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- Author or Editor: R. M. Samelson x
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Abstract
A simple theory is presented for steady geostrophic circulation of a stratified fluid in a rectangular basin with a circumpolar connection. The interior flow obeys the β-plane Sverdrup vorticity balance, and the circulation is closed by geostrophic boundary currents. The circulation is forced by surface thermal gradients and wind-driven Ekman transport near the latitudes of the circumpolar connection. A thermal circumpolar current arises in response to imposed surface thermal gradients and northward Ekman transport across the gap latitudes. The transport of this model circumpolar current depends on the imposed surface thermal gradients and the gap geometry, but not on the strength of the wind forcing. In contrast, the circulation induced in a related reduced-gravity model by Sverdrup transport into the gap latitudes has zero zonally integrated zonal transport. The thermal current arises as a consequence of the geostrophic constraint, which requires that the northern region fill with warm fluid until it reaches the sill depth, where return geostrophic flow can be supported. Thus, the structure of the middepth, midlatitude thermocline is directly influenced by the geometry of the gap. A similar constraint evidently operates in the Southern Ocean.
Abstract
A simple theory is presented for steady geostrophic circulation of a stratified fluid in a rectangular basin with a circumpolar connection. The interior flow obeys the β-plane Sverdrup vorticity balance, and the circulation is closed by geostrophic boundary currents. The circulation is forced by surface thermal gradients and wind-driven Ekman transport near the latitudes of the circumpolar connection. A thermal circumpolar current arises in response to imposed surface thermal gradients and northward Ekman transport across the gap latitudes. The transport of this model circumpolar current depends on the imposed surface thermal gradients and the gap geometry, but not on the strength of the wind forcing. In contrast, the circulation induced in a related reduced-gravity model by Sverdrup transport into the gap latitudes has zero zonally integrated zonal transport. The thermal current arises as a consequence of the geostrophic constraint, which requires that the northern region fill with warm fluid until it reaches the sill depth, where return geostrophic flow can be supported. Thus, the structure of the middepth, midlatitude thermocline is directly influenced by the geometry of the gap. A similar constraint evidently operates in the Southern Ocean.
Abstract
A recently proposed reduced-gravity model of the warm-water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence. The model describes the balance between gain of warm water through northward Ekman advection across the circumpolar current, loss of warm water through eddy fluxes southward across the current, net gain or loss of warm water through diabatic processes north of the current, and changes in the thickness of the warm-water layer. The steady solutions are the same as those found previously, when the previous parameterization of diabatic fluxes is used. Time-dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed. An initial adjustment occurs through a combination of boundary and equatorial adjustment, followed by planetary wave propagation. The longer-term adjustment to equilibrium consists primarily of the slow change in eastern boundary thickness of the warm layer, which controls the mean depth of the entire layer. An approximate analytical solution of the time-dependent equations yields an explicit expression for the intrinsic time scale of the long-term adjustment, which depends on the eddy and diabatic flux parameters and on the equilibrium solution toward which the time-dependent solution adjusts. Numerical solutions are also considered with a second, advective–diffusive diabatic flux parameterization. These solutions differ quantitatively but not qualitatively from those with the original parameterization. For the range of parameter values considered, the adjustment time scale has dimensional values of several decades to more than a century, but the meridional flux of warm water may respond to changes in external parameters or forcing much more rapidly than this time scale for equilibration of the eastern boundary thickness and thermocline structure.
Abstract
A recently proposed reduced-gravity model of the warm-water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence. The model describes the balance between gain of warm water through northward Ekman advection across the circumpolar current, loss of warm water through eddy fluxes southward across the current, net gain or loss of warm water through diabatic processes north of the current, and changes in the thickness of the warm-water layer. The steady solutions are the same as those found previously, when the previous parameterization of diabatic fluxes is used. Time-dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed. An initial adjustment occurs through a combination of boundary and equatorial adjustment, followed by planetary wave propagation. The longer-term adjustment to equilibrium consists primarily of the slow change in eastern boundary thickness of the warm layer, which controls the mean depth of the entire layer. An approximate analytical solution of the time-dependent equations yields an explicit expression for the intrinsic time scale of the long-term adjustment, which depends on the eddy and diabatic flux parameters and on the equilibrium solution toward which the time-dependent solution adjusts. Numerical solutions are also considered with a second, advective–diffusive diabatic flux parameterization. These solutions differ quantitatively but not qualitatively from those with the original parameterization. For the range of parameter values considered, the adjustment time scale has dimensional values of several decades to more than a century, but the meridional flux of warm water may respond to changes in external parameters or forcing much more rapidly than this time scale for equilibration of the eastern boundary thickness and thermocline structure.
