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- Author or Editor: David C. Chapman x
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Abstract
The frontal trapping mechanism is remarkably robust, in fact so robust that the presence of a shelf break has little effect on the final location of the front. Bottom stress is necessary for the frontal trapping mechanism, but the trapping isobath is relatively insensitive to the magnitude of the bottom friction coefficient. The near-surface part of the front is sometimes unstable, but it can be stabilized either by ambient stratification or by a weak background current in the direction of the buoyant inflow.
Abstract
The frontal trapping mechanism is remarkably robust, in fact so robust that the presence of a shelf break has little effect on the final location of the front. Bottom stress is necessary for the frontal trapping mechanism, but the trapping isobath is relatively insensitive to the magnitude of the bottom friction coefficient. The near-surface part of the front is sometimes unstable, but it can be stabilized either by ambient stratification or by a weak background current in the direction of the buoyant inflow.
Abstract
Based on a limited set of available oxygen isotope measurements, it is hypothesized that the mean now in the Middle Atlantic Bight is part of a 5000 km-long buoyancy-driven, coastal current which originates along the southern coast of Greenland. This idea is consistent with most features of the known circulation of the region.
Abstract
Based on a limited set of available oxygen isotope measurements, it is hypothesized that the mean now in the Middle Atlantic Bight is part of a 5000 km-long buoyancy-driven, coastal current which originates along the southern coast of Greenland. This idea is consistent with most features of the known circulation of the region.
Abstract
No abstract available.
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Abstract
The effect of steady, deep-ocean forcing on the flow over a continental slope and shelf region is examined using a linear and time-independent numerical model which includes continuous stratification, vertical and horizontal diffusion of momentum and density and linear bottom friction. The penetration of the pressure forcing is measured by the vertically averaged kinetic and potential energy as a function of cross-shore location. The most important factor governing the penetration of energy across the continental slope is the vertical structure of the imposed forcing: a surface-intensified pressure perturbation can penetrate easily onto the upper slope. Increasing the stratification also increases the energy penetration but not as effectively. Diffusion is relatively unimportant. The velocity field over the continental shelf is depth-independent regardless of the stratification or the location or vertical structure of the forcing function, and relatively little energy penetrates shoreward of the shelf break.
Abstract
The effect of steady, deep-ocean forcing on the flow over a continental slope and shelf region is examined using a linear and time-independent numerical model which includes continuous stratification, vertical and horizontal diffusion of momentum and density and linear bottom friction. The penetration of the pressure forcing is measured by the vertically averaged kinetic and potential energy as a function of cross-shore location. The most important factor governing the penetration of energy across the continental slope is the vertical structure of the imposed forcing: a surface-intensified pressure perturbation can penetrate easily onto the upper slope. Increasing the stratification also increases the energy penetration but not as effectively. Diffusion is relatively unimportant. The velocity field over the continental shelf is depth-independent regardless of the stratification or the location or vertical structure of the forcing function, and relatively little energy penetrates shoreward of the shelf break.
Abstract
An analytical solution is presented for the scattering of a free shelf wave incident upon a discontinuity in shelf width in a barotropic ocean. The discussion of solutions relying on backscattered free-waves with large wavenumbers which may not exist in a realistically stratified ocean is avoided by considering only the range of parameters over which energy transmission is nearly 100%. There is a substantial transfer of energy to modes other than that of the incident wave. The mode most readily excited is that which has the cross-shelf structure most closely coinciding with that of the incident wave. The resultant presence of multiple modes produces a strong alongshelf modulation in flow intensity and phase progression downstream of the scattering region which may affect the interpretation of shelf wave observations. A nondispersive long shelf wave pulse is shown to scatter into a train of pulses of differing mode number, each propagating at its own flee wave speed.
Abstract
An analytical solution is presented for the scattering of a free shelf wave incident upon a discontinuity in shelf width in a barotropic ocean. The discussion of solutions relying on backscattered free-waves with large wavenumbers which may not exist in a realistically stratified ocean is avoided by considering only the range of parameters over which energy transmission is nearly 100%. There is a substantial transfer of energy to modes other than that of the incident wave. The mode most readily excited is that which has the cross-shelf structure most closely coinciding with that of the incident wave. The resultant presence of multiple modes produces a strong alongshelf modulation in flow intensity and phase progression downstream of the scattering region which may affect the interpretation of shelf wave observations. A nondispersive long shelf wave pulse is shown to scatter into a train of pulses of differing mode number, each propagating at its own flee wave speed.
