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- Author or Editor: C. L. Tang x
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Abstract
CTD data obtained from three transects across a controlled quasi-permanent density front in the Gulf of St. Lawrence were analyzed for the purpose of investigating cross-front mixing, mechanisms for frontal convergence, secondary circulation induced by the front and relationship between surface mixed-layer properties and frontal structure. Water mass analysis indicates that mixing takes place mainly in the ambient water, from the lower boundary of the frontal layer down to ∼100 m. On the side of heavier water, there is a region of low surface temperature. The water masses have a distribution suggestive of upwelling in the low surface temperature region. The thickness of the surface mixed layer varies across he front. Outside the frontal zone there is a well-developed mixed layer of a thickness of about 25 m. It disappears completely in the low surface temperature zone and is re-established in the frontal layer with a reduced thickness. Horizontal intrusions below the frontal layer and interleaving of thin layers in the intermediate cold layer (40–100 m) were observed. A cross-front circulation is proposed to explain the observations. Two mechanisms to generate the cross-front flow and upwelling, i.e. the centripetal acceleration of water parcels flowing along a curved density surface and suction of subsurface water by an internal Ekman flow beneath the frontal layer, are discussed.
Abstract
CTD data obtained from three transects across a controlled quasi-permanent density front in the Gulf of St. Lawrence were analyzed for the purpose of investigating cross-front mixing, mechanisms for frontal convergence, secondary circulation induced by the front and relationship between surface mixed-layer properties and frontal structure. Water mass analysis indicates that mixing takes place mainly in the ambient water, from the lower boundary of the frontal layer down to ∼100 m. On the side of heavier water, there is a region of low surface temperature. The water masses have a distribution suggestive of upwelling in the low surface temperature region. The thickness of the surface mixed layer varies across he front. Outside the frontal zone there is a well-developed mixed layer of a thickness of about 25 m. It disappears completely in the low surface temperature zone and is re-established in the frontal layer with a reduced thickness. Horizontal intrusions below the frontal layer and interleaving of thin layers in the intermediate cold layer (40–100 m) were observed. A cross-front circulation is proposed to explain the observations. Two mechanisms to generate the cross-front flow and upwelling, i.e. the centripetal acceleration of water parcels flowing along a curved density surface and suction of subsurface water by an internal Ekman flow beneath the frontal layer, are discussed.
Abstract
From current meter and satellite data, an unusual pattern of movement of the Gaspe Current was identified on two occasions in the summer of 1978. On the first occasion, the axis of the current moved offshore and the deep current reversed its normal seaward direction. In the satellite pictures, a wavelike structure of the Gaspe Current began to appear when the current was moving toward its normal nearshore position after a several-day excursion offshore. In a few days, it developed into a form resembling an overgrown wave, which eventually broke. The time span for the event is about 10 days. During the second episode, the appearance of a wavelike structure was also preceded by a shift in the position of the Gaspe Current, but the changes in the current direction and temperature were much less drastic than in the first event. Results of calculations based on Niiler and Mysak's model of barotropic instability for a coastal jet suggest that the wavelike motion may be triggered by the increased instability of the Gaspe Current when it is away from the coast, since the effect of the coast is stabilizing. The model predicts an e-folding time of 1 day, a wavelength of 52 km and a period of 4 days, which compare favorably with the observed wavelength of 60 km and period of 3–5 days.
Abstract
From current meter and satellite data, an unusual pattern of movement of the Gaspe Current was identified on two occasions in the summer of 1978. On the first occasion, the axis of the current moved offshore and the deep current reversed its normal seaward direction. In the satellite pictures, a wavelike structure of the Gaspe Current began to appear when the current was moving toward its normal nearshore position after a several-day excursion offshore. In a few days, it developed into a form resembling an overgrown wave, which eventually broke. The time span for the event is about 10 days. During the second episode, the appearance of a wavelike structure was also preceded by a shift in the position of the Gaspe Current, but the changes in the current direction and temperature were much less drastic than in the first event. Results of calculations based on Niiler and Mysak's model of barotropic instability for a coastal jet suggest that the wavelike motion may be triggered by the increased instability of the Gaspe Current when it is away from the coast, since the effect of the coast is stabilizing. The model predicts an e-folding time of 1 day, a wavelength of 52 km and a period of 4 days, which compare favorably with the observed wavelength of 60 km and period of 3–5 days.