Abstract
The influence of localized regions of intensified vertical mixing on the stratification and circulation in a large-scale ocean model is investigated with idealized numerical experiments. Numerical solutions are obtained of a closed-basin, single-hemisphere ocean model based on the planetary geostrophic equations. Mesoscale eddy effects are minimized, and vertical mixing at the turbulent microscale is represented by a vertical diffusivity κ υ . Solutions with uniform κ υ are contrasted with a “localized mixing” solution, in which κ υ increases by two orders of magnitude from its interior value (0.2 × 10−4 m2 s−1) in a region 500 km wide adjacent to the vertical eastern boundary. When κ υ is uniform, the stratification beneath the ventilated thermocline is characterized by a single vertical scale. In contrast, the localized vertical mixing supports a deep diffusive thermocline with two distinct vertical scales: an internal boundary layer centered at the base of the ventilated thermocline (roughly 1000-m depth) and an abyssal thermocline whose vertical scale is set in the region of large κ υ . This stratification is qualitatively similar to observed deep ocean stratification. In contrast to the Stommel–Arons meridional abyssal flow that arises in the model when κ υ is uniform and small, the localized mixing solution has primarily zonal flow in the abyssal interior, with meridional motion confined to boundary layers. An advective–diffusive balance is established in the region of enhanced mixing. The near-surface circulation is dominated by westward zonal flow in the southern half of the interior, northward flow along the western boundary, and eastward flow in the northern half of the interior, while the pattern of flow in the abyssal interior is essentially the reverse. The circulation is closed by upwelling in the mixing region and downwelling along the northern boundary. Meridional motion in the mixing region is consistent with the Sverdrup vorticity balance, with northward flow at depth and southward flow near the surface. The source water for the deep circulation is confined to a narrow range of the coldest temperature classes in the basin, while the middepth subtropical thermocline is filled with warmer deep water that enters the gyre as cold deep water and then is modified in the eastern mixing region.
Abstract
The influence of localized regions of intensified vertical mixing on the stratification and circulation in a large-scale ocean model is investigated with idealized numerical experiments. Numerical solutions are obtained of a closed-basin, single-hemisphere ocean model based on the planetary geostrophic equations. Mesoscale eddy effects are minimized, and vertical mixing at the turbulent microscale is represented by a vertical diffusivity κ υ . Solutions with uniform κ υ are contrasted with a “localized mixing” solution, in which κ υ increases by two orders of magnitude from its interior value (0.2 × 10−4 m2 s−1) in a region 500 km wide adjacent to the vertical eastern boundary. When κ υ is uniform, the stratification beneath the ventilated thermocline is characterized by a single vertical scale. In contrast, the localized vertical mixing supports a deep diffusive thermocline with two distinct vertical scales: an internal boundary layer centered at the base of the ventilated thermocline (roughly 1000-m depth) and an abyssal thermocline whose vertical scale is set in the region of large κ υ . This stratification is qualitatively similar to observed deep ocean stratification. In contrast to the Stommel–Arons meridional abyssal flow that arises in the model when κ υ is uniform and small, the localized mixing solution has primarily zonal flow in the abyssal interior, with meridional motion confined to boundary layers. An advective–diffusive balance is established in the region of enhanced mixing. The near-surface circulation is dominated by westward zonal flow in the southern half of the interior, northward flow along the western boundary, and eastward flow in the northern half of the interior, while the pattern of flow in the abyssal interior is essentially the reverse. The circulation is closed by upwelling in the mixing region and downwelling along the northern boundary. Meridional motion in the mixing region is consistent with the Sverdrup vorticity balance, with northward flow at depth and southward flow near the surface. The source water for the deep circulation is confined to a narrow range of the coldest temperature classes in the basin, while the middepth subtropical thermocline is filled with warmer deep water that enters the gyre as cold deep water and then is modified in the eastern mixing region.
