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- Author or Editor: A. L. Kurapov x
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Abstract
Internal tides on the continental shelf can be intermittent as a result of changing hydrographic conditions associated with wind-driven upwelling. In turn, the internal tide can affect transports associated with upwelling. To study these processes, simulations in an idealized, alongshore uniform setup are performed utilizing the hydrostatic Regional Ocean Modeling System (ROMS) with conditions corresponding, as closely as possible, to the central Oregon shelf. “Wind only” (WO), “tide only” (TO), and “tide and wind” (TW) solutions are compared, utilizing cases with constant upwelling-favorable wind stress as well as with time-variable observed stress. The tide is forced by applying cross-shore barotropic flow at the offshore boundary with intensity sufficient to generate an internal tide with horizontal velocity amplitudes near 0.15 m s−1, corresponding to observed levels. The internal tide affects the subinertial circulation, mostly through the changes in the bottom boundary layer variability, resulting in a larger bottom stress and a weaker depth-averaged alongshore current in the TW case compared to WO. The spatial variability of the cross-shore and vertical volume transport is also affected. Divergence in the Reynolds stress associated with the baroclinic tidal flow contributes to the tidally averaged cross-shore momentum balance. Internal waves cause high-frequency variability in the turbulent kinetic energy in both the bottom and surface boundary layers, causing periodic restratification of the inner shelf in the area of the upwelling front. Increased vertical shear in the horizontal velocity resulting from the superposition of the upwelling jet and the internal tide results in intermittent patches of intensified turbulence in the mid–water column. Variability in stratification associated with upwelling can affect not only the propagation of the internal tide on the shelf, but also the barotropic-to-baroclinic energy conversion on the continental slope, in this case changing the classification of the slope from nearly critical to supercritical such that less barotropic tidal energy is converted to baroclinic and a larger fraction of the baroclinic energy is radiated into the open ocean.
Abstract
Internal tides on the continental shelf can be intermittent as a result of changing hydrographic conditions associated with wind-driven upwelling. In turn, the internal tide can affect transports associated with upwelling. To study these processes, simulations in an idealized, alongshore uniform setup are performed utilizing the hydrostatic Regional Ocean Modeling System (ROMS) with conditions corresponding, as closely as possible, to the central Oregon shelf. “Wind only” (WO), “tide only” (TO), and “tide and wind” (TW) solutions are compared, utilizing cases with constant upwelling-favorable wind stress as well as with time-variable observed stress. The tide is forced by applying cross-shore barotropic flow at the offshore boundary with intensity sufficient to generate an internal tide with horizontal velocity amplitudes near 0.15 m s−1, corresponding to observed levels. The internal tide affects the subinertial circulation, mostly through the changes in the bottom boundary layer variability, resulting in a larger bottom stress and a weaker depth-averaged alongshore current in the TW case compared to WO. The spatial variability of the cross-shore and vertical volume transport is also affected. Divergence in the Reynolds stress associated with the baroclinic tidal flow contributes to the tidally averaged cross-shore momentum balance. Internal waves cause high-frequency variability in the turbulent kinetic energy in both the bottom and surface boundary layers, causing periodic restratification of the inner shelf in the area of the upwelling front. Increased vertical shear in the horizontal velocity resulting from the superposition of the upwelling jet and the internal tide results in intermittent patches of intensified turbulence in the mid–water column. Variability in stratification associated with upwelling can affect not only the propagation of the internal tide on the shelf, but also the barotropic-to-baroclinic energy conversion on the continental slope, in this case changing the classification of the slope from nearly critical to supercritical such that less barotropic tidal energy is converted to baroclinic and a larger fraction of the baroclinic energy is radiated into the open ocean.
Abstract
Weak constraint data assimilation involves a certain number of weighting and smoothing parameters. The authors present an approach to estimate them based on maximizing the entropy. Because application of this rigorous scheme to large-dimensional data assimilation problems is a tedious task, the authors also consider a simplified version of the entropy method, which assumes maximizing a data cost as a function of relative data weights. It is proven to be equivalent to maximizing the entropy under certain assumptions. In the scope of this method, the authors have also proposed a smoothing procedure necessary for very fine grids. The schemes have been checked using a tidal channel model for Tatarsky Strait.
