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- Author or Editor: Tatsuo Motoi x
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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
Interannual SST variability in a coupled atmosphere–mixed layer ocean model is investigated. This model has no El Niño but shows a large interannual SST variability in the tropical Pacific. The basin-scale feature of SST variation has some common characteristics shared with that obtained by a global ocean–atmosphere coupled GCM and observational data in the subtropical to the midlatitude Pacific. Both the latent heat flux and shortwave radiation have their roles in producing the SST anomalies. There is no large contrast in the total heat flux between the eastern and the western Pacific. However, their main components, the shortwave radiation and the latent heat flux, have a remarkable contrast between the cold tongue in the east and the warm pool region in the west. In the east, the ocean is warmed by shortwave radiation and cooled by latent heat. This shortwave radiation is negatively correlated with low-level clouds. When the SST is warmer than normal in the eastern Pacific, there is less low-level stratus cloud cover and more shortwave radiation reaching the surface. In the western Pacific, the ocean is warmed by less evaporation due to weaker winds. When the ocean becomes warm, it is cooled by less shortwave radiation due to stronger activity in cumulus convection.
Abstract
Interannual SST variability in a coupled atmosphere–mixed layer ocean model is investigated. This model has no El Niño but shows a large interannual SST variability in the tropical Pacific. The basin-scale feature of SST variation has some common characteristics shared with that obtained by a global ocean–atmosphere coupled GCM and observational data in the subtropical to the midlatitude Pacific. Both the latent heat flux and shortwave radiation have their roles in producing the SST anomalies. There is no large contrast in the total heat flux between the eastern and the western Pacific. However, their main components, the shortwave radiation and the latent heat flux, have a remarkable contrast between the cold tongue in the east and the warm pool region in the west. In the east, the ocean is warmed by shortwave radiation and cooled by latent heat. This shortwave radiation is negatively correlated with low-level clouds. When the SST is warmer than normal in the eastern Pacific, there is less low-level stratus cloud cover and more shortwave radiation reaching the surface. In the western Pacific, the ocean is warmed by less evaporation due to weaker winds. When the ocean becomes warm, it is cooled by less shortwave radiation due to stronger activity in cumulus convection.
Abstract
Modulation of El Niño–Southern Oscillation at the mid-Holocene [6000 yr before present (6 ka)] is investigated with a coupled ocean–atmosphere general circulation model. The model is integrated for 300 yr with 6-ka and present (0 ka) insolation both with and without flux adjustment, and the effect of flux adjustment on the simulation of El Niño is investigated. The response in the equatorial Pacific Ocean in 6 ka is in favor of weaker El Niño variability resulting from lowered sea surface temperature (SST) and a more diffuse thermocline. Atmospheric sensitivity in 6 ka is larger than that in 0 ka because of increased trade winds, while oceanic sensitivity in 6 ka is weaker than that in 0 ka, resulting from destabilization of the upper ocean, both in the flux- and non-flux-adjusted experiments. However, the use of flux adjustment causes a difference in the total response. El Niño variability in 6 ka does not change much from that in 0 ka with the flux-adjusted case, while the 6-ka El Niño variability is weaker without flux adjustment. Because the observed proxy data suggest weaker El Niño variability in the mid-Holocene, the non-flux-adjusted version gives a more reasonable response despite a larger bias in its basic states, implying that nondistortion of sensitivity to forcing is more important.
Abstract
Modulation of El Niño–Southern Oscillation at the mid-Holocene [6000 yr before present (6 ka)] is investigated with a coupled ocean–atmosphere general circulation model. The model is integrated for 300 yr with 6-ka and present (0 ka) insolation both with and without flux adjustment, and the effect of flux adjustment on the simulation of El Niño is investigated. The response in the equatorial Pacific Ocean in 6 ka is in favor of weaker El Niño variability resulting from lowered sea surface temperature (SST) and a more diffuse thermocline. Atmospheric sensitivity in 6 ka is larger than that in 0 ka because of increased trade winds, while oceanic sensitivity in 6 ka is weaker than that in 0 ka, resulting from destabilization of the upper ocean, both in the flux- and non-flux-adjusted experiments. However, the use of flux adjustment causes a difference in the total response. El Niño variability in 6 ka does not change much from that in 0 ka with the flux-adjusted case, while the 6-ka El Niño variability is weaker without flux adjustment. Because the observed proxy data suggest weaker El Niño variability in the mid-Holocene, the non-flux-adjusted version gives a more reasonable response despite a larger bias in its basic states, implying that nondistortion of sensitivity to forcing is more important.
Abstract
The formation of the Weddell Polynya is investigated with a one-dimensional, convective mixed-layer model, using in situ data from late summer 1974 as initial conditions. We propose that a high-salinity mixed layer in summer resulted in the formation of the Weddell Polynya in 1974. The model results show that the high salinity allows deep convection driven by surface cooling alone, and the resulting upward transfer of heat and salt prohibits sea-ice formation throughout the winter, subject to the freshwater input less than 0.4 m yr−1. Because of the uncertainty of amount of the actual freshwater input in 1974, whether sea ice formed is ambiguous. However, the occurrence of the deep convection during the 1974 polynya formation is confirmed by summer data from 1975, showing the modification of deep water to be colder, fresher and richer in oxygen than in 1974. The upward heat flux through the deep convection resulted in the formation of the Weddell Polynya in 1974.
