroclinic response to both wind stress curl EOF1 and EOF2 (western North Pacific anticyclone mode) via forced baroclinic Rossby waves. SSH EOF2 explains 39.6% of the SSH variance in the SVOC dipole and is more important for the SVOC formation than SSH EOF1, which explains 28.2% of the SVOC dipole SSH variance. SSH EOF2 principal component time series indicate that the summer dipole mode in 1995, 1998, 2010, and 2015 is associated with an anti-cyclone to the north and a cyclone to the south, opposite to the normal SVOC dipole. In 2007, the SSH EOF2 is very weak with its principal component close to zero.

Classification and Causes of East Asian Marine Heatwaves during Boreal Summer

Abstract Marine heatwaves (MHWs) are among the most severe climatic events under global warming conditions; they damage marine ecosystems and result in socioeconomic losses. In East Asia, the sea surface temperature (SST) has increased significantly and is projected to increase further. Considering such increases in SST over East Asia, studies on MHWs are needed to minimize the damage caused. In this study, we classified the spatiotemporal characteristics of East Asian MHWs during boreal summer with a self-organizing map for 39 years (1982–2020) and determined their unexplored possible mechanisms. Four dominant modes of MHWs across East Asia were identified: the 1) Subtropical Gyre, 2) East China Sea, 3) East Sea/Japan Sea, and 4) Yellow Sea modes. The total annual number of MHW days has increased by about 1.23 days yr−1. The significant trend is mainly determined by the Subtropical Gyre mode, whereas other modes do not exhibit significant trends. We found that the East China Sea mode could be attributed to a strong East Asian summer monsoon via ocean stratification by the low-salinity water from the Yangtze (Changjiang) River. The East Sea/Japan Sea and Yellow Sea modes could be linked to remote forcings over the subtropics, leading to persistent anticyclonic circulation over East Asia. The anomalous anticyclone contributed to the MHW occurrences by enhancing downward shortwave radiation and reducing entrainment cooling caused by Ekman downwelling. These findings have important implications for distinguishing the characteristics of East Asian MHWs and reveal the possibility of predicting them by identifying their precursors. Significance Statement This study aimed to better understand East Asian marine heatwaves (MHWs) and their causes. We utilized a self-organizing map to classify four patterns of MHWs during boreal summer across East Asia—the Subtropical Gyre, East China Sea, East Sea/Japan Sea, and Yellow Sea modes. We found that enhanced shortwave radiation and latent heat flux play an important role in modulating MHWs in East Asia. A reduced vertical-mixing-amplified ocean warming, and the ocean dynamics drove different temporal evolutions of the MHWs among the four modes. We also identified the potential precursors, which would help in improving the future prediction of MHWs in East Asia.

The Role of an Intense Jet in the Tsugaru Strait in the Formation of the Outflow Gyre Revealed Using High‐Frequency Radar Data

AbstractThe seasonal features of the eastward jet through the Tsugaru Strait was investigated using surface velocity data between 2014 and 2019 obtained from high‐frequency radar located in the eastern part of the strait, as well as data obtained from repeated shipboard observations from 2009 to 2019. The potential‐vorticity‐front‐model was used, and the volume transport of low potential vorticity (PV) water and the intensity of the PV gap at the front in the eastern part of the strait were estimated from these data. The influence of these parameters on the seasonal flow pattern of the Tsugaru Warm Current out to the Pacific Ocean (the summer/autumn “gyre mode” or winter/spring “coastal mode”) was examined. The distribution of the parameters was consistent with the theoretical understanding, showing that in addition to the volume transport, an increase in the local PV gap can also contribute to the development of the gyre.

