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Abstract
Drifters released offshore of Oregon during predominantly downwelling favorable alongshore winds during three different deployments (October 1994, January 1998, and September 1998) display similar behavior: after being advected around in the offshore eddy field, they move onshore to a particular isobath and are advected poleward alongshore, without coming ashore. Numerical modeling results suggest that this may be due to downwelling circulation creating a marginally stable density gradient on the shelf inshore of the downwelling front, thereby increasing the vertical eddy diffusivity, which reduces the effective cross-shelf Ekman transport to nearly zero. The downwelling front itself is accompanied by a poleward jet, which carries drifters rapidly to the north. This behavior is consistent with previous modeling results.
Abstract
Drifters released offshore of Oregon during predominantly downwelling favorable alongshore winds during three different deployments (October 1994, January 1998, and September 1998) display similar behavior: after being advected around in the offshore eddy field, they move onshore to a particular isobath and are advected poleward alongshore, without coming ashore. Numerical modeling results suggest that this may be due to downwelling circulation creating a marginally stable density gradient on the shelf inshore of the downwelling front, thereby increasing the vertical eddy diffusivity, which reduces the effective cross-shelf Ekman transport to nearly zero. The downwelling front itself is accompanied by a poleward jet, which carries drifters rapidly to the north. This behavior is consistent with previous modeling results.
Abstract
Upwelling jets flow alongshore in approximate geostrophic balance with the onshore pressure gradient induced by coastal upwelling. Observations of such jets have shown that they often move offshore downstream of capes, leaving a pool of upwelled water inshore. Comparisons are made between this behavior and the hydraulic transition of a potential-vorticity-conserving coastal current as it passes a topographic anomaly at which it is exactly critical to long coastal-trapped waves. An analytic 1.5-layer model of coastal hydraulics with constant potential vorticity in each layer predicts flow fields (i.e., jet separation) in critical situations that resemble observations. When scales approximate Cape Blanco on the Oregon coast, separation occurs at a jet transport of around 0.76 × 106 m3 s−1, similar to observed transports. Time-dependent, semigeostrophic calculations suggest that, during an upwelling season, the jet would evolve from a weak flow, which was subcritical everywhere and symmetric about the cape, to an exactly critical state that made a transition from subcritical to supercritical structure at the head of the cape. The predicted flow field at critical transition consists of a narrow upwelling jet upstream of the cape that moves offshore and broadens at the cape. This critical state would be accompanied by a downstream jump back to subcritical conditions. Further upwelling-favorable winds would lead to transient waves that propagated upstream and downstream, modifying the upstream and downstream conditions and restoring criticality. Thus, the head of the cape exerts hydraulic control on the flow and prevents the jet transport from increasing above its critical level.
Inherent in the hydraulic approach is the assumption that alongshore scales are large. For realistic alongshore scales, solutions modified by coastline curvature suggest that the convexity of the head of a cape slightly inhibits the transition to a strongly upwelled downstream state by increasing the required critical transport. In the presence of topographic features with finite alongshore scale, the hydraulic approach can be used to construct a flow field, although this flow field has an inherent error arising from the implicit assumptions regarding scales. Estimation of this error for topography representing Cape Blanco suggests that in places the cape is rather abrupt for hydraulic theory to be valid.
Abstract
Upwelling jets flow alongshore in approximate geostrophic balance with the onshore pressure gradient induced by coastal upwelling. Observations of such jets have shown that they often move offshore downstream of capes, leaving a pool of upwelled water inshore. Comparisons are made between this behavior and the hydraulic transition of a potential-vorticity-conserving coastal current as it passes a topographic anomaly at which it is exactly critical to long coastal-trapped waves. An analytic 1.5-layer model of coastal hydraulics with constant potential vorticity in each layer predicts flow fields (i.e., jet separation) in critical situations that resemble observations. When scales approximate Cape Blanco on the Oregon coast, separation occurs at a jet transport of around 0.76 × 106 m3 s−1, similar to observed transports. Time-dependent, semigeostrophic calculations suggest that, during an upwelling season, the jet would evolve from a weak flow, which was subcritical everywhere and symmetric about the cape, to an exactly critical state that made a transition from subcritical to supercritical structure at the head of the cape. The predicted flow field at critical transition consists of a narrow upwelling jet upstream of the cape that moves offshore and broadens at the cape. This critical state would be accompanied by a downstream jump back to subcritical conditions. Further upwelling-favorable winds would lead to transient waves that propagated upstream and downstream, modifying the upstream and downstream conditions and restoring criticality. Thus, the head of the cape exerts hydraulic control on the flow and prevents the jet transport from increasing above its critical level.
