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- Author or Editor: Clinton Winant x
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Abstract
Coastal downwelling events, induced by tropical storms which travel up along the coast, occur regularly during the summer over the shelf of Southern California. Large vertical velocities (0.5 cm s−1) are observed over the very narrow (3.6 km) shelf. Simultaneous observations of longshore current and cross-shelf pressure gradient indicate the cross-shelf momentum balance is geostrophic. Heat balance computations reveal that the increase in mean temperature over the shelf is mostly caused by cross-shelf advection of heat. Large longshore accelerations occurring simultaneously at all depths in the shallower part of the shelf may be explained by longshore sea surface slopes contributing, along with the wind stress, to the longshore momentum balance. Profiles of temperature and velocity are consistent with a two-layer description of the vertical structure, these layers being separated by a thin, turbulent mixing layer.
Abstract
Coastal downwelling events, induced by tropical storms which travel up along the coast, occur regularly during the summer over the shelf of Southern California. Large vertical velocities (0.5 cm s−1) are observed over the very narrow (3.6 km) shelf. Simultaneous observations of longshore current and cross-shelf pressure gradient indicate the cross-shelf momentum balance is geostrophic. Heat balance computations reveal that the increase in mean temperature over the shelf is mostly caused by cross-shelf advection of heat. Large longshore accelerations occurring simultaneously at all depths in the shallower part of the shelf may be explained by longshore sea surface slopes contributing, along with the wind stress, to the longshore momentum balance. Profiles of temperature and velocity are consistent with a two-layer description of the vertical structure, these layers being separated by a thin, turbulent mixing layer.
Abstract
The wind-driven circulation in lakes, lagoons, estuaries, or coastal embayments is described with a linear, steady, three-dimensional barotropic model in an elongated basin of arbitrary depth distribution, on an f plane. With rotation, the vertically averaged velocity scales with the Ekman depth rather than the maximum depth h 0 as in the case without rotation. Near the closed ends of the basin, the flow turns in viscous boundary layers. Because the length of the turning areas depends on the sign of the bottom slope and on δ, the ratio of the Ekman depth to h 0, there is a striking contrast between the turning areas on either side of an observer looking toward the end of the basin. In the Northern Hemisphere, the turning area on the left is broad, of order δ −1 B*, where B* is the basin half-width. The turning area on the right is narrow, of order δB*, and dynamically equivalent to the western boundary current in models of the wind-driven ocean circulation. Ekman solutions are used to describe the vertical structure of the corresponding three-dimensional flow. The axial flow is qualitatively similar to the flow without rotation, but with reduced amplitude. The lateral circulation consists of two superposed gyres. The upper gyre rotates in the sense expected for Ekman transport: the surface flow is to the right of the wind. In the lower gyre, the circulation is in the opposite sense, driven by the veering in the bottom Ekman layer. The largest horizontal and vertical velocities occur in the narrow boundary layer near the end of the basin. Near midbasin, fluid parcels spiral downwind in a sheath surrounding a central core that rotates in the lateral plane, in the sense expected from Ekman dynamics. After turning at the end of the basin, some parcels travel upwind in the central core, while others return in the lower gyre.
Abstract
The wind-driven circulation in lakes, lagoons, estuaries, or coastal embayments is described with a linear, steady, three-dimensional barotropic model in an elongated basin of arbitrary depth distribution, on an f plane. With rotation, the vertically averaged velocity scales with the Ekman depth rather than the maximum depth h 0 as in the case without rotation. Near the closed ends of the basin, the flow turns in viscous boundary layers. Because the length of the turning areas depends on the sign of the bottom slope and on δ, the ratio of the Ekman depth to h 0, there is a striking contrast between the turning areas on either side of an observer looking toward the end of the basin. In the Northern Hemisphere, the turning area on the left is broad, of order δ −1 B*, where B* is the basin half-width. The turning area on the right is narrow, of order δB*, and dynamically equivalent to the western boundary current in models of the wind-driven ocean circulation. Ekman solutions are used to describe the vertical structure of the corresponding three-dimensional flow. The axial flow is qualitatively similar to the flow without rotation, but with reduced amplitude. The lateral circulation consists of two superposed gyres. The upper gyre rotates in the sense expected for Ekman transport: the surface flow is to the right of the wind. In the lower gyre, the circulation is in the opposite sense, driven by the veering in the bottom Ekman layer. The largest horizontal and vertical velocities occur in the narrow boundary layer near the end of the basin. Near midbasin, fluid parcels spiral downwind in a sheath surrounding a central core that rotates in the lateral plane, in the sense expected from Ekman dynamics. After turning at the end of the basin, some parcels travel upwind in the central core, while others return in the lower gyre.
