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- Author or Editor: Robert S. Pickart x
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Abstract
Twelve historical CTD/oxygen sections between the Grand Banks and Cape Hatteras, occupied over the time period 1981–85, are analyzed to investigate the variability of the Deep Western Boundary Current (DWBC) property core. The sections are transformed into a coordinate system of bottom depth versus height above the bottom, then interpolated onto a regular grid in order to facilitate an inter-sectional comparison. The average sections show a high-oxygen core near 3200-m depth, θ = 2.2°C, which corresponds to a region of upward sloping isotherms against the boundary, inshore of the deep Gulf Stream. As the DWBC progresses toward Cape Hatteras it shoals significantly and becomes less dense as a result of mixing with surrounding fluid along its path. There is, however, much scatter about this general alongstream trend and large density fluctuations are correlated with changes in bottom depth and cross-sectional area of the DWBC core. This variability is most likely the downstream response to changes in the overflow source waters of the DWBC. Some of the DWBC appears to recirculate with the deep Gulf Stream near Cape Hatteras (where the two currents cross each other), forming a weaker offshore oxygen core.
Abstract
Twelve historical CTD/oxygen sections between the Grand Banks and Cape Hatteras, occupied over the time period 1981–85, are analyzed to investigate the variability of the Deep Western Boundary Current (DWBC) property core. The sections are transformed into a coordinate system of bottom depth versus height above the bottom, then interpolated onto a regular grid in order to facilitate an inter-sectional comparison. The average sections show a high-oxygen core near 3200-m depth, θ = 2.2°C, which corresponds to a region of upward sloping isotherms against the boundary, inshore of the deep Gulf Stream. As the DWBC progresses toward Cape Hatteras it shoals significantly and becomes less dense as a result of mixing with surrounding fluid along its path. There is, however, much scatter about this general alongstream trend and large density fluctuations are correlated with changes in bottom depth and cross-sectional area of the DWBC core. This variability is most likely the downstream response to changes in the overflow source waters of the DWBC. Some of the DWBC appears to recirculate with the deep Gulf Stream near Cape Hatteras (where the two currents cross each other), forming a weaker offshore oxygen core.
Abstract
An inverse ray tracing model is applied to observations of 40-day topographic Rossby waves on the continental slope off of Cape Hatteras, North Carolina, to determine their origin. The rays are traced seaward and extend into the deep Gulf Stream, where the bottom slope remains strong enough that topographic β dominates planetary β. This enable coupling to occur between eastward propagating Gulf Stream meanders and topographic waves with eastward phase speed and matching zonal wavelength. Previous satellite observations indicate that the most frequently occurring Gulf Stream meanders have a period of 40 days and the model reveals that, as these meanders pass a topographic bend near 71°–72°W, they are able to couple to the observed topographic waves traced back from Cape Hatteras. The 40-day Gulf Stream meanders occur in bursts, which leads to associated bursts of the topographic waves.
Abstract
An inverse ray tracing model is applied to observations of 40-day topographic Rossby waves on the continental slope off of Cape Hatteras, North Carolina, to determine their origin. The rays are traced seaward and extend into the deep Gulf Stream, where the bottom slope remains strong enough that topographic β dominates planetary β. This enable coupling to occur between eastward propagating Gulf Stream meanders and topographic waves with eastward phase speed and matching zonal wavelength. Previous satellite observations indicate that the most frequently occurring Gulf Stream meanders have a period of 40 days and the model reveals that, as these meanders pass a topographic bend near 71°–72°W, they are able to couple to the observed topographic waves traced back from Cape Hatteras. The 40-day Gulf Stream meanders occur in bursts, which leads to associated bursts of the topographic waves.
Abstract
The hydrographic properties of the bottom boundary layer (BBL) are investigated in a synoptic cross section of the Middle Atlantic Bight shelfbreak frontal jet. The dataset consists of closely spaced conductivity–temperature–depth stations and concurrent shipboard acoustic Doppler current profiler measurements. An extremum in BBL properties occurs in the frontal region where the layer becomes thinner (disappearing briefly), more stratified, and more strongly capped. These changes are apparently related to the significant cross-slope variation in interior stratification. Where the BBL vanishes, at the shoreward edge of the front, it detaches into the interior along an (nearly) isopycnal layer. This is revealed both by weak vertical stratification as well as weak isopycnal gradients of potential temperature (θ) and salinity (S) along the layer. An advective–diffusive model of the detachment in density space is used to explain the observed θ, S distribution as well as estimate the pumping speed along the detached BBL. The detided ADCP velocity fields are analyzed in light of the observed detached BBL. The mechanism of detachment is discussed in relation to existing models, and the secondary circulation in the cross-stream plane is inferred. This reveals a deep interior upwelling cell, apparently tied to the local bathymetry, which enhances the flow along the detached BBL.
