Search Results
You are looking at 11 - 20 of 29 items for
- Author or Editor: Richard E. Thomson x
- Refine by Access: All Content x
Abstract
In tropical regions, and for applications where the alongshore scale k −1 of the forcing is large, the assumption of constant Coriolis parameters f in Csanady's Arrested Topographic Wave (ATW) model is invalid. Here we generalize the ATW model for study wind-driven coastal circulation by allowing f to vary according to the β-plane approximation f = 0 + βy, and by deriving solutions for finite width shelves. Bottom friction is assumed to be linear in the depth-averaged velocity with coefficient r and the depth h(x) = sx is assumed to increase linearly with distance x offshore. The generalization includes the ATW solutions as a subset; however, theoretical and numerical calculations show that the dimensionless parameter β/f 0 k plays a key role in the flow structure. In particular, for infinitely wide shelves and nonzero values of β/f 0 k, enhanced trapping occurs for coastal circulation off an east cost while trapped solutions cease to exist for circulation off a west coast. For finite width shelves, specification of zero sea level anomaly at the shelf break allows solutions for wind-driven circulation on both eat and west costs. Inclusion of the β effect results in a smaller trapping scale for coastal flows on east coasts (western ocean boundaries) and a larger trapping scale for coastal flows on west coasts. Asymptotic solutions for geographically varying wind stress with oscillatory form are presented as examples.
Abstract
In tropical regions, and for applications where the alongshore scale k −1 of the forcing is large, the assumption of constant Coriolis parameters f in Csanady's Arrested Topographic Wave (ATW) model is invalid. Here we generalize the ATW model for study wind-driven coastal circulation by allowing f to vary according to the β-plane approximation f = 0 + βy, and by deriving solutions for finite width shelves. Bottom friction is assumed to be linear in the depth-averaged velocity with coefficient r and the depth h(x) = sx is assumed to increase linearly with distance x offshore. The generalization includes the ATW solutions as a subset; however, theoretical and numerical calculations show that the dimensionless parameter β/f 0 k plays a key role in the flow structure. In particular, for infinitely wide shelves and nonzero values of β/f 0 k, enhanced trapping occurs for coastal circulation off an east cost while trapped solutions cease to exist for circulation off a west coast. For finite width shelves, specification of zero sea level anomaly at the shelf break allows solutions for wind-driven circulation on both eat and west costs. Inclusion of the β effect results in a smaller trapping scale for coastal flows on east coasts (western ocean boundaries) and a larger trapping scale for coastal flows on west coasts. Asymptotic solutions for geographically varying wind stress with oscillatory form are presented as examples.
Abstract
Current and sea level data from the Coastal Oceanic Dynamics Experiment conducted of Vancouver Island from May 1979 to September 1980 reveal shelf waves of diurnal period whose motions dominate the flow over the continental shelf. In this paper, current meter data from mooring lines off Brooks Peninsula, Estevan Point and Carmanah Point are compared with theoretical solutions for a combination of free shelf waves and barotropic Kelvin waves over uniform alongshore topography. Use of Brink's scheme for baroclinic shelf waves gives close agreement between the modeled and observed K1-period horizontal currents over the continental margin off central Vancouver Island and accurately predicts vertical velocities and sea levels. The shelf waves am shown to be entirely dominated by the first baroclinic mode. Wavelength derived from the model closely approximate wavelenghts computed from the alongshore change in phase of the observed diurnal current ellipses. The baroclinic model successfully predicts the longer wavelength off southern Vancouver Island and the shorter wavelength at K1 compared to O1 periods. The model's principal failure is its inability to account for the seasonal variation in observed wavelengths. Incorporation of a mean alongshore current with cross-shelf shear into a model for barotropic shelf waves suggests that this variation arises through Doppler shifting of the wave frequency.
A comparison with currents off Carmanah Point is less satisfactory presumably because of the proximity of the current meters to shallow banks and a canyon. Similarly, irregular bottom topography near Brooks Peninsula precludes meaningful comparisons.