Abstract
The three-dimensional tidal circulation in an elongated basin of arbitrary depth is described with a linear, constant-density model on the f plane. Rotation fundamentally alters the lateral flow, introducing a lateral recirculation comparable in magnitude to the axial flow, as long as friction is not too large. This circulation is due to the imbalance between the cross-channel sea level gradient, which is in near-geostrophic balance with the Coriolis acceleration associated with the vertically averaged axial flow, and the Coriolis acceleration associated with the vertically sheared axial flow. During flood condition, for example, the lateral Coriolis acceleration near the surface exceeds the pressure gradient, tending to accelerate the lateral flow, while the converse is true near the bottom. As a result, with rotation, fluid parcels tend to corkscrew into and out of the basin in a tidal period. The axial flow is only weakly modified by rotation. When friction is small, the axial velocity is uniform in each section, except in a narrow bottom boundary layer where it decreases to zero. The boundary layer thickness increases with friction, so that with moderate or large friction, axial velocities are sheared from bottom to surface. When friction is large, the local and Coriolis accelerations are both small and the dynamics are governed by a balance between friction and the pressure gradient.
Abstract
The three-dimensional tidal circulation in an elongated basin of arbitrary depth is described with a linear, constant-density model on the f plane. Rotation fundamentally alters the lateral flow, introducing a lateral recirculation comparable in magnitude to the axial flow, as long as friction is not too large. This circulation is due to the imbalance between the cross-channel sea level gradient, which is in near-geostrophic balance with the Coriolis acceleration associated with the vertically averaged axial flow, and the Coriolis acceleration associated with the vertically sheared axial flow. During flood condition, for example, the lateral Coriolis acceleration near the surface exceeds the pressure gradient, tending to accelerate the lateral flow, while the converse is true near the bottom. As a result, with rotation, fluid parcels tend to corkscrew into and out of the basin in a tidal period. The axial flow is only weakly modified by rotation. When friction is small, the axial velocity is uniform in each section, except in a narrow bottom boundary layer where it decreases to zero. The boundary layer thickness increases with friction, so that with moderate or large friction, axial velocities are sheared from bottom to surface. When friction is large, the local and Coriolis accelerations are both small and the dynamics are governed by a balance between friction and the pressure gradient.
Abstract
Local velocities and the trajectories of fluid parcels forced by wind blowing over a continental shelf, in the vicinity of a headland, are described with a linear, steady, three-dimensional barotropic model. The dynamical balance that governs the transport is similar to the wind-driven general circulation, because the varying bottom depth acts in the same way as meridional variation in the rotation rate. Far from the headland the circulation is independent of alongshore position, and the transport is parallel to the coast. The alongshore pressure gradient is a significant term in the alongshore momentum balance. Near the headland, the amplitude of the circulation, including the vertical motion, is larger on the upwave side (the side toward which a Kelvin wave would travel) than on the downwave side. On the upwave side, the flow adjusts to the presence of the headland over a distance of order δEB*, where δE is the ratio of the Ekman depth to the maximum shelf depth and B* is the width of the shelf. Fluid parcels that upwell on the upwave side are drawn from deeper depths than parcels that upwell at other alongshore locations. On the downwave side the flow adjusts over a relatively long distance, of order δ −1 EB*.