Inherent in the hydraulic approach is the assumption that alongshore scales are large. For realistic alongshore scales, solutions modified by coastline curvature suggest that the convexity of the head of a cape slightly inhibits the transition to a strongly upwelled downstream state by increasing the required critical transport. In the presence of topographic features with finite alongshore scale, the hydraulic approach can be used to construct a flow field, although this flow field has an inherent error arising from the implicit assumptions regarding scales. Estimation of this error for topography representing Cape Blanco suggests that in places the cape is rather abrupt for hydraulic theory to be valid.
Abstract
A high-resolution numerical model with idealized topography is used to investigate the degree to which a coastal upwelling jet separates from the shelf as it flows around a submarine bank depending on the wind strength and the horizontal scale of the bank. Experiments were run using several wind forcing magnitudes and submarine banks with different geometries, so as to explore a wide range of the flow strength as measured by the Rossby number (Ro) and the ratio of the squares of the internal Rossby radius of deformation and curvature of the topography as denoted by the Burger number (Bu). The intensity of the jet separation is strongly dependent on both parameters, with maximum separation with increasing Ro and Bu close to 1, when large amounts of upwelled water are exported toward deeper waters. For small Bu, separation is minimal and independent of Ro. For high Ro, the degree of separation decreases at large Bu since the bank acts only as a small perturbation to the flow. Term balances in the along-shelf momentum equation reveal that the primary balance over the bank is between the nonlinear and the ageostrophic terms. In an asymmetric bank, the radius of curvature in the upstream half of the bank dominates in terms of determining the offshore deflection of a water particle at the surface. The asymmetry increases the cross-isobath transport but not the offshore deflection of the jet.
Abstract
A high-resolution numerical model with idealized topography is used to investigate the degree to which a coastal upwelling jet separates from the shelf as it flows around a submarine bank depending on the wind strength and the horizontal scale of the bank. Experiments were run using several wind forcing magnitudes and submarine banks with different geometries, so as to explore a wide range of the flow strength as measured by the Rossby number (Ro) and the ratio of the squares of the internal Rossby radius of deformation and curvature of the topography as denoted by the Burger number (Bu). The intensity of the jet separation is strongly dependent on both parameters, with maximum separation with increasing Ro and Bu close to 1, when large amounts of upwelled water are exported toward deeper waters. For small Bu, separation is minimal and independent of Ro. For high Ro, the degree of separation decreases at large Bu since the bank acts only as a small perturbation to the flow. Term balances in the along-shelf momentum equation reveal that the primary balance over the bank is between the nonlinear and the ageostrophic terms. In an asymmetric bank, the radius of curvature in the upstream half of the bank dominates in terms of determining the offshore deflection of a water particle at the surface. The asymmetry increases the cross-isobath transport but not the offshore deflection of the jet.
Abstract
A high-resolution numerical model is used to study the importance of spatial variability in the wind forcing to the separation of a coastal upwelling jet at a cape. An idealized topography and wind field based on observations from the Cape Blanco (Oregon) region are used. Several simulations are investigated, with both the intensity and the spatial structure of the wind forcing varied to isolate the importance of the observed intensification in the wind stress and wind stress curl magnitudes to the separation process. A simulation using a straight coast confirms that the presence of the cape is crucial for separation. Wind stress intensification by itself, with zero curl, does not aid separation. The wind stress curl intensification south of the cape, on the other hand, is important for controlling details of the process. Because the positive wind stress curl drives upwelling, isotherms in the offshore region tilt upward, creating a pressure gradient that sustains an intensification of the southward velocities via the thermal wind balance. This aids jet separation via continuity and by creating potential vorticity contours that track far offshore of the cape. The timing of the separation is dependent on the intensity of the wind stress curl (stronger curl leads to earlier separation), while how far offshore the jet is deflected depends on the offshore extent of the region of positive curl close to the coast (increasing the extent increases the deflection).