Abstract
Current and sea level data from the Coastal Oceanic Dynamics Experiment conducted of Vancouver Island from May 1979 to September 1980 reveal shelf waves of diurnal period whose motions dominate the flow over the continental shelf. In this paper, current meter data from mooring lines off Brooks Peninsula, Estevan Point and Carmanah Point are compared with theoretical solutions for a combination of free shelf waves and barotropic Kelvin waves over uniform alongshore topography. Use of Brink's scheme for baroclinic shelf waves gives close agreement between the modeled and observed K1-period horizontal currents over the continental margin off central Vancouver Island and accurately predicts vertical velocities and sea levels. The shelf waves am shown to be entirely dominated by the first baroclinic mode. Wavelength derived from the model closely approximate wavelenghts computed from the alongshore change in phase of the observed diurnal current ellipses. The baroclinic model successfully predicts the longer wavelength off southern Vancouver Island and the shorter wavelength at K1 compared to O1 periods. The model's principal failure is its inability to account for the seasonal variation in observed wavelengths. Incorporation of a mean alongshore current with cross-shelf shear into a model for barotropic shelf waves suggests that this variation arises through Doppler shifting of the wave frequency.
A comparison with currents off Carmanah Point is less satisfactory presumably because of the proximity of the current meters to shallow banks and a canyon. Similarly, irregular bottom topography near Brooks Peninsula precludes meaningful comparisons.
Abstract
Continental shelf waves of diurnal period are shown to be generated by tidally induced Reynolds stresses within a bottom Stokes-scale boundary layer. The theory is applicable to a uniformly rotating, homogeneous ocean of two-dimensional depth variability in which alongshore variations occur over scales large compared to the shelf width. Explicit solutions are derived for the shelf wave velocity components and for the cross-shelf sea surface slope in the case of a traveling Kelvin wave forcing. Numerical values are presented for an exponential depth profile H = H0 exp(−2αx), where x is the coordinate normal to the coast. Results indicate that the amplitude of the shelf wave current can exceed that of the astronomical tidal current and that the alongshore component of the shelf wave current will consistently lead the alongshore component of the tidal current by 180° to 0° over one, wavelength in the direction of phase propagation.
Abstract
Continental shelf waves of diurnal period are shown to be generated by tidally induced Reynolds stresses within a bottom Stokes-scale boundary layer. The theory is applicable to a uniformly rotating, homogeneous ocean of two-dimensional depth variability in which alongshore variations occur over scales large compared to the shelf width. Explicit solutions are derived for the shelf wave velocity components and for the cross-shelf sea surface slope in the case of a traveling Kelvin wave forcing. Numerical values are presented for an exponential depth profile H = H0 exp(−2αx), where x is the coordinate normal to the coast. Results indicate that the amplitude of the shelf wave current can exceed that of the astronomical tidal current and that the alongshore component of the shelf wave current will consistently lead the alongshore component of the tidal current by 180° to 0° over one, wavelength in the direction of phase propagation.
Abstract
Tide, current and water property data collected in the western basin of Johnstone Strait, British Columbia, are compared with analytical models for M2 semidiurnal motions in a stratified, rotating channel of uniform depth. We show that, for a rectangular basin in which there is a depth-independent mean flow, the tidal fluctuations are readily expressible in terms of a landward (eastward) propagating Kelvin wave and exponentially damped, seaward (westward) propagating baroclinic Kelvin waves. The internal M2 tide appears to be generated through interaction of the surface tide with the shallow sill at the eastern end of the basin and is dominated by a first-mode baroclinic wave whose amplitude undergoes attenuation over an e-folding distance of one wavelength (∼26 km). Near the sill the baroclinic current speed may exceed 50% of the barotropic flow speed. The along-channel energy flux (power) of the barotropic tide is comparatively uniform at ∼1.4×109 W throughout the western portion of the Strait but decreases by a factor of 2 across the sill. Less than 0.3% of this power loss is needed to account for the baroclinic energy flux of ∼2×106 W near the sill; dissipation due to bottom friction is the main sink for the barotropic tidal energy. Attenuation of the baroclinic waves appears to result from a combination of turbulent horizontal friction and bottom boundary layer friction. Based on the first mode amplitudes, we find a horizontal eddy viscosity of ∼4×106 cm2 s−1 and a vertical eddy viscosity of ∼103 cm2 s−1. Our analysis further indicates that higher modes may be subject to attenuation by critical-layer absorption in the presence of the mean estuarine circulation in the basin. The M2 tides and currents are shown to possess considerable variability over periods of weeks and to have seasonal trends that, qualitatively at least, are related to changes in the mean density structure.
