Browse
Abstract
This study adopts a curvature dynamics approach to understand and predict the trajectory of an idealized depth-averaged barotropic outflow onto a slope in shallow water. A novel equation for streamwise curvature dynamics was derived from the barotropic vorticity equation and applied to a momentum jet subject to bottom friction, topographic slope, and planetary rotation. The terms in the curvature dynamics equation have a natural geometric interpretation whereby each physical process can influence the flow direction. It is shown that a weakly spreading jet onto a steep slope admits the formulation of a 1D ordinary differential equation system in a streamline coordinate system, yielding an integrable ordinary differential equation system that predicts the kinematical behavior of the jet. The 1D model was compared with a set of high-resolution idealized depth-averaged circulation model simulations where bottom friction, planetary rotation, and bottom slope were varied. Favorable performance of the 1D reduced physics model was found, especially in the near field of the outflow. The effect of nonlinear processes such as topographic stretching and bottom torque on the fate of the jet outflow is explained using curvature dynamics. Even in the tropics, planetary rotation can have a surprisingly strong influence on the near-field deflection of an intermediate-scale jet, provided that it flows across steep topography.
Abstract
This study adopts a curvature dynamics approach to understand and predict the trajectory of an idealized depth-averaged barotropic outflow onto a slope in shallow water. A novel equation for streamwise curvature dynamics was derived from the barotropic vorticity equation and applied to a momentum jet subject to bottom friction, topographic slope, and planetary rotation. The terms in the curvature dynamics equation have a natural geometric interpretation whereby each physical process can influence the flow direction. It is shown that a weakly spreading jet onto a steep slope admits the formulation of a 1D ordinary differential equation system in a streamline coordinate system, yielding an integrable ordinary differential equation system that predicts the kinematical behavior of the jet. The 1D model was compared with a set of high-resolution idealized depth-averaged circulation model simulations where bottom friction, planetary rotation, and bottom slope were varied. Favorable performance of the 1D reduced physics model was found, especially in the near field of the outflow. The effect of nonlinear processes such as topographic stretching and bottom torque on the fate of the jet outflow is explained using curvature dynamics. Even in the tropics, planetary rotation can have a surprisingly strong influence on the near-field deflection of an intermediate-scale jet, provided that it flows across steep topography.
Abstract
The Atlantic meridional overturning circulation (MOC) is traditionally monitored in terms of zonally integrated transport either in depth space or in density space. While this view has the advantage of simplicity, it obscures the rich and complex three-dimensional structure, so that the exact physics of the downwelling and upwelling branch remains poorly understood. The near-equivalence of the depth- and density-space MOC in the subtropics suggests that vertical and diapycnal volumes transports are intimately coupled, whereas the divergence of these two metrics at higher latitudes indicates that any such coupling is neither instantaneous nor local. Previous work has characterized the surface buoyancy forcing and mixing processes which drive diapycnal volume transport. Here, we develop a new analytical decomposition of vertical volume transport based on the vorticity budget. We show that most terms can be estimated from observations and provide additional insights from a high-resolution numerical simulation of the North Atlantic. Our analysis highlights the roles of 1) relative vorticity advection for the sinking of overflow water at the northern subpolar North Atlantic boundaries and 2) the geostrophic β effect for the sinking of dense waters in the intergyre region. These results provide insights into the coupling between density- and depth-space overturning circulations.
Significance Statement
The purpose of this study is to better understand where and why dense water sinks in the North Atlantic. This is important because dense water sinking in the North Atlantic is a crucial component of the global thermohaline circulation. Our results reveal the primary controls on dense water sinking at a regional level and highlight the importance of mesoscale processes at high latitudes in shaping the circulation and heat distribution throughout the Atlantic Ocean.