Abstract
The diffusivity dependence of internal boundary layers in solutions of the continuously stratified, diffusive thermocline equations is revisited. If a solution exists that approaches a two-layer solution of the ideal thermocline equations in the limit of small vertical diffusivity κ
υ
, it must contain an internal boundary layer that collapses to a discontinuity as κ
υ
→ 0. An asymptotic internal boundary layer equation is derived for this case, and the associated boundary layer thickness is proportional to
Abstract
The diffusivity dependence of internal boundary layers in solutions of the continuously stratified, diffusive thermocline equations is revisited. If a solution exists that approaches a two-layer solution of the ideal thermocline equations in the limit of small vertical diffusivity κ
υ
, it must contain an internal boundary layer that collapses to a discontinuity as κ
υ
→ 0. An asymptotic internal boundary layer equation is derived for this case, and the associated boundary layer thickness is proportional to
Abstract
A time-dependent, inviscid, linear theory for the generation of poleward undercurrent flow under upwelling conditions along midlatitude ocean eastern boundaries is proposed. The theory relies on a conceptual separation of time scales between the rapid, coastal-trapped wave response to upwelling winds and the subsequent slow, interior, quasigeostrophic evolution. Solutions are obtained under idealized conditions in which the coastal boundary and the continental-slope topography are uniform alongshore, and the time-dependent wind-stress forcing is applied over a limited meridional range, uniform cross-shore, and directed alongshore. A time-dependent coastal boundary condition on the slow-time-scale interior flow, consisting of the low-frequency, geostrophically balanced sea surface height disturbance over the outer shelf, is obtained from consideration of the fast-time-scale, coastal-trapped response. A quasigeostrophic potential vorticity equation is then solved to determine the interior response to this time-dependent boundary condition. Under upwelling conditions, the results show the formation of a localized region of subsurface poleward flow over the upper continental slope that is qualitatively consistent in amplitude, location, and timing with observations of poleward undercurrents on eastern boundaries. Despite its origin as a sea surface height anomaly, the coastal-boundary condition drives a baroclinic planetary wave response, in which the poleward subsurface flow evolves in planetary vorticity balance with induced subsurface upwelling.
Abstract
A time-dependent, inviscid, linear theory for the generation of poleward undercurrent flow under upwelling conditions along midlatitude ocean eastern boundaries is proposed. The theory relies on a conceptual separation of time scales between the rapid, coastal-trapped wave response to upwelling winds and the subsequent slow, interior, quasigeostrophic evolution. Solutions are obtained under idealized conditions in which the coastal boundary and the continental-slope topography are uniform alongshore, and the time-dependent wind-stress forcing is applied over a limited meridional range, uniform cross-shore, and directed alongshore. A time-dependent coastal boundary condition on the slow-time-scale interior flow, consisting of the low-frequency, geostrophically balanced sea surface height disturbance over the outer shelf, is obtained from consideration of the fast-time-scale, coastal-trapped response. A quasigeostrophic potential vorticity equation is then solved to determine the interior response to this time-dependent boundary condition. Under upwelling conditions, the results show the formation of a localized region of subsurface poleward flow over the upper continental slope that is qualitatively consistent in amplitude, location, and timing with observations of poleward undercurrents on eastern boundaries. Despite its origin as a sea surface height anomaly, the coastal-boundary condition drives a baroclinic planetary wave response, in which the poleward subsurface flow evolves in planetary vorticity balance with induced subsurface upwelling.