Prediction of dominant intraseasonal modes in the East Asian-western North Pacific summer monsoon

Intraseasonal monsoon prediction is the most imperative task, but there remains an enduring challenge in climate science. The present study aims to provide a physical understanding of the sources for prediction of dominant intraseasonal modes in the East Asian-western North Pacific summer monsoon (EA-WNPSM): pre-Meiyu&Baiu, Changma&Meiyu, WNPSM, and monsoon gyre modes classified by the self-organizing map analysis. Here, we use stepwise regression to determine the predictors for the four modes in the EA-WNPSM. The selected predictors are based on the persistent and tendency signals of the sea surface temperature (SST)/2m air temperature and sea level pressure fields, which reflect the asymmetric response to the El Nino Southern Oscillation (ENSO) and the ocean and land surface anomalous conditions. For the pre-Meiyu&Baiu mode, the SST cooling tendency over the western North Pacific (WNP), which persists into summer, is the distinguishing contributor that results in strong baroclinic instability. A major precursor for the Changma&Meiyu mode is related to the WNP subtropical high, induced by the persistent SST difference between the Indian Ocean and the western Pacific. The WNPSM mode is mostly affected by the Pacific-Japan pattern, and monsoon gyre mode is primarily associated with a persistent SST cooling over the tropical Indian Ocean by the preceding ENSO signal. This study carries important implications for prediction by establishing valuable precursors of the four modes including nonlinear characteristics.

Assimilation of the seabird and ship drift data in the north-eastern sea of Japan into an operational ocean nowcast/forecast system.

At the present time, ocean current is being operationally monitored mainly by combined use of numerical ocean nowcast/forecast models and satellite remote sensing data. Improvement in the accuracy of the ocean current nowcast/forecast requires additional measurements with higher spatial and temporal resolution as expected from the current observation network. Here we show feasibility of assimilating high-resolution seabird and ship drift data into an operational ocean forecast system. Data assimilation of geostrophic current contained in the observed drift leads to refinement in the gyre mode events of the Tsugaru warm current in the north-eastern sea of Japan represented by the model. Fitting the observed drift to the model depends on ability of the drift representing geostrophic current compared to that representing directly wind driven components. A preferable horizontal scale of 50 km indicated for the seabird drift data assimilation implies their capability of capturing eddies with smaller horizontal scale than the minimum scale of 100 km resolved by the satellite altimetry. The present study actually demonstrates that transdisciplinary approaches combining bio-/ship- logging and numerical modeling could be effective for enhancement in monitoring the ocean current.

Interactions between Typhoon Megi (2010) and a Low-Frequency Monsoon Gyre

The ARW Model is used to investigate the sharp northward turn of Super Typhoon Megi (2010) after it moved westward and crossed the Philippines. The NCEP analyzed fields during this period are separated into a slowly varying background-flow component, a 10–60-day low-frequency component representing the monsoon gyre, and a 10-day high-pass-filtered component representing Megi and other synoptic-scale motion. It appears that the low-frequency (10–60 day) monsoon gyre interacted with Megi and affected its track. To investigate the effect of the low-frequency mode on Megi, numerical experiments were designed. In the control experiment, the total fields of the analysis are retained in the initial and boundary conditions, and the model is able to simulate Megi’s sharp northward turn. In the second experiment, the 10–60-day monsoon gyre mode is removed from the initial and lateral boundary fields, and Megi moves westward and slightly northwestward without turning north. Tracks of the relative positions between the Megi and the monsoon gyre centers suggest that a Fujiwhara effect may exist between the monsoon gyre and Megi. The northward turning of both Megi and the monsoon gyre occurred when the two centers were close to each other and the beta drift was enhanced. A vorticity budget analysis was conducted. It is noted that the Megi moves toward the maximum wavenumber-1 vorticity tendency. The sharp change of the maximum vorticity tendency direction before and after the track turning point is primarily attributed to the change of the horizontal vorticity advection. A further diagnosis shows that the steering of the vertically integrated low-frequency flow is crucial for the change of the horizontal advection tendency.

The Role of Closed Gyres in Setting the Zonal Transport of the Antarctic Circumpolar Current

AbstractEddy-permitting simulations are used to show that basinlike gyres can be observed in the large-scale barotropic flow of a wind-driven channel with a meridional topographic ridge. This is confirmed using both two-layer quasigeostrophic and 25-level primitive equation models at high horizontal resolution. Comparing results from simulations with and without the topographic ridge, it is shown that the zonal baroclinic transport in the channel increases with increasing wind stress when the bottom topography is flat but not when there is a meridional ridge. The saturation of transport for increasing wind occurs in conjunction with the development of recirculating gyres in the large-scale barotropic streamfunction. This suggests that the total circulation can be thought of as a superposition of a gyre mode (which has zero circumpolar transport) and a free circumpolar mode (which contains all of the transport). Basinlike gyres arise in the channel because the topography steers the barotropic streamlines and supports a frictional boundary layer similar to the more familiar ones observed along western boundaries. The gyre mode is thus closely linked with the bottom form stress exerted by the along-ridge flow and provides the sink for the wind momentum input. In this framework, any increase in wind forcing spins a stronger gyre as opposed to feeding the circumpolar transport. This hypothesis is supported with a suite of experiments where key parameters are carried over a wide range: wind stress, wind stress curl, ridge height, channel length, and bottom friction.