Abstract
The Atlantic meridional overturning circulation (MOC) is traditionally monitored in terms of zonally integrated transport either in depth space or in density space. While this view has the advantage of simplicity, it obscures the rich and complex three-dimensional structure, so that the exact physics of the downwelling and upwelling branch remains poorly understood. The near-equivalence of the depth- and density-space MOC in the subtropics suggests that vertical and diapycnal volumes transports are intimately coupled, whereas the divergence of these two metrics at higher latitudes indicates that any such coupling is neither instantaneous nor local. Previous work has characterized the surface buoyancy forcing and mixing processes which drive diapycnal volume transport. Here, we develop a new analytical decomposition of vertical volume transport based on the vorticity budget. We show that most terms can be estimated from observations and provide additional insights from a high-resolution numerical simulation of the North Atlantic. Our analysis highlights the roles of 1) relative vorticity advection for the sinking of overflow water at the northern subpolar North Atlantic boundaries and 2) the geostrophic β effect for the sinking of dense waters in the intergyre region. These results provide insights into the coupling between density- and depth-space overturning circulations.
Significance Statement
The purpose of this study is to better understand where and why dense water sinks in the North Atlantic. This is important because dense water sinking in the North Atlantic is a crucial component of the global thermohaline circulation. Our results reveal the primary controls on dense water sinking at a regional level and highlight the importance of mesoscale processes at high latitudes in shaping the circulation and heat distribution throughout the Atlantic Ocean.
Abstract
Measurements collected by a Remote Environmental Monitoring Units (REMUS) 600 autonomous underwater vehicle (AUV) off the coast of southern California demonstrate large-scale coherent wave-driven vortices, consistent with Langmuir turbulence (LT), and played a dominant role in structuring turbulent dissipation within the oceanic surface boundary layer. During a 10-h period with sustained wind speeds of 10 m s−1, Langmuir circulations were limited to the upper third of the surface mixed layer by persistent stratification within the water column. The ensemble-averaged circulation, calculated using conditional averaging of acoustic Doppler dual current profile (AD2CP) velocity profiles using elevated backscattering intensity associated with subsurface bubble clouds, indicates that LT vortex pairs were characterized by an energetic downwelling zone flanked by broader, weaker upwelling regions with vertical velocity magnitudes similar to previous numerical studies of LT. Horizontally distributed microstructure estimates of turbulent kinetic energy dissipation rates were lognormally distributed near the surface in the wave mixing layer with the majority of values falling between wall layer scaling and wave transport layer scaling. Partitioning dissipation rates between downwelling centers and ambient conditions suggests that LT may play a dominant role in elevating dissipation rates in the ocean surface boundary layer (OSBL) by redistributing wave-breaking turbulence.
Abstract
Measurements collected by a Remote Environmental Monitoring Units (REMUS) 600 autonomous underwater vehicle (AUV) off the coast of southern California demonstrate large-scale coherent wave-driven vortices, consistent with Langmuir turbulence (LT), and played a dominant role in structuring turbulent dissipation within the oceanic surface boundary layer. During a 10-h period with sustained wind speeds of 10 m s−1, Langmuir circulations were limited to the upper third of the surface mixed layer by persistent stratification within the water column. The ensemble-averaged circulation, calculated using conditional averaging of acoustic Doppler dual current profile (AD2CP) velocity profiles using elevated backscattering intensity associated with subsurface bubble clouds, indicates that LT vortex pairs were characterized by an energetic downwelling zone flanked by broader, weaker upwelling regions with vertical velocity magnitudes similar to previous numerical studies of LT. Horizontally distributed microstructure estimates of turbulent kinetic energy dissipation rates were lognormally distributed near the surface in the wave mixing layer with the majority of values falling between wall layer scaling and wave transport layer scaling. Partitioning dissipation rates between downwelling centers and ambient conditions suggests that LT may play a dominant role in elevating dissipation rates in the ocean surface boundary layer (OSBL) by redistributing wave-breaking turbulence.