Abstract
A high-resolution numerical model is used to study the importance of spatial variability in the wind forcing to the separation of a coastal upwelling jet at a cape. An idealized topography and wind field based on observations from the Cape Blanco (Oregon) region are used. Several simulations are investigated, with both the intensity and the spatial structure of the wind forcing varied to isolate the importance of the observed intensification in the wind stress and wind stress curl magnitudes to the separation process. A simulation using a straight coast confirms that the presence of the cape is crucial for separation. Wind stress intensification by itself, with zero curl, does not aid separation. The wind stress curl intensification south of the cape, on the other hand, is important for controlling details of the process. Because the positive wind stress curl drives upwelling, isotherms in the offshore region tilt upward, creating a pressure gradient that sustains an intensification of the southward velocities via the thermal wind balance. This aids jet separation via continuity and by creating potential vorticity contours that track far offshore of the cape. The timing of the separation is dependent on the intensity of the wind stress curl (stronger curl leads to earlier separation), while how far offshore the jet is deflected depends on the offshore extent of the region of positive curl close to the coast (increasing the extent increases the deflection).
Abstract
The event-scale variability of across-shelf transport was investigated using observations made in 15 m of water on the central Oregon inner shelf. In a study area with intermittently upwelling-favorable winds and significant density stratification, hydrographic and velocity observations show rapid across-shelf movement of water masses over event time scales of 2–7 days. To understand the time variability of across-shelf exchange, an inverse calculation was used to estimate eddy viscosity and the vertical turbulent diffusion of momentum from velocity profiles and wind forcing. Depth-averaged eddy viscosity varied over a large dynamic range, but averaged 1.3 × 10−3 m2 s−1 during upwelling winds and 2.1 × 10−3 m2 s−1 during downwelling winds. The fraction of full Ekman transport present in the surface layer, a measure of the efficiency of across-shelf exchange at this water depth, was a strong function of eddy viscosity and wind forcing, but not stratification. Transport fractions ranged from 60%, during times of weak or variable wind forcing and low eddy viscosity, to 10%–20%, during times of strong downwelling and high eddy viscosity. The difference in eddy viscosities between upwelling and downwelling led to varying across-shelf exchange efficiencies and, potentially, increased net upwelling over time. These results quantify the variability of across-shelf transport efficiency and have significant implications for ecological processes (e.g., larval transport) in the inner shelf.
Abstract
The event-scale variability of across-shelf transport was investigated using observations made in 15 m of water on the central Oregon inner shelf. In a study area with intermittently upwelling-favorable winds and significant density stratification, hydrographic and velocity observations show rapid across-shelf movement of water masses over event time scales of 2–7 days. To understand the time variability of across-shelf exchange, an inverse calculation was used to estimate eddy viscosity and the vertical turbulent diffusion of momentum from velocity profiles and wind forcing. Depth-averaged eddy viscosity varied over a large dynamic range, but averaged 1.3 × 10−3 m2 s−1 during upwelling winds and 2.1 × 10−3 m2 s−1 during downwelling winds. The fraction of full Ekman transport present in the surface layer, a measure of the efficiency of across-shelf exchange at this water depth, was a strong function of eddy viscosity and wind forcing, but not stratification. Transport fractions ranged from 60%, during times of weak or variable wind forcing and low eddy viscosity, to 10%–20%, during times of strong downwelling and high eddy viscosity. The difference in eddy viscosities between upwelling and downwelling led to varying across-shelf exchange efficiencies and, potentially, increased net upwelling over time. These results quantify the variability of across-shelf transport efficiency and have significant implications for ecological processes (e.g., larval transport) in the inner shelf.