Abstract
Tide, current and water property data collected in the western basin of Johnstone Strait, British Columbia, are compared with analytical models for M2 semidiurnal motions in a stratified, rotating channel of uniform depth. We show that, for a rectangular basin in which there is a depth-independent mean flow, the tidal fluctuations are readily expressible in terms of a landward (eastward) propagating Kelvin wave and exponentially damped, seaward (westward) propagating baroclinic Kelvin waves. The internal M2 tide appears to be generated through interaction of the surface tide with the shallow sill at the eastern end of the basin and is dominated by a first-mode baroclinic wave whose amplitude undergoes attenuation over an e-folding distance of one wavelength (∼26 km). Near the sill the baroclinic current speed may exceed 50% of the barotropic flow speed. The along-channel energy flux (power) of the barotropic tide is comparatively uniform at ∼1.4×109 W throughout the western portion of the Strait but decreases by a factor of 2 across the sill. Less than 0.3% of this power loss is needed to account for the baroclinic energy flux of ∼2×106 W near the sill; dissipation due to bottom friction is the main sink for the barotropic tidal energy. Attenuation of the baroclinic waves appears to result from a combination of turbulent horizontal friction and bottom boundary layer friction. Based on the first mode amplitudes, we find a horizontal eddy viscosity of ∼4×106 cm2 s−1 and a vertical eddy viscosity of ∼103 cm2 s−1. Our analysis further indicates that higher modes may be subject to attenuation by critical-layer absorption in the presence of the mean estuarine circulation in the basin. The M2 tides and currents are shown to possess considerable variability over periods of weeks and to have seasonal trends that, qualitatively at least, are related to changes in the mean density structure.
Abstract
Satellite-tracked surface drifters deployed in September 1993 in the vicinity of the Kuril–Kamchatka Trench were advected onto the Pacific continental shelf of the Kuril Islands where they encountered strong (40–50 cm s−1) diurnal tidal currents. One of the drifters subsequently passed through Friz Strait into the Sea of Okhotsk, experiencing intense (>100 cm s−1) diurnal currents in the strait and strong (35–40 cm s−1) diurnal currents over the Okhotsk shelf of the Kuril Islands. The across-shelf structure of the diurnal tidal currents is shown to be consistent with that of free, topographically trapped subinertial waves propagating along the continental margin of the islands. Of the three continental shelf wave models considered (a barotropic model with zero mean flow, a barotropic model with nonzero alongshore mean flow, and a baroclinic model based on the observed density structure), only the baroclinic model accurately explains the main features of the diurnal currents for the Pacific and Okhotsk shelves. Both first and second mode waves contribute to the diurnal currents.
Abstract
Satellite-tracked surface drifters deployed in September 1993 in the vicinity of the Kuril–Kamchatka Trench were advected onto the Pacific continental shelf of the Kuril Islands where they encountered strong (40–50 cm s−1) diurnal tidal currents. One of the drifters subsequently passed through Friz Strait into the Sea of Okhotsk, experiencing intense (>100 cm s−1) diurnal currents in the strait and strong (35–40 cm s−1) diurnal currents over the Okhotsk shelf of the Kuril Islands. The across-shelf structure of the diurnal tidal currents is shown to be consistent with that of free, topographically trapped subinertial waves propagating along the continental margin of the islands. Of the three continental shelf wave models considered (a barotropic model with zero mean flow, a barotropic model with nonzero alongshore mean flow, and a baroclinic model based on the observed density structure), only the baroclinic model accurately explains the main features of the diurnal currents for the Pacific and Okhotsk shelves. Both first and second mode waves contribute to the diurnal currents.