Abstract
The solution from linear theory for the barotropic-to-baroclinic tidal energy conversion into vertical modes is validated with numerical simulations and analytical results. The main result is the translation of the traditional critical slope condition into a modewise condition on the topographic height only. Our findings are then used for estimates of the global M2 tidal conversion into the first 10 vertical modes in the open ocean (excluding the continental shelves and slopes). We observe a rapid increase with mode number of the fraction of the World Ocean where linear theory is invalid. In terms of conversion, which is highly variable in space, this corresponds to an even more rapid increase with mode number of the fraction of the converted energy that is strongly affected by nonlinear effects. Out of the 373.6 GW of the globally integrated conversion into modes 1–10, only 241.7 GW occur in locations where linear theory is valid. While it represents 95% for mode 1, this fraction rapidly drops with mode number to reach 27% for mode 10. Moreover, for the conversion into a single mode, we show that capping the linear solution at supercritical topography is inappropriate. Hence, linear theory appears unfit to directly quantify the role played by high-mode internal tides in the internal wave energy budget.
Abstract
The solution from linear theory for the barotropic-to-baroclinic tidal energy conversion into vertical modes is validated with numerical simulations and analytical results. The main result is the translation of the traditional critical slope condition into a modewise condition on the topographic height only. Our findings are then used for estimates of the global M2 tidal conversion into the first 10 vertical modes in the open ocean (excluding the continental shelves and slopes). We observe a rapid increase with mode number of the fraction of the World Ocean where linear theory is invalid. In terms of conversion, which is highly variable in space, this corresponds to an even more rapid increase with mode number of the fraction of the converted energy that is strongly affected by nonlinear effects. Out of the 373.6 GW of the globally integrated conversion into modes 1–10, only 241.7 GW occur in locations where linear theory is valid. While it represents 95% for mode 1, this fraction rapidly drops with mode number to reach 27% for mode 10. Moreover, for the conversion into a single mode, we show that capping the linear solution at supercritical topography is inappropriate. Hence, linear theory appears unfit to directly quantify the role played by high-mode internal tides in the internal wave energy budget.
Abstract
A large part of the variability in the Atlantic meridional overturning circulation (AMOC) and thus uncertainty in its estimates on interannual time scales comes from atmospheric synoptic eddies and mesoscale processes. In this study, a suite of experiments with a 1/12° regional configuration of the MITgcm is performed where low-pass filtering is applied to surface wind forcing to investigate the impact of subsynoptic (<2 days) and synoptic (2–10 days) atmospheric processes on the ocean circulation. Changes in the wind magnitude and hence the wind energy input in the region have a significant effect on the strength of the overturning; once this is accounted for, the magnitude of the overturning in all sensitivity experiments is very similar to that of the control run. Synoptic and subsynoptic variability in atmospheric winds reduce the surface heat loss in the Labrador Sea, resulting in anomalous advection of warm and salty waters into the Irminger Sea and lower upper-ocean densities in the eastern subpolar North Atlantic. Other effects of high-frequency variability in surface winds on the AMOC are associated with changes in Ekman convergence in the midlatitudes. Synoptic and subsynoptic winds also impact the strength of the boundary currents and density structure in the subpolar North Atlantic. In the Labrador Sea, the overturning strength is more sensitive to the changes in density structure, whereas in the eastern subpolar North Atlantic, the role of density is comparable to that of the strength of the East Greenland Current.
Significance Statement
A key issue in understanding how well the Atlantic meridional overturning circulation is simulated in climate models is determining the impact of synoptic (2–10 days) and subsynoptic (shorter) wind variability on ocean circulation. We find that the greatest impact of wind changes on the strength of the overturning is through changes in energy input from winds to the ocean. Variations in winds have a more modest impact via changes in heat loss over the Labrador Sea, alongside changes in wind-driven surface currents. This study highlights the importance of accurately representing the density in the Labrador Sea, and both the strength and density structure of the East Greenland Current, for the correct representation of overturning circulation in climate models.