Abstract
The spatial and temporal variability of inner-shelf circulation along the central Oregon coast during the 2004 upwelling season is described using a 70-km-long array of moorings along the 15-m isobath. Circulation at three stations located onshore of a submarine bank differed from that of a station north of the bank, despite the relatively uniform wind forcing and inner-shelf bathymetry present. During upwelling-favorable winds, strong southward alongshelf flow occurred north of the bank, no alongshelf flow occurred onshore of the northern part of the bank, and increasing southward flow occurred onshore of the southern part of the bank. During downwelling-favorable winds, strong northward flow occurred in the inner shelf onshore of the bank while weak flow occurred north of the bank. These alongshelf differences in inner-shelf circulation were due to the effects of the bank, which isolated the inner shelf onshore of the bank from the regional upwelling circulation that was evident at the northernmost station. As a result, circulation onshore of the bank was driven primarily by local wind forcing, while flow north of the bank was only partially driven by local winds. A secondary mode of variability, attributed to the movement of the regional upwelling jet due to remote forcings, contributed the bulk of the variability observed north of the bank. With the time-dependent wind forcing present, acceleration was an important term in the depth-averaged alongshelf momentum equation at all stations. During upwelling, bottom stress and acceleration opposed the wind stress north of the bank, while bottom stress was weaker onshore of the bank where the across-shelf momentum flux and the alongshelf pressure gradient balanced the residual of the acceleration and stresses. During downwelling, waters onshore of the bank surged northward at magnitudes much larger than that found north of the bank. These spatial variations developed as the season progressed and the regional upwelling circulation intensified, explaining known variations in growth and recruitment of nearshore invertebrate species.
Abstract
The spatial and temporal variability of inner-shelf circulation along the central Oregon coast during the 2004 upwelling season is described using a 70-km-long array of moorings along the 15-m isobath. Circulation at three stations located onshore of a submarine bank differed from that of a station north of the bank, despite the relatively uniform wind forcing and inner-shelf bathymetry present. During upwelling-favorable winds, strong southward alongshelf flow occurred north of the bank, no alongshelf flow occurred onshore of the northern part of the bank, and increasing southward flow occurred onshore of the southern part of the bank. During downwelling-favorable winds, strong northward flow occurred in the inner shelf onshore of the bank while weak flow occurred north of the bank. These alongshelf differences in inner-shelf circulation were due to the effects of the bank, which isolated the inner shelf onshore of the bank from the regional upwelling circulation that was evident at the northernmost station. As a result, circulation onshore of the bank was driven primarily by local wind forcing, while flow north of the bank was only partially driven by local winds. A secondary mode of variability, attributed to the movement of the regional upwelling jet due to remote forcings, contributed the bulk of the variability observed north of the bank. With the time-dependent wind forcing present, acceleration was an important term in the depth-averaged alongshelf momentum equation at all stations. During upwelling, bottom stress and acceleration opposed the wind stress north of the bank, while bottom stress was weaker onshore of the bank where the across-shelf momentum flux and the alongshelf pressure gradient balanced the residual of the acceleration and stresses. During downwelling, waters onshore of the bank surged northward at magnitudes much larger than that found north of the bank. These spatial variations developed as the season progressed and the regional upwelling circulation intensified, explaining known variations in growth and recruitment of nearshore invertebrate species.
Abstract
Semidiurnal velocity and density oscillations are examined over the mid- and inner continental shelf near Heceta Bank on the Oregon coast. Measurements from two long-term observation networks with sites on and off the submarine bank reveal that both baroclinic velocities and displacements are dominated by the first mode, with larger velocities on the midshelf and northern parts of the bank. Midshelf sites have current ellipses that are near the theoretical value for single, progressive internal tidal waves compared to more linearly polarized currents over the inner shelf. Baroclinic current variability is not correlated to the spring–neap cycle and is uncorrelated between mooring locations. An idealized model of two internal waves propagating from different directions reproduces some of the observed variability in semidiurnal ellipse parameters. At times, the phasing between moorings along a cross-shelf transect are consistent with onshelf wave propagation, a characteristic also present in the output of a three-dimensional regional circulation model. Regional wind-driven upwelling/downwelling influences stratification at all shelf moorings. At locations north of the bank, stronger baroclinic velocities were found during periods of higher background stratification.