Abstract
We use bottom pressure records from 59 sites of the global tsunami warning system to examine the nonisostatic response of the World Ocean to surface air pressure forcing within the 4–6-day band. It is within this narrow “5-day” band that sea level fluctuations strongly depart from the isostatic inverted barometer response. Numerical simulations of the observed bottom pressures were conducted using a two-dimensional Princeton Ocean Model forced at the upper boundary by two versions of the air pressure loading: (i) an analytical version having the form of the westward propagating, 5-day Rossby–Haurwitz air pressure mode; and (ii) an observational version based on a 16-yr record of global-scale atmospheric reanalysis data with a spatial resolution of 2.5°. Simulations from the two models—consisting of barotropic standing waves of millibar amplitudes and near uniform phases in the Pacific, Atlantic, and Indian Oceans—are in close agreement and closely reproduce the observed bottom pressures. The marked similarity of the outputs from the two models and the ability of both models to accurately reproduce the seafloor pressure records indicate a pronounced dynamic response of the World Ocean to nonstationary air pressure fields resembling the theoretical Rossby–Haurwitz air pressure mode.
Abstract
We use bottom pressure records from 59 sites of the global tsunami warning system to examine the nonisostatic response of the World Ocean to surface air pressure forcing within the 4–6-day band. It is within this narrow “5-day” band that sea level fluctuations strongly depart from the isostatic inverted barometer response. Numerical simulations of the observed bottom pressures were conducted using a two-dimensional Princeton Ocean Model forced at the upper boundary by two versions of the air pressure loading: (i) an analytical version having the form of the westward propagating, 5-day Rossby–Haurwitz air pressure mode; and (ii) an observational version based on a 16-yr record of global-scale atmospheric reanalysis data with a spatial resolution of 2.5°. Simulations from the two models—consisting of barotropic standing waves of millibar amplitudes and near uniform phases in the Pacific, Atlantic, and Indian Oceans—are in close agreement and closely reproduce the observed bottom pressures. The marked similarity of the outputs from the two models and the ability of both models to accurately reproduce the seafloor pressure records indicate a pronounced dynamic response of the World Ocean to nonstationary air pressure fields resembling the theoretical Rossby–Haurwitz air pressure mode.
Abstract
The authors examine barotropic nondivergent shelf waves generated on an exponential continental shelf that has an abrupt change in width. Three types of forcing are considered: 1) a tidal period volume flux through a gap in the coastline located along the discontinuity, 2) an alongshore propagating wind stress over the continental shelf, and 3) an alongshore propagating perturbation in the streamfunction at the edge of the continental slope. Dimensional results for the linearized models are derived for the northwest coast of Vancouver Island, British Columbia, where the shelf abruptly widens into Queen Charlotte Sound. Because of the change in wave scales and numbers of shelf modes possible on either side of the coastline discontinuity, the response for the discontinuities width shelf differs markedly from that for a uniform width shelf. Results show that shelf wave energy generated by fortnightly tidal flow through the gap (or coastal strait) is radiated in a narrow “beam” across the broader portion of the shelf. Diurnal period motions are trapped near the mouth of the strait and do not contribute significantly to the shelf current. The use of realistic periods and wavelengths (5–10 days and 500–1000 km) for the alongshore forcing terms yields propagating eddylike circulation patterns that closely resemble the flow patterns commonly seen in satellite thermal imagery over the narrow portion of the Vancouver Island shelf. At low forcing frequencies a distinct “shadow zone” with relatively weak barotropic response is found over the wide portion of the shelf in the vicinity of the coastal discontinuity.
