Search Results
Abstract
The jet structure of the Antarctic Circumpolar Current (ACC) simulated by two general circulation models (GCMs), FRAM (Fine Resolution Antarctic Model) and POP (Parallel Ocean Program), is examined in relation to the bottom topography field. Despite differences in configuration both GCMs display similar behavior: the model ACC consists of a number of distinct current cores superimposed on broader-scale flow. The jets display temporal and spatial (including vertical) coherence with maximum velocities occurring at the surface. It is shown that multiple jets can arise in wind-forced baroclinic quasigeostrophic flow. The main factors influencing the number and spacing of jets are found to be the bottom topography and the proximity of lateral boundaries. The meridional spacing of jets on a flat-bottomed β plane is consistent with the Rhines scaling criterion for barotropic β-plane turbulence with a small modification due to baroclinicity and the presence of meridional boundaries. When a zonally oriented ridge is present, the meridional spacing decreases. This is explained by postulating that the β effect is augmented by a factor related to the topographic slope. Smaller-scale roughness alters the magnitude of the mean flow and mass transport but does not necessarily alter the meridional scaling. The number and meridional spacing of multiple jets in FRAM are also found to be broadly consistent with this hypothesis, although other effects such as topographic steering may also be important. The POP model generally exhibits shorter length scales than would be expected from the topographically modified Rhines scaling alone, and it is likely that other factors are present.
Abstract
The jet structure of the Antarctic Circumpolar Current (ACC) simulated by two general circulation models (GCMs), FRAM (Fine Resolution Antarctic Model) and POP (Parallel Ocean Program), is examined in relation to the bottom topography field. Despite differences in configuration both GCMs display similar behavior: the model ACC consists of a number of distinct current cores superimposed on broader-scale flow. The jets display temporal and spatial (including vertical) coherence with maximum velocities occurring at the surface. It is shown that multiple jets can arise in wind-forced baroclinic quasigeostrophic flow. The main factors influencing the number and spacing of jets are found to be the bottom topography and the proximity of lateral boundaries. The meridional spacing of jets on a flat-bottomed β plane is consistent with the Rhines scaling criterion for barotropic β-plane turbulence with a small modification due to baroclinicity and the presence of meridional boundaries. When a zonally oriented ridge is present, the meridional spacing decreases. This is explained by postulating that the β effect is augmented by a factor related to the topographic slope. Smaller-scale roughness alters the magnitude of the mean flow and mass transport but does not necessarily alter the meridional scaling. The number and meridional spacing of multiple jets in FRAM are also found to be broadly consistent with this hypothesis, although other effects such as topographic steering may also be important. The POP model generally exhibits shorter length scales than would be expected from the topographically modified Rhines scaling alone, and it is likely that other factors are present.
Abstract
The question of whether the coefficient of diffusivity of potential vorticity by mesoscale eddies is positive is studied for a zonally reentrant barotropic channel using the quasigeostrophic approach. The topography is limited to the first mode in the meridional direction but is unlimited in the zonal direction. We derive an analytic solution for the stationary (time independent) solution. New terms associated with parameterized eddy fluxes of potential vorticity appear both in the equations for the mean zonal momentum balance and in the kinetic energy balance. These terms are linked with the topographic form stress exerted by parameterized eddies. It is demonstrated that in regimes with zonal flow (analogous to the Antarctic Circumpolar Current), the coefficient of eddy potential vorticity diffusivity must be positive.
Abstract
The question of whether the coefficient of diffusivity of potential vorticity by mesoscale eddies is positive is studied for a zonally reentrant barotropic channel using the quasigeostrophic approach. The topography is limited to the first mode in the meridional direction but is unlimited in the zonal direction. We derive an analytic solution for the stationary (time independent) solution. New terms associated with parameterized eddy fluxes of potential vorticity appear both in the equations for the mean zonal momentum balance and in the kinetic energy balance. These terms are linked with the topographic form stress exerted by parameterized eddies. It is demonstrated that in regimes with zonal flow (analogous to the Antarctic Circumpolar Current), the coefficient of eddy potential vorticity diffusivity must be positive.
Abstract
Integral constraints for momentum and energy impose restrictions on parameterizations of eddy potential vorticity (PV) fluxes. The impact of these constraints is studied for a wind-forced quasigeostrophic two-layer zonal channel model with variable bottom topography. The presence of a small parameter, given by the ratio of Rossby radius to the width of the channel, makes it possible to find an analytical/asymptotic solution for the zonally and time-averaged flow, given diffusive parameterizations for the eddy PV fluxes. This solution, when substituted in the constraints, leads to nontrivial explicit restrictions on diffusivities. The system is characterized by four dimensionless governing parameters with a clear physical interpretation. The bottom form stress, the major term balancing the external force of wind stress, depends on the governing parameters and fundamentally modifies the restrictions compared to the flat bottom case. While the analytical solution bears an illustrative character, it helps to see certain nontrivial connections in the system that will be useful in the analysis of more complicated models of ocean circulation. A numerical solution supports the analytical study and confirms that the presence of topography strongly modifies the eddy fluxes.
Abstract
Integral constraints for momentum and energy impose restrictions on parameterizations of eddy potential vorticity (PV) fluxes. The impact of these constraints is studied for a wind-forced quasigeostrophic two-layer zonal channel model with variable bottom topography. The presence of a small parameter, given by the ratio of Rossby radius to the width of the channel, makes it possible to find an analytical/asymptotic solution for the zonally and time-averaged flow, given diffusive parameterizations for the eddy PV fluxes. This solution, when substituted in the constraints, leads to nontrivial explicit restrictions on diffusivities. The system is characterized by four dimensionless governing parameters with a clear physical interpretation. The bottom form stress, the major term balancing the external force of wind stress, depends on the governing parameters and fundamentally modifies the restrictions compared to the flat bottom case. While the analytical solution bears an illustrative character, it helps to see certain nontrivial connections in the system that will be useful in the analysis of more complicated models of ocean circulation. A numerical solution supports the analytical study and confirms that the presence of topography strongly modifies the eddy fluxes.
Abstract
An integral constraint for eddy fluxes of potential vorticity (PV), corresponding to global momentum conservation, is applied to two-layer zonal quasigeostrophic channel flow. This constraint must be satisfied for any type of parameterization of eddy PV fluxes. Bottom topography strongly influences the integral constraint compared to a flat bottom channel. An analytical solution for the mean flow solution has been found by using asymptotic expansion in a small parameter, which is the ratio of the Rossby radius to the meridional extent of the channel. Applying the integral constraint to this solution, one can find restrictions for eddy PV transfer coefficients that relate the eddy fluxes of PV to the mean flow. These restrictions strongly deviate from restrictions for the channel with flat bottom topography.
Abstract
An integral constraint for eddy fluxes of potential vorticity (PV), corresponding to global momentum conservation, is applied to two-layer zonal quasigeostrophic channel flow. This constraint must be satisfied for any type of parameterization of eddy PV fluxes. Bottom topography strongly influences the integral constraint compared to a flat bottom channel. An analytical solution for the mean flow solution has been found by using asymptotic expansion in a small parameter, which is the ratio of the Rossby radius to the meridional extent of the channel. Applying the integral constraint to this solution, one can find restrictions for eddy PV transfer coefficients that relate the eddy fluxes of PV to the mean flow. These restrictions strongly deviate from restrictions for the channel with flat bottom topography.