Northern North Atlantic sea surface height and ocean heat content variability

The evolution of nearly 20 years of altimetric sea surface height (SSH) is investigated to understand its association with decadal to multidecadal variability of the North Atlantic heat content. Altimetric SSH is dominated by an increase of about 14 cm in the Labrador and Irminger seas from 1993 to 2011, while the opposite has occurred over the Gulf Stream region over the same time period. During the altimeter period the observed 0-700 m ocean heat content (OHC) in the subpolar gyre mirrors the increased SSH by its dominantly positive trend. Over a longer period, 1955-2011, fluctuations in the subpolar OHC reflect Atlantic multidecadal variability (AMV) and can be attributed to advection driven by the wind stress ''gyre mode'' bringing more subtropical waters into the subpolar gyre. The extended subpolar warming evident in SSH and OHC during the altimeter period represents transition of the AMV from cold to warm phase. In addition to the dominant trend, the first empirical orthogonal function SSH time series shows an abrupt change 2009-2010 reaching a new minimum in 2010. The change coincides with the change in the meridional overturning circulation at 26.5N as observed by the RAPID (Rapid Climate Change) project, and with extreme behavior of the wind stress gyre mode and of atmospheric blocking. While the general relationship between northern warming and Atlantic meridional overturning circulation (AMOC) volume transport remains undetermined, the meridional heat and salt transport carried by AMOC's arteries are rich with decade-to-century timescale variability.

Elongation and Contraction of the Western Boundary Current Extension in a Shallow-Water Model: A Bifurcation Analysis

Abstract The double-gyre circulation, formulated in terms of the quasigeostrophic equations, has a symmetry about the basin midlatitude (y → −y, ψ → −ψ), which is absent in a formulation based on the shallow-water equations. As a result, the shallow-water model does not have the pitchfork bifurcation structures that, in the case of the quasigeostrophic model, connect together multiple solution branches with elongated and contracted recirculation gyres. For the shallow-water model, solution branches with elongated recirculation gyres are disconnected, and a one-parameter bifurcation analysis is unable to detect their existence. The deeply penetrating jet solution branches do, however, continue to exist, and can be found using a bifurcation analysis couched in terms of two parameters. An effective pair of parameters is the viscosity and a parameter controlling the symmetry of the wind stress profile. A bifurcation analysis with these parameters reveals the existence of new solution branches that were not found in previous bifurcation analyses of the shallow-water model. The new solutions have a jet extension that penetrates farther eastward and that is more stable than the jet-up and jet-down solutions found in previous studies. Furthermore, the origin of the low-frequency variability at low viscosities is associated with a sequence of bifurcations originating from one of the new steady-state solution branches. In particular, the eigenmode analysis of the new branch reveals that a so-called gyre mode is at the origin of the model’s low-frequency variability at decadal time scales.

Wind-driven, double-gyre, ocean circulation in a reduced-gravity, 2.5-layer, lattice boltzmann model

A coupled lattice Boltzmann (LB) model with second-order accuracy is applied to the reduced-gravity, shallow water, 2.5-layer model for wind-driven double-gyre ocean circulation. By introducing the second-order integral approximation for the collision operator, the model becomes fully explicit. The Coriolis force and other external forces are included in the model with second-order accuracy, which is consistent with the discretization accuracy of the LB equation. The feature of the multiple equilibria solutions is found in the numerical experiments under different Reynolds numbers based on this LB scheme. With the Reynolds number increasing from 3000 to 4000, the solution of this model is destabilized from the anti-symmetric double-gyre solution to the subtropic gyre solution and then to the subpolar gyre solution. The transitions between these equilibria states are also found in some parameter ranges. The time-dependent variability of the circulation based on this LB simulation is also discussed for varying viscosity regimes. The flow of this model exhibits oscillations with different timescales varying from subannual to interannual. The corresponding statistical oscillation modes are obtained by spectral analysis. By analyzing the spatiotemporal structures of these modes, it is found that the subannual oscillation with a 9-month period originates from the barotropic Rossby basin mode, and the interannual oscillations with periods ranging from 1.5 years to 4.6 years originate from the recirculation gyre modes, which include the barotropic and the baroclinic recirculation gyre modes.