Abstract
Semidiurnal velocity and density oscillations are examined over the mid- and inner continental shelf near Heceta Bank on the Oregon coast. Measurements from two long-term observation networks with sites on and off the submarine bank reveal that both baroclinic velocities and displacements are dominated by the first mode, with larger velocities on the midshelf and northern parts of the bank. Midshelf sites have current ellipses that are near the theoretical value for single, progressive internal tidal waves compared to more linearly polarized currents over the inner shelf. Baroclinic current variability is not correlated to the spring–neap cycle and is uncorrelated between mooring locations. An idealized model of two internal waves propagating from different directions reproduces some of the observed variability in semidiurnal ellipse parameters. At times, the phasing between moorings along a cross-shelf transect are consistent with onshelf wave propagation, a characteristic also present in the output of a three-dimensional regional circulation model. Regional wind-driven upwelling/downwelling influences stratification at all shelf moorings. At locations north of the bank, stronger baroclinic velocities were found during periods of higher background stratification.
Abstract
Observations of hypoxia, dissolved oxygen (DO) concentrations < 1.4 ml L−1, off the central Oregon coast vary in duration and spatial extent throughout each upwelling season. Underwater glider measurements along the Newport hydrographic line (NH-Line) reveal cross-shelf DO gradients at a horizontal resolution nearly 30 times greater than previous ship-based station sampling. Two prevalent hypoxic locations are identified along the NH-Line, as is a midshelf region with less severe hypoxia north of Stonewall Bank. Intraseasonal cross-shelf variability is investigated with 10 sequential glider lines and a midshelf mooring time series during the 2011 upwelling season. The cross-sectional area of hypoxia observed in the glider lines ranges from 0 to 1.41 km2. The vertical extent of hypoxia in the water column agrees well with the bottom mixed layer height. Midshelf mooring water velocities show that cross-shelf advection cannot account for the increase in outer-shelf hypoxia observed in the glider sequence. This change is attributed to an along-shelf DO gradient of −0.72 ml L−1 over 2.58 km or 0.28 ml L−1 km−1. In early July of the 2011 upwelling season, near-bottom cross-shelf currents reverse direction as an onshore flow at 30-m depth is observed. This shoaling of the return flow depth throughout the season, as the equatorward coastal jet moves offshore, results in a more retentive near-bottom environment more vulnerable to hypoxia. Slope Burger numbers calculated across the season do not reconcile this return flow depth change, providing evidence that simplified two-dimensional upwelling model assumptions do not hold in this location.
Abstract
Observations of hypoxia, dissolved oxygen (DO) concentrations < 1.4 ml L−1, off the central Oregon coast vary in duration and spatial extent throughout each upwelling season. Underwater glider measurements along the Newport hydrographic line (NH-Line) reveal cross-shelf DO gradients at a horizontal resolution nearly 30 times greater than previous ship-based station sampling. Two prevalent hypoxic locations are identified along the NH-Line, as is a midshelf region with less severe hypoxia north of Stonewall Bank. Intraseasonal cross-shelf variability is investigated with 10 sequential glider lines and a midshelf mooring time series during the 2011 upwelling season. The cross-sectional area of hypoxia observed in the glider lines ranges from 0 to 1.41 km2. The vertical extent of hypoxia in the water column agrees well with the bottom mixed layer height. Midshelf mooring water velocities show that cross-shelf advection cannot account for the increase in outer-shelf hypoxia observed in the glider sequence. This change is attributed to an along-shelf DO gradient of −0.72 ml L−1 over 2.58 km or 0.28 ml L−1 km−1. In early July of the 2011 upwelling season, near-bottom cross-shelf currents reverse direction as an onshore flow at 30-m depth is observed. This shoaling of the return flow depth throughout the season, as the equatorward coastal jet moves offshore, results in a more retentive near-bottom environment more vulnerable to hypoxia. Slope Burger numbers calculated across the season do not reconcile this return flow depth change, providing evidence that simplified two-dimensional upwelling model assumptions do not hold in this location.