Abstract
A simple dynamical model is proposed for the near-surface drift current in a homogeneous, equilibrium sea. The momentum balance is formulated for a mass-weighted mean in curvilinear surface-conforming coordinates. Stokes drifts computed analytically for small wave slopes by this approach for inviscid linear sinusoidal and Pollard–Gerstner waves agree with the corresponding Lagrangian means, consistent with a mean momentum balance that determines mean parcel motion. A wave-modified mixing length model is proposed, with a depth-dependent eddy viscosity that depends on an effective ocean surface roughness length z 0 o , distinct from the atmospheric bulk-flux roughness length z 0 a , and an additional wave-correction factor ϕw . Kinematic Stokes drift profiles are computed for two sets of quasi-equilibrium sea states and are interpreted as mean wind drift profiles to provide calibration references for the model. A third calibration reference, for surface drift only, is provided by the traditional 3%-of-wind rule. For 10-m neutral wind U 10 N ≤ 20 m s−1, the empirical z 0 o ranges from 10−4 to 10 m, while ϕw ranges from 1.0 to 0.1. The model profiles show a shallow log-layer structure at the smaller wind speeds and a nearly uniform near-surface shear at the larger wind speeds. Surface velocities are oriented 10°–20° from downwind for U 10 N ≤ 10 m s−1 and 20°–35° from downwind for 10 ≤ U 10 N ≤ 20 m s−1. A small correction to the drag coefficient is implied. The model predictions show a basic consistency with several sets of previously published near-surface current measurements, but the comparison is not definitive.
Abstract
A simple dynamical model is proposed for the near-surface drift current in a homogeneous, equilibrium sea. The momentum balance is formulated for a mass-weighted mean in curvilinear surface-conforming coordinates. Stokes drifts computed analytically for small wave slopes by this approach for inviscid linear sinusoidal and Pollard–Gerstner waves agree with the corresponding Lagrangian means, consistent with a mean momentum balance that determines mean parcel motion. A wave-modified mixing length model is proposed, with a depth-dependent eddy viscosity that depends on an effective ocean surface roughness length z 0 o , distinct from the atmospheric bulk-flux roughness length z 0 a , and an additional wave-correction factor ϕw . Kinematic Stokes drift profiles are computed for two sets of quasi-equilibrium sea states and are interpreted as mean wind drift profiles to provide calibration references for the model. A third calibration reference, for surface drift only, is provided by the traditional 3%-of-wind rule. For 10-m neutral wind U 10 N ≤ 20 m s−1, the empirical z 0 o ranges from 10−4 to 10 m, while ϕw ranges from 1.0 to 0.1. The model profiles show a shallow log-layer structure at the smaller wind speeds and a nearly uniform near-surface shear at the larger wind speeds. Surface velocities are oriented 10°–20° from downwind for U 10 N ≤ 10 m s−1 and 20°–35° from downwind for 10 ≤ U 10 N ≤ 20 m s−1. A small correction to the drag coefficient is implied. The model predictions show a basic consistency with several sets of previously published near-surface current measurements, but the comparison is not definitive.
Abstract
The motion of fluid parcels in a two-dimensional kinematic model of a meandering jet is investigated using Melnikov's method. The study is motivated by a recent analysis of float trajectories in the Gulf Stream. The results indicate that the efficiency of cross-jet exchange induced by fluctuating meander amplitudes depends strongly on the frequency of the fluctuations. For high frequencies (≳0.04 cpd), exchange between the core of the jet and regions of “trapped” fluid moving with the meander is preferred, while for low frequencies (≲0.04 cpd), exchange between the “trapped” fluid and the slow-moving fluid surrounding the jet is preferred. Propagating waves superimposed on the meandering jet can efficiently cause exchange between regimes when their phase speeds roughly match the basic flow velocities along the regime boundaries. Numerical results suggest that exchange across the center of the jet is less efficient than exchange between adjacent regimes so that the meandering jet will tend to stir fluid along each of its sides but preserve gradients across the jet core.