Abstract
The hydrographic properties of the bottom boundary layer (BBL) are investigated in a synoptic cross section of the Middle Atlantic Bight shelfbreak frontal jet. The dataset consists of closely spaced conductivity–temperature–depth stations and concurrent shipboard acoustic Doppler current profiler measurements. An extremum in BBL properties occurs in the frontal region where the layer becomes thinner (disappearing briefly), more stratified, and more strongly capped. These changes are apparently related to the significant cross-slope variation in interior stratification. Where the BBL vanishes, at the shoreward edge of the front, it detaches into the interior along an (nearly) isopycnal layer. This is revealed both by weak vertical stratification as well as weak isopycnal gradients of potential temperature (θ) and salinity (S) along the layer. An advective–diffusive model of the detachment in density space is used to explain the observed θ, S distribution as well as estimate the pumping speed along the detached BBL. The detided ADCP velocity fields are analyzed in light of the observed detached BBL. The mechanism of detachment is discussed in relation to existing models, and the secondary circulation in the cross-stream plane is inferred. This reveals a deep interior upwelling cell, apparently tied to the local bathymetry, which enhances the flow along the detached BBL.
Abstract
Twelve years of historical hydrographic data, spanning the period 1990–2001, are analyzed to examine the along-stream evolution of the western North Atlantic Ocean shelfbreak front and current, following its path between the west coast of Greenland and the Middle Atlantic Bight. Over 700 synoptic sections are used to construct a mean three-dimensional description of the summer shelfbreak front and to quantify the along-stream evolution in properties, including frontal strength and grounding position. Results show that there are actually two fronts in the northern part of the domain—a shallow front located near the shelf break and a deeper front centered in the core of Irminger Water over the upper slope. The properties of the deeper Irminger front erode gradually to the south, and the front disappears entirely near the Grand Banks of Newfoundland. The shallow shelfbreak front is identifiable throughout the domain, and its properties exhibit large variations from north to south, with the largest changes occurring near the Tail of the Grand Banks. Despite these structural changes, and large variations in topography, the foot of the shelfbreak front remains within 20 km of the shelf break. The hydrographic sections are also used to examine the evolution of the baroclinic velocity field and its associated volume transport. The baroclinic velocity structure consists of a single velocity core that is stronger and penetrates deeper where the Irminger front is present. The baroclinic volume transport decreases by equal amounts at the southern end of the Labrador Shelf and at the Tail of the Grand Banks. Overall, the results suggest that the Grand Banks is a geographically critical location in the North Atlantic shelfbreak system.
Abstract
Twelve years of historical hydrographic data, spanning the period 1990–2001, are analyzed to examine the along-stream evolution of the western North Atlantic Ocean shelfbreak front and current, following its path between the west coast of Greenland and the Middle Atlantic Bight. Over 700 synoptic sections are used to construct a mean three-dimensional description of the summer shelfbreak front and to quantify the along-stream evolution in properties, including frontal strength and grounding position. Results show that there are actually two fronts in the northern part of the domain—a shallow front located near the shelf break and a deeper front centered in the core of Irminger Water over the upper slope. The properties of the deeper Irminger front erode gradually to the south, and the front disappears entirely near the Grand Banks of Newfoundland. The shallow shelfbreak front is identifiable throughout the domain, and its properties exhibit large variations from north to south, with the largest changes occurring near the Tail of the Grand Banks. Despite these structural changes, and large variations in topography, the foot of the shelfbreak front remains within 20 km of the shelf break. The hydrographic sections are also used to examine the evolution of the baroclinic velocity field and its associated volume transport. The baroclinic velocity structure consists of a single velocity core that is stronger and penetrates deeper where the Irminger front is present. The baroclinic volume transport decreases by equal amounts at the southern end of the Labrador Shelf and at the Tail of the Grand Banks. Overall, the results suggest that the Grand Banks is a geographically critical location in the North Atlantic shelfbreak system.