Abstract
Weak constraint data assimilation involves a certain number of weighting and smoothing parameters. The authors present an approach to estimate them based on maximizing the entropy. Because application of this rigorous scheme to large-dimensional data assimilation problems is a tedious task, the authors also consider a simplified version of the entropy method, which assumes maximizing a data cost as a function of relative data weights. It is proven to be equivalent to maximizing the entropy under certain assumptions. In the scope of this method, the authors have also proposed a smoothing procedure necessary for very fine grids. The schemes have been checked using a tidal channel model for Tatarsky Strait.
Abstract
The performance of data assimilation methods in an idealized three-dimensional time-dependent coastal baroclinic model is assessed by computing ensemble error statistics. The analytical representer solution allows for computation of posterior error statistics for the variational generalized inverse method (GIM) as well as sequential methods such as the Kalman filter (KF) and optimal interpolation (OI). Computations can be made in a straightforward way, given the statistics of errors in the model equations and data. The GIM yields solutions with significantly smaller variance than that given by KF or OI if the data contain valuable information about the past flow. This is the case, for instance, when a large fraction of the model error is due to uncertainty in the wind stress. In the scope of the model presented here, the plausibility of simplifications made in a practical OI scheme is analyzed. The unified study of the GIM, KF, and OI allows for the demonstratation of how the forecast error covariance used in a practical OI sequential scheme may be optimized with the use of lagged covariances for the model solution. The effect of the misspecified input error statistics on the solution quality is also assessed. In some practically relevant cases the use of future data by the GIM, in contrast to KF and OI, compensates for incorrectly specified input error covariances.
Abstract
The performance of data assimilation methods in an idealized three-dimensional time-dependent coastal baroclinic model is assessed by computing ensemble error statistics. The analytical representer solution allows for computation of posterior error statistics for the variational generalized inverse method (GIM) as well as sequential methods such as the Kalman filter (KF) and optimal interpolation (OI). Computations can be made in a straightforward way, given the statistics of errors in the model equations and data. The GIM yields solutions with significantly smaller variance than that given by KF or OI if the data contain valuable information about the past flow. This is the case, for instance, when a large fraction of the model error is due to uncertainty in the wind stress. In the scope of the model presented here, the plausibility of simplifications made in a practical OI scheme is analyzed. The unified study of the GIM, KF, and OI allows for the demonstratation of how the forecast error covariance used in a practical OI sequential scheme may be optimized with the use of lagged covariances for the model solution. The effect of the misspecified input error statistics on the solution quality is also assessed. In some practically relevant cases the use of future data by the GIM, in contrast to KF and OI, compensates for incorrectly specified input error covariances.
Abstract
Intensified diurnal tides are found along portions of the Oregon shelf (U.S. West Coast) based on analyses of high-frequency (HF) radar surface current data and outputs of a 1-km resolution ocean circulation model. The K 1 tidal currents with magnitudes near 0.07 m s−1 over a wider part of the shelf (Heceta Bank complex; 44°–44.5°N), previously predicted by Erofeeva et al., are confirmed here by newly available HF radar data. Intensified diurnal tides are also found along the narrow shelf south of Heceta Bank. In the close vicinity of Cape Blanco (42.8°N), diurnal tidal currents (K 1 and O 1 constituents combined) may reach 0.3 m s−1. Appreciable differences in diurnal tide intensity are found depending on whether the model is forced with tides and winds (TW) or only tides. Also, diurnal variability in wind forcing is found to affect diurnal surface velocities. For the case forced by tides alone, results strongly depend on whether the model ocean is stratified [tides only, stratified (TOS)] or not [tides only, no stratification (TONS)]. In case TONS, coastal-trapped waves at diurnal frequencies do not occur over the narrow shelf south of 43.5°N, consistent with the dispersion analysis of a linear shallow-water model. However, in case TOS, diurnal tides are intensified in that area, associated with the presence of coastal-trapped waves. Case TW produces the strongest modeled diurnal tidal motions over the entire Oregon shelf, partially due to cross-shore tidal displacement (advection) of alongshore subinertial currents. At Cape Blanco, diurnal tidal variability dominates the modeled relative vorticity spectrum, suggesting that tides may influence the separation of the alongshore coastal jet at that location.