Abstract
The authors examine barotropic nondivergent shelf waves generated on an exponential continental shelf that has an abrupt change in width. Three types of forcing are considered: 1) a tidal period volume flux through a gap in the coastline located along the discontinuity, 2) an alongshore propagating wind stress over the continental shelf, and 3) an alongshore propagating perturbation in the streamfunction at the edge of the continental slope. Dimensional results for the linearized models are derived for the northwest coast of Vancouver Island, British Columbia, where the shelf abruptly widens into Queen Charlotte Sound. Because of the change in wave scales and numbers of shelf modes possible on either side of the coastline discontinuity, the response for the discontinuities width shelf differs markedly from that for a uniform width shelf. Results show that shelf wave energy generated by fortnightly tidal flow through the gap (or coastal strait) is radiated in a narrow “beam” across the broader portion of the shelf. Diurnal period motions are trapped near the mouth of the strait and do not contribute significantly to the shelf current. The use of realistic periods and wavelengths (5–10 days and 500–1000 km) for the alongshore forcing terms yields propagating eddylike circulation patterns that closely resemble the flow patterns commonly seen in satellite thermal imagery over the narrow portion of the Vancouver Island shelf. At low forcing frequencies a distinct “shadow zone” with relatively weak barotropic response is found over the wide portion of the shelf in the vicinity of the coastal discontinuity.
Abstract
This paper presents a simple diagnostic model for estimating mixed layer depth based solely on the one-dimensional heat balance equation, the surface heat flux, and the sea surface temperature. The surface fluxes drive heating or cooling of the upper layer whereas the surface temperature acts as a “thermostat” that regulates the vertical extent of the layer. Daily mixed layer depth estimates from the diagnostic model (and two standard bulk mixed layer models) are compared with depths obtained from oceanic profiles collected during the 1956–80 Canadian Weathership program at Station P and more recent (2001–07) profiles from the vicinity of this station from Argo drifters. Summer mixed layer depths from the diagnostic model agree more closely with observed depths and are less sensitive to heat flux errors than those from bulk models. For the Weathership monitoring period, the root-mean-square difference between modeled and observed monthly mean mixed layer depths is ∼6 m for the diagnostic model and ∼10 m for the bulk models. The diagnostic model is simpler to apply than bulk models and sidesteps the need for wind data and turbulence parameterization required by these models. Mixed layer depths obtained from the diagnostic model using NCEP–NCAR reanalysis data reveal that—contrary to reports for late winter—there has been no significant trend in the summer mixed layer depth in the central northeast Pacific over the past 52 yr.
Abstract
This paper presents a simple diagnostic model for estimating mixed layer depth based solely on the one-dimensional heat balance equation, the surface heat flux, and the sea surface temperature. The surface fluxes drive heating or cooling of the upper layer whereas the surface temperature acts as a “thermostat” that regulates the vertical extent of the layer. Daily mixed layer depth estimates from the diagnostic model (and two standard bulk mixed layer models) are compared with depths obtained from oceanic profiles collected during the 1956–80 Canadian Weathership program at Station P and more recent (2001–07) profiles from the vicinity of this station from Argo drifters. Summer mixed layer depths from the diagnostic model agree more closely with observed depths and are less sensitive to heat flux errors than those from bulk models. For the Weathership monitoring period, the root-mean-square difference between modeled and observed monthly mean mixed layer depths is ∼6 m for the diagnostic model and ∼10 m for the bulk models. The diagnostic model is simpler to apply than bulk models and sidesteps the need for wind data and turbulence parameterization required by these models. Mixed layer depths obtained from the diagnostic model using NCEP–NCAR reanalysis data reveal that—contrary to reports for late winter—there has been no significant trend in the summer mixed layer depth in the central northeast Pacific over the past 52 yr.
Abstract
It has been suggested that low-frequency current fluctuations in the southern Strait of Georgia are the result of baroclinic instability. However, data extracted from cyclesonde and fixed current meter moorings suggest that the conditions for baroclinic instability are highly variable in space and time. It has been recently discovered that there are summertime bottom-intensified gravity currents with fortnightly and monthly periods associated with the introduction of salty waters from the Juan de Fuca Strait during periods of neap tides. These currents are the dominant mechanism for deep-water renewal in the Strait of Georgia. It is argued that these currents are baroclinically unstable and that the stability characteristics are reasonably consistent with the observed structure of the low-frequency current fluctuations. The episodic nature of these unstable bottom flows may help to explain the spatial and temporal variability of the low-frequency current fluctuations observed in the Strait of Georgia.
