Browse
Abstract
Submesoscale baroclinic instabilities have been shown to restratify the surface mixed layer and to seasonally energize submesoscale turbulence in the upper ocean. But do these instabilities also affect the large-scale circulation and stratification of the upper thermocline? This question is addressed for the North Atlantic Subtropical Mode Water region with a series of numerical simulations at varying horizontal grid spacings (16, 8, 4, and 2 km). These simulations are realistically forced and integrated long enough for the thermocline to adjust to the presence or absence of submesoscales. Linear stability analysis indicates that a 2-km grid spacing is sufficient to resolve the most unstable mode of the wintertime mixed layer instability. As the resolution is increased, spectral slopes of horizontal kinetic energy flatten and vertical velocities increase in magnitude, consistent with previous regional and short-time simulations. The equilibrium stratification of the thermocline changes drastically as the grid spacing is refined from 16 to 8 km and mesoscale eddies are fully resolved. The thermocline stratification remains largely unchanged, however, between the 8-, 4-, and 2-km runs. This robustness is argued to arise from a mesoscale constraint on the buoyancy variance budget. Once mesoscale processes are resolved, the rate of mesoscale variance production is largely fixed. This constrains the variance destruction by submesoscale vertical buoyancy fluxes, which thus remain invariant across resolutions. The bulk impact of mixed layer instabilities on upper-ocean stratification in the Subtropical Mode Water region through an enhanced vertical buoyancy flux is therefore captured at 8-km grid spacing, even though the instabilities are severely underresolved.
Abstract
Submesoscale baroclinic instabilities have been shown to restratify the surface mixed layer and to seasonally energize submesoscale turbulence in the upper ocean. But do these instabilities also affect the large-scale circulation and stratification of the upper thermocline? This question is addressed for the North Atlantic Subtropical Mode Water region with a series of numerical simulations at varying horizontal grid spacings (16, 8, 4, and 2 km). These simulations are realistically forced and integrated long enough for the thermocline to adjust to the presence or absence of submesoscales. Linear stability analysis indicates that a 2-km grid spacing is sufficient to resolve the most unstable mode of the wintertime mixed layer instability. As the resolution is increased, spectral slopes of horizontal kinetic energy flatten and vertical velocities increase in magnitude, consistent with previous regional and short-time simulations. The equilibrium stratification of the thermocline changes drastically as the grid spacing is refined from 16 to 8 km and mesoscale eddies are fully resolved. The thermocline stratification remains largely unchanged, however, between the 8-, 4-, and 2-km runs. This robustness is argued to arise from a mesoscale constraint on the buoyancy variance budget. Once mesoscale processes are resolved, the rate of mesoscale variance production is largely fixed. This constrains the variance destruction by submesoscale vertical buoyancy fluxes, which thus remain invariant across resolutions. The bulk impact of mixed layer instabilities on upper-ocean stratification in the Subtropical Mode Water region through an enhanced vertical buoyancy flux is therefore captured at 8-km grid spacing, even though the instabilities are severely underresolved.
Abstract
In addition to their well-known seasonal cycle, eastern boundary upwelling systems (EBUS) undergo modulation on shorter synoptic to intraseasonal time scales. Energetic intensifications and relaxations of upwelling-favorable winds with 5–10-day typical time scales can impact the EBUS dynamics and biogeochemical functioning. In this work the dynamical effects of wind-forced synoptic fluctuations on the South Senegalese Upwelling Sector (SSUS) are characterized. The region geomorphology is unique with its wide continental shelf and a major coastline discontinuity at its northern edge. The ocean response to synoptic events is explored using a modeling framework that involves applying idealized synoptic wind intensification or relaxation to a five-member climatological SSUS ensemble run. Model evaluation against sparse midshelf in situ observations indicates qualitative agreement in terms of synoptic variability of temperature, stratification, and ocean currents, despite a moderate but systematic bias in current intensity. Modeled synoptic wind and heat flux fluctuations produce clear modulations of all dynamical variables with robust SSUS-scale and mesoscale spatial patterns. A mixed layer heat budget analysis is performed over the continental shelf to uncover the dominant processes involved in SSUS synoptic variability. Modulations of horizontal advection and atmospheric forcing are the leading-order drivers of heat changes during either wind intensification or relaxation while vertical dynamics is of primary importance only in a very localized area. Also, modest asymmetries in the oceanic responses to upwelling intensification and relaxation are only identified for meridional velocities. This brings partial support to the hypothesis that synoptic variability has a modest net effect on the climatological state and functioning of upwelling systems dynamics.