Abstract
The motion of fluid parcels in a two-dimensional kinematic model of a meandering jet is investigated using Melnikov's method. The study is motivated by a recent analysis of float trajectories in the Gulf Stream. The results indicate that the efficiency of cross-jet exchange induced by fluctuating meander amplitudes depends strongly on the frequency of the fluctuations. For high frequencies (≳0.04 cpd), exchange between the core of the jet and regions of “trapped” fluid moving with the meander is preferred, while for low frequencies (≲0.04 cpd), exchange between the “trapped” fluid and the slow-moving fluid surrounding the jet is preferred. Propagating waves superimposed on the meandering jet can efficiently cause exchange between regimes when their phase speeds roughly match the basic flow velocities along the regime boundaries. Numerical results suggest that exchange across the center of the jet is less efficient than exchange between adjacent regimes so that the meandering jet will tend to stir fluid along each of its sides but preserve gradients across the jet core.
Abstract
The generation of continental shelf currents by wind forcing is investigated by analytical and numerical methods. The investigation is motivated by observations from the Coastal Ocean Dynamics Experiment. A central assumption is that the vertical structure of the response over the inshore half of the shelf is controlled by the vertical distribution of the turbulent stress. This suggests a two-layer model of the wind-driven circulation, in which the upper layer represents a surface wind-mixed layer, and the lower layer represents the remainder of the fluid. The response of this idealized dynamical model to wind forcing is examined and compared with observations in the 2–7-day period band. For the alongshore velocity gain relative to local wind stress, an onshore surface maximum and an offshore interior maximum are robustly reproduced by the model. These features are evidently related to a dynamical transition over the inner half of the shelf, in which the alongshore wind stress is balanced more by acceleration of near-surface alongshore flow and less by time-dependent Ekman transport as the coast is approached. This differs from a previous hypothesis, based on a linear model in which the turbulent stress was confined to infinitesimally thin surface and bottom boundary layers, which related the alongshore flow structure to the cross-shore profile of the alongshore wind amplitude. In the present model, the cross-shore velocity variances are roughly comparable to those observed over the onshore half of the shelf. This also contrasts with the previous model results, which underpredicted cross-shore velocity variances by more than an order of magnitude. However, the present agreement is probably fortuitous, as the enhanced lower-layer cross-shore flow is frictionally driven, and should probably be confined to a bottom boundary layer as it was in the previous model. The results demonstrate that the response of these models over the inner half of the shelf depends strongly on a poorly understood coastal boundary condition.
Abstract
The generation of continental shelf currents by wind forcing is investigated by analytical and numerical methods. The investigation is motivated by observations from the Coastal Ocean Dynamics Experiment. A central assumption is that the vertical structure of the response over the inshore half of the shelf is controlled by the vertical distribution of the turbulent stress. This suggests a two-layer model of the wind-driven circulation, in which the upper layer represents a surface wind-mixed layer, and the lower layer represents the remainder of the fluid. The response of this idealized dynamical model to wind forcing is examined and compared with observations in the 2–7-day period band. For the alongshore velocity gain relative to local wind stress, an onshore surface maximum and an offshore interior maximum are robustly reproduced by the model. These features are evidently related to a dynamical transition over the inner half of the shelf, in which the alongshore wind stress is balanced more by acceleration of near-surface alongshore flow and less by time-dependent Ekman transport as the coast is approached. This differs from a previous hypothesis, based on a linear model in which the turbulent stress was confined to infinitesimally thin surface and bottom boundary layers, which related the alongshore flow structure to the cross-shore profile of the alongshore wind amplitude. In the present model, the cross-shore velocity variances are roughly comparable to those observed over the onshore half of the shelf. This also contrasts with the previous model results, which underpredicted cross-shore velocity variances by more than an order of magnitude. However, the present agreement is probably fortuitous, as the enhanced lower-layer cross-shore flow is frictionally driven, and should probably be confined to a bottom boundary layer as it was in the previous model. The results demonstrate that the response of these models over the inner half of the shelf depends strongly on a poorly understood coastal boundary condition.
