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Ryo Furue

Abstract

New equatorial boundary layer solutions are found in a reduced-gravity model with large horizontal diffusivity. Analytical and semianalytical solutions are obtained and the structure of the boundary layer is discussed. The equatorial “stacked jets” often found in thermally driven coarse-resolution OGCMs are identified as corresponding to these boundary layer solutions. The width of the boundary layer is essentially that of the frictional (Stommel or Munk) boundary layer when the horizontal diffusivity is sufficiently large. The flow of the equatorial boundary layer is primarily a jetlike zonal flow, but there is significant meridional velocity, which has meridional convergence and divergence. The resultant vertical velocity manifests itself as meridional overturning cells that are also often found in OGCMs.

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Ryo Furue

Abstract

Three-dimensional numerical experiments are conducted to examine energy transfer within the small-scale portion of the Garrett–Munk model spectrum of oceanic internal waves. The rate of energy transfer in the experiments is a significant fraction of observed total transfer rate in the interior main thermocline. This transfer may supplement the previous estimate by the eikonal theory. Because nonlinearity is strong in this spectral region, wavenumber-local interactions dominate the energy transfer rather than scale-separated ones. Transfer to higher horizontal wavenumbers is robust, whereas that to higher vertical wavenumbers seems to depend strongly on the spectral shape. Vortical motions seem to be enhancing energy transfer. All of these suggest that further investigation of this spectral region is important and necessary by means of three-dimensional, fully nonlinear analysis.

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Ryo Furue
and
Masahiro Endoh

Abstract

Numerical experiments are conducted using an idealized basin to investigate roles of the deep vertical diffusivity and wind stress of the Pacific Ocean in the global and Pacific meridional overturning circulation. The Pacific middepth diffusivity is found to be enhancing the global meridional overturning circulation; when this part of diffusivity is reduced to the background value, not only is the layered circulation of the Pacific greatly weakened, but also the production of the North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) is significantly reduced. The deeper part of the Pacific diffusivity is found to be enhancing the production of the AABW in the model. When the wind stress is turned off in the Pacific, the deep meridional overturning circulation of the Pacific is reduced and the production of the NADW and AABW is also significantly reduced. This is likely due to the reduction of the wind-enhanced upwelling in the subpolar and equatorial regions. These results suggest the importance of the diapycnal diffusion and sea surface conditions in the Pacific not only to the circulation within the Pacific but also to the global meridional overturning circulation.

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Yasuhiro Yamanaka
,
Ryo Furue
,
Hiroyasu Hasumi
, and
Nobuo Suginohara

Abstract

The authors compare two classical advection schemes, the centered difference and weighted upcurrent, for coarse-resolution OGCMs, using an idealized ocean basin and a realistic World Ocean topography. For the idealized basin, three experiments are run, one with 12 vertical levels and the centered difference scheme, one with 12 levels and the weighted upcurrent scheme, and the other with 800 levels and the centered scheme. The last experiment perfectly satisfies the grid Péclet number stability criterion and is regarded as the “true solution.” Comparison of the coarse vertical resolution experiments with the true solution indicates 1) that with the centered scheme, when strong vertical motion crosses a strong stratification, false density values are created in the coarse resolution model and this leads to false convective adjustment, which transports those false density values downward; and 2) that because of computational diffusion, the weighted upcurrent scheme leads to a less dense deep water with a stronger stratification than those of the true solution. These characteristics also apply even to the World Ocean model with relatively small grid Péclet numbers (moderately high vertical resolution and relatively large vertical diffusivity): the centered scheme leads to artificial convective adjustment near the surface in the equatorial Pacific, creating an artificial circulation, and the weighted upcurrent scheme leads to a warmer deep water and more diffuse thermocline. Deep equatorial “stacked jets” are found in all idealized-basin experiments, in particular, in the super-high vertical resolution case. Horizontal diffusion is found to dominate the density balance at the bottom jet in the super-high-resolution model, as previously found in an OGCM with a moderately high vertical resolution. This is consistent with the hypothesis that the jets exist owing to diapycnal mixing.