Abstract
A geostrophic velocity section across the Gulf Stream and deep western boundary current near 35°N is referenced four different ways: using a POGO float (acoustically tracked transport float), shipboard acoustic Doppler current profiler (ADCP), and bottom current meters, and by assuming an isotherm level of no motion. The comparison between the first two techniques is emphasized because they are most easily applied. In general, reference velocities calculated using the float data agree well with those obtained from the ADCP data. However, there is disagreement at locations where the ADCP velocity is not in thermal wind balance, in which case the POGO value is deemed more accurate because the float samples deeper into the subsurface geostrophic flow. Disagreement is also caused by insufficient cross-stream POCSO spacing (although this could be avoided). The isotherm- and current meter-referenced sections, while similar to each other, both show unrealistic features. it is argued that the POGO method is preferable to the shipboard ADCP for a deep-water hydrographic experiment.
Abstract
A geostrophic velocity section across the Gulf Stream and deep western boundary current near 35°N is referenced four different ways: using a POGO float (acoustically tracked transport float), shipboard acoustic Doppler current profiler (ADCP), and bottom current meters, and by assuming an isotherm level of no motion. The comparison between the first two techniques is emphasized because they are most easily applied. In general, reference velocities calculated using the float data agree well with those obtained from the ADCP data. However, there is disagreement at locations where the ADCP velocity is not in thermal wind balance, in which case the POGO value is deemed more accurate because the float samples deeper into the subsurface geostrophic flow. Disagreement is also caused by insufficient cross-stream POCSO spacing (although this could be avoided). The isotherm- and current meter-referenced sections, while similar to each other, both show unrealistic features. it is argued that the POGO method is preferable to the shipboard ADCP for a deep-water hydrographic experiment.
Abstract
The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1 Sv (1 Sv ≡ 106 m3 s−1), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea.
Abstract
The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1 Sv (1 Sv ≡ 106 m3 s−1), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea.
Abstract
The methodology for converting the travel time measurement of the inverted echo sounder (IES) into an amplitude of the first baroclinic dynamical mode, A 1, is presented. For a Gulf Stream IES record the so-generated A 1(t) time series is used to compute a vertical profile of first mode temperature versus time by perturbing a basic state temperature profile. The basic state is constructed by averaging together historical CTD data collected near the IES site. Similarly the first mode amplitudes are used to perturb a basic state dynamic height profile, and, using neighboring IESs, a profile of alongstream geostrophic velocity is obtained at the same location. The resulting IES-derived temperatures and velocities compare favorably to independent current meter results, exhibiting most of the variability observed in both the current meter temperature and alongstream velocity.
Abstract
The methodology for converting the travel time measurement of the inverted echo sounder (IES) into an amplitude of the first baroclinic dynamical mode, A 1, is presented. For a Gulf Stream IES record the so-generated A 1(t) time series is used to compute a vertical profile of first mode temperature versus time by perturbing a basic state temperature profile. The basic state is constructed by averaging together historical CTD data collected near the IES site. Similarly the first mode amplitudes are used to perturb a basic state dynamic height profile, and, using neighboring IESs, a profile of alongstream geostrophic velocity is obtained at the same location. The resulting IES-derived temperatures and velocities compare favorably to independent current meter results, exhibiting most of the variability observed in both the current meter temperature and alongstream velocity.
Abstract
It is demonstrated that recently observed cyclonic recirculation gyres in the Irminger and Labrador Seas may be forced by the strong cyclonic wind stress curl that develops each winter seaward of the east coast of Greenland. Idealized analytical and numerical models forced with such variable winds over a sloping bottom reproduce the essential aspects of the observed gyres (strength, location, and horizontal and vertical length scales). The communication between the forcing region in the Irminger Sea and the recirculation to the west is achieved by baroclinic topographic Rossby wave propagation along potential vorticity contours. The circulation is characterized as a time-dependent, stratified, topographic beta plume. For weak stratification, as found in the subpolar North Atlantic, the recirculation strength exhibits only weak seasonal variability, consistent with the observations, even though the forcing is active only during the winter. Baroclinic Rossby waves that develop when the wind forcing ceases in springtime interact with the bottom to provide a source of cyclonic vorticity that maintains the circulation until the wind strengthens again in the following winter.
Abstract
It is demonstrated that recently observed cyclonic recirculation gyres in the Irminger and Labrador Seas may be forced by the strong cyclonic wind stress curl that develops each winter seaward of the east coast of Greenland. Idealized analytical and numerical models forced with such variable winds over a sloping bottom reproduce the essential aspects of the observed gyres (strength, location, and horizontal and vertical length scales). The communication between the forcing region in the Irminger Sea and the recirculation to the west is achieved by baroclinic topographic Rossby wave propagation along potential vorticity contours. The circulation is characterized as a time-dependent, stratified, topographic beta plume. For weak stratification, as found in the subpolar North Atlantic, the recirculation strength exhibits only weak seasonal variability, consistent with the observations, even though the forcing is active only during the winter. Baroclinic Rossby waves that develop when the wind forcing ceases in springtime interact with the bottom to provide a source of cyclonic vorticity that maintains the circulation until the wind strengthens again in the following winter.
