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- Author or Editor: R. C. Beardsley x
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Abstract
The viscous decay of a fluid in solid body rotation in a circular two-dimensional container is studied using a vorticity finite-difference scheme. A comparison is made with the well-known analytic solution to determine the accuracy of several finite-difference expressions for the wall vorticity. The most accurate results are obtained with a first-order scheme that conserves the net integrated vorticity.
Abstract
The viscous decay of a fluid in solid body rotation in a circular two-dimensional container is studied using a vorticity finite-difference scheme. A comparison is made with the well-known analytic solution to determine the accuracy of several finite-difference expressions for the wall vorticity. The most accurate results are obtained with a first-order scheme that conserves the net integrated vorticity.
Abstract
Two possible mechanisms which may drive the observed mean alongshelf flow in the Mid-Atlantic Bight are described. Runoff from concentrated sources could conceivably force this flow; however, the one-layer homogeneous model results of Csanady (1978) and Beardsley and Hart (1978) imply that the observed shelf flow is not driven by runoff alone. On the other hand. the Semtner and Mintz (1977) numerical model of the North Atlantic strongly suggests that the shelf circulation is just a boundary layer component of the ocean circulation and thus driven by the large-scale wind stress and heat flux distributions. This model result supports Csanady's (1978) conclusion that the physical mechanism which creates the alongshelf pressure gradient thought to drive the alongshelf flow must be of oceanic origin.
Abstract
Two possible mechanisms which may drive the observed mean alongshelf flow in the Mid-Atlantic Bight are described. Runoff from concentrated sources could conceivably force this flow; however, the one-layer homogeneous model results of Csanady (1978) and Beardsley and Hart (1978) imply that the observed shelf flow is not driven by runoff alone. On the other hand. the Semtner and Mintz (1977) numerical model of the North Atlantic strongly suggests that the shelf circulation is just a boundary layer component of the ocean circulation and thus driven by the large-scale wind stress and heat flux distributions. This model result supports Csanady's (1978) conclusion that the physical mechanism which creates the alongshelf pressure gradient thought to drive the alongshelf flow must be of oceanic origin.
Abstract
Four sets of current measurements made in water depths ranging between 28 and 38 m over periods ranging from three to five weeks are examined and compared. The response of the water column to wind forcing is examined by computing regression coefficients between the surface wind stress and two different parameterizations of bottom stress in terms of measured currents. Coefficients computed for the different data sets vary by as much as a factor of 4. While such variations might be due to instrumental differences, it seems more likely that the assumed dynamical balance between surface and bottom stress is incomplete, i.e., other forces such as the alongshore pressure gradient are quantitatively important even when the water depth is comparable to the turbulent Ekman layer thickness.
Abstract
Four sets of current measurements made in water depths ranging between 28 and 38 m over periods ranging from three to five weeks are examined and compared. The response of the water column to wind forcing is examined by computing regression coefficients between the surface wind stress and two different parameterizations of bottom stress in terms of measured currents. Coefficients computed for the different data sets vary by as much as a factor of 4. While such variations might be due to instrumental differences, it seems more likely that the assumed dynamical balance between surface and bottom stress is incomplete, i.e., other forces such as the alongshore pressure gradient are quantitatively important even when the water depth is comparable to the turbulent Ekman layer thickness.
Abstract
This paper examines the two-dimensional convective motion of a nonrotating incompressible Boussinesq fluid heated non-uniformly from below. The fluid container is rectangular; the side and top boundaries are insulating and rigid. A linear temperature field is maintained along the bottom boundary. Using the DuFort-Frankel scheme for diffusion and the Arakawa scheme for advection, the governing vorticity and temperature equations are integrated numerically for two cases, the first having a stress-free bottom boundary and the second having a constant stress along the bottom boundary.
In the first case, a single convective cell develops; an intense buoyant jet of fluid rises from the warmer section of the bottom while there is a more uniform sinking motion over the cooler section of the bottom. The cell asymmetry, the circulation, and the convective heat transfer increase markedly with increasing Rayleigh number (based here on fluid properties, cell height, and the horizontal temperature difference along the bottom). The flow is insensitive to changes in the Prandtl number σ, provided σ>1. A review of previous literature on this convective problem is given.
