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- Author or Editor: M. Susan Lozier x
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Abstract
Historical hydrographic data in the eastern North Atlantic are used to suggest a connection between the northward penetration of Mediterranean Overflow Water (MOW) and the location of the subpolar front, the latter of which is shown to vary with the North Atlantic Oscillation (NAO). During persistent high-NAO periods, when the subpolar front moves eastward, waters in the subpolar gyre essentially block the northward-flowing MOW, preventing its entry into the subpolar gyre. Conversely, during low NAO periods, the subpolar front moves westward, allowing MOW to penetrate past Porcupine Bank into the subpolar gyre. The impacts of an intermittent penetration of MOW into the subpolar gyre, including the possible effect on water mass transformations, remain to be investigated.
Abstract
Historical hydrographic data in the eastern North Atlantic are used to suggest a connection between the northward penetration of Mediterranean Overflow Water (MOW) and the location of the subpolar front, the latter of which is shown to vary with the North Atlantic Oscillation (NAO). During persistent high-NAO periods, when the subpolar front moves eastward, waters in the subpolar gyre essentially block the northward-flowing MOW, preventing its entry into the subpolar gyre. Conversely, during low NAO periods, the subpolar front moves westward, allowing MOW to penetrate past Porcupine Bank into the subpolar gyre. The impacts of an intermittent penetration of MOW into the subpolar gyre, including the possible effect on water mass transformations, remain to be investigated.
Abstract
Although proxies have generally been used to study deep ocean convection and overturning circulation in the Labrador Sea, their efficacy has not been explicitly evaluated because observations that directly measure those variables are scarce. In this study, the volume of newly formed Labrador Sea Water (LSW) and the overturning circulation in the Labrador Sea are estimated using observational data and output from a high-resolution ocean model and then compared to proxies used to represent those variables. The comparisons reveal the limitations of proxies, highlighting the desirability of robust estimates derived from direct monitoring in the region [i.e., from Argo and Overturning in the Subpolar North Atlantic Program (OSNAP)]. A linkage among LSW formation, overturning circulation in the Labrador Sea, and the export of LSW from the basin on interannual time scales is found in the model.
Abstract
Although proxies have generally been used to study deep ocean convection and overturning circulation in the Labrador Sea, their efficacy has not been explicitly evaluated because observations that directly measure those variables are scarce. In this study, the volume of newly formed Labrador Sea Water (LSW) and the overturning circulation in the Labrador Sea are estimated using observational data and output from a high-resolution ocean model and then compared to proxies used to represent those variables. The comparisons reveal the limitations of proxies, highlighting the desirability of robust estimates derived from direct monitoring in the region [i.e., from Argo and Overturning in the Subpolar North Atlantic Program (OSNAP)]. A linkage among LSW formation, overturning circulation in the Labrador Sea, and the export of LSW from the basin on interannual time scales is found in the model.
Abstract
The trajectories of 95 isopycnal floats deployed in the Gulf Stream in the last decade have shown that a substantial amount of particle exchange takes place between the Gulf Stream and the surrounding fluid at the level of the main thermocline. This exchange is suggestive of significant cross-stream eddy mixing, but in order to accurately interpret the float exchange in terms of property exchange the location of float deployment was assessed relative to the strong potential vorticity front associated with the Gulf Stream. The basic result of this analysis is that most of the observed float exchange is not representative of cross-frontal exchange. At the level where a strong potential vorticity front is present, some fluid particles escape from the jet, but most of them stay on the same side of the front. In the deep main thermocline, significant particle exchange is observed between the Gulf Stream and fluid on both sides of the jet, but this exchange is indicative of particles circulating in a relatively homogeneous pool of potential vorticity and thus does not signify a cross-stream property flux. These characteristics of particle exchange in the Gulf Stream are found to be generally compatible with the results from a study of particle behavior in a quasigeostrophic eddy-resolving GCM.