Abstract
It is demonstrated that recently observed cyclonic recirculation gyres in the Irminger and Labrador Seas may be forced by the strong cyclonic wind stress curl that develops each winter seaward of the east coast of Greenland. Idealized analytical and numerical models forced with such variable winds over a sloping bottom reproduce the essential aspects of the observed gyres (strength, location, and horizontal and vertical length scales). The communication between the forcing region in the Irminger Sea and the recirculation to the west is achieved by baroclinic topographic Rossby wave propagation along potential vorticity contours. The circulation is characterized as a time-dependent, stratified, topographic beta plume. For weak stratification, as found in the subpolar North Atlantic, the recirculation strength exhibits only weak seasonal variability, consistent with the observations, even though the forcing is active only during the winter. Baroclinic Rossby waves that develop when the wind forcing ceases in springtime interact with the bottom to provide a source of cyclonic vorticity that maintains the circulation until the wind strengthens again in the following winter.
Abstract
It is demonstrated that recently observed cyclonic recirculation gyres in the Irminger and Labrador Seas may be forced by the strong cyclonic wind stress curl that develops each winter seaward of the east coast of Greenland. Idealized analytical and numerical models forced with such variable winds over a sloping bottom reproduce the essential aspects of the observed gyres (strength, location, and horizontal and vertical length scales). The communication between the forcing region in the Irminger Sea and the recirculation to the west is achieved by baroclinic topographic Rossby wave propagation along potential vorticity contours. The circulation is characterized as a time-dependent, stratified, topographic beta plume. For weak stratification, as found in the subpolar North Atlantic, the recirculation strength exhibits only weak seasonal variability, consistent with the observations, even though the forcing is active only during the winter. Baroclinic Rossby waves that develop when the wind forcing ceases in springtime interact with the bottom to provide a source of cyclonic vorticity that maintains the circulation until the wind strengthens again in the following winter.
Abstract
It is proposed that a dominant component of the downwelling limb of the thermohaline circulation takes place in regions where convective mixing is found adjacent to steep topography. A simple theoretical estimate of the overturning forced by such boundary convection is derived that depends only on the properties of the oceanic mixed layer along the boundary. Scaling estimates indicate that sinking forced by boundary convection is an order of magnitude greater than sinking in the open ocean resulting from large-scale dynamics or baroclinic instability of deep convective sites. Recent hydrographic observations in the Labrador Sea are used to estimate the downwelling due to these different mechanisms and support the notion that boundary sinking dominates. The theory compares well with the overturning rates diagnosed in a noneddy-resolving general circulation model over a wide range of parameters. As a direct consequence of these dynamics, the high-latitude hydrography and overturning circulation in the model are very sensitive to the presence of cyclonic rim currents. Lateral density advection by the rim currents in the subpolar gyre increases the stratification and limits the mixing near the boundaries, thus reducing the maximum downwelling. As a result, most of the high-latitude meridional heat transport is carried by the horizontal circulation instead of the overturning circulation. Such rim currents are found in different configurations of the model, including 1) a continental slope and standard diffusion parameters and 2) zero horizontal diffusion and a flat bottom.
Abstract
It is proposed that a dominant component of the downwelling limb of the thermohaline circulation takes place in regions where convective mixing is found adjacent to steep topography. A simple theoretical estimate of the overturning forced by such boundary convection is derived that depends only on the properties of the oceanic mixed layer along the boundary. Scaling estimates indicate that sinking forced by boundary convection is an order of magnitude greater than sinking in the open ocean resulting from large-scale dynamics or baroclinic instability of deep convective sites. Recent hydrographic observations in the Labrador Sea are used to estimate the downwelling due to these different mechanisms and support the notion that boundary sinking dominates. The theory compares well with the overturning rates diagnosed in a noneddy-resolving general circulation model over a wide range of parameters. As a direct consequence of these dynamics, the high-latitude hydrography and overturning circulation in the model are very sensitive to the presence of cyclonic rim currents. Lateral density advection by the rim currents in the subpolar gyre increases the stratification and limits the mixing near the boundaries, thus reducing the maximum downwelling. As a result, most of the high-latitude meridional heat transport is carried by the horizontal circulation instead of the overturning circulation. Such rim currents are found in different configurations of the model, including 1) a continental slope and standard diffusion parameters and 2) zero horizontal diffusion and a flat bottom.