Quantization of the Low-Frequency Variability of the Double-Gyre Circulation

AbstractThe low-frequency dynamics of the double-gyre wind-driven circulation in large midlatitude oceanic basins is investigated. It is shown that for quasigeostrophic models linear (Rayleigh) friction is necessary to obtain realistic recirculation gyres and elongated jet streams with small meridional-to-zonal aspect ratio. It is also found that the use of either no-slip or free-slip boundary conditions does not change the drastic effects of bottom drag on the large scales. These long oceanic jets are alternatively destabilized and restabilized through successive (subcritical) supercritical symmetry-breaking bifurcations that are linked to the (non) existence of stationary Rossby waves. These waves are strongly localized along the oceanic front and are thus hardly affected by the basin geometry. Numerical and analytical results show that these waves are “quantized” with respect to the length of the jet, and an explicit dispersion relation is given. Numerical computations of branches of steady states, together with linear and nonlinear analysis, indicate that two classes of regimes characterize the low-frequency dynamics of the flow. The first class corresponds to supercritical regimes, which are associated with oceanic jets that have been destabilized by undamped stationary Rossby waves. In particular, these regimes allow the formation of gyre modes responsible for low-frequency relaxation oscillations of the jet. The second class corresponds to subcritical regimes, which are either quiescent or dominated by high-frequency instabilities and are characterized by jets that do not allow the formation of both stationary Rossby waves and gyre modes. Each of these regimes is characterized by typical spatial and time scales that are both quantized. The number n of “bumps” of the jet, which is related to the zonal wavenumber of the stationary Rossby waves, is used to distinguish between these regimes either in their supercritical or subcritical phase. For instance, the supercritical n = 2 regime is associated with a class of interdecadal gyre modes that extend up to 3000 km in the zonal direction. The quantization of the low-frequency dynamics and the existence of these regimes are also found to survive severe modifications of the basin geometry. These quantized regimes suggest that the low-frequency dynamics in turbulent regimes is likely to be autosimilar to the low-frequency dynamics found in a weakly nonlinear “ground” regime corresponding to n = 0.

Homoclinic bifurcations in the quasi-geostrophic double-gyre circulation

The wind-driven double-gyre circulation in a rectangular basin goes through several dynamical regimes as the amount of lateral friction is decreased. This paper studies the transition to irregular flow in the double-gyre circulation by applying dynamical systems methodology to a quasigeostrophic, equivalent-barotropic model with a 10-km resolution. The origin of the irregularities, in space and time, is the occurrence of homoclinic bifurcations that involve phase-space behavior far from stationary solutions. The connection between these homoclinic bifurcations and earlier transitions, which occur at larger lateral friction, is explained. The earlier transitions, such as pitchfork and asymmetric Hopf bifurcation, only involve the nonlinear saturation of linear instabilities, while the homoclinic bifurcations are associated with genuinely nonlinear behavior. The sequence of bifurcations—pitchfork, Hopf, and homoclinic—is independent of the lateral friction and may be described as the unfolding of a singularity that occurs in the frictionless, Hamiltonian limit of the governing equations. Two distinct chaotic regimes are identified: Lorenz chaos at relatively large lateral friction versus Shilnikov chaos at relatively small lateral friction. Both types of homoclinic bifurcations induce chaotic behavior of the recirculation gyres that is dominated by relaxation oscillations with a well-defined period. The relevance of these results to the mid-latitude oceans’ observed low-frequency variations is discussed. A previously documented 7-year peak in observed North-Atlantic variability is shown to exist across a hierarchy of models that share the gyre modes and homoclinic bifurcations discussed herein.