Abstract
Two idealized, three-dimensional, analytical models of middepth meridional overturning in a basin with a Southern Hemisphere circumpolar connection are described. In the first, the overturning circulation can be understood as a “pump and valve” system, in which the wind forcing at the latitudes of the circumpolar connection is the pump and surface thermodynamic exchange at high northern latitudes is the valve. When the valve is on, the overturning circulation extends to the extreme northern latitudes of the basin, and the middepth thermocline is cold. When the valve is off, the overturning circulation is short-circuited and confined near the circumpolar connection, and the middepth thermocline is warm. The meridional overturning cell in this first model is not driven by turbulent mixing, and the subsurface circulation is adiabatic. In contrast, the pump that primarily drives the overturning cell in the second model is turbulent mixing, at low and midlatitudes, in the ocean interior. In both models, however, the depth of the midlatitude deep layer is controlled by the sill depth of the circumpolar gap. The thermocline structures in these two models are nearly indistinguishable. These models suggest that Northern Hemisphere wind and surface buoyancy forcing may influence the strength and structure of the circumpolar current in the Southern Hemisphere.
Abstract
Two idealized, three-dimensional, analytical models of middepth meridional overturning in a basin with a Southern Hemisphere circumpolar connection are described. In the first, the overturning circulation can be understood as a “pump and valve” system, in which the wind forcing at the latitudes of the circumpolar connection is the pump and surface thermodynamic exchange at high northern latitudes is the valve. When the valve is on, the overturning circulation extends to the extreme northern latitudes of the basin, and the middepth thermocline is cold. When the valve is off, the overturning circulation is short-circuited and confined near the circumpolar connection, and the middepth thermocline is warm. The meridional overturning cell in this first model is not driven by turbulent mixing, and the subsurface circulation is adiabatic. In contrast, the pump that primarily drives the overturning cell in the second model is turbulent mixing, at low and midlatitudes, in the ocean interior. In both models, however, the depth of the midlatitude deep layer is controlled by the sill depth of the circumpolar gap. The thermocline structures in these two models are nearly indistinguishable. These models suggest that Northern Hemisphere wind and surface buoyancy forcing may influence the strength and structure of the circumpolar current in the Southern Hemisphere.
Abstract
A reduced-gravity model is presented of the warm-water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection. The model describes the balance between production of warm water by Ekman advection across the circumpolar current, dissipation of warm water by eddy fluxes southward across the current, and the net production or dissipation of warm water by diabatic processes north of the current. The results emphasize the role of the eastern boundary condition in setting the thermocline structure north of the current and the nonlinear interactions between wind forcing, eddy fluxes, and diabatic mixing, which together control the structure and amplitude of the model meridional overturning circulation. Solutions are shown to exist in which the northward Ekman transport across the circumpolar current is completely compensated by southward eddy fluxes and the meridional overturning north of the current is entirely driven by diabatic forcing and interior upwelling through the base of the layer. Other solutions are shown to exist in which the interior upwelling into the warm layer at midlatitudes is negligible and the meridional overturning circulation consists of a continuous cell that carried the fluid delivered by the northward Ekman transport across the circumpolar current through midlatitudes to the Northern Hemisphere subpolar gyre, where it cools and returns to depth. The results emphasize that the coupled elements of wind driving, eddy fluxes, and diabatic processes are inextricably intertwined in the middepth meridional overturning circulation.
Abstract
A reduced-gravity model is presented of the warm-water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection. The model describes the balance between production of warm water by Ekman advection across the circumpolar current, dissipation of warm water by eddy fluxes southward across the current, and the net production or dissipation of warm water by diabatic processes north of the current. The results emphasize the role of the eastern boundary condition in setting the thermocline structure north of the current and the nonlinear interactions between wind forcing, eddy fluxes, and diabatic mixing, which together control the structure and amplitude of the model meridional overturning circulation. Solutions are shown to exist in which the northward Ekman transport across the circumpolar current is completely compensated by southward eddy fluxes and the meridional overturning north of the current is entirely driven by diabatic forcing and interior upwelling through the base of the layer. Other solutions are shown to exist in which the interior upwelling into the warm layer at midlatitudes is negligible and the meridional overturning circulation consists of a continuous cell that carried the fluid delivered by the northward Ekman transport across the circumpolar current through midlatitudes to the Northern Hemisphere subpolar gyre, where it cools and returns to depth. The results emphasize that the coupled elements of wind driving, eddy fluxes, and diabatic processes are inextricably intertwined in the middepth meridional overturning circulation.