Abstract
In addition to their well-known seasonal cycle, eastern boundary upwelling systems (EBUS) undergo modulation on shorter synoptic to intraseasonal time scales. Energetic intensifications and relaxations of upwelling-favorable winds with 5–10-day typical time scales can impact the EBUS dynamics and biogeochemical functioning. In this work the dynamical effects of wind-forced synoptic fluctuations on the South Senegalese Upwelling Sector (SSUS) are characterized. The region geomorphology is unique with its wide continental shelf and a major coastline discontinuity at its northern edge. The ocean response to synoptic events is explored using a modeling framework that involves applying idealized synoptic wind intensification or relaxation to a five-member climatological SSUS ensemble run. Model evaluation against sparse midshelf in situ observations indicates qualitative agreement in terms of synoptic variability of temperature, stratification, and ocean currents, despite a moderate but systematic bias in current intensity. Modeled synoptic wind and heat flux fluctuations produce clear modulations of all dynamical variables with robust SSUS-scale and mesoscale spatial patterns. A mixed layer heat budget analysis is performed over the continental shelf to uncover the dominant processes involved in SSUS synoptic variability. Modulations of horizontal advection and atmospheric forcing are the leading-order drivers of heat changes during either wind intensification or relaxation while vertical dynamics is of primary importance only in a very localized area. Also, modest asymmetries in the oceanic responses to upwelling intensification and relaxation are only identified for meridional velocities. This brings partial support to the hypothesis that synoptic variability has a modest net effect on the climatological state and functioning of upwelling systems dynamics.
Abstract
Eddies in the northwestern tropical Atlantic Ocean play a crucial role in transporting the South Atlantic Upper Ocean Water to the North Atlantic and connect the Atlantic and the Caribbean Sea. Although surface characteristics of those eddies have been well studied, their vertical structures and governing mechanisms are much less known. Here, using a time-dependent energetics framework based on the multiscale window transform, we examine the seasonal variability of the eddy kinetic energy (EKE) in the northwestern tropical Atlantic. Both altimeter-based data and ocean reanalyses show a substantial EKE seasonal cycle in the North Brazil Current Retroflection (NBCR) region that is mostly trapped in the upper 200 m. In the most energetic NBCR region, the EKE reaches its minimum in April–June and maximum in July–September. By analyzing six ocean reanalysis products, we find that barotropic instability is the controlling mechanism for the seasonal eddy variability in the NBCR region. Nonlocal processes, including advection and pressure work, play opposite roles in the EKE seasonal cycle. In the eastern part of the NBCR region, the EKE seasonal evolution is similar to the NBCR region. However, it is the nonlocal processes that control the EKE seasonality. In the western part of the NBCR region, the EKE magnitude is one order of magnitude smaller than in the NBCR region and shows a different seasonal cycle, which peaks in March and reaches its minimum in October–November. Our results highlight the complex mechanisms governing eddy variability in the northwestern tropical Atlantic and provide insights into their potential changes with changing background conditions.
Abstract
Eddies in the northwestern tropical Atlantic Ocean play a crucial role in transporting the South Atlantic Upper Ocean Water to the North Atlantic and connect the Atlantic and the Caribbean Sea. Although surface characteristics of those eddies have been well studied, their vertical structures and governing mechanisms are much less known. Here, using a time-dependent energetics framework based on the multiscale window transform, we examine the seasonal variability of the eddy kinetic energy (EKE) in the northwestern tropical Atlantic. Both altimeter-based data and ocean reanalyses show a substantial EKE seasonal cycle in the North Brazil Current Retroflection (NBCR) region that is mostly trapped in the upper 200 m. In the most energetic NBCR region, the EKE reaches its minimum in April–June and maximum in July–September. By analyzing six ocean reanalysis products, we find that barotropic instability is the controlling mechanism for the seasonal eddy variability in the NBCR region. Nonlocal processes, including advection and pressure work, play opposite roles in the EKE seasonal cycle. In the eastern part of the NBCR region, the EKE seasonal evolution is similar to the NBCR region. However, it is the nonlocal processes that control the EKE seasonality. In the western part of the NBCR region, the EKE magnitude is one order of magnitude smaller than in the NBCR region and shows a different seasonal cycle, which peaks in March and reaches its minimum in October–November. Our results highlight the complex mechanisms governing eddy variability in the northwestern tropical Atlantic and provide insights into their potential changes with changing background conditions.