Abstract
The trajectories of 95 isopycnal floats deployed in the Gulf Stream in the last decade have shown that a substantial amount of particle exchange takes place between the Gulf Stream and the surrounding fluid at the level of the main thermocline. This exchange is suggestive of significant cross-stream eddy mixing, but in order to accurately interpret the float exchange in terms of property exchange the location of float deployment was assessed relative to the strong potential vorticity front associated with the Gulf Stream. The basic result of this analysis is that most of the observed float exchange is not representative of cross-frontal exchange. At the level where a strong potential vorticity front is present, some fluid particles escape from the jet, but most of them stay on the same side of the front. In the deep main thermocline, significant particle exchange is observed between the Gulf Stream and fluid on both sides of the jet, but this exchange is indicative of particles circulating in a relatively homogeneous pool of potential vorticity and thus does not signify a cross-stream property flux. These characteristics of particle exchange in the Gulf Stream are found to be generally compatible with the results from a study of particle behavior in a quasigeostrophic eddy-resolving GCM.
Abstract
Potential vorticity dynamics for a quasi-geostrophic eddy-resolving general circulation model (EGCM) are studied in order to determine the effects of mesoscale variability on the potential vorticity distribution of a wind-driven ocean. The study employs both Eulerian and Lagrangian analyses in the effort to describe the potential vorticity gain/loss cycle along the path of a particle. While the mean wind stress curl is the dominant potential vorticity source for the interior of the upper layer, a redistribution of eddy potential vorticity creates sources of potential vorticity for the multiple gyres in the lower layers. This redistribution is a result of the local generation of eddies via baroclinic instabilities. These eddies are advected by the western boundary current into the midlatitude jet where they are responsible for a cross-gyre potential vorticity exchange. This exchange is concentrated at the entrance to the eastward jets where northward and southward boundary currents converge. From a Lagrangian viewpoint the vorticity exchange is accomplished via dissipative meandering rather than particle exchange across gyre fronts.
Abstract
Potential vorticity dynamics for a quasi-geostrophic eddy-resolving general circulation model (EGCM) are studied in order to determine the effects of mesoscale variability on the potential vorticity distribution of a wind-driven ocean. The study employs both Eulerian and Lagrangian analyses in the effort to describe the potential vorticity gain/loss cycle along the path of a particle. While the mean wind stress curl is the dominant potential vorticity source for the interior of the upper layer, a redistribution of eddy potential vorticity creates sources of potential vorticity for the multiple gyres in the lower layers. This redistribution is a result of the local generation of eddies via baroclinic instabilities. These eddies are advected by the western boundary current into the midlatitude jet where they are responsible for a cross-gyre potential vorticity exchange. This exchange is concentrated at the entrance to the eastward jets where northward and southward boundary currents converge. From a Lagrangian viewpoint the vorticity exchange is accomplished via dissipative meandering rather than particle exchange across gyre fronts.
Abstract
A one-dimensional model of baroclinic instability is used to reconcile two differing interpretations on why neutrally buoyant floats cross the Gulf Stream more readily at subthermocline depths. The study compares the location of the steering level, where the particle speed matches the speed of a propagating meander, to the location of the minimum in the meridional potential vorticity gradient The former locale is the expected site of a maxima in particle exchange, based on kinematic arguments, while the latter is the expected site based on potential vorticity dynamics. Model results show that the two levels are not coincident; in general, the steering level is deeper in the water column than the minimum in the potential vorticity gradient. Additionally, it is the steering level where the largest cross-stream particle exchange is expected to occur.
Abstract
A one-dimensional model of baroclinic instability is used to reconcile two differing interpretations on why neutrally buoyant floats cross the Gulf Stream more readily at subthermocline depths. The study compares the location of the steering level, where the particle speed matches the speed of a propagating meander, to the location of the minimum in the meridional potential vorticity gradient The former locale is the expected site of a maxima in particle exchange, based on kinematic arguments, while the latter is the expected site based on potential vorticity dynamics. Model results show that the two levels are not coincident; in general, the steering level is deeper in the water column than the minimum in the potential vorticity gradient. Additionally, it is the steering level where the largest cross-stream particle exchange is expected to occur.
Abstract
Boundary layer potential vorticity dynamics for a quasi-geostrophic, eddy-resolving general circulation ocean model are studied using both Lagrangian and Eulerian analyses. Active western boundary layers are found not only in the upper wind-driven layer but also in the lower layers, despite the lack of a direct vorticity input to the deep ocean. At the western wall dissipative and inertial boundary regimes are exclusively controlled by the time-mean dynamics except for the deepest layer where eddy fluxes drive the mean flow across mean potential vorticity contours. Boundary layers formed at the southern wall in this model are dynamically distinct from the western boundary layers; they are controlled solely by the eddy flux of potential vorticity found in this region of active baroclinic instability. Basin-integrated vorticity balances reveal a strong contribution to the vorticity cycle by the lateral boundaries with such input overshadowed by the vorticity exchange across the midbasin gyre boundary in the surface layer.