In the second case, a uniform stress is applied along the heated bottom in opposition to the thermal driving. The magnitude of the stress is varied while the horizontal Rayleigh and Prandtl numbers are held constant.
For weak stresses, the flow is that of the first case. A two-cell circulation pattern containing both thermal and stress-driven sections emerges for a moderate stress, while for large stresses the predominantly thermally driven cell disappears.
Abstract
This paper examines the two-dimensional convective motion of a nonrotating incompressible Boussinesq fluid heated non-uniformly from below. The fluid container is rectangular; the side and top boundaries are insulating and rigid. A linear temperature field is maintained along the bottom boundary. Using the DuFort-Frankel scheme for diffusion and the Arakawa scheme for advection, the governing vorticity and temperature equations are integrated numerically for two cases, the first having a stress-free bottom boundary and the second having a constant stress along the bottom boundary.
In the first case, a single convective cell develops; an intense buoyant jet of fluid rises from the warmer section of the bottom while there is a more uniform sinking motion over the cooler section of the bottom. The cell asymmetry, the circulation, and the convective heat transfer increase markedly with increasing Rayleigh number (based here on fluid properties, cell height, and the horizontal temperature difference along the bottom). The flow is insensitive to changes in the Prandtl number σ, provided σ>1. A review of previous literature on this convective problem is given.
In the second case, a uniform stress is applied along the heated bottom in opposition to the thermal driving. The magnitude of the stress is varied while the horizontal Rayleigh and Prandtl numbers are held constant.
For weak stresses, the flow is that of the first case. A two-cell circulation pattern containing both thermal and stress-driven sections emerges for a moderate stress, while for large stresses the predominantly thermally driven cell disappears.
Abstract
A description is given of the K1 tide over the northeast continental shelf off North America from Nova Scotia to Cape Hatteras. Analyzed pressure data obtained from W. Brown and J. Irish (University of New Hampshire) have been used to draw up the K1 cotidal map and existing current data have been analyzed to give the associated velocity map. Offshore, there is a sweep of the tide from north to south, in general agreement with what is known of the oceanic K1 tide in the North Atlantic. On the shelf, there is a trapping of phase lines to the coast, creating, in particular, a virtual amphidrome south of Cape Cod. Maximum amplitudes of around 15 cm are found in the Gulf of Maine, lowest around 7 cm south of Cape Cod. The K1 currents are generally barotropic and current ellipses are aligned with the local topography. Maximum currents of about 10 cm s−1 are found south of Cape Cod.
A simple model for the K1 pressure field is developed using the free and forced inviscid barotropic waves on a two-dimensional shelf. The theoretical solutions are fitted to the K1 pressure data using a least-squares method. The model results confirm that the K1 tide is composed of both a Kelvin wave and a shelf wave, with the Kelvin wave dominating the pressure field, and the shelf wave dominating the current field. The two free waves account for 99% of the variance of the difference of the observed pressures and the calculated forced wave, but unfortunately some of the observed features are not accurately reproduced. Possible model improvements should include the addition of bottom friction and longshore topographic variations (especially the changes in shelf geometry associated with the Gulf of Maine).
Abstract
A description is given of the K1 tide over the northeast continental shelf off North America from Nova Scotia to Cape Hatteras. Analyzed pressure data obtained from W. Brown and J. Irish (University of New Hampshire) have been used to draw up the K1 cotidal map and existing current data have been analyzed to give the associated velocity map. Offshore, there is a sweep of the tide from north to south, in general agreement with what is known of the oceanic K1 tide in the North Atlantic. On the shelf, there is a trapping of phase lines to the coast, creating, in particular, a virtual amphidrome south of Cape Cod. Maximum amplitudes of around 15 cm are found in the Gulf of Maine, lowest around 7 cm south of Cape Cod. The K1 currents are generally barotropic and current ellipses are aligned with the local topography. Maximum currents of about 10 cm s−1 are found south of Cape Cod.
A simple model for the K1 pressure field is developed using the free and forced inviscid barotropic waves on a two-dimensional shelf. The theoretical solutions are fitted to the K1 pressure data using a least-squares method. The model results confirm that the K1 tide is composed of both a Kelvin wave and a shelf wave, with the Kelvin wave dominating the pressure field, and the shelf wave dominating the current field. The two free waves account for 99% of the variance of the difference of the observed pressures and the calculated forced wave, but unfortunately some of the observed features are not accurately reproduced. Possible model improvements should include the addition of bottom friction and longshore topographic variations (especially the changes in shelf geometry associated with the Gulf of Maine).