Abstract
We show that in the presence of a randomly perturbed stability frequency, a coherent internal wave (f<σ<N 0) or gyroscopic wave (N 0<σ<f) is always attenuated in the direction of energy propagation, where σ and f denote respectively the wave angular frequency and Coriolis parameter, and N 0 denotes the mean stability frequency. For internal waves, the vertical e-folding attenuation scale due to the random perturbations is estimated to be 1–2 km. The effect of the random perturbations on the wave phase and group velocities appears to be insignificant, however.
Abstract
We show that in the presence of a randomly perturbed stability frequency, a coherent internal wave (f<σ<N 0) or gyroscopic wave (N 0<σ<f) is always attenuated in the direction of energy propagation, where σ and f denote respectively the wave angular frequency and Coriolis parameter, and N 0 denotes the mean stability frequency. For internal waves, the vertical e-folding attenuation scale due to the random perturbations is estimated to be 1–2 km. The effect of the random perturbations on the wave phase and group velocities appears to be insignificant, however.
Abstract
Current meter data for six mouths from the Grand Bank are analyzed to study inertial currents generated by moving storms. It is found that during periods of strong winds, but no well-defined storm system, the inertial motion exhibits no simple relationship to the local wind. During intense storms inertial currents up to 0.5 m s−1 were observed both in and below the mixed layer. Upper and lower layer currents are roughly equal in amplitude, but are 180° out of phase. To explain this observation, a two-layer, one-dimensional model is developed that successfully simulates the observed inertial currents. We show that under the conditions encountered during the storms only baroclinic inertial motion can be generated. The pressure gradient effect is not important, and the current below the mixed layer is produced by mass continuity. Wavelength computed from the continuity equation is consistent with that predicted by first-order linear theory. For inertial motion generated during periods of strong wind but no cyclone, pressure gradients and barotropic response can be important and should not be neglected.
Abstract
Current meter data for six mouths from the Grand Bank are analyzed to study inertial currents generated by moving storms. It is found that during periods of strong winds, but no well-defined storm system, the inertial motion exhibits no simple relationship to the local wind. During intense storms inertial currents up to 0.5 m s−1 were observed both in and below the mixed layer. Upper and lower layer currents are roughly equal in amplitude, but are 180° out of phase. To explain this observation, a two-layer, one-dimensional model is developed that successfully simulates the observed inertial currents. We show that under the conditions encountered during the storms only baroclinic inertial motion can be generated. The pressure gradient effect is not important, and the current below the mixed layer is produced by mass continuity. Wavelength computed from the continuity equation is consistent with that predicted by first-order linear theory. For inertial motion generated during periods of strong wind but no cyclone, pressure gradients and barotropic response can be important and should not be neglected.
Abstract
The interannual variations of volume transport in the western Labrador Sea are estimated using six years of TOPEX/Poseidon altimeter data and hydrographic data from a WOCE repeat section and a method based on the linear momentum equation in which the sea surface is the level of known motion. The interannual variation of the total transport in spring/summer has a range of 6.2 Sv (Sv ≡ 106 m3 s−1) and is positively correlated with the fall/winter North Atlantic Oscillation (NAO) index and wind patterns in the northwest North Atlantic. The total transport anomaly is decomposed into a barotropic and a baroclinic component. The interannual change of the barotropic transport is similar to that of the total transport, and is positively correlated with the fall/winter NAO index. The baroclinic transport anomaly, in comparison, has a smaller magnitude and the opposite sign. The authors conjecture that the deepened Icelandic low in high index years generates a strong cyclonic wind stress curl, which in turn creates a strong divergence and a large upward sea surface slope toward the Greenland coast, resulting in an intensified Labrador Sea circulation.
Abstract
The interannual variations of volume transport in the western Labrador Sea are estimated using six years of TOPEX/Poseidon altimeter data and hydrographic data from a WOCE repeat section and a method based on the linear momentum equation in which the sea surface is the level of known motion. The interannual variation of the total transport in spring/summer has a range of 6.2 Sv (Sv ≡ 106 m3 s−1) and is positively correlated with the fall/winter North Atlantic Oscillation (NAO) index and wind patterns in the northwest North Atlantic. The total transport anomaly is decomposed into a barotropic and a baroclinic component. The interannual change of the barotropic transport is similar to that of the total transport, and is positively correlated with the fall/winter NAO index. The baroclinic transport anomaly, in comparison, has a smaller magnitude and the opposite sign. The authors conjecture that the deepened Icelandic low in high index years generates a strong cyclonic wind stress curl, which in turn creates a strong divergence and a large upward sea surface slope toward the Greenland coast, resulting in an intensified Labrador Sea circulation.