Abstract
Intensified diurnal tides are found along portions of the Oregon shelf (U.S. West Coast) based on analyses of high-frequency (HF) radar surface current data and outputs of a 1-km resolution ocean circulation model. The K 1 tidal currents with magnitudes near 0.07 m s−1 over a wider part of the shelf (Heceta Bank complex; 44°–44.5°N), previously predicted by Erofeeva et al., are confirmed here by newly available HF radar data. Intensified diurnal tides are also found along the narrow shelf south of Heceta Bank. In the close vicinity of Cape Blanco (42.8°N), diurnal tidal currents (K 1 and O 1 constituents combined) may reach 0.3 m s−1. Appreciable differences in diurnal tide intensity are found depending on whether the model is forced with tides and winds (TW) or only tides. Also, diurnal variability in wind forcing is found to affect diurnal surface velocities. For the case forced by tides alone, results strongly depend on whether the model ocean is stratified [tides only, stratified (TOS)] or not [tides only, no stratification (TONS)]. In case TONS, coastal-trapped waves at diurnal frequencies do not occur over the narrow shelf south of 43.5°N, consistent with the dispersion analysis of a linear shallow-water model. However, in case TOS, diurnal tides are intensified in that area, associated with the presence of coastal-trapped waves. Case TW produces the strongest modeled diurnal tidal motions over the entire Oregon shelf, partially due to cross-shore tidal displacement (advection) of alongshore subinertial currents. At Cape Blanco, diurnal tidal variability dominates the modeled relative vorticity spectrum, suggesting that tides may influence the separation of the alongshore coastal jet at that location.
Abstract
A 1-km-horizontal-resolution model based on the Regional Ocean Modeling System is implemented along the Oregon coast to study average characteristics and intermittency of the M 2 internal tide during summer upwelling. Wind-driven and tidally driven flows are simulated in combination, using realistic bathymetry, atmospheric forcing, and boundary conditions. The study period is April through August 2002, when mooring velocities are available for comparison. Modeled subtidal and tidal variability on the shelf are in good quantitative agreement with moored velocity time series observations. Depth-integrated baroclinic tidal energy flux (EF), its divergence, and topographic energy conversion (TEC) from the barotropic to baroclinic tide are computed from high-pass-filtered, harmonically analyzed model results in a series of 16-day time windows. Model results reveal several “hot spots” of intensive TEC on the slope. At these locations, TEC is well balanced by EF divergence. Changes in background stratification and currents associated with wind-driven upwelling and downwelling do not appreciably affect TEC hot spot locations but may affect intensity of internal tide generation at those locations. Relatively little internal tide is generated on the shelf. Areas of supercritical slope near the shelf break partially reflect baroclinic tidal energy to deeper water, contributing to spatial variability in seasonally averaged on-shelf EF. Despite significant temporal and spatial variability in the internal tide, the alongshore-integrated flux of internal tide energy onto the Oregon shelf, where it is dissipated, does not vary much with time. Approximately 65% of the M 2 baroclinic tidal energy generated on the slope is dissipated there, and the rest is radiated toward the shelf and interior ocean in roughly equal proportions. An experiment with smoother bathymetry reveals that slope-integrated TEC is more sensitive to bathymetric roughness than on-shelf EF.