Abstract
A depth-averaged model with no density variations was used by Chapman to describe the formation of a passive tracer front at a shelfbreak. The relevance of this frontogenesis mechanism to cases that allow vertical variations is examined by considering the three-dimensional structure of a passive tracer front with explicit finite vertical mixing and bottom boundary layer dynamics. A three-dimensional primitive-equation numerical model is configured in a channel with a continental shelf, slope, and abyssal plain running the length of the channel. A vertically and horizontally uniform inflow is imposed over the shelf, with a large horizontal velocity shear near the shelfbreak. In the primitive-equation model, the offshore flow is concentrated in the bottom boundary layer while the alongshelf flow distribution is similar to the depth-averaged case; the presence of the bottom topography maintains a strong horizontal shear near the shelfbreak above the bottom boundary layer. This velocity shear causes a smooth passive tracer distribution imposed at the inflow boundary to develop strong cross-shelf gradients near the shelfbreak (i.e., a passive tracer front) within a rather short downstream distance, as in the depth-averaged model. Neutrally buoyant Lagrangian particles initialized above the bottom boundary layer are rapidly advected along the shelf with little cross-shelf motion. However, particles initialized within the bottom boundary layer move quickly offshore toward the shelfbreak and beyond while being advected alongshelf relatively slowly. The shelfbreak does not act as a barrier to the offshore transport of neutrally buoyant particles despite the presence of the passive tracer front. This results in a continuous net offshore transport from the shelf to the deep ocean due to the effects of bottom friction.
Abstract
A depth-averaged model with no density variations was used by Chapman to describe the formation of a passive tracer front at a shelfbreak. The relevance of this frontogenesis mechanism to cases that allow vertical variations is examined by considering the three-dimensional structure of a passive tracer front with explicit finite vertical mixing and bottom boundary layer dynamics. A three-dimensional primitive-equation numerical model is configured in a channel with a continental shelf, slope, and abyssal plain running the length of the channel. A vertically and horizontally uniform inflow is imposed over the shelf, with a large horizontal velocity shear near the shelfbreak. In the primitive-equation model, the offshore flow is concentrated in the bottom boundary layer while the alongshelf flow distribution is similar to the depth-averaged case; the presence of the bottom topography maintains a strong horizontal shear near the shelfbreak above the bottom boundary layer. This velocity shear causes a smooth passive tracer distribution imposed at the inflow boundary to develop strong cross-shelf gradients near the shelfbreak (i.e., a passive tracer front) within a rather short downstream distance, as in the depth-averaged model. Neutrally buoyant Lagrangian particles initialized above the bottom boundary layer are rapidly advected along the shelf with little cross-shelf motion. However, particles initialized within the bottom boundary layer move quickly offshore toward the shelfbreak and beyond while being advected alongshelf relatively slowly. The shelfbreak does not act as a barrier to the offshore transport of neutrally buoyant particles despite the presence of the passive tracer front. This results in a continuous net offshore transport from the shelf to the deep ocean due to the effects of bottom friction.
Abstract
A simple linear model of the abyssal circulation is studied in which a north-south topographic slope influences the interior and boundary-layer flow. As in an earlier study, the reversals of the meridional velocity in the abyssal interior are related to the longitudinal variation of upwelling into the main thermocline.
When the topography slopes in the anti-β sense (down to the north in the northern hemisphere) an eastern boundary current appears regardless of the magnitude of the slope. If the slope is weak, the eastern boundary current is broad and bottom trapped. As the slope becomes steeper, the current narrows and stretches vertically. At a critical value of the slope, for which the barotropic potential vorticity gradient changes sign, the eastern boundary current metamorphoses into a modified Munk layer.
For all values of the slope, a system of broad, baroclinic western boundary currents exist whose effects reach rather far into the interior.
Abstract
A simple linear model of the abyssal circulation is studied in which a north-south topographic slope influences the interior and boundary-layer flow. As in an earlier study, the reversals of the meridional velocity in the abyssal interior are related to the longitudinal variation of upwelling into the main thermocline.
When the topography slopes in the anti-β sense (down to the north in the northern hemisphere) an eastern boundary current appears regardless of the magnitude of the slope. If the slope is weak, the eastern boundary current is broad and bottom trapped. As the slope becomes steeper, the current narrows and stretches vertically. At a critical value of the slope, for which the barotropic potential vorticity gradient changes sign, the eastern boundary current metamorphoses into a modified Munk layer.
For all values of the slope, a system of broad, baroclinic western boundary currents exist whose effects reach rather far into the interior.
Abstract
A dynamical mechanism for the generation of coastal sciches by deep-sea internal waves is investigated using a linear, two-layer coastal model in which internal waves from the deep ocean impinge upon a step-shelf bottom topography. For periodic incident waves, a pronounced peak in the shelf response occurs at each coastal seiche frequency. The maximum amplitude over the shelf is almost directly proportional to the degree of stratification, suggesting that sciche activity should vary with seasonal changes in the stratification.