Abstract
The Agulhas Current (AC) is a critical component of global ocean circulation. However, due to a lack of multidecadal observations, it is not clear how the AC has changed in response to anthropogenic forcing. A recent observational study suggests a broadening and slight weakening of the AC in the past few decades, while others suggest a strengthening of the AC during the historical period. In this paper, we find substantial internal variability of the AC on decadal to multidecadal time scales in high-resolution models. We show that the AC consistently exhibits two modes of decadal and multidecadal (i.e., low frequency) variability in a series of high-resolution climate models: a uniform mode that is largely associated with changes in the AC strength and a dipole mode that is mainly related to width changes of the AC. We demonstrate that the uniform mode is mainly forced externally by the decadal variations of the wind field and presents a decline under global warming, suggesting a weakening of the AC in response to anthropogenic forcing. The dipole mode, on the other hand, is mainly due to internal dynamics and does not show a trend during the historical period. Using a quasigeostrophic model that captures the dipole mode, we attribute the dipole mode to low-frequency potential vorticity changes in the western boundary, driven by a divergence of relative potential vorticity due to eddy activity. Thus, our results present further context for the interpretation of the AC responses in a changing climate based on a short observational record.
Abstract
The Agulhas Current (AC) is a critical component of global ocean circulation. However, due to a lack of multidecadal observations, it is not clear how the AC has changed in response to anthropogenic forcing. A recent observational study suggests a broadening and slight weakening of the AC in the past few decades, while others suggest a strengthening of the AC during the historical period. In this paper, we find substantial internal variability of the AC on decadal to multidecadal time scales in high-resolution models. We show that the AC consistently exhibits two modes of decadal and multidecadal (i.e., low frequency) variability in a series of high-resolution climate models: a uniform mode that is largely associated with changes in the AC strength and a dipole mode that is mainly related to width changes of the AC. We demonstrate that the uniform mode is mainly forced externally by the decadal variations of the wind field and presents a decline under global warming, suggesting a weakening of the AC in response to anthropogenic forcing. The dipole mode, on the other hand, is mainly due to internal dynamics and does not show a trend during the historical period. Using a quasigeostrophic model that captures the dipole mode, we attribute the dipole mode to low-frequency potential vorticity changes in the western boundary, driven by a divergence of relative potential vorticity due to eddy activity. Thus, our results present further context for the interpretation of the AC responses in a changing climate based on a short observational record.
Abstract
Internal solitary waves in the ocean are characterized by the surface roughness signature of smooth and rough bands that are observable in synthetic aperture radar satellite imagery, which is caused by the interaction between surface gravity waves and internal wave–induced surface currents. In this work, we study the surface signature of an internal wave packet in deep water over a large range of spatial scales using an improved wave–current interaction model that supports a moving surface current corresponding to a propagating internal gravity wave. After validating the model by comparison to previously published numerical results by Hao and Shen, we investigate a realistic case based on a recent comprehensive field campaign conducted by Lenain and Pizzo. Distinct surface manifestation caused by internal waves can be directly observed from the surface waves and the associated surface wave steepness. Consistent with observations, the surface is relatively rough where the internal wave–induced surface current is convergent (∂U/∂x < 0), while it is relatively smooth where the surface current is divergent (∂U/∂x > 0). The spatial modulation of the surface wave spectrum is rapid as a function of along-propagation distance, showing a remarkable redistribution of energy under the influence of the propagating internal wave packet. The directional wavenumber spectra computed in the smooth and rough regions show that the directional properties of the surface wave spectra are also rapidly modulated through strong wave–current interactions. Good agreement is found between the model results and the field observations, demonstrating the robustness of the present model in studying the impact of internal waves on surface gravity waves.
Significance Statement
The purpose of this study is to better understand the physical processes leading to the bands of rough and smooth surface waves arising from internal gravity waves. The surface manifestation of internal gravity waves allows them to be measured remotely via surface imagery, which can provide insight into their nonlinear behavior and sources and fate and which can ultimately inform the local stratification for assimilation into larger-scale models. Our results highlight the application of wave–current interaction models to the study of the interaction of surface waves with internal gravity waves and indicate strong modulation of the surface wave spectra over relatively short time scales despite the long time scales associated with the internal wave propagation.