Abstract
A 1-km-horizontal-resolution model based on the Regional Ocean Modeling System is implemented along the Oregon coast to study average characteristics and intermittency of the M 2 internal tide during summer upwelling. Wind-driven and tidally driven flows are simulated in combination, using realistic bathymetry, atmospheric forcing, and boundary conditions. The study period is April through August 2002, when mooring velocities are available for comparison. Modeled subtidal and tidal variability on the shelf are in good quantitative agreement with moored velocity time series observations. Depth-integrated baroclinic tidal energy flux (EF), its divergence, and topographic energy conversion (TEC) from the barotropic to baroclinic tide are computed from high-pass-filtered, harmonically analyzed model results in a series of 16-day time windows. Model results reveal several “hot spots” of intensive TEC on the slope. At these locations, TEC is well balanced by EF divergence. Changes in background stratification and currents associated with wind-driven upwelling and downwelling do not appreciably affect TEC hot spot locations but may affect intensity of internal tide generation at those locations. Relatively little internal tide is generated on the shelf. Areas of supercritical slope near the shelf break partially reflect baroclinic tidal energy to deeper water, contributing to spatial variability in seasonally averaged on-shelf EF. Despite significant temporal and spatial variability in the internal tide, the alongshore-integrated flux of internal tide energy onto the Oregon shelf, where it is dissipated, does not vary much with time. Approximately 65% of the M 2 baroclinic tidal energy generated on the slope is dissipated there, and the rest is radiated toward the shelf and interior ocean in roughly equal proportions. An experiment with smoother bathymetry reveals that slope-integrated TEC is more sensitive to bathymetric roughness than on-shelf EF.
Abstract
Results from a model of wind-driven circulation are analyzed to study spatial and temporal variability in the bottom mixed layer (BML) on the mid-Oregon shelf in summer 2001. The model assimilates acoustic Doppler profiler velocities from two cross-shore lines of moorings 90 km apart to provide improved accuracy of near-bottom velocities and turbulence variables in the area between the mooring lines. Model results suggest that the response of the BML thickness to upwelling- and downwelling-favorable winds differs qualitatively between an area of “simple” bathymetric slope at 45°N and a wider shelf area east of Stonewall Bank (44.5°N). At 45°N, the BML grows in response to downwelling-favorable conditions, in agreement with known theories. East of Stonewall Bank, the BML thickness is increased following upwelling events. In this area, the southward upwelling jet detaches from the coast and flows over a wider part of the Oregon shelf, creating conditions for Ekman pumping near the bottom. Based on computations of bottom stress curl, the vertical pumping velocity in this area may reach 15 m day−1 following periods of intensified upwelling-favorable winds. A column of denser, near-bottom water upwelled over the Ekman flow convergence area is tilted as a result of vertical shear in horizontal velocities, causing unstable stratification and convective overturning. As a result of this process, BML thickness values east of Stonewall Bank can be in excess of 20 m following upwelling, comparable to maximum values at 45°N following downwelling.
Abstract
Results from a model of wind-driven circulation are analyzed to study spatial and temporal variability in the bottom mixed layer (BML) on the mid-Oregon shelf in summer 2001. The model assimilates acoustic Doppler profiler velocities from two cross-shore lines of moorings 90 km apart to provide improved accuracy of near-bottom velocities and turbulence variables in the area between the mooring lines. Model results suggest that the response of the BML thickness to upwelling- and downwelling-favorable winds differs qualitatively between an area of “simple” bathymetric slope at 45°N and a wider shelf area east of Stonewall Bank (44.5°N). At 45°N, the BML grows in response to downwelling-favorable conditions, in agreement with known theories. East of Stonewall Bank, the BML thickness is increased following upwelling events. In this area, the southward upwelling jet detaches from the coast and flows over a wider part of the Oregon shelf, creating conditions for Ekman pumping near the bottom. Based on computations of bottom stress curl, the vertical pumping velocity in this area may reach 15 m day−1 following periods of intensified upwelling-favorable winds. A column of denser, near-bottom water upwelled over the Ekman flow convergence area is tilted as a result of vertical shear in horizontal velocities, causing unstable stratification and convective overturning. As a result of this process, BML thickness values east of Stonewall Bank can be in excess of 20 m following upwelling, comparable to maximum values at 45°N following downwelling.