Abstract
It has been suggested that low-frequency current fluctuations in the southern Strait of Georgia are the result of baroclinic instability. However, data extracted from cyclesonde and fixed current meter moorings suggest that the conditions for baroclinic instability are highly variable in space and time. It has been recently discovered that there are summertime bottom-intensified gravity currents with fortnightly and monthly periods associated with the introduction of salty waters from the Juan de Fuca Strait during periods of neap tides. These currents are the dominant mechanism for deep-water renewal in the Strait of Georgia. It is argued that these currents are baroclinically unstable and that the stability characteristics are reasonably consistent with the observed structure of the low-frequency current fluctuations. The episodic nature of these unstable bottom flows may help to explain the spatial and temporal variability of the low-frequency current fluctuations observed in the Strait of Georgia.
Abstract
A three-dimensional finite element model is used to calculate the barotropic tides and seasonal buoyancy flows off the western and northern coasts of Vancouver Island. The model buoyancy currents and the harmonics of eight tidal constituents are compared with those from previous models, and those from tide gauge and current meter observations. The rms differences between observed and calculated sea level tidal amplitudes are within 2.3 cm for all constituents, whereas the rms differences between observed and calculated phases are, with the exception of Q 1, within 3.5°. The model currents are more accurate than those from previous models.
Of particular interest are the diurnal continental shelf waves. It is shown that these waves are generated through the conservation of potental vorticity arising when the strong diurnal tidal currents in Juan de Fuca Strait encounter the abrupt topography near the entrance to the strait. These waves do not appear to propagate beyond Brooks Peninsula, a large promontory cutting across the continental shelf. A power budget analysis reveals that the reason for this is not the blocking effect of the peninsula but rather there is little energy left in the waves when they reach that point. This energy loss is primarily through frictional dissipation in a series of trapped eddies along the shelf break. The location of these eddies varies with the forcing frequency and appears to be related to the spacing of canyons. It is also demonstrated that the strong diurnal currents observed over the shallow banks in Queen Charlotte Sound to the north of Brooks Peninsula do not arise from the oscillatory diurnal flows in Queen Charlotte Strait. Unlike the case for Juan de Fuca Strait, the region offshore of Queen Charlotte Strait does not support diurnal coastally trapped waves. Seasonal changes in the wavelengths of the Vancouver Island shelf waves are shown to arise through an advective interaction (Doppler shift) with the buoyancy-driven Vancouver Island coastal current and the wind-driven shelf break current.
Abstract
A three-dimensional finite element model is used to calculate the barotropic tides and seasonal buoyancy flows off the western and northern coasts of Vancouver Island. The model buoyancy currents and the harmonics of eight tidal constituents are compared with those from previous models, and those from tide gauge and current meter observations. The rms differences between observed and calculated sea level tidal amplitudes are within 2.3 cm for all constituents, whereas the rms differences between observed and calculated phases are, with the exception of Q 1, within 3.5°. The model currents are more accurate than those from previous models.
Of particular interest are the diurnal continental shelf waves. It is shown that these waves are generated through the conservation of potental vorticity arising when the strong diurnal tidal currents in Juan de Fuca Strait encounter the abrupt topography near the entrance to the strait. These waves do not appear to propagate beyond Brooks Peninsula, a large promontory cutting across the continental shelf. A power budget analysis reveals that the reason for this is not the blocking effect of the peninsula but rather there is little energy left in the waves when they reach that point. This energy loss is primarily through frictional dissipation in a series of trapped eddies along the shelf break. The location of these eddies varies with the forcing frequency and appears to be related to the spacing of canyons. It is also demonstrated that the strong diurnal currents observed over the shallow banks in Queen Charlotte Sound to the north of Brooks Peninsula do not arise from the oscillatory diurnal flows in Queen Charlotte Strait. Unlike the case for Juan de Fuca Strait, the region offshore of Queen Charlotte Strait does not support diurnal coastally trapped waves. Seasonal changes in the wavelengths of the Vancouver Island shelf waves are shown to arise through an advective interaction (Doppler shift) with the buoyancy-driven Vancouver Island coastal current and the wind-driven shelf break current.