Abstract
MicroSoar, an undulating profiler capable of measuring turbulence parameters such as Thorpe scales and thermal dissipation rate while being towed at speeds of up to 4 m s−1, offers the possibility of obtaining a close-to-synoptic image of mixing over large spatial areas. In this paper, the method of calculating Thorpe scales from the high-frequency MicroSoar data is developed, and results from data taken off the coast of Oregon during the summer of 2001 are presented. Large Thorpe scales and elevated measurements of the thermal dissipation rate are shown to be associated with shear at the edge of the coastal jet. It is further shown that using MicroSoar data rather than Sea-Bird conductivity–temperature–depth data extends the range of measurable overturns to smaller scales, particularly in regions of low stratification.
Abstract
MicroSoar, an undulating profiler capable of measuring turbulence parameters such as Thorpe scales and thermal dissipation rate while being towed at speeds of up to 4 m s−1, offers the possibility of obtaining a close-to-synoptic image of mixing over large spatial areas. In this paper, the method of calculating Thorpe scales from the high-frequency MicroSoar data is developed, and results from data taken off the coast of Oregon during the summer of 2001 are presented. Large Thorpe scales and elevated measurements of the thermal dissipation rate are shown to be associated with shear at the edge of the coastal jet. It is further shown that using MicroSoar data rather than Sea-Bird conductivity–temperature–depth data extends the range of measurable overturns to smaller scales, particularly in regions of low stratification.
Abstract
Accurate ship velocity is important for determining absolute currents from acoustic Doppler current profiler (ADCP) measurements. In this paper, the authors describe the application of two methods to improve the quality of ship velocity estimates. The first uses wide-area differential global positioning system (WADGPS) navigation to improve ship positioning. During the cruise, raw global positioning system (GPS) pseudorange data are collected. The pseudorange measurement is the difference between satellite transmission time and receiver reception time of a GPS signal. A few days after the cruise, satellite clock corrections from the Canadian Active Control System and orbital parameters from the U.S. Coast Guard Navigation Center are used to derive WADGPS positions that remove the position degradation effects of selective availability. Two-dimensional root-mean-square (rms) position accuracies reduce from ±34 to ±9 m. The authors’ second method of improving the ship velocity applies an adaptive local third-order polynomial smoother to the raw ship velocities. This smoothing method is particularly effective at handling the nonstationary nature of the signal when the ship is starting, stopping, or turning, which is typical of oceanographic cruises. Application of the smoother in this case reduces overall rms noise in the ship velocity by 16%. The combination of both methods reduces the uncertainty due to navigation of a 20-min ADCP absolute velocity from ±0.063 to ±0.038 m s−1—a 40% reduction. These methods also improve the calibration for sensitivity error and ADCP–gyrocompass misalignment angle.
Abstract
Accurate ship velocity is important for determining absolute currents from acoustic Doppler current profiler (ADCP) measurements. In this paper, the authors describe the application of two methods to improve the quality of ship velocity estimates. The first uses wide-area differential global positioning system (WADGPS) navigation to improve ship positioning. During the cruise, raw global positioning system (GPS) pseudorange data are collected. The pseudorange measurement is the difference between satellite transmission time and receiver reception time of a GPS signal. A few days after the cruise, satellite clock corrections from the Canadian Active Control System and orbital parameters from the U.S. Coast Guard Navigation Center are used to derive WADGPS positions that remove the position degradation effects of selective availability. Two-dimensional root-mean-square (rms) position accuracies reduce from ±34 to ±9 m. The authors’ second method of improving the ship velocity applies an adaptive local third-order polynomial smoother to the raw ship velocities. This smoothing method is particularly effective at handling the nonstationary nature of the signal when the ship is starting, stopping, or turning, which is typical of oceanographic cruises. Application of the smoother in this case reduces overall rms noise in the ship velocity by 16%. The combination of both methods reduces the uncertainty due to navigation of a 20-min ADCP absolute velocity from ±0.063 to ±0.038 m s−1—a 40% reduction. These methods also improve the calibration for sensitivity error and ADCP–gyrocompass misalignment angle.