Abstract
A large part of the variability in the Atlantic meridional overturning circulation (AMOC) and thus uncertainty in its estimates on interannual time scales comes from atmospheric synoptic eddies and mesoscale processes. In this study, a suite of experiments with a 1/12° regional configuration of the MITgcm is performed where low-pass filtering is applied to surface wind forcing to investigate the impact of subsynoptic (<2 days) and synoptic (2–10 days) atmospheric processes on the ocean circulation. Changes in the wind magnitude and hence the wind energy input in the region have a significant effect on the strength of the overturning; once this is accounted for, the magnitude of the overturning in all sensitivity experiments is very similar to that of the control run. Synoptic and subsynoptic variability in atmospheric winds reduce the surface heat loss in the Labrador Sea, resulting in anomalous advection of warm and salty waters into the Irminger Sea and lower upper-ocean densities in the eastern subpolar North Atlantic. Other effects of high-frequency variability in surface winds on the AMOC are associated with changes in Ekman convergence in the midlatitudes. Synoptic and subsynoptic winds also impact the strength of the boundary currents and density structure in the subpolar North Atlantic. In the Labrador Sea, the overturning strength is more sensitive to the changes in density structure, whereas in the eastern subpolar North Atlantic, the role of density is comparable to that of the strength of the East Greenland Current.
Significance Statement
A key issue in understanding how well the Atlantic meridional overturning circulation is simulated in climate models is determining the impact of synoptic (2–10 days) and subsynoptic (shorter) wind variability on ocean circulation. We find that the greatest impact of wind changes on the strength of the overturning is through changes in energy input from winds to the ocean. Variations in winds have a more modest impact via changes in heat loss over the Labrador Sea, alongside changes in wind-driven surface currents. This study highlights the importance of accurately representing the density in the Labrador Sea, and both the strength and density structure of the East Greenland Current, for the correct representation of overturning circulation in climate models.
Abstract
We report novel observations of the onset and growth of Langmuir circulations (LCs) from simultaneous airborne and subsurface in situ measurements. Under weak, fetch-limited wind–wave forcing with stabilizing buoyancy forcing, the onset of LCs is observed for wind speeds greater than about 1 m s−1. LCs appear nonuniformly in space, consistent with previous laboratory experiments and suggestive of coupled wave–turbulence interaction. Following an increase in wind speed from <1 m s−1 to sustained 3 m s−1 winds, a shallow (<0.7 m) diurnal warm layer is observed to deepen at 1 m h−1, while the cross-cell scales of LCs grow at 2 m h−1, as observed in sea surface temperature collected from a research aircraft. Subsurface temperature structures show temperature intrusions into the base of the diurnal warm layer of the same scale as bubble entrainment depth during the deepening period and are comparable to temperature structures observed during strong wind forcing with a deep mixed layer that is representative of previous LC studies. We show that an LES run with observed initial conditions and forcing is able to reproduce the onset and rate of boundary layer deepening. The surface temperature expression however is significantly different from observations, and the model exhibits large sensitivity to the numerical representation of surface radiative heating. These novel observations of Langmuir circulations offer a benchmark for further improvement of numerical models.
Significance Statement
The purpose of this study is to better understand the structure and dynamics of Langmuir circulations (LCs), coherent turbulent vortices in the surface ocean. Using observations of the ocean surface boundary layer from aircraft and autonomous instruments, we show the onset and growth of LCs. We compare the observations to a numerical model and find that while the model can reproduce the deepening of a shallow surface warm layer, the representation of coherent vortices differs from observations. Future studies can improve on the numerical representation of coherent upper ocean structures which are important to modeling upper ocean turbulence, air–sea exchanges, biology, ocean acoustics, and the distribution of anthropogenic pollutants like oil and microplastics.
