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- Author or Editor: Hiroshi Ishizaki x
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Abstract
A simulation was made of the abyssal circulation in the North Pacific Ocean. The model covers the whole Pacific, has realistic coastal geometry and bottom topography, and is driven by the annual mean surface wind stress and the annual mean differential heating at the surface and the southern boundary. About 6.5 Sv (Sv ≡ 106 m3 s−1) of bottom water crosses the equator from the South to the North Pacific below 3500 m and rises there, with 2.8 Sv of the 6.5 Sv returning to the South Pacific in the layer between 1750 and 3250 m. The abyssal water below 1750 m is divided into three layers: the upper deep layer (1750–3250 m), the lower deep layer (3250–4250 m), and the bottom layer (below 4250 m), based on the large-scale horizontal circulation pattern. In the bottom layer, the western boundary current flows to the north without any stagnation point, and the interior flow is essentially zonal and eastward, especially over the gentle slope of the western flank of the East Pacific Rise. In the upper and lower deep layers, on the other hand, a single anticyclonic gyre is dominant over the whole North Pacific excluding the Philippine Sea. In particular, the horizontal circulation pattern in the upper deep layer can be regarded as a reversed pattern of the Stommel-Arons circulation, driven by the outflux from the North to the South Pacific in the layer and consequent vertical shrinking of the water column.
The present results are compared with recent observations including deep-water property distributions, trans-Pacific hydrographic data analyses, and deep current meter records. There is a general similarity between the simulation and observations, with some discrepancies, particularly in the equatorial regions.
Abstract
A simulation was made of the abyssal circulation in the North Pacific Ocean. The model covers the whole Pacific, has realistic coastal geometry and bottom topography, and is driven by the annual mean surface wind stress and the annual mean differential heating at the surface and the southern boundary. About 6.5 Sv (Sv ≡ 106 m3 s−1) of bottom water crosses the equator from the South to the North Pacific below 3500 m and rises there, with 2.8 Sv of the 6.5 Sv returning to the South Pacific in the layer between 1750 and 3250 m. The abyssal water below 1750 m is divided into three layers: the upper deep layer (1750–3250 m), the lower deep layer (3250–4250 m), and the bottom layer (below 4250 m), based on the large-scale horizontal circulation pattern. In the bottom layer, the western boundary current flows to the north without any stagnation point, and the interior flow is essentially zonal and eastward, especially over the gentle slope of the western flank of the East Pacific Rise. In the upper and lower deep layers, on the other hand, a single anticyclonic gyre is dominant over the whole North Pacific excluding the Philippine Sea. In particular, the horizontal circulation pattern in the upper deep layer can be regarded as a reversed pattern of the Stommel-Arons circulation, driven by the outflux from the North to the South Pacific in the layer and consequent vertical shrinking of the water column.
The present results are compared with recent observations including deep-water property distributions, trans-Pacific hydrographic data analyses, and deep current meter records. There is a general similarity between the simulation and observations, with some discrepancies, particularly in the equatorial regions.
Abstract
A theoretical rationale is given for the simulated abyssal circulation in the North Pacific detailed in Part I. A simple theory clarifies the importance of the vertical change in area of the horizontal cross section of the basin owing to the existence of bottom topography, or basin shape. With general upward vertical transport in the basin, a water column horizontally diverges and vertically shrinks as it rises in spite of the general upwelling, resulting in the tendency to produce an anticyclonic circulation. This hypsometric effect is detailed by Rhines and MacCready. In the present case, this effect is present in the lower deep layer (3250–4250 m) and in the bottom layer (below 4250 m) of the North Pacific. In the lower deep layer, a single anticyclonic circulation owing to this effect appears explicitly. In the bottom layer, however, this effect almost balances the cyclogenesis owing to the bottom water influx, resulting in essentially eastward interior flow. On the other hand, an anticyclonic gyre in the upper deep layer (1750–3250 m) is maintained by the vertical shrinking of the water column owing to the outflux from the North to the South Pacific, resulting in a reversed pattern of the Stommel-Arons circulation.
Abstract
A theoretical rationale is given for the simulated abyssal circulation in the North Pacific detailed in Part I. A simple theory clarifies the importance of the vertical change in area of the horizontal cross section of the basin owing to the existence of bottom topography, or basin shape. With general upward vertical transport in the basin, a water column horizontally diverges and vertically shrinks as it rises in spite of the general upwelling, resulting in the tendency to produce an anticyclonic circulation. This hypsometric effect is detailed by Rhines and MacCready. In the present case, this effect is present in the lower deep layer (3250–4250 m) and in the bottom layer (below 4250 m) of the North Pacific. In the lower deep layer, a single anticyclonic circulation owing to this effect appears explicitly. In the bottom layer, however, this effect almost balances the cyclogenesis owing to the bottom water influx, resulting in essentially eastward interior flow. On the other hand, an anticyclonic gyre in the upper deep layer (1750–3250 m) is maintained by the vertical shrinking of the water column owing to the outflux from the North to the South Pacific, resulting in a reversed pattern of the Stommel-Arons circulation.