Abstract
Small-scale waves have been observed near 40°N, 68°30′W in specially enhanced satellite imagery of a strong temperature front formed in May 1979, between the shelf/slope-water front and warm core ring 79-B. Thew frontal waves had a wavelength of 23±4 km, an eastward propagation speed of 32 ± 4 km day−1, and a growth rate (e-folding time) of 17.5 h (12–19 h). These satellite data plus current-velocity and hydro-graphical data gathered from the Nantucket Shoals Flux Experiment (NSFE79) allow comparison of the observed growth rate with theoretical predictions based on the assumption of a Margules front and the instability model of Orlanski (1968). This comparison suggests that the observed frontal waves were due primarily to horizontal-shear instability and derived their energy from the mean flow structure across the front in the presence of ring 79-B.
Abstract
Small-scale waves have been observed near 40°N, 68°30′W in specially enhanced satellite imagery of a strong temperature front formed in May 1979, between the shelf/slope-water front and warm core ring 79-B. Thew frontal waves had a wavelength of 23±4 km, an eastward propagation speed of 32 ± 4 km day−1, and a growth rate (e-folding time) of 17.5 h (12–19 h). These satellite data plus current-velocity and hydro-graphical data gathered from the Nantucket Shoals Flux Experiment (NSFE79) allow comparison of the observed growth rate with theoretical predictions based on the assumption of a Margules front and the instability model of Orlanski (1968). This comparison suggests that the observed frontal waves were due primarily to horizontal-shear instability and derived their energy from the mean flow structure across the front in the presence of ring 79-B.
Abstract
The wintertime circulation in the western Gulf of Maine has been studied with a moored current, temperature and pressure array which was deployed from November 1974 to January 1975. These observations have been interpreted with three additional data sets: coastal sea level records, Portland Lightship meteorological data, and offshore hydrographic transect data which describe the evolution of the density field on weekly time scales. The observed mean currents are consistent with the idea of a cyclonic Gulf of Maine gyre. The subtidal current fluctuations were coherent in the vertical at each mooring but incoherent between the moorings which were separated by about 50 km in both the alongshore and offshore direction. Furthermore, the currents showed only weak coherence with the winds.
The pressure field was highly coherent over the whole Gulf of Maine. Therefore, estimates of the pressure gradient vector inside and outside the 100 m isobath were made using coastal subsurface and bottom pressure records. The alongshore pressure gradient for the deeper water was found to be quite coherent with the winds for periods between 35 and 200 h. The relation of the pressure gradients and the winds in the shallower water suggests the development of a transient coastal boundary layer.
The incoherence between the observed current and pressure gradient fields is due in part to the existence of geostrophic currents associated with a highly variable density field. The density field variability is caused by incomplete mixing of three water masses: advected Scotian shelf water, deeper more saline slope water, and local winter water which is formed in the region of the experiment.
Abstract
The wintertime circulation in the western Gulf of Maine has been studied with a moored current, temperature and pressure array which was deployed from November 1974 to January 1975. These observations have been interpreted with three additional data sets: coastal sea level records, Portland Lightship meteorological data, and offshore hydrographic transect data which describe the evolution of the density field on weekly time scales. The observed mean currents are consistent with the idea of a cyclonic Gulf of Maine gyre. The subtidal current fluctuations were coherent in the vertical at each mooring but incoherent between the moorings which were separated by about 50 km in both the alongshore and offshore direction. Furthermore, the currents showed only weak coherence with the winds.
The pressure field was highly coherent over the whole Gulf of Maine. Therefore, estimates of the pressure gradient vector inside and outside the 100 m isobath were made using coastal subsurface and bottom pressure records. The alongshore pressure gradient for the deeper water was found to be quite coherent with the winds for periods between 35 and 200 h. The relation of the pressure gradients and the winds in the shallower water suggests the development of a transient coastal boundary layer.