Abstract
Internal solitary waves in the ocean are characterized by the surface roughness signature of smooth and rough bands that are observable in synthetic aperture radar satellite imagery, which is caused by the interaction between surface gravity waves and internal wave–induced surface currents. In this work, we study the surface signature of an internal wave packet in deep water over a large range of spatial scales using an improved wave–current interaction model that supports a moving surface current corresponding to a propagating internal gravity wave. After validating the model by comparison to previously published numerical results by Hao and Shen, we investigate a realistic case based on a recent comprehensive field campaign conducted by Lenain and Pizzo. Distinct surface manifestation caused by internal waves can be directly observed from the surface waves and the associated surface wave steepness. Consistent with observations, the surface is relatively rough where the internal wave–induced surface current is convergent (∂U/∂x < 0), while it is relatively smooth where the surface current is divergent (∂U/∂x > 0). The spatial modulation of the surface wave spectrum is rapid as a function of along-propagation distance, showing a remarkable redistribution of energy under the influence of the propagating internal wave packet. The directional wavenumber spectra computed in the smooth and rough regions show that the directional properties of the surface wave spectra are also rapidly modulated through strong wave–current interactions. Good agreement is found between the model results and the field observations, demonstrating the robustness of the present model in studying the impact of internal waves on surface gravity waves.
Significance Statement
The purpose of this study is to better understand the physical processes leading to the bands of rough and smooth surface waves arising from internal gravity waves. The surface manifestation of internal gravity waves allows them to be measured remotely via surface imagery, which can provide insight into their nonlinear behavior and sources and fate and which can ultimately inform the local stratification for assimilation into larger-scale models. Our results highlight the application of wave–current interaction models to the study of the interaction of surface waves with internal gravity waves and indicate strong modulation of the surface wave spectra over relatively short time scales despite the long time scales associated with the internal wave propagation.
Abstract
Western boundary currents (WBCs) under no-slip boundary conditions tend to separate from the coast prematurely (without reaching the intergyre boundary) and form eastward jets. This study theoretically considers the meridional structure and location of a prematurely separated WBC extension jet using a two-layer quasigeostrophic model. Assuming homogenized potential vorticity (PV) regions on both sides of and below the jet, we constructed a simple model for the meridional profiles of the zonal flows in the western subtropical gyre. This work clarifies that the meridional structure can be determined if two variables, such as the strength of the PV front (difference in PV across the jet) and the value of the streamfunction at the jet’s center, are given in addition to the meridional profile of the Sverdrup zonal flow and the vertical stratification. The zonal velocity profiles in both layers agreed well with those obtained by numerical experiments. When the jet is close to the intergyre boundary, the meridional location of the jet depends only on the front’s strength. When the northern recirculation gyre is detached from the intergyre boundary and the local wind effect on the jet is negligible, comparisons with the numerical experiments suggest that the jet’s central streamline connects to the central streamline of the eastward Sverdrup flow. We also found that a downward Ekman pumping velocity shifts the jet southward.
Abstract
Western boundary currents (WBCs) under no-slip boundary conditions tend to separate from the coast prematurely (without reaching the intergyre boundary) and form eastward jets. This study theoretically considers the meridional structure and location of a prematurely separated WBC extension jet using a two-layer quasigeostrophic model. Assuming homogenized potential vorticity (PV) regions on both sides of and below the jet, we constructed a simple model for the meridional profiles of the zonal flows in the western subtropical gyre. This work clarifies that the meridional structure can be determined if two variables, such as the strength of the PV front (difference in PV across the jet) and the value of the streamfunction at the jet’s center, are given in addition to the meridional profile of the Sverdrup zonal flow and the vertical stratification. The zonal velocity profiles in both layers agreed well with those obtained by numerical experiments. When the jet is close to the intergyre boundary, the meridional location of the jet depends only on the front’s strength. When the northern recirculation gyre is detached from the intergyre boundary and the local wind effect on the jet is negligible, comparisons with the numerical experiments suggest that the jet’s central streamline connects to the central streamline of the eastward Sverdrup flow. We also found that a downward Ekman pumping velocity shifts the jet southward.
Abstract
We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone-trapped river plume. We quantify mixing based on the rate of change of salinity DS/Dt in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kz = (2.2 ± 0.6) × 10−3 m2 s−1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1 × 10−2 m2 s−1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage, or offshore wave height; as a result, we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions.
Significance Statement
River outflows are important sources of pollutants, sediment, and nutrients to the coastal ocean. Small rivers often meet large breaking waves in the surf zone close to shore, trapping river water and river-borne material near the beach. Such trapped material can influence coastal public health, beach morphology, and nearshore ecology. This study investigates how trapped fresh river water mixes with salty ocean water in the presence of large breaking waves by using high-resolution measurements of waves, salinity, and turbulence. We find that the surf zone is often fresh and stratified, which could have significant implications for the fate of riverine material. Wave breaking provides a constant source of turbulence, and the amount of mixing is limited by the degree of vertical salt stratification; more mixing occurs when stratification is higher.