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Ryo Furue
,
Julian P. McCreary Jr.
, and
Zuojun Yu

Abstract

The Tsuchiya jets (TJs) are narrow eastward currents located along thermal fronts at the poleward edges of thermostad water in the Pacific Ocean. In this study, an oceanic general circulation model (OGCM) is used to explore the dynamics of the northern TJ. Solutions are found in a rectangular basin, extending 100° zonally and from 40°S to 40°N. They are forced by three idealized forcings: several patches of idealized wind fields, including one that simulates the strong Ekman pumping region in the vicinity of the Costa Rica Dome (CRD); surface heating that warms the ocean in the tropics; and a prescribed interocean circulation (IOC) that enters the basin through the southern boundary and exits through the western boundary from 2° to 6°N (the model’s Indonesian passages).

Solutions forced by all the aforementioned processes and with minimal diffusion resemble the observed flow field in the tropical North Pacific. A narrow eastward current, the model’s northern TJ, flows across the basin along the northern edge of a thick equatorial thermostad. Part of the TJ water upwells at the CRD upwelling region and the rest returns westward in the lower part of the North Equatorial Current. The deeper part of the TJ is supplied by water that leaves the western boundary current somewhat north of the equator. Its shallower part originates from water that diverges from the deep portion of the Equatorial Undercurrent (EUC); as a result, the TJ transport increases to the east and the TJ warms as it flows across the basin. These and other properties suggest that the dynamics of the model’s TJ are those of an arrested front, which in a 2½-layer model are generated when characteristics of the flow converge strongly or intersect.

Eddy form stress, due to instability waves generated at the CRD region, extends the TJ circulation to deeper levels. When diffusivity is increased to commonly used values, the thermostad is less well defined and the TJ is weak. In a solution without the IOC, the TJ is shifted to higher temperatures with its water supplied by the subtropical cell. When horizontal viscosity is reduced, the TJ becomes narrower and is flanked by a westward current on its equatorward side.

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Kunihiro Aoki
,
Atsushi Kubokawa
,
Ryo Furue
, and
Hideharu Sasaki

Abstract

This study explores the role of the momentum flux divergence due to mesoscale eddies for the maintenance of the Kuroshio Extension (KE) jet. For that purpose, the zonal momentum budget in a high-resolution ocean general circulation model is examined on the basis of the temporal residual mean framework. The momentum budget analysis is performed for two control volumes: the upstream region of the KE jet flanked by the robust recirculations (33°–38°N and 142.2°–149.4°E) and the downstream region to the east (33°–38°N and 149.4°–160.0°E), both fully covering the meridional width of the KE jet. In both regions the KE jet decelerates to the east, which can be well accounted for by sum of zonal Reynolds stress and Coriolis force on mean ageostrophic flow; the former tends to decelerate the KE jet and the latter to accelerate it in the upstream region, respectively, but these effects are switched in the downstream region. The mean ageostrophic Coriolis force is partially balanced by the horizontal gradient of the eddy kinetic energy, which is the isotropic component of the Reynolds stress. The difference between these terms, that is, net ageostrophic Coriolis force, leads to the final deceleration of the KE jet in the downstream region, overwhelming the acceleration tendency of the anisotropic Reynolds stress. The authors also reinterpret the downstream decay process of an eastward jet in a previous quasigeostrophic experiment in terms of momentum and show that the same features as described above are also likely to be included in that experiment.

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Toru Miyama
,
Humio Mitsudera
,
Hajime Nishigaki
, and
Ryo Furue

ABSTRACT

The dynamics of a quasi-stationary jet along the Subarctic Front in the North Pacific Ocean (the Western Isoguchi Jet) were investigated using an idealized two-layer model. The experiments suggested that a seafloor topography, which is 500 m high, produces a jet along its eastern flank. The formation mechanism of the jet can be explained via baroclinic Rossby wave characteristics. Baroclinic Rossby waves propagate along characteristic curves, which are significantly distorted by anticyclonic barotropic flow on the seafloor topography. A baroclinic surface jet is formed where a characteristic curve originating in the subtropical gyre and one originating in the subpolar gyre meet because the pycnocline depth varies discontinuously at this location. The barotropic flow on the seafloor topography is induced by eddies.