Abstract
Boundary layer potential vorticity dynamics for a quasi-geostrophic, eddy-resolving general circulation ocean model are studied using both Lagrangian and Eulerian analyses. Active western boundary layers are found not only in the upper wind-driven layer but also in the lower layers, despite the lack of a direct vorticity input to the deep ocean. At the western wall dissipative and inertial boundary regimes are exclusively controlled by the time-mean dynamics except for the deepest layer where eddy fluxes drive the mean flow across mean potential vorticity contours. Boundary layers formed at the southern wall in this model are dynamically distinct from the western boundary layers; they are controlled solely by the eddy flux of potential vorticity found in this region of active baroclinic instability. Basin-integrated vorticity balances reveal a strong contribution to the vorticity cycle by the lateral boundaries with such input overshadowed by the vorticity exchange across the midbasin gyre boundary in the surface layer.
Abstract
Surface drifters at 10 and 40 m are analyzed to assess cross-frontal exchange characteristics within the Middle Atlantic Bight. This Lagrangian analysis shows a shelfbreak jet characterized by strong and ubiquitous meandering. The drifters collectively demonstrate the continuity of the shelfbreak frontal jet from Georges Bank to Cape Hatteras. Along the length of the shelf break the drifters are detrained both onshore and offshore, yet offshore detrainment is predominant. The sites for offshore detrainment are distributed along the Bight, precluding the possibility that localized bathymetric features are the primary conduits for near-surface to mid-depth cross-frontal exchange. Finally, a strong seasonal asymmetry is noted in the drifter exchange pattern, with more offshore exchange in the winter than in the summer. However, the available data limits our interpretation of this feature.
Abstract
Surface drifters at 10 and 40 m are analyzed to assess cross-frontal exchange characteristics within the Middle Atlantic Bight. This Lagrangian analysis shows a shelfbreak jet characterized by strong and ubiquitous meandering. The drifters collectively demonstrate the continuity of the shelfbreak frontal jet from Georges Bank to Cape Hatteras. Along the length of the shelf break the drifters are detrained both onshore and offshore, yet offshore detrainment is predominant. The sites for offshore detrainment are distributed along the Bight, precluding the possibility that localized bathymetric features are the primary conduits for near-surface to mid-depth cross-frontal exchange. Finally, a strong seasonal asymmetry is noted in the drifter exchange pattern, with more offshore exchange in the winter than in the summer. However, the available data limits our interpretation of this feature.
Abstract
One way to measure the skill of an ocean general circulation model is to evaluate its ability to simulate observed property distributions. Pressure, temperature, and salinity distributions generated by a ⅙° Atlantic Ocean general circulation model are compared with climatological fields on three potential density surfaces, representative of the upper, middepth, and deep ocean waters. The upper ocean property fields are relatively well simulated, a testimony to the model's ability to generally reproduce the wind-driven circulation in the North Atlantic. However, in the middepth and deep ocean, where wind forcing is negligible and buoyant flows associated with deep-water formation play a major role in establishing property distributions, the fields are poorly represented in the ⅙° Atlantic Ocean model. The comparison between the observed and modeled fields indicates several model deficiencies in the representation of intermediate and deep waters and their pathways. Possible model improvements to reduce the mismatch between model and data are proposed.
Abstract
One way to measure the skill of an ocean general circulation model is to evaluate its ability to simulate observed property distributions. Pressure, temperature, and salinity distributions generated by a ⅙° Atlantic Ocean general circulation model are compared with climatological fields on three potential density surfaces, representative of the upper, middepth, and deep ocean waters. The upper ocean property fields are relatively well simulated, a testimony to the model's ability to generally reproduce the wind-driven circulation in the North Atlantic. However, in the middepth and deep ocean, where wind forcing is negligible and buoyant flows associated with deep-water formation play a major role in establishing property distributions, the fields are poorly represented in the ⅙° Atlantic Ocean model. The comparison between the observed and modeled fields indicates several model deficiencies in the representation of intermediate and deep waters and their pathways. Possible model improvements to reduce the mismatch between model and data are proposed.