Abstract
In the Takano and Oonishi models the finite-difference analog of the nonlinear momentum advection contains the concept of diagonally upward/downward mass and momentum fluxes along the bottom slope, and the generalized Arakawa scheme for the horizontal advection, modified to be fit to arbitrary coastal shape. It has been said to have a good performance, but is not widely used, largely because of its complicated expression.
The purpose of this paper is to reevaluate the Takano–Oonishi scheme for the momentum advection to put it to more practical use by using the redefinition of it in a simple, generalized form and the confirmation of its good performance through a comparison with other schemes.
Based on the definition of mass continuity for a momentum cell (U cell) in terms of that for tracer cells (T cell), the vertical and horizontal mass and momentum fluxes for the U cell are generalized on arbitrary bottom relief in simple forms. Although the grid spacing of the present model is different from that of the Geophysical Fluid Dynamics Laboratory model, applicability of the present scheme to the latter grid spacing is discussed.
Then, the present scheme is tested in an eddy-resolving ocean model and its results are compared with those of a traditional scheme. The present scheme shows good performance in computational efficiency as well as reality of the simulated flow field.
Abstract
In the Takano and Oonishi models the finite-difference analog of the nonlinear momentum advection contains the concept of diagonally upward/downward mass and momentum fluxes along the bottom slope, and the generalized Arakawa scheme for the horizontal advection, modified to be fit to arbitrary coastal shape. It has been said to have a good performance, but is not widely used, largely because of its complicated expression.
The purpose of this paper is to reevaluate the Takano–Oonishi scheme for the momentum advection to put it to more practical use by using the redefinition of it in a simple, generalized form and the confirmation of its good performance through a comparison with other schemes.
Based on the definition of mass continuity for a momentum cell (U cell) in terms of that for tracer cells (T cell), the vertical and horizontal mass and momentum fluxes for the U cell are generalized on arbitrary bottom relief in simple forms. Although the grid spacing of the present model is different from that of the Geophysical Fluid Dynamics Laboratory model, applicability of the present scheme to the latter grid spacing is discussed.
Then, the present scheme is tested in an eddy-resolving ocean model and its results are compared with those of a traditional scheme. The present scheme shows good performance in computational efficiency as well as reality of the simulated flow field.
Abstract
Numerical experiments were conducted to clarify the processes through which the Southern Ocean wind affects the meridional overturning (NA cell) associated with North Atlantic Deep Water production. These were based on idealized single- and twin-basin (idealized Atlantic and Pacific Ocean) models with a periodically connected passage under various forcings at the surface. Relationships among the wind stresses, the NA cell, and the buoyancy fluxes were investigated. Increased westerly wind stresses increase the surface buoyancy gains in the Southern Ocean under the density-restoring boundary condition. The buoyancy anomalies excited in the Southern Ocean propagate as baroclinic waves into the northern North Atlantic, modify the density field, and enhance the NA cell, which increases buoyancy losses there until the global buoyancy flux budget balances. The results from experiments using a realistically configured global ocean model confirm that the Southern Ocean wind effects on the NA cell can be understood consistently through thermodynamics and that the wind stresses outside the channel latitudes, as well as those at the Cape Horn latitude, affect the global buoyancy fluxes and the NA cell.
Abstract
Numerical experiments were conducted to clarify the processes through which the Southern Ocean wind affects the meridional overturning (NA cell) associated with North Atlantic Deep Water production. These were based on idealized single- and twin-basin (idealized Atlantic and Pacific Ocean) models with a periodically connected passage under various forcings at the surface. Relationships among the wind stresses, the NA cell, and the buoyancy fluxes were investigated. Increased westerly wind stresses increase the surface buoyancy gains in the Southern Ocean under the density-restoring boundary condition. The buoyancy anomalies excited in the Southern Ocean propagate as baroclinic waves into the northern North Atlantic, modify the density field, and enhance the NA cell, which increases buoyancy losses there until the global buoyancy flux budget balances. The results from experiments using a realistically configured global ocean model confirm that the Southern Ocean wind effects on the NA cell can be understood consistently through thermodynamics and that the wind stresses outside the channel latitudes, as well as those at the Cape Horn latitude, affect the global buoyancy fluxes and the NA cell.