Abstract
We report novel observations of the onset and growth of Langmuir circulations (LCs) from simultaneous airborne and subsurface in situ measurements. Under weak, fetch-limited wind–wave forcing with stabilizing buoyancy forcing, the onset of LCs is observed for wind speeds greater than about 1 m s−1. LCs appear nonuniformly in space, consistent with previous laboratory experiments and suggestive of coupled wave–turbulence interaction. Following an increase in wind speed from <1 m s−1 to sustained 3 m s−1 winds, a shallow (<0.7 m) diurnal warm layer is observed to deepen at 1 m h−1, while the cross-cell scales of LCs grow at 2 m h−1, as observed in sea surface temperature collected from a research aircraft. Subsurface temperature structures show temperature intrusions into the base of the diurnal warm layer of the same scale as bubble entrainment depth during the deepening period and are comparable to temperature structures observed during strong wind forcing with a deep mixed layer that is representative of previous LC studies. We show that an LES run with observed initial conditions and forcing is able to reproduce the onset and rate of boundary layer deepening. The surface temperature expression however is significantly different from observations, and the model exhibits large sensitivity to the numerical representation of surface radiative heating. These novel observations of Langmuir circulations offer a benchmark for further improvement of numerical models.
Significance Statement
The purpose of this study is to better understand the structure and dynamics of Langmuir circulations (LCs), coherent turbulent vortices in the surface ocean. Using observations of the ocean surface boundary layer from aircraft and autonomous instruments, we show the onset and growth of LCs. We compare the observations to a numerical model and find that while the model can reproduce the deepening of a shallow surface warm layer, the representation of coherent vortices differs from observations. Future studies can improve on the numerical representation of coherent upper ocean structures which are important to modeling upper ocean turbulence, air–sea exchanges, biology, ocean acoustics, and the distribution of anthropogenic pollutants like oil and microplastics.
Abstract
The transport of warm deep water (WDW) onto the Weddell Sea continental shelf is associated with heat flux and strongly contributes to the melting of Antarctic ice shelves. The small radius of deformation at high latitudes makes it difficult to accurately represent the eddy-driven component of onshore WDW transport in coarse-resolution ocean models so that parameterization becomes necessary. The Gent and McWilliams/Redi (GM/Redi) scheme was designed to parameterize mesoscale eddies in the open ocean. Here, it is assessed to what extent the GM/Redi scheme can generate a realistic transport of WDW across the Weddell Sea continental slope. To this end, the eddy parameterization is applied to a coarse-resolution idealized model of the Weddell Sea continental shelf and slope, and its performance is evaluated against a high-resolution reference simulation. With the GM/Redi parameterization applied, the coarse model simulates a shoreward WDW transport with a heat transport that matches the high-resolution reference and both the hydrographic mean fields and the mean slopes of the isopycnals are improved. A successful application of the GM/Redi parameterization is only possible by reducing the GM diffusivity over the continental slope by an order of magnitude compared to the open ocean value to account for the eddy-suppressing effect of the topographic slope. When the influence of topography on the GM diffusivity is neglected, the coarse model with the parameterization either under- or overestimates the shoreward heat flux. These results motivate the incorporation of slope-aware eddy parameterizations into regional and global ocean models.
Significance Statement
Mesoscale eddies drive warm water across the continental slope and onto the continental shelf of the Weddell Sea, where it melts the adjacent Antarctic ice shelves. This process is not resolved in ocean models employing a coarse horizontal resolution akin to state-of-the-art climate models. This work addresses this issue by modifying and applying a well-established eddy parameterization to this specific case. The parameterization works particularly well when it accounts for the effect of sloping topography, over which eddy transports are weaker. We expect this modification also to be of benefit to regional and global models.
Abstract
The transport of warm deep water (WDW) onto the Weddell Sea continental shelf is associated with heat flux and strongly contributes to the melting of Antarctic ice shelves. The small radius of deformation at high latitudes makes it difficult to accurately represent the eddy-driven component of onshore WDW transport in coarse-resolution ocean models so that parameterization becomes necessary. The Gent and McWilliams/Redi (GM/Redi) scheme was designed to parameterize mesoscale eddies in the open ocean. Here, it is assessed to what extent the GM/Redi scheme can generate a realistic transport of WDW across the Weddell Sea continental slope. To this end, the eddy parameterization is applied to a coarse-resolution idealized model of the Weddell Sea continental shelf and slope, and its performance is evaluated against a high-resolution reference simulation. With the GM/Redi parameterization applied, the coarse model simulates a shoreward WDW transport with a heat transport that matches the high-resolution reference and both the hydrographic mean fields and the mean slopes of the isopycnals are improved. A successful application of the GM/Redi parameterization is only possible by reducing the GM diffusivity over the continental slope by an order of magnitude compared to the open ocean value to account for the eddy-suppressing effect of the topographic slope. When the influence of topography on the GM diffusivity is neglected, the coarse model with the parameterization either under- or overestimates the shoreward heat flux. These results motivate the incorporation of slope-aware eddy parameterizations into regional and global ocean models.