Abstract
Local velocities and the trajectories of fluid parcels forced by wind blowing over a continental shelf, in the vicinity of a headland, are described with a linear, steady, three-dimensional barotropic model. The dynamical balance that governs the transport is similar to the wind-driven general circulation, because the varying bottom depth acts in the same way as meridional variation in the rotation rate. Far from the headland the circulation is independent of alongshore position, and the transport is parallel to the coast. The alongshore pressure gradient is a significant term in the alongshore momentum balance. Near the headland, the amplitude of the circulation, including the vertical motion, is larger on the upwave side (the side toward which a Kelvin wave would travel) than on the downwave side. On the upwave side, the flow adjusts to the presence of the headland over a distance of order δEB*, where δE is the ratio of the Ekman depth to the maximum shelf depth and B* is the width of the shelf. Fluid parcels that upwell on the upwave side are drawn from deeper depths than parcels that upwell at other alongshore locations. On the downwave side the flow adjusts over a relatively long distance, of order δ −1 EB*.
Abstract
The three-dimensional tidal circulation in an elongated basin of arbitrary depth is described with a coupled barotropic and baroclinic two-layer model on the f plane. As long as friction is not dominant, near-standing waves are present on the interface as well as on the surface. The surface pattern is principally determined by the product of the tidal barotropic wavenumber by the basin length. The interface deformation is determined by a baroclinic equivalent, usually a much larger number. As a result, the shape of the interface is characterized by horizontally smaller features than the surface. If the product of the tidal baroclinic wavenumber by the basin width is greater than one, both lateral and axial modes can be excited at the interface. If these modes are near resonant, large internal tides can be forced directly by the co-oscillating surface tide at the basin entrance. The amplitude and phase of the baroclinic component are sensitive functions of the density anomaly and the interface depth. As a result, the phase and amplitude of the interface vary by large amounts with comparatively small changes in those parameters. The model behavior is qualitatively consistent with observations in fjords and straits.
Abstract
The three-dimensional tidal circulation in an elongated basin of arbitrary depth is described with a coupled barotropic and baroclinic two-layer model on the f plane. As long as friction is not dominant, near-standing waves are present on the interface as well as on the surface. The surface pattern is principally determined by the product of the tidal barotropic wavenumber by the basin length. The interface deformation is determined by a baroclinic equivalent, usually a much larger number. As a result, the shape of the interface is characterized by horizontally smaller features than the surface. If the product of the tidal baroclinic wavenumber by the basin width is greater than one, both lateral and axial modes can be excited at the interface. If these modes are near resonant, large internal tides can be forced directly by the co-oscillating surface tide at the basin entrance. The amplitude and phase of the baroclinic component are sensitive functions of the density anomaly and the interface depth. As a result, the phase and amplitude of the interface vary by large amounts with comparatively small changes in those parameters. The model behavior is qualitatively consistent with observations in fjords and straits.
Abstract
The three-dimensional residual circulation driven by tides in an elongated basin of arbitrary depth is described with a small amplitude, constant density model on the f plane. The inclusion of rotation fundamentally alters the residual flow. With rotation, fluid is drawn into the basin on the right side of an observer looking toward the closed end (in the Northern Hemisphere) and the return flow is on the opposite side. A lateral circulation is superposed on the axial flow, with upwelling over the deeper part of each section and downwelling near the sides. The residual flow is driven by a combination of advective terms, including the lateral advection of axial momentum associated with the Coriolis acceleration and Stokes forcing. Tidally averaged fluid parcel trajectories are determined by integrating the Lagrangian mean velocities. With or without rotation these trajectories vary considerably depending on small differences in initial position as well as on basin shape and other parameters of the problem.