Abstract
We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone-trapped river plume. We quantify mixing based on the rate of change of salinity DS/Dt in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kz = (2.2 ± 0.6) × 10−3 m2 s−1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1 × 10−2 m2 s−1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage, or offshore wave height; as a result, we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions.
Significance Statement
River outflows are important sources of pollutants, sediment, and nutrients to the coastal ocean. Small rivers often meet large breaking waves in the surf zone close to shore, trapping river water and river-borne material near the beach. Such trapped material can influence coastal public health, beach morphology, and nearshore ecology. This study investigates how trapped fresh river water mixes with salty ocean water in the presence of large breaking waves by using high-resolution measurements of waves, salinity, and turbulence. We find that the surf zone is often fresh and stratified, which could have significant implications for the fate of riverine material. Wave breaking provides a constant source of turbulence, and the amount of mixing is limited by the degree of vertical salt stratification; more mixing occurs when stratification is higher.
Abstract
The influence of a large-scale circulation (LSC) in a marginal sea on a hysteresis western boundary current (WBC) flowing across a gap is studied using a nonlinear 1.5-layer ocean model. Results show that both single-gyre LSC and double-gyre LSC are able to induce the critical-state WBC transition from the eddy-shedding regime to the leaping regime, while only double-gyre LSC is able to induce the critical-state WBC transition from the leaping regime to the eddy-shedding regime. The dynamics of WBC transition suggests that the meridional advection enhanced by the perturbation of the LSC is responsible for the regime shift from penetration to leap and that the meridional advection reduced by the perturbation of the LSC is responsible for the regime shift from leap to penetration. We also present the parameter space of the critical LSC that can induce the regime shift of WBC far away from the critical state. When the WBC is in the eddy-shedding regime, the critical strength of the single-gyre LSC increases as the WBC transport decreases regardless of the island’s presence in the gap. The critical strength of the double-gyre LSC increases as the WBC transport decreases in the no-island case, while the critical parameter increases as the WBC transport at first decreases and then increases in the island case. When the WBC is in the leaping regime, the critical strength of the double-gyre LSC increases as the WBC transport increases. These results help to explain the observed fact that the Kuroshio flows across the Luzon Strait in the leaping regime or the penetrating regime.
Abstract
The influence of a large-scale circulation (LSC) in a marginal sea on a hysteresis western boundary current (WBC) flowing across a gap is studied using a nonlinear 1.5-layer ocean model. Results show that both single-gyre LSC and double-gyre LSC are able to induce the critical-state WBC transition from the eddy-shedding regime to the leaping regime, while only double-gyre LSC is able to induce the critical-state WBC transition from the leaping regime to the eddy-shedding regime. The dynamics of WBC transition suggests that the meridional advection enhanced by the perturbation of the LSC is responsible for the regime shift from penetration to leap and that the meridional advection reduced by the perturbation of the LSC is responsible for the regime shift from leap to penetration. We also present the parameter space of the critical LSC that can induce the regime shift of WBC far away from the critical state. When the WBC is in the eddy-shedding regime, the critical strength of the single-gyre LSC increases as the WBC transport decreases regardless of the island’s presence in the gap. The critical strength of the double-gyre LSC increases as the WBC transport decreases in the no-island case, while the critical parameter increases as the WBC transport at first decreases and then increases in the island case. When the WBC is in the leaping regime, the critical strength of the double-gyre LSC increases as the WBC transport increases. These results help to explain the observed fact that the Kuroshio flows across the Luzon Strait in the leaping regime or the penetrating regime.
Abstract
Ocean bottom pressure pB is an important oceanic variable that is dynamically related to the abyssal ocean circulation through geostrophy. In this study we examine the seasonal pB variability in the North Pacific Ocean by analyzing satellite gravimetric observations from the GRACE program and a data-assimilated ocean-state estimate from ECCOv4. The seasonal pB variability is characterized by alternations of low and high anomalies among three regions, the subpolar and subtropical basins as well as the equatorial region. A linear two-layer wind-driven model is used to examine forcing mechanisms and topographic effects on seasonal pB variations. The model control run, which uses a realistic topography, is able to simulate a basinwide seasonal pB variability that is remarkably similar to that from GRACE and ECCOv4. Since the model is driven by wind stress alone, the good model–data agreement indicates that wind stress is the leading forcing for seasonal changes in pB . An additional model simulation was conducted by setting the water depth uniformly at 5000 m. The magnitude of seasonal pB anomaly is amplified significantly in the flat-bottom simulation as compared with that in the control run. The difference can be explained in terms of the topographic Sverdrup balance. In addition, the spatial pattern of the seasonal pB variability is also profoundly affected by topography especially on continental margins, ridges, and trenches. Such differences are due to topographic effects on the propagation pathways of Rossby waves.