Significance Statement
Mesoscale eddies drive warm water across the continental slope and onto the continental shelf of the Weddell Sea, where it melts the adjacent Antarctic ice shelves. This process is not resolved in ocean models employing a coarse horizontal resolution akin to state-of-the-art climate models. This work addresses this issue by modifying and applying a well-established eddy parameterization to this specific case. The parameterization works particularly well when it accounts for the effect of sloping topography, over which eddy transports are weaker. We expect this modification also to be of benefit to regional and global models.
Abstract
Being situated in the estuary of the flood-dominated Hooghly River system, the macrotidal Indian Sundarban Delta (ISD) has become one of the most complex, dynamic, and rapidly changing landforms on Earth’s surface. To study the horizontal areal shifting of shoreline and its impact on mangrove cover in the region, the U.S. Geological Survey (USGS) satellite data of 1980, 1990, 2000, 2010, and 2021 were used. Remote sensing and geographic information system (GIS) techniques were employed in the investigation. Simultaneous prograding and retrograding shoreline shifting was distinguished almost in all the parts, although sediment-starved eastern and macrotidally more active southern lobes experienced dominantly retreating shift, and the sediment-engorged western lobe was demonstrated to be more dynamic. Net areal change over north–south tracks followed the trend of decreasing accretion to increasing erosion while going from west to east, whereas that over west–east tracks followed the trend of exponentially increasing erosion while going from north to south. Overall accretion of ∼91 km2 in the ISD accounted for the augmentation of sparse vegetation of ∼13 km2, whereas ∼243 km2 erosion called for the depletion of sparse and moderate vegetation of ∼18 and ∼174 km2, respectively, over the 41-yr period. Various oceanographic and riparian forces and actions, episodic natural events, etc., vis-a-vis several anthropogenic interventions—all together contributed to such changes. The findings may help the coastal environmentalists, professionals, planners, decision-makers, and implementers in formulating and taking up of suitable strategic measures for integrated and effective coastal zone management in this estuarine wetland forest.
Abstract
Being situated in the estuary of the flood-dominated Hooghly River system, the macrotidal Indian Sundarban Delta (ISD) has become one of the most complex, dynamic, and rapidly changing landforms on Earth’s surface. To study the horizontal areal shifting of shoreline and its impact on mangrove cover in the region, the U.S. Geological Survey (USGS) satellite data of 1980, 1990, 2000, 2010, and 2021 were used. Remote sensing and geographic information system (GIS) techniques were employed in the investigation. Simultaneous prograding and retrograding shoreline shifting was distinguished almost in all the parts, although sediment-starved eastern and macrotidally more active southern lobes experienced dominantly retreating shift, and the sediment-engorged western lobe was demonstrated to be more dynamic. Net areal change over north–south tracks followed the trend of decreasing accretion to increasing erosion while going from west to east, whereas that over west–east tracks followed the trend of exponentially increasing erosion while going from north to south. Overall accretion of ∼91 km2 in the ISD accounted for the augmentation of sparse vegetation of ∼13 km2, whereas ∼243 km2 erosion called for the depletion of sparse and moderate vegetation of ∼18 and ∼174 km2, respectively, over the 41-yr period. Various oceanographic and riparian forces and actions, episodic natural events, etc., vis-a-vis several anthropogenic interventions—all together contributed to such changes. The findings may help the coastal environmentalists, professionals, planners, decision-makers, and implementers in formulating and taking up of suitable strategic measures for integrated and effective coastal zone management in this estuarine wetland forest.