Abstract
The three-dimensional residual circulation driven by tides in an elongated basin of arbitrary depth is described with a small amplitude, constant density model on the f plane. The inclusion of rotation fundamentally alters the residual flow. With rotation, fluid is drawn into the basin on the right side of an observer looking toward the closed end (in the Northern Hemisphere) and the return flow is on the opposite side. A lateral circulation is superposed on the axial flow, with upwelling over the deeper part of each section and downwelling near the sides. The residual flow is driven by a combination of advective terms, including the lateral advection of axial momentum associated with the Coriolis acceleration and Stokes forcing. Tidally averaged fluid parcel trajectories are determined by integrating the Lagrangian mean velocities. With or without rotation these trajectories vary considerably depending on small differences in initial position as well as on basin shape and other parameters of the problem.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Long time series of atmospheric parameters and limited oceanographic parameters such as near-surface temperature and wave statistics have been available for some time. There is, however, a need for similar observations of currents in the coastal ocean. In December 1991, the National Data Buoy Center (NDBC) deployed two meteorological buoys in the Southern California Bight on a transect between San Diego and San Clemente Island. Each buoy consisted of a 10-m discus hull instrumented to measure a suite of meteorological parameters, and, for the first time in the NDBC buoy program, acoustic Doppler current profilers (ADCPs) were included to gather hourly current profiles beneath the two buoys. Moorings instrumented with seven vector-measuring current meters (VMCMs) were deployed adjacent to the NDBC buoys for several months and provided current observations for comparison with the ADCP measurements.
When the situation is such that the wave-induced buoy motion is not overly large, the observations of horizontal current made by the ADCP and the VMCM are highly correlated. Time series of differences between ADCP and VMCM measurements are characterized by a mean difference (bias error) of about 0.01 m s−1 and standard deviation of about 0.035 m s−1 for 1-h observations. Estimates of current spectra from ADCP and VMCM records suggest that the ADCP system can be characterized by a white noise level of 2 × 10−3 m2 s−2 (cph)−1. However, when the in situ environment is such that large surface waves are present (including breaking waves and whitecaps), erroneous current values are usually reported by the ADCP.
Mean values of vertical velocities reported by the ADCP appear to be much larger than what could be physically expected and are therefore deemed unreliable. As previously reported in the literature, the vertical velocities are contaminated by vertically migrating organisms and, while effective in detecting these diel migrations, the ADCP does not appear to yield useful observations of vertical water velocity in any of the frequency bands resolved by this set of observations.
Abstract
Long time series of atmospheric parameters and limited oceanographic parameters such as near-surface temperature and wave statistics have been available for some time. There is, however, a need for similar observations of currents in the coastal ocean. In December 1991, the National Data Buoy Center (NDBC) deployed two meteorological buoys in the Southern California Bight on a transect between San Diego and San Clemente Island. Each buoy consisted of a 10-m discus hull instrumented to measure a suite of meteorological parameters, and, for the first time in the NDBC buoy program, acoustic Doppler current profilers (ADCPs) were included to gather hourly current profiles beneath the two buoys. Moorings instrumented with seven vector-measuring current meters (VMCMs) were deployed adjacent to the NDBC buoys for several months and provided current observations for comparison with the ADCP measurements.
When the situation is such that the wave-induced buoy motion is not overly large, the observations of horizontal current made by the ADCP and the VMCM are highly correlated. Time series of differences between ADCP and VMCM measurements are characterized by a mean difference (bias error) of about 0.01 m s−1 and standard deviation of about 0.035 m s−1 for 1-h observations. Estimates of current spectra from ADCP and VMCM records suggest that the ADCP system can be characterized by a white noise level of 2 × 10−3 m2 s−2 (cph)−1. However, when the in situ environment is such that large surface waves are present (including breaking waves and whitecaps), erroneous current values are usually reported by the ADCP.
Mean values of vertical velocities reported by the ADCP appear to be much larger than what could be physically expected and are therefore deemed unreliable. As previously reported in the literature, the vertical velocities are contaminated by vertically migrating organisms and, while effective in detecting these diel migrations, the ADCP does not appear to yield useful observations of vertical water velocity in any of the frequency bands resolved by this set of observations.
Abstract
Synthetic subsurface pressure (SSP) can be formed from tide gauge records and from bottom pressure measurements to provide a consistent and convenient basis for comparison of these two different types of observations. Common methods for this estimation are reviewed, and their accuracy is evaluated. Calculations show that subtidal SSP estimates from sea level (SSPSL) and from bottom pressure observations (SSPBP) at close sites agree only in a finite band of frequencies, corresponding to periods between 3.5 and 30 days. At the lower frequencies (periods longer than 30 days), sea level observations are subject to errors induced by the daily measure of staff height. At higher frequencies (periods between 1.5 and 3.5 days), the amplitude of fluctuations is too small to be resolved by a sea level gauge.