The incoherence between the observed current and pressure gradient fields is due in part to the existence of geostrophic currents associated with a highly variable density field. The density field variability is caused by incomplete mixing of three water masses: advected Scotian shelf water, deeper more saline slope water, and local winter water which is formed in the region of the experiment.
Abstract
During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern California continental shelf, only interrupted for periods of a few days, when weak or southerly winds occur. In the course of the Coastal Ocean Dynamics Experiment (CODE), fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast. These observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories: the surface wind can be everywhere weak (Pattern 1), it can blow at large speeds in a uniform pattern (Pattern 2), or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales (Pattern 3), The latter pattern, which occurs with generally northerly winds, is characterized by a strong low-level inversion and the spatial structure of the surface wind is correlated with the coastal topography. The inversion acts as a material interface, and the marine layer behaves as a supercritical channel flow, when the Froude number is greater than one: oblique expansion waves and hydraulic jumps, associated with changes in the orientation of the coastline, account for the observed spatial structure of the flow. Observations from mid-latitudes on the eastern side of other ocean basins suggest that similar supercritical conditions in the marine layer may prevail there also.
Abstract
During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern California continental shelf, only interrupted for periods of a few days, when weak or southerly winds occur. In the course of the Coastal Ocean Dynamics Experiment (CODE), fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast. These observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories: the surface wind can be everywhere weak (Pattern 1), it can blow at large speeds in a uniform pattern (Pattern 2), or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales (Pattern 3), The latter pattern, which occurs with generally northerly winds, is characterized by a strong low-level inversion and the spatial structure of the surface wind is correlated with the coastal topography. The inversion acts as a material interface, and the marine layer behaves as a supercritical channel flow, when the Froude number is greater than one: oblique expansion waves and hydraulic jumps, associated with changes in the orientation of the coastline, account for the observed spatial structure of the flow. Observations from mid-latitudes on the eastern side of other ocean basins suggest that similar supercritical conditions in the marine layer may prevail there also.
Abstract
The Nantucket Shoals Flux Experiment (NSFE79) was conducted across the continental shelf and upper slope south of Nantucket from March 1979 to April 1980 to study the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The experiment included a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope spanning a depth range from 46 to 810 m, and supporting hydrographic observations. A basic description of the moored current and temperature data is given here with an emphasis on the low-frequency variability.
In the summer period (April–August) when the local vertical stratification reached a maximum due to increased surface heating and reduced wind mixing, the mean flow over the shelf at all instruments was primarily along 1ocal isobaths towards the west. The subtidal current fluctuations were coherent both horizontally and vertically over the shelf, but not with current fluctuations observed over the upper slope. The wind stress during summer was weak and only moderately correlated with the subtidal current fluctuations.
In the winter period (October–March), when the seasonal thermocline was destroyed and the shelf water locally homogenized by increased surface cooling and wind mixing, the mean currents observed over the shelf were also primarily alongshelf towards the west at speeds comparable to those measured in summer. However, the low-frequency current fluctuations over the shelf were much more energetic in winter. These subtidal current fluctuations were highly coherent horizontally and vertically over the shelf and with surface wind-stress fluctuations (which increased in magnitude by a factor of 5 over the summer period). The most energetic subtidal current events observed over the shelf also tended to extend into the upper slope region.
The subtidal currents observed over the upper slope in summer were dominated by three bursts of large eastward currents which correspond to the passage of anticyclonic Gulf Stream warm-core rings near or through the moored army. The effect of these rings on the current field does not appear to penetrate shoreward of the shelf break. In winter only two rings passed near the array and their influence on the observed upper slope currents was unclear owing, in part, to the increased subtidal current variability caused by the stronger synoptic wind forcing in winter.
Multiple regression analysis was used to identify possible annual variations in the NSFE79 moored current and temperature data. Significant annual variations were found in the temperature field over the shelf and upper slope and in the low-frequency current variability over the shelf. No significant annual variation was observed in the alongshelf current over the shelf, however, suggesting that there is, at least on time scales of one month and more, a continuous flow of shelf water into the Middle Atlantic Bight from the Georges Bank/Gulf of Maine region. The mean westward volume flux between the 40 and 120 m isobaths observed in NSFE79 was 38.3 ± 6.9 × 104 m3 s−1.