Mechanisms of Decadal Variability of the Wind-Driven Ocean Circulation

Abstract Eddy-resolving quasigeostrophic simulations of wind-driven circulation in a large ocean basin are presented. The results show that strong modes of low-frequency variability arise in many parameter regimes and that the strength of these modes depends upon the presence of inertial recirculations in the flow field. The inertial recirculations arise through advection of anomalous potential vorticity by the western boundary current and are barotropized by the effect of baroclinic eddies in the flow. The mechanism of low-frequency oscillations is explored with reference to previous studies, and it is found that the observed mode can be linked to the gyre mode but is strongly modified by the effect of eddies.

Regimes of low-frequency variability in a three-layer quasi-geostrophic ocean model

The temporal variability of the midlatitude double-gyre wind-driven ocean circulation is studied in a three-layer quasi-geostrophic model over a broad range in parameter space. Four different types of flow regimes are found, each characterized by a specific time-mean state and spatio-temporal variability. As the lateral friction is decreased, these regimes are encountered in the following order: the viscous antisymmetric regime, the asymmetric regime, the quasi-homoclinic regime and the inertial antisymmetric regime. The variability in the viscous and the inertial antisymmetric regimes (at high and low lateral friction, respectively) is mainly caused by Rossby basin modes. Lowfrequency variability, i.e. on interannual to decadal time-scales, is present in the asymmetric and quasi-homoclinic regime and can be related to relaxation oscillations originating from low-frequency gyre modes. The focus of this paper is on the mechanisms of the transitions between the different regimes. The transition from the viscous antisymmetric regime to the asymmetric regime occurs through a symmetry-breaking pitchfork bifurcation. There are strong indications that the quasihomoclinic regime is introduced through the existence of a homoclinic orbit. The transition to the inertial antisymmetric regime is due to the symmetrization of the time-mean state zonal velocity field through rectification effects.

Spontaneous Generation of Low-Frequency Modes of Variability in the Wind-Driven Ocean Circulation

In idealized models that aim to understand the temporal variability of the wind-driven ocean circulation, lowfrequency instabilities associated with so-called oscillatory gyre modes have been found. For the double-gyre case, the spectral origin of these modes as well as the physical mechanism of the instability is explained. In a barotropic quasigeostrophic model, the low-frequency modes arise spontaneously from the merging between two nonoscillatory eigenmodes. Of the latter two, one is called here the P-mode and is responsible for the existence of multiple steady states. The other is called the L-mode and it controls the intensity of the gyres. This merging turns out to be robust over a hierarchy of models and can even be found in a low-order truncated quasigeostrophic model. The latter model is used to determine the physical mechanism of the instability. The low-frequency oscillation results from the conjugate effects of shear- and symmetry-breaking instabilities and is free of Rossby wave dynamics.

Development of Ocean Pollution Prediction System for Shimokita Region Model Development and Verification

A three-dimensional model system has been developed to predict oceanic dispersions of radionuclides released into the off Shimokita region. This prediction system is a combination of the Princeton Ocean Model (POM) for predicting ocean currents and a particle random walk model for oceanic dispersion of radionuclides. The model has been verified by using measured currents, temperature and salinity at the coastal area of Shimokita, Aomori-ken, Japan, where a nuclear fuel reprocessing plant is under construction. The results obtained from simulations are as follows; (1) Sea surface wind and the Tsugaru Warm Current (TWC) entering into the objective region through the Tsugaru Strait significantly affect the structure of current field over the region. (2) POM can represent seasonal variations of the TWC well with the input of monthly inflow data of the TWC. The calculation succeeded in reproducing the coastal mode from winter to spring and the gyre mode from summer to autumn.