Abstract
The formation of the Weddell Polynya is investigated with a one-dimensional, convective mixed-layer model, using in situ data from late summer 1974 as initial conditions. We propose that a high-salinity mixed layer in summer resulted in the formation of the Weddell Polynya in 1974. The model results show that the high salinity allows deep convection driven by surface cooling alone, and the resulting upward transfer of heat and salt prohibits sea-ice formation throughout the winter, subject to the freshwater input less than 0.4 m yr−1. Because of the uncertainty of amount of the actual freshwater input in 1974, whether sea ice formed is ambiguous. However, the occurrence of the deep convection during the 1974 polynya formation is confirmed by summer data from 1975, showing the modification of deep water to be colder, fresher and richer in oxygen than in 1974. The upward heat flux through the deep convection resulted in the formation of the Weddell Polynya in 1974.
Abstract
The Coastal Oyashio (CO) carries the cold, fresh, and relatively light water mass called the Coastal Oyashio Water (COW) westward along the southeastern coast of Hokkaido in winter and spring. To investigate dynamics of the CO and its seasonal variation, model experiments are executed using a western North Pacific general circulation model with horizontal resolutions of approximately 2 and 6 km. The 2-km resolution model reproduces the properties of COW with temperature of 0°–2°C and salinity of 32.2–32.6 and reproduces its distribution. COW is less dense than offshore water by 0.2 kg m−3, and it forms a surface-to-bottom density front with a width of 10 km near the shelf break. The CO appears as a baroclinic jet current along the front with a maximum velocity of approximately 40 cm s−1. The velocity and density structures and the front location relative to bathymetry indicate that the CO can be understood in terms of a simplified dynamical model developed for the shelfbreak front in the Middle Atlantic Bight. In contrast to the 2-km resolution model, the 6-km model cannot realistically reproduce the COW distribution. This is because only the 2-km model can represent the sharp density structure of the shelfbreak front and the accompanying CO. The CO exists during the limited period from January to April. This is directly connected with seasonal variation of the COW inflow from the Okhotsk Sea to the North Pacific Ocean through the Nemuro and Kunashiri Straits, indicating that the seasonal variation of the CO is ultimately controlled by the variation of the circulation in the Okhotsk Sea induced by the monsoon.
Abstract
The Coastal Oyashio (CO) carries the cold, fresh, and relatively light water mass called the Coastal Oyashio Water (COW) westward along the southeastern coast of Hokkaido in winter and spring. To investigate dynamics of the CO and its seasonal variation, model experiments are executed using a western North Pacific general circulation model with horizontal resolutions of approximately 2 and 6 km. The 2-km resolution model reproduces the properties of COW with temperature of 0°–2°C and salinity of 32.2–32.6 and reproduces its distribution. COW is less dense than offshore water by 0.2 kg m−3, and it forms a surface-to-bottom density front with a width of 10 km near the shelf break. The CO appears as a baroclinic jet current along the front with a maximum velocity of approximately 40 cm s−1. The velocity and density structures and the front location relative to bathymetry indicate that the CO can be understood in terms of a simplified dynamical model developed for the shelfbreak front in the Middle Atlantic Bight. In contrast to the 2-km resolution model, the 6-km model cannot realistically reproduce the COW distribution. This is because only the 2-km model can represent the sharp density structure of the shelfbreak front and the accompanying CO. The CO exists during the limited period from January to April. This is directly connected with seasonal variation of the COW inflow from the Okhotsk Sea to the North Pacific Ocean through the Nemuro and Kunashiri Straits, indicating that the seasonal variation of the CO is ultimately controlled by the variation of the circulation in the Okhotsk Sea induced by the monsoon.
Abstract
Using oceanographic observations and an eddy-resolving ice–ocean coupled model simulation from 1955 to 2004, the effects of the wind-driven ocean circulation change that occurred in the late 1970s during multidecadal-scale freshening of the North Pacific Intermediate Water (NPIW) at salinity minimum density (~26.8 σ θ ) were investigated. An analysis of the observations revealed that salinity decreased significantly at the density range of 26.6–26.8 σ θ in the western subtropical gyre, including the mixed water region (MWR). The temporal variability of the salinity is dominated by the marked change in the late 1970s. With results similar to the observations, the model, selectively forced by the interannual variability of the wind-driven ocean circulation, simulated significant freshening of the intermediate layer over the subtropical gyre. The significant freshening is related to the increase in southward transport of the Oyashio associated with the intensification of the Aleutian low. Accompanying these changes, the intrusion of fresh and low potential vorticity water, originating in the Okhotsk Sea, to the MWR increased, and the freshening signal propagated farther southward in the western subtropical gyre during the subsequent 6 yr, crossing the Kuroshio Extension. These results indicate that the multidecadal-scale freshening of the NPIW is partly caused by intensification of the wind-driven cross-gyre transport of the subarctic water to the subtropical gyre.
Abstract
Using oceanographic observations and an eddy-resolving ice–ocean coupled model simulation from 1955 to 2004, the effects of the wind-driven ocean circulation change that occurred in the late 1970s during multidecadal-scale freshening of the North Pacific Intermediate Water (NPIW) at salinity minimum density (~26.8 σ θ ) were investigated. An analysis of the observations revealed that salinity decreased significantly at the density range of 26.6–26.8 σ θ in the western subtropical gyre, including the mixed water region (MWR). The temporal variability of the salinity is dominated by the marked change in the late 1970s. With results similar to the observations, the model, selectively forced by the interannual variability of the wind-driven ocean circulation, simulated significant freshening of the intermediate layer over the subtropical gyre. The significant freshening is related to the increase in southward transport of the Oyashio associated with the intensification of the Aleutian low. Accompanying these changes, the intrusion of fresh and low potential vorticity water, originating in the Okhotsk Sea, to the MWR increased, and the freshening signal propagated farther southward in the western subtropical gyre during the subsequent 6 yr, crossing the Kuroshio Extension. These results indicate that the multidecadal-scale freshening of the NPIW is partly caused by intensification of the wind-driven cross-gyre transport of the subarctic water to the subtropical gyre.