Abstract
Ocean bottom pressure pB is an important oceanic variable that is dynamically related to the abyssal ocean circulation through geostrophy. In this study we examine the seasonal pB variability in the North Pacific Ocean by analyzing satellite gravimetric observations from the GRACE program and a data-assimilated ocean-state estimate from ECCOv4. The seasonal pB variability is characterized by alternations of low and high anomalies among three regions, the subpolar and subtropical basins as well as the equatorial region. A linear two-layer wind-driven model is used to examine forcing mechanisms and topographic effects on seasonal pB variations. The model control run, which uses a realistic topography, is able to simulate a basinwide seasonal pB variability that is remarkably similar to that from GRACE and ECCOv4. Since the model is driven by wind stress alone, the good model–data agreement indicates that wind stress is the leading forcing for seasonal changes in pB . An additional model simulation was conducted by setting the water depth uniformly at 5000 m. The magnitude of seasonal pB anomaly is amplified significantly in the flat-bottom simulation as compared with that in the control run. The difference can be explained in terms of the topographic Sverdrup balance. In addition, the spatial pattern of the seasonal pB variability is also profoundly affected by topography especially on continental margins, ridges, and trenches. Such differences are due to topographic effects on the propagation pathways of Rossby waves.
Abstract
In this study, modifications of the Held scaling (Held) are proposed and tested against simulations of f-plane, two-layer quasigeostrophic turbulence. The aim is to better constrain the eddy mixing length and rms barotropic velocity in response to varied quadratic and linear bottom drag. The proposed modifications allow eddies to be partially barotropized, to relax the commonly invoked barotropization approximation, and consider a drag-dependent cascade rate per energy input, to account for the lack of an inertial range. Quantitative comparisons with the vortex gas scaling are also carried out. It is shown that the progressively weakened sensitivity in eddy scales to increased drag strength is mainly a result of eddy partial barotropization. For both drag forms except toward the limit of weak linear drag, accounting for partial barotropization alone leads to good predictions of eddy velocity, although not of mixing length. It also partly resolves the degeneracy of balance constraints for linear drag because partial barotropization acts like scale-dependent damping. Adding a cascade correction, which is interpreted as allowing for changes in spectral room for cascade, further improves the mixing length representation. Overall, the proposed theory can augment the existing scalings by extending the eddy scale predictions to O(1) quadratic drag and has skills generally comparable to the vortex gas scaling for linear drag. However, toward the weak linear drag limit where eddies approach complete barotropization, the proposed theory breaks down but the vortex gas performs well. Potential issues concerning the applicability of vortex gas to this limit are discussed.
Abstract
In this study, modifications of the Held scaling (Held) are proposed and tested against simulations of f-plane, two-layer quasigeostrophic turbulence. The aim is to better constrain the eddy mixing length and rms barotropic velocity in response to varied quadratic and linear bottom drag. The proposed modifications allow eddies to be partially barotropized, to relax the commonly invoked barotropization approximation, and consider a drag-dependent cascade rate per energy input, to account for the lack of an inertial range. Quantitative comparisons with the vortex gas scaling are also carried out. It is shown that the progressively weakened sensitivity in eddy scales to increased drag strength is mainly a result of eddy partial barotropization. For both drag forms except toward the limit of weak linear drag, accounting for partial barotropization alone leads to good predictions of eddy velocity, although not of mixing length. It also partly resolves the degeneracy of balance constraints for linear drag because partial barotropization acts like scale-dependent damping. Adding a cascade correction, which is interpreted as allowing for changes in spectral room for cascade, further improves the mixing length representation. Overall, the proposed theory can augment the existing scalings by extending the eddy scale predictions to O(1) quadratic drag and has skills generally comparable to the vortex gas scaling for linear drag. However, toward the weak linear drag limit where eddies approach complete barotropization, the proposed theory breaks down but the vortex gas performs well. Potential issues concerning the applicability of vortex gas to this limit are discussed.