Abstract
A recent study of the eastern North Atlantic detailed significant increases in the temperature and salinity of the Mediterranean Overflow Water (MOW) from 1950 to 2000. To examine the degree to which the source waters, which spill over the sill at the Strait of Gibraltar, could be responsible for these observations in the open Atlantic, a box model of water mass transformation by marginal seas was employed. Time series for the salinity of the inflowing North Atlantic surface waters, freshwater fluxes in the Mediterranean (evaporation and precipitation and river runoff), and the volumetric flow rates for the inflow and outflow across the Strait of Gibraltar were used to predict the salinity of the source waters to the North Atlantic from 1950 to 2000. Results from this calculation reveal that source water changes have minimal impact on MOW property changes on interannual and decadal time scales. It is suggested instead that circulation changes within the open Atlantic alter the advective–diffusive pathways of MOW such that property changes within the MOW reservoir are created.
Abstract
A recent study of the eastern North Atlantic detailed significant increases in the temperature and salinity of the Mediterranean Overflow Water (MOW) from 1950 to 2000. To examine the degree to which the source waters, which spill over the sill at the Strait of Gibraltar, could be responsible for these observations in the open Atlantic, a box model of water mass transformation by marginal seas was employed. Time series for the salinity of the inflowing North Atlantic surface waters, freshwater fluxes in the Mediterranean (evaporation and precipitation and river runoff), and the volumetric flow rates for the inflow and outflow across the Strait of Gibraltar were used to predict the salinity of the source waters to the North Atlantic from 1950 to 2000. Results from this calculation reveal that source water changes have minimal impact on MOW property changes on interannual and decadal time scales. It is suggested instead that circulation changes within the open Atlantic alter the advective–diffusive pathways of MOW such that property changes within the MOW reservoir are created.
Abstract
Deep water formation in the northern North Atlantic has been of long-standing interest because the resultant water masses, along with those that flow over the Greenland–Scotland Ridge, constitute the lower limb of the Atlantic meridional overturning circulation (AMOC), which carries these cold, deep waters southward to the subtropical region and beyond. It has long been assumed that an increase in deep water formation would result in a larger southward export of newly formed deep water masses. However, recent observations of Lagrangian floats have raised questions about this linkage. Motivated by these observations, the relationship between convective activity in the Labrador Sea and the export of newly formed Labrador Sea Water (LSW), the shallowest component of the deep AMOC, to the subtropics is explored. This study uses simulated Lagrangian pathways of synthetic floats produced with output from a global ocean–sea ice model. It is shown that substantial recirculation of newly formed LSW in the subpolar gyre leads to a relatively small fraction of this water exported to the subtropical gyre: 40 years after release, only 46% of the floats are able to reach the subtropics. Furthermore, waters produced from any one particular convection event are not collectively and contemporaneously exported to the subtropical gyre, such that the waters that are exported to the subtropical gyre have a wide distribution in age.
Abstract
Deep water formation in the northern North Atlantic has been of long-standing interest because the resultant water masses, along with those that flow over the Greenland–Scotland Ridge, constitute the lower limb of the Atlantic meridional overturning circulation (AMOC), which carries these cold, deep waters southward to the subtropical region and beyond. It has long been assumed that an increase in deep water formation would result in a larger southward export of newly formed deep water masses. However, recent observations of Lagrangian floats have raised questions about this linkage. Motivated by these observations, the relationship between convective activity in the Labrador Sea and the export of newly formed Labrador Sea Water (LSW), the shallowest component of the deep AMOC, to the subtropics is explored. This study uses simulated Lagrangian pathways of synthetic floats produced with output from a global ocean–sea ice model. It is shown that substantial recirculation of newly formed LSW in the subpolar gyre leads to a relatively small fraction of this water exported to the subtropical gyre: 40 years after release, only 46% of the floats are able to reach the subtropics. Furthermore, waters produced from any one particular convection event are not collectively and contemporaneously exported to the subtropical gyre, such that the waters that are exported to the subtropical gyre have a wide distribution in age.