Based on the periodic solutions, Fourier transforms are used to determine the response to one or more internal-wave pulses, and the results are qualitatively consistent with observations. For geometry and gratification which are representative of the Caribbean coast of Puerto Rico, reasonably realistic incident pulses preferentially excite the basic seiche frequency, and a rather small amplitude pulse (10 m) can easily generate currents at the shelf break of 8–10 cm s−1. Further, as is typical of the observed sciches, the time history of the modeled motions over the shelf can be rather irregular, depending on the pulse shape and the time delay between pulses.
Abstract
A dynamical mechanism for the generation of coastal sciches by deep-sea internal waves is investigated using a linear, two-layer coastal model in which internal waves from the deep ocean impinge upon a step-shelf bottom topography. For periodic incident waves, a pronounced peak in the shelf response occurs at each coastal seiche frequency. The maximum amplitude over the shelf is almost directly proportional to the degree of stratification, suggesting that sciche activity should vary with seasonal changes in the stratification.
Based on the periodic solutions, Fourier transforms are used to determine the response to one or more internal-wave pulses, and the results are qualitatively consistent with observations. For geometry and gratification which are representative of the Caribbean coast of Puerto Rico, reasonably realistic incident pulses preferentially excite the basic seiche frequency, and a rather small amplitude pulse (10 m) can easily generate currents at the shelf break of 8–10 cm s−1. Further, as is typical of the observed sciches, the time history of the modeled motions over the shelf can be rather irregular, depending on the pulse shape and the time delay between pulses.
Abstract
A mechanism is described for the formation or a front at the edge of a continental shelf in an initially linearly stratified fluid lacking horizontal density gradients. A primitive equation numerical model is used with a specified vertically uniform inflow imposed at the upstream boundary, and the flow is allowed to evolve alongshelf under the influence of bottom friction. As the flow progresses downstream, the shelf water moves steadily offshore due to the Ekman flux concentrated in the bottom boundary layer. This offshore flow transports light water under heavier water, which leads to convective overturning and ultimately a vertically well-mixed density field over the shelf. Large cross-shelf density gradients appear along the bottom at the shelf break where the vertically well-mixed shelf water abuts the linearly stratified water on the upper slope. At the shelf break, the bottom boundary layer detaches and continues offshore along upward sloping isopycnals. Neutrally buoyant particles in the bottom boundary layer over the shelf move offshore until reaching the shelf break, where they ascend along isopycnal surfaces. Near-surface particles over the outer shelf move alongshelf rapidly with small offshore velocities, while near-surface particles over the inner shelf are drawn shoreward and downwell at the coastal boundary to feed the bottom boundary layer. The basic frontogenesis process is quite robust over a wide range of values of model parameters. The frontogenesis mechanism has important implications for observations of shelf-break fronts as well as for the biology and chemistry of these fronts.
Abstract
A mechanism is described for the formation or a front at the edge of a continental shelf in an initially linearly stratified fluid lacking horizontal density gradients. A primitive equation numerical model is used with a specified vertically uniform inflow imposed at the upstream boundary, and the flow is allowed to evolve alongshelf under the influence of bottom friction. As the flow progresses downstream, the shelf water moves steadily offshore due to the Ekman flux concentrated in the bottom boundary layer. This offshore flow transports light water under heavier water, which leads to convective overturning and ultimately a vertically well-mixed density field over the shelf. Large cross-shelf density gradients appear along the bottom at the shelf break where the vertically well-mixed shelf water abuts the linearly stratified water on the upper slope. At the shelf break, the bottom boundary layer detaches and continues offshore along upward sloping isopycnals. Neutrally buoyant particles in the bottom boundary layer over the shelf move offshore until reaching the shelf break, where they ascend along isopycnal surfaces. Near-surface particles over the outer shelf move alongshelf rapidly with small offshore velocities, while near-surface particles over the inner shelf are drawn shoreward and downwell at the coastal boundary to feed the bottom boundary layer. The basic frontogenesis process is quite robust over a wide range of values of model parameters. The frontogenesis mechanism has important implications for observations of shelf-break fronts as well as for the biology and chemistry of these fronts.
Abstract
Each year the hydrography of the Middle Atlantic Bight changes dramatically from winter conditions with strong horizontal gradients of temperature, salinity, and density at the shelf break separating shelf and slope waters to the summer stratification with a sharp pycnocline at about 20-m depth across both the shelf and slope and weak horizontal density gradients. We use a simple one-dimensional diffusion model to demonstrate that this change could result from a uniform surface heating provided that the nonlinearity of the equation of state is taken into account.
Abstract
Each year the hydrography of the Middle Atlantic Bight changes dramatically from winter conditions with strong horizontal gradients of temperature, salinity, and density at the shelf break separating shelf and slope waters to the summer stratification with a sharp pycnocline at about 20-m depth across both the shelf and slope and weak horizontal density gradients. We use a simple one-dimensional diffusion model to demonstrate that this change could result from a uniform surface heating provided that the nonlinearity of the equation of state is taken into account.