Abstract
Synthetic subsurface pressure (SSP) can be formed from tide gauge records and from bottom pressure measurements to provide a consistent and convenient basis for comparison of these two different types of observations. Common methods for this estimation are reviewed, and their accuracy is evaluated. Calculations show that subtidal SSP estimates from sea level (SSPSL) and from bottom pressure observations (SSPBP) at close sites agree only in a finite band of frequencies, corresponding to periods between 3.5 and 30 days. At the lower frequencies (periods longer than 30 days), sea level observations are subject to errors induced by the daily measure of staff height. At higher frequencies (periods between 1.5 and 3.5 days), the amplitude of fluctuations is too small to be resolved by a sea level gauge.
Abstract
Temperature and horizontal current observations at three water depths (15, 30 and 60 m) over the Southern California shelf are reported for four discrete periods during 1978–79, roughly corresponding to each of the principal seasons. The vertical structure of temperature changes markedly during the year; the water over the shelf is weakly stratified in the winter (N = 50 cpd) but stratification is stronger in the summer (N = 250 cpd). Seasonal changes in vertically averaged temperature are comparatively unimportant. Long-term averages of the longshore currents are to the south near the surface in all seasons, with amplitudes ranging up to 10 cm s−1 in the winter. During spring and summer, the stratification is accompanied by shear in the vertical structure of these long-term current averages, with surface currents sweeping to the south, but with deeper, colder water flowing in the opposite direction. Currents fluctuating at subtidal frequencies are predominantly alongshore and are strongest during the winter. The major fluctuations in this frequency band may he decomposed into barotropic and baroclinic components; the latter reach their maximum amplitudes during the summer. Relations between the barotropic currents, longshore wind stress, and synthetic bottom pressure are remarkably similar to those defined previously off Oregon, although the amplitude of currents is observed to increase with distance offshore. At tidal frequencies, both cross-shelf and longshore modes of fluctuation are important. Neither is well correlated to tidal sea surface elevation over long periods. The principal mode of variability associated with longshore tidal currents is barotropic, while that associated with cross-shelf currents is baroclinic. The motion in the cross-shelf plane resembles that due to a standing gravest-mode internal wave. At supratidal frequencies, internal waves travel onshore during those seasons when the water column is strongly stratified. The propagation characteristics of these high-frequency currents are similar to those expected for shoaling interfacial waves.
Abstract
Temperature and horizontal current observations at three water depths (15, 30 and 60 m) over the Southern California shelf are reported for four discrete periods during 1978–79, roughly corresponding to each of the principal seasons. The vertical structure of temperature changes markedly during the year; the water over the shelf is weakly stratified in the winter (N = 50 cpd) but stratification is stronger in the summer (N = 250 cpd). Seasonal changes in vertically averaged temperature are comparatively unimportant. Long-term averages of the longshore currents are to the south near the surface in all seasons, with amplitudes ranging up to 10 cm s−1 in the winter. During spring and summer, the stratification is accompanied by shear in the vertical structure of these long-term current averages, with surface currents sweeping to the south, but with deeper, colder water flowing in the opposite direction. Currents fluctuating at subtidal frequencies are predominantly alongshore and are strongest during the winter. The major fluctuations in this frequency band may he decomposed into barotropic and baroclinic components; the latter reach their maximum amplitudes during the summer. Relations between the barotropic currents, longshore wind stress, and synthetic bottom pressure are remarkably similar to those defined previously off Oregon, although the amplitude of currents is observed to increase with distance offshore. At tidal frequencies, both cross-shelf and longshore modes of fluctuation are important. Neither is well correlated to tidal sea surface elevation over long periods. The principal mode of variability associated with longshore tidal currents is barotropic, while that associated with cross-shelf currents is baroclinic. The motion in the cross-shelf plane resembles that due to a standing gravest-mode internal wave. At supratidal frequencies, internal waves travel onshore during those seasons when the water column is strongly stratified. The propagation characteristics of these high-frequency currents are similar to those expected for shoaling interfacial waves.