Abstract
TOPEX/Poseidon (T/P) altimeter data over the period 1992–98 have been analyzed to examine annual variability of sea surface elevation and currents over the Scotian Shelf and Slope. A modified orthogonal response analysis is used to derive the annual cycle while simultaneously removing the residual tides and other dynamical processes at the appropriate T/P alias periods. An evaluation of the M 2 and K 1 alias variations is carried out, suggesting notable tidal correction errors off Cape Cod and over Georges Bank. The along-track sea surface slopes, which represent surface geostrophic current components normal to the track, are estimated on selected T/P ascending and descending ground tracks. The annual altimetric sea level harmonic is compared with steric height anomalies and wind-driven setup. The comparison indicates that the altimetric sea surface elevation variability is dominated by the baroclinic (and associated barotropic) component and supplemented by the wind-driven and remotely forced components. Altimetric elevations agree favorably with tide-gauge data at Halifax, Nova Scotia, and well with those at St. John's, Newfoundland. Wintertime intensification of the shelf-break flows is indicated in the altimetric surface currents, consistent with the solutions of regional diagnostic model forced by baroclinicity and boundary flows. Altimetric results clearly demonstrate seasonal variability of northeastward slope current stronger in fall and winter and weaker in spring and summer, which is less well resolved in the model. Assimilation of altimetric data into regional circulation models could help improve their prognostic ability to hindcast and nowcast seasonal variability of shelf-edge and slope water circulation. This study also implies a demand for better shelf tidal models to detide altimetric data for extraction of semiannual and shorter-period processes.
Abstract
TOPEX/Poseidon (T/P) altimeter data over the period 1992–98 have been analyzed to examine annual variability of sea surface elevation and currents over the Scotian Shelf and Slope. A modified orthogonal response analysis is used to derive the annual cycle while simultaneously removing the residual tides and other dynamical processes at the appropriate T/P alias periods. An evaluation of the M 2 and K 1 alias variations is carried out, suggesting notable tidal correction errors off Cape Cod and over Georges Bank. The along-track sea surface slopes, which represent surface geostrophic current components normal to the track, are estimated on selected T/P ascending and descending ground tracks. The annual altimetric sea level harmonic is compared with steric height anomalies and wind-driven setup. The comparison indicates that the altimetric sea surface elevation variability is dominated by the baroclinic (and associated barotropic) component and supplemented by the wind-driven and remotely forced components. Altimetric elevations agree favorably with tide-gauge data at Halifax, Nova Scotia, and well with those at St. John's, Newfoundland. Wintertime intensification of the shelf-break flows is indicated in the altimetric surface currents, consistent with the solutions of regional diagnostic model forced by baroclinicity and boundary flows. Altimetric results clearly demonstrate seasonal variability of northeastward slope current stronger in fall and winter and weaker in spring and summer, which is less well resolved in the model. Assimilation of altimetric data into regional circulation models could help improve their prognostic ability to hindcast and nowcast seasonal variability of shelf-edge and slope water circulation. This study also implies a demand for better shelf tidal models to detide altimetric data for extraction of semiannual and shorter-period processes.
Abstract
The authors investigate the interannual variations of freshwater content (FWC) and sea surface height (SSH) in the Beaufort Sea, particularly their increases during 2004–09, using a coupled ice–ocean model (CIOM), adapted for the Arctic Ocean to simulate the interannual variations. The CIOM simulation exhibits a (relative) salinity minimum in the Beaufort Sea and a warm Atlantic water layer in the Arctic Ocean, which is similar to the Polar Hydrographic Climatology (PHC), and captures the observed FWC maximum in the central Beaufort Sea, and the observed variation and rapid decline of total ice concentration, over the last 30 years. The model simulations of SSH and FWC suggest a significant increase in the central Beaufort Sea during 2004–09. The simulated SSH increase is about 8 cm, while the FWC increase is about 2.5 m, with most of these increases occurring in the center of the Beaufort gyre. The authors show that these increases are due to an increased surface wind stress curl during 2004–09, which increased the FWC in the Beaufort Sea by about 0.63 m yr−1 through Ekman pumping. Moreover, the increased surface wind is related to the interannual variation of the Arctic polar vortex at 500 hPa. During 2004–09, the polar vortex had significant weakness, which enhanced the Beaufort Sea high by affecting the frequency of synoptic weather systems in the region. In addition to the impacts of the polar vortex, enhanced melting of sea ice also contributes to the FWC increase by about 0.3 m yr−1 during 2004–09.
