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- Author or Editor: Jin Huang x
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Abstract
The deep circulation in the South Atlantic is studied through numerical experiments, using an oceanic general circulation model based on z coordinates. The new feature of these numerical experiments is that diapycnal mixing is idealized as strong bottom-intensified mixing on both sides of the midocean ridge; elsewhere diapycnal mixing is set at a low background value.
The bottom-intensified diapycnal mixing induces strong equatorward (poleward) flow along the western (eastern) slope of the midocean ridge. In addition, the strong vertical gradient of diapycnal mixing rate induces downwelling in the basin interior and intensifies the upwelling near the axis of the midocean ridge. The strong ridge-following currents induce anticlockwise circulation in both the eastern and western deep basins, and such circulation is opposite to the classical Stommel–Arons circulation. This study indicates that bottom topography, strong mixing over the midocean ridge, and strong localized mixing associated with overflows can have major impact on the strength of the deep meridional overturning cell and deep water properties in the whole deep basin.
Abstract
The deep circulation in the South Atlantic is studied through numerical experiments, using an oceanic general circulation model based on z coordinates. The new feature of these numerical experiments is that diapycnal mixing is idealized as strong bottom-intensified mixing on both sides of the midocean ridge; elsewhere diapycnal mixing is set at a low background value.
The bottom-intensified diapycnal mixing induces strong equatorward (poleward) flow along the western (eastern) slope of the midocean ridge. In addition, the strong vertical gradient of diapycnal mixing rate induces downwelling in the basin interior and intensifies the upwelling near the axis of the midocean ridge. The strong ridge-following currents induce anticlockwise circulation in both the eastern and western deep basins, and such circulation is opposite to the classical Stommel–Arons circulation. This study indicates that bottom topography, strong mixing over the midocean ridge, and strong localized mixing associated with overflows can have major impact on the strength of the deep meridional overturning cell and deep water properties in the whole deep basin.
Abstract
Sea surface elevation and bottom pressure anomalies due to thermohaline forcing are examined through analytical and numerical models, including Boussinesq and non-Boussinesq models. It is shown that Boussinesq approximations can introduce noticeable errors, depending on the spatial and temporal scales of the perturbations. According to the theory of geostrophic adjustment, when the initial perturbations have horizontal scales comparable to the barotropic radius of deformation, the initial pressure perturbations will be basically retained through the adjustment. On the other hand, if the initial perturbations have horizontal scales much smaller than the barotropic radius of deformation, the initial pressure perturbations will be largely lost. Precipitation has horizontal scales on the order of 10–100 km, much smaller than the barotropic radius of deformation. Thus, for timescales longer than days, the contribution from individual precipitation events to the local free surface elevation and bottom pressure is small and is difficult to identify from satellite data. On the other hand, thermal forcing has horizontal scales comparable to the barotropic radius of deformation, so its long-term contribution to sea surface height anomaly is noticeable and is easily identified from satellite data. Because Boussinesq models induce faulty sea surface height and bottom pressure signals, the errors introduced by these models are noticeable for anomalies in large-scale [O(1000 km)] thermohaline forcing.
Abstract
Sea surface elevation and bottom pressure anomalies due to thermohaline forcing are examined through analytical and numerical models, including Boussinesq and non-Boussinesq models. It is shown that Boussinesq approximations can introduce noticeable errors, depending on the spatial and temporal scales of the perturbations. According to the theory of geostrophic adjustment, when the initial perturbations have horizontal scales comparable to the barotropic radius of deformation, the initial pressure perturbations will be basically retained through the adjustment. On the other hand, if the initial perturbations have horizontal scales much smaller than the barotropic radius of deformation, the initial pressure perturbations will be largely lost. Precipitation has horizontal scales on the order of 10–100 km, much smaller than the barotropic radius of deformation. Thus, for timescales longer than days, the contribution from individual precipitation events to the local free surface elevation and bottom pressure is small and is difficult to identify from satellite data. On the other hand, thermal forcing has horizontal scales comparable to the barotropic radius of deformation, so its long-term contribution to sea surface height anomaly is noticeable and is easily identified from satellite data. Because Boussinesq models induce faulty sea surface height and bottom pressure signals, the errors introduced by these models are noticeable for anomalies in large-scale [O(1000 km)] thermohaline forcing.
Abstract
The gravitational potential energy balance of the thermal circulation in a simple rectangular model basin is diagnosed from numerical experiments based on a mass-conserving oceanic general circulation model. The vertical mixing coefficient is assumed to be a given constant. The model ocean is heated/cooled from the upper surface or bottom, and the equation of state is linear or nonlinear. Although the circulation patterns obtained from these cases look rather similar, the energetics of the circulation may be very different. For cases of differential heating from the bottom with a nonlinear equation of state, the circulation is driven by mechanical energy generated by heating from the bottom. On the other hand, circulation for three other cases is driven by external mechanical energy, which is implicitly provided by tidal dissipation and wind stress. The major balance of gravitational energy in this model ocean is between the source of energy due to vertical mixing and the conversion from kinetic energy at low latitudes and the sink of energy due to convection adjustment and conversion to kinetic energy at high latitudes.
Abstract
The gravitational potential energy balance of the thermal circulation in a simple rectangular model basin is diagnosed from numerical experiments based on a mass-conserving oceanic general circulation model. The vertical mixing coefficient is assumed to be a given constant. The model ocean is heated/cooled from the upper surface or bottom, and the equation of state is linear or nonlinear. Although the circulation patterns obtained from these cases look rather similar, the energetics of the circulation may be very different. For cases of differential heating from the bottom with a nonlinear equation of state, the circulation is driven by mechanical energy generated by heating from the bottom. On the other hand, circulation for three other cases is driven by external mechanical energy, which is implicitly provided by tidal dissipation and wind stress. The major balance of gravitational energy in this model ocean is between the source of energy due to vertical mixing and the conversion from kinetic energy at low latitudes and the sink of energy due to convection adjustment and conversion to kinetic energy at high latitudes.
Abstract
The structure of the Equatorial Undercurrent in a two-hemisphere ocean is studied, using a simple ideal-fluid model in which both potential vorticity and the Bernoulli function are conserved along streamlines. For the case of a symmetric forcing, the solution is reduced to the case discussed in previous studies. For the case of asymmetric forcing, the western boundary current from the hemisphere with stronger forcing overshoots the equator where the two western boundary currents merge and form an undercurrent that is asymmetric with respect to the equator. Layer thicknesses are continuous across the matching streamline, but zonal velocity can be discontinuous. Both the wind stress pattern and the Indonesian Throughflow are the most important factors dictating the asymmetric nature of the undercurrent.
Abstract
The structure of the Equatorial Undercurrent in a two-hemisphere ocean is studied, using a simple ideal-fluid model in which both potential vorticity and the Bernoulli function are conserved along streamlines. For the case of a symmetric forcing, the solution is reduced to the case discussed in previous studies. For the case of asymmetric forcing, the western boundary current from the hemisphere with stronger forcing overshoots the equator where the two western boundary currents merge and form an undercurrent that is asymmetric with respect to the equator. Layer thicknesses are continuous across the matching streamline, but zonal velocity can be discontinuous. Both the wind stress pattern and the Indonesian Throughflow are the most important factors dictating the asymmetric nature of the undercurrent.