Abstract
The Nantucket Shoals Flux Experiment (NSFE79) was conducted across the continental shelf and upper slope south of Nantucket from March 1979 to April 1980 to study the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The experiment included a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope spanning a depth range from 46 to 810 m, and supporting hydrographic observations. A basic description of the moored current and temperature data is given here with an emphasis on the low-frequency variability.
In the summer period (April–August) when the local vertical stratification reached a maximum due to increased surface heating and reduced wind mixing, the mean flow over the shelf at all instruments was primarily along 1ocal isobaths towards the west. The subtidal current fluctuations were coherent both horizontally and vertically over the shelf, but not with current fluctuations observed over the upper slope. The wind stress during summer was weak and only moderately correlated with the subtidal current fluctuations.
In the winter period (October–March), when the seasonal thermocline was destroyed and the shelf water locally homogenized by increased surface cooling and wind mixing, the mean currents observed over the shelf were also primarily alongshelf towards the west at speeds comparable to those measured in summer. However, the low-frequency current fluctuations over the shelf were much more energetic in winter. These subtidal current fluctuations were highly coherent horizontally and vertically over the shelf and with surface wind-stress fluctuations (which increased in magnitude by a factor of 5 over the summer period). The most energetic subtidal current events observed over the shelf also tended to extend into the upper slope region.
The subtidal currents observed over the upper slope in summer were dominated by three bursts of large eastward currents which correspond to the passage of anticyclonic Gulf Stream warm-core rings near or through the moored army. The effect of these rings on the current field does not appear to penetrate shoreward of the shelf break. In winter only two rings passed near the array and their influence on the observed upper slope currents was unclear owing, in part, to the increased subtidal current variability caused by the stronger synoptic wind forcing in winter.
Multiple regression analysis was used to identify possible annual variations in the NSFE79 moored current and temperature data. Significant annual variations were found in the temperature field over the shelf and upper slope and in the low-frequency current variability over the shelf. No significant annual variation was observed in the alongshelf current over the shelf, however, suggesting that there is, at least on time scales of one month and more, a continuous flow of shelf water into the Middle Atlantic Bight from the Georges Bank/Gulf of Maine region. The mean westward volume flux between the 40 and 120 m isobaths observed in NSFE79 was 38.3 ± 6.9 × 104 m3 s−1.
Abstract
Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 4°N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv ≡ 106 m3 s−1) in July–August to a minimum of 13 Sv in April–May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by flow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3–5 Sv and appears to be fairly constant throughout the year, based on the available current meter records and shipboard ADCP surveys. The NBC transport cycle is in good agreement with linear wind-driven models and appears to be in near-equilibrium with remote wind stress curl forcing across the tropical Atlantic for much of the year. However, the mean transport of the NBC is 15 Sv larger than can be explained by wind forcing alone, indicating a strong thermohaline component. Mesoscale variability in the region is dominated by fluctuations with periods near 25–40 days and 60–90 days. The 25–40-day fluctuations are strongly surface trapped and are most energetic in early summer during the acceleration phase of the NBC. The lower-frequency fluctuations have a deeper reaching baroclinic structure, are present year-round, and are associated with the propagation of large anticyclonic eddies northwestward along the coast. It is hypothesized that these features may serve as a catalyst for the eddy shedding process seen in the NBC retroflection in earlier observations.
Abstract
Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 4°N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv ≡ 106 m3 s−1) in July–August to a minimum of 13 Sv in April–May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by flow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3–5 Sv and appears to be fairly constant throughout the year, based on the available current meter records and shipboard ADCP surveys. The NBC transport cycle is in good agreement with linear wind-driven models and appears to be in near-equilibrium with remote wind stress curl forcing across the tropical Atlantic for much of the year. However, the mean transport of the NBC is 15 Sv larger than can be explained by wind forcing alone, indicating a strong thermohaline component. Mesoscale variability in the region is dominated by fluctuations with periods near 25–40 days and 60–90 days. The 25–40-day fluctuations are strongly surface trapped and are most energetic in early summer during the acceleration phase of the NBC. The lower-frequency fluctuations have a deeper reaching baroclinic structure, are present year-round, and are associated with the propagation of large anticyclonic eddies northwestward along the coast. It is hypothesized that these features may serve as a catalyst for the eddy shedding process seen in the NBC retroflection in earlier observations.