Abstract
The authors investigate the interannual variations of freshwater content (FWC) and sea surface height (SSH) in the Beaufort Sea, particularly their increases during 2004–09, using a coupled ice–ocean model (CIOM), adapted for the Arctic Ocean to simulate the interannual variations. The CIOM simulation exhibits a (relative) salinity minimum in the Beaufort Sea and a warm Atlantic water layer in the Arctic Ocean, which is similar to the Polar Hydrographic Climatology (PHC), and captures the observed FWC maximum in the central Beaufort Sea, and the observed variation and rapid decline of total ice concentration, over the last 30 years. The model simulations of SSH and FWC suggest a significant increase in the central Beaufort Sea during 2004–09. The simulated SSH increase is about 8 cm, while the FWC increase is about 2.5 m, with most of these increases occurring in the center of the Beaufort gyre. The authors show that these increases are due to an increased surface wind stress curl during 2004–09, which increased the FWC in the Beaufort Sea by about 0.63 m yr−1 through Ekman pumping. Moreover, the increased surface wind is related to the interannual variation of the Arctic polar vortex at 500 hPa. During 2004–09, the polar vortex had significant weakness, which enhanced the Beaufort Sea high by affecting the frequency of synoptic weather systems in the region. In addition to the impacts of the polar vortex, enhanced melting of sea ice also contributes to the FWC increase by about 0.3 m yr−1 during 2004–09.
Abstract
The barotropic response of the Labrador and Newfoundland shelves to a moving storm over the Labrador Sea is investigated using a linear barotropic ocean model with realistic coastline and topography. The model results show that the storm generates motions of different time–space scales. Four types of motions are identified:directly wind-forced motion, shelf waves with distinctive frequency and wavelength, low-frequency shelf waves, and trapped inertio–gravity waves. The strongest currents are directly wind-forced currents occurring in areas of maximum wind stress over the shelf. The spatial pattern and temporal change of the current field are strongly influenced by the time history of the storm and the geometry of the coastline. Continental shelf waves are generated in the shelf region south of the storm track. Maximum amplitude occurs along the shelf edge at a wavelength of 800 km and a period of 20 h. This wavelength and period are close to the maximum frequency point of the dispersion curve for the first-mode shelf waves. On the northeast Newfoundland shelf and Grand Banks, the most energetic motion is associated with low-frequency shelf waves with no definitive frequency and wavelength. The currents are rectilinear and parallel to the bathymetry contours at the shelf break and clockwise circular in the shelf interior. Inertio–gravity waves with signatures in both current and sea surface elevation are trapped in the northern Labrador Sea and the Davis Strait. The implications of the model results for current observation on the shelf are discussed.
Abstract
The barotropic response of the Labrador and Newfoundland shelves to a moving storm over the Labrador Sea is investigated using a linear barotropic ocean model with realistic coastline and topography. The model results show that the storm generates motions of different time–space scales. Four types of motions are identified:directly wind-forced motion, shelf waves with distinctive frequency and wavelength, low-frequency shelf waves, and trapped inertio–gravity waves. The strongest currents are directly wind-forced currents occurring in areas of maximum wind stress over the shelf. The spatial pattern and temporal change of the current field are strongly influenced by the time history of the storm and the geometry of the coastline. Continental shelf waves are generated in the shelf region south of the storm track. Maximum amplitude occurs along the shelf edge at a wavelength of 800 km and a period of 20 h. This wavelength and period are close to the maximum frequency point of the dispersion curve for the first-mode shelf waves. On the northeast Newfoundland shelf and Grand Banks, the most energetic motion is associated with low-frequency shelf waves with no definitive frequency and wavelength. The currents are rectilinear and parallel to the bathymetry contours at the shelf break and clockwise circular in the shelf interior. Inertio–gravity waves with signatures in both current and sea surface elevation are trapped in the northern Labrador Sea and the Davis Strait. The implications of the model results for current observation on the shelf are discussed.