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Ryo Furue
,
Julian P. McCreary Jr.
,
Zuojun Yu
, and
Dailin Wang

Abstract

The Tsuchiya jets (TJs) are narrow eastward currents, located a few degrees on either side of the equator at depths from 200 to 500 m in the Pacific Ocean. In this study, non-eddy-resolving, oceanic general circulation models (OGCMs) are used to investigate the dynamics of the southern TJ. Most solutions are found in a rectangular basin extending 100° zonally and from 40°S to 10°N. They are forced by idealized zonal and meridional winds representing the trades and the southerly winds near the South American coast, by a prescribed interocean circulation (IOC) that enters the basin through the southern boundary and exits through the western boundary from 2° to 6°N (the model’s Indonesian passages), and by surface heating that warms the ocean in the Tropics. A suite of solutions is presented to isolate effects of each forcing and mixing process. A few solutions are also found to a global OGCM driven by realistic forcings. Solutions forced by all of the aforementioned processes and with minimal diffusion resemble the observed flow field in the tropical South Pacific. A narrow eastward current, the model southern TJ, flows across the basin along the southern edge of a thick equatorial thermostad, and upwells at the eastern boundary. Its deeper part is supplied by water that leaves the western boundary current somewhat south of the equator. Its shallower part originates from water that diverges from the deep portion of the Equatorial Undercurrent (EUC); as a result, the TJ transport increases to the east and the TJ warms as it flows across the basin. A major part of the water that upwells at the eastern boundary is supplied by the TJ with a minor contribution from the southern boundary region. In idealized-basin solutions without forcing either by the IOC or meridional wind, the TJ is weak or absent. These, and other, properties suggest that the dynamics of the model’s TJ are those of an arrested front, which in a 2½-layer model are generated when characteristics of the flow merge or intersect. When diffusivity is increased to commonly used values, the thermostad is less well defined or even absent and the TJ is weak, suggesting that excessive diffusion is the reason why TJs are not present in many previous OGCMs. In the solution to a global OGCM, the southern TJ still exists without the IOC, although it is warmed by 1°C, indicating that much of its water is supplied by an overturning cell confined within the Pacific basin.

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Zuojun Yu
,
Julian P. McCreary Jr.
,
Max Yaremchuk
, and
Ryo Furue

Abstract

The South China Sea (SCS) is often treated as a semienclosed water body, with the Luzon Strait as its only connection to the Pacific Ocean. A branch of the Kuroshio flows northwestward across the Luzon Strait to enter the SCS, carrying North Pacific Tropical Water (NPTW) into the basin. Using the subsurface salinity maximum as a tracer for NPTW, the authors show how important three secondary straits—the Taiwan Strait to the north and the Karimata and Mindoro Straits to the south—are to the NPTW intrusion at the Luzon Strait. The authors demonstrate that the SCS cannot reach an equilibrium state that is consistent with the observed subsurface salinity distribution unless all of the following components are in place: the Kuroshio, transports through the three secondary straits, downward mixing of freshwater, horizontal mixing induced by mesoscale eddies, and forcing by the local monsoonal winds.

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Max Yaremchuk
,
Julian McCreary Jr.
,
Zuojun Yu
, and
Ryo Furue

Abstract

The salinity distribution in the South China Sea (SCS) has a pronounced subsurface maximum from 150–220 m throughout the year. This feature can only be maintained by the existence of a mean flow through the SCS, consisting of a net inflow of salty North Pacific tropical water through the Luzon Strait and outflow through the Mindoro, Karimata, and Taiwan Straits. Using an inverse modeling approach, the authors show that the magnitude and space–time variations of the SCS thermohaline structure, particularly for the salinity maximum, allow a quantitative estimate of the SCS throughflow and its distribution among the three outflow straits. Results from the inversion are compared with available observations and output from a 50-yr simulation of a highly resolved ocean general circulation model.

The annual-mean Luzon Strait transport is found to be 2.4 ± 0.6 Sv (Sv ≡ 106 m3 s−1). This inflow is balanced by the outflows from the Karimata (0.3 ± 0.5 Sv), Mindoro (1.5 ± 0.4), and Taiwan (0.6 ± 0.5 Sv) Straits. Results of the inversion suggest that the Karimata transport tends to be overestimated in numerical models. The Mindoro Strait provides the only passage from the SCS deeper than 100 m, and half of the SCS throughflow (1.2 ± 0.3 Sv) exits the basin below 100 m in the Mindoro Strait, a result that is consistent with a climatological run of a 0.1° global ocean general circulation model.

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