Four Decadal Ocean-Atmosphere Modes in the North Pacific Revealed by Various Analysis Methods

Various statistical methods (empirical orthogonal function (EOF), rotated EOF, singular value decomposition (SVD), principal oscillation pattern (POP), complex EOF (CEOF) and joint CEOF) were applied to low-pass filtered (>7 years) sea surface temperature (SST), subsurface temperature and 500 hPa geopotential height in order to reveal standing and propagating features of decadal variations in the North Pacific. Four decadal ocean-atmosphere covariant modes were found in this study. The first mode is the well-known ENSO-like mode associated with the “Pacific-North American” atmospheric pattern, showing SST variations reversed between the tropics and the extratropics. In the western tropical Pacific, subsurface temperature variations were found to be out of phase with the SST variations. The other three modes are related to the oceanic general circulation composed of the subtropical gyre, the Alaskan gyre and the subpolar gyre, respectively. The 1988/89 event in the northern North Pacific was found to be closely associated with the subtropical gyre mode, and the atmospheric pattern associated with this mode is the Arctic Oscillation. An upper ocean heat budget analysis suggests that the surface net heat flux and mean gyre advection are important to the Alaskan gyre mode. For the subpolar gyre mode, the mean gyre advection, local Ekman pumping and surface net heat flux play important roles. Possible air-sea interactions in the North Pacific are also discussed. The oceanic signals for these decadal modes occupy a thick layer in the North Pacific, so that accumulated heat content may in turn support long-term climate variations.

Development of Ocean Pollution Prediction System for Shimokita Region. Model Development and Verification.

A three-dimensional model system has been developed to predict oceanic dispersions of radionuclides released into the off Shimokita region. This prediction system is a combination of the Princeton Ocean Model (POM) for predicting ocean currents and a particle random walk model for oceanic dispersion of radionuclides. The model has been verified by using measured currents, temperature and salinity at the coastal area of Shimokita, Aomori-ken, Japan, where a nuclear fuel reprocessing plant is under construction. The results obtained from simulations are as follows; (1) Sea surface wind and the Tsugaru Warm Current (TWC) entering into the objective region through the Tsugaru Strait significantly affect the structure of current field over the region. (2) POM can represent seasonal variations of the TWC well with the input of monthly inflow data of the TWC. The calculation succeeded in reproducing the coastal mode from winter to spring and the gyre mode from summer to autumn.

Analytical Prototypes for Ocean–Atmosphere Interaction at Midlatitudes. Part II: Mechanisms for Coupled Gyre Modes*

A simple midlatitude coupled model for idealized ocean basins is used to investigate processes of ocean‐ atmosphere interaction and its role in interdecadal climate variability at midlatitudes. The ocean model consists of a linearized quasigeostrophic upper ocean layer and a sea surface temperature (SST) equation for an embedded surface mixed layer. The atmospheric response to the ocean is through wind stress and heat flux feedbacks associated with SST. Eigenvalue analysis of both coupled and uncoupled models presented here complements previous work on the stochastically forced system. Comparison of the eigenspectrum of coupled and uncoupled cases shows that coupling creates an oscillatory interdecadal mode whose properties are distinct from any other mode in the system. This mode exists whether the atmospheric feedbacks are weak or strong, and is stable even in the strong feedback case. The weak decay rate makes it possible for the mode to be maintained by atmospheric stochastic forcing. Analytic approximations to the dispersion relation show how the spatial structure of the atmospheric feedback tends to select a large-scale spatial pattern for this eigenmode. The oscillation involves westward Rossby wave propagation in the ocean with the atmosphere carrying information back eastward into the interior of the basin in response to SST anomalies produced by advection. SST modes are also found, which purely decay in most cases due to both local and nonlocal negative heat flux feedbacks. A case with large positive heat flux feedback can produce a purely growing SST mode but does not greatly impact the interdecadal mode.

On the behavior of outflows with low potential vorticity from a sea strait

Behaviour of outflows from a sea strait contained in a shallow upper layer is investigated by using a simple quasigeostrophic model. The outflow is assumed to have two piecewise-uniform potential vorticities and the higher one to be the same value as that in the open ocean. It is shown that this simple model can reproduce both the gyre mode and the coastal mode similar to those observed near the Tsugaru Strait, which connects the Sea of Japan with the North Pacific, and in laboratory experiments. In addition to these two modes, a coastal mode in which the current at the downstream temporally widens can also occur in the present model. The existence of these three modes can be attributed to the limited capacity of the downstream coastal current to advect the low potential vorticity fluid and the characteristics of frontal waves. The responses of the flow pattern to the changes in the outflow conditions are also briefly examined. DOI: 10.1034/j.1600-0870.1991.t01-1-00007.x