Abstract
A linear three-dimensional diagnostic model is used to study several aspects of the mean circulation of the Labrador Sea and the adjacent shelves: volume transport, three-dimensional structure of currents, mesoscale features produced by JEBAR, winter surface current derived from ice drift data, difference between summer and winter circulation, and effects of buoyancy and local wind on the circulation. The summer circulation is forced by a sea surface elevation at the northern boundary, tuned to produce a 35 Sv (Sv≡106 m3 s−1) total southward transport (including the shelf, the Labrador Current, and the deep sea transports) across the Hamilton Bank section. A comparison of the model results with data collected over the shelves indicates that the model is able to reproduce the major features of the observations. Buoyancy effects on the circulation are found to be more important in the southern Labrador Sea than in the northern Labrador Sea. The subbasin-scale currents give rise to an alongshelf variation of the volume transport of the Labrador Current, 25 Sv in the northern Labrador Sea, 12 Sv at the Hamilton Bank section, and 25 Sv at the northeast Newfoundland shelf section. The winter circulation is calibrated with surface currents derived from ice drift data. The basin-scale transport at the Hamilton Bank section in the winter is 17–29 Sv greater than that in the summer. The effects of seasonally averaged local winds on the volume transport are found to be small.
Abstract
A linear three-dimensional diagnostic model is used to study several aspects of the mean circulation of the Labrador Sea and the adjacent shelves: volume transport, three-dimensional structure of currents, mesoscale features produced by JEBAR, winter surface current derived from ice drift data, difference between summer and winter circulation, and effects of buoyancy and local wind on the circulation. The summer circulation is forced by a sea surface elevation at the northern boundary, tuned to produce a 35 Sv (Sv≡106 m3 s−1) total southward transport (including the shelf, the Labrador Current, and the deep sea transports) across the Hamilton Bank section. A comparison of the model results with data collected over the shelves indicates that the model is able to reproduce the major features of the observations. Buoyancy effects on the circulation are found to be more important in the southern Labrador Sea than in the northern Labrador Sea. The subbasin-scale currents give rise to an alongshelf variation of the volume transport of the Labrador Current, 25 Sv in the northern Labrador Sea, 12 Sv at the Hamilton Bank section, and 25 Sv at the northeast Newfoundland shelf section. The winter circulation is calibrated with surface currents derived from ice drift data. The basin-scale transport at the Hamilton Bank section in the winter is 17–29 Sv greater than that in the summer. The effects of seasonally averaged local winds on the volume transport are found to be small.
Abstract
Ocean models usually estimate surface currents without explicit modeling of the ocean waves. To consider the impact of waves on surface currents, here a wave model is used in a modified Ekman layer model, which is imbedded in a diagnostic ocean model. Thus wave effects, for example, Stokes drift and wave-breaking dissipation, are explicitly considered in conjunction with the Ekman current, mean currents, and wind-driven pressure gradient currents. It is shown that the wave effect on currents is largest in rapidly developing intense storms, when wave-modified currents can exceed the usual Ekman currents by as much as 40%. A large part of this increase in velocity can be attributed to the Stokes drift. Reductions in momentum transfer to the ocean due to wind input to waves and enhancements due to wave dissipation are each of the order 20%–30%. Model results are compared with measurements from the Labrador Sea Deep Convection Experiment of 1997.
Abstract
Ocean models usually estimate surface currents without explicit modeling of the ocean waves. To consider the impact of waves on surface currents, here a wave model is used in a modified Ekman layer model, which is imbedded in a diagnostic ocean model. Thus wave effects, for example, Stokes drift and wave-breaking dissipation, are explicitly considered in conjunction with the Ekman current, mean currents, and wind-driven pressure gradient currents. It is shown that the wave effect on currents is largest in rapidly developing intense storms, when wave-modified currents can exceed the usual Ekman currents by as much as 40%. A large part of this increase in velocity can be attributed to the Stokes drift. Reductions in momentum transfer to the ocean due to wind input to waves and enhancements due to wave dissipation are each of the order 20%–30%. Model results are compared with measurements from the Labrador Sea Deep Convection Experiment of 1997.
