Abstract Recent observations and numerical simulations have demonstrated the potential for significant interactions between mesoscale eddies and smaller-scale tidally generated internal waves — also known as internal tides. Here we develop a simple theoretical model that predicts the one-way upscale transfer of energy from internal tides to mesoscale eddies through a critical level mechanism. We find that — in the presence of a critical level — the internal tide energy flux into an eddy is partitioned according to the wave frequency Ω and local inertial frequency f : a fraction of 1 – f /Ω is transferred to the eddy kinetic energy while the remainder is viscously dissipated or supports mixing. These predictions are validated by comparison with a suite of numerical simulations. The simulations further show that the wave-driven energisation of the eddies also accelerates the onset of hydrodynamical instabilities and the break down of the eddies, thereby increasing eddy kinetic energy, but reducing eddy lifetimes. Our estimates suggest that in regions of the ocean with both significant eddy fields and internal tides—such as parts of the Gulf Stream and Antarctic Circumpolar Current—the critical level effect could drive a ∼10% per month increase in the kinetic energy of a typical eddy. Our results provide a basis for parameterising internal tide-eddy interactions in global ocean models where they are currently unrepresented.
最近的观测和数值模拟表明，中尺度涡旋和较小尺度潮汐产生的内波(也称为内潮)之间可能存在显著的相互作用。在这里，我们建立了一个简单的理论模型，预测能量通过一个临界水平机制从内部潮汐向中尺度漩涡的单向高级转移。我们发现，在存在临界水平时，进入涡流的内部潮汐能量通量根据波频率Ω和局部惯性频率f进行了划分:1 - f /Ω的一小部分被转移到涡流动能中，而其余部分则被粘性耗散或支持混合。这些预测通过与一系列数值模拟的比较得到了验证。模拟进一步表明，波浪驱动的涡流能量也加速了流体动力不稳定性的发生和涡流的破裂，从而增加了涡流动能，但减少了涡流寿命。我们的估计表明，在具有显著涡旋场和内部潮汐的海洋区域，如墨西哥湾流和南极环极流的部分地区，临界水平效应可以使典型涡旋的动能每月增加约10%。我们的研究结果为参数化全球海洋模型中的内部潮汐-涡流相互作用提供了基础，而这些模型目前还没有得到代表。
{"title":"Eddy acceleration and decay driven by internal tides","authors":"Callum J. Shakespeare","doi":"10.1175/jpo-d-23-0127.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0127.1","url":null,"abstract":"Abstract Recent observations and numerical simulations have demonstrated the potential for significant interactions between mesoscale eddies and smaller-scale tidally generated internal waves — also known as internal tides. Here we develop a simple theoretical model that predicts the one-way upscale transfer of energy from internal tides to mesoscale eddies through a critical level mechanism. We find that — in the presence of a critical level — the internal tide energy flux into an eddy is partitioned according to the wave frequency Ω and local inertial frequency f : a fraction of 1 – f /Ω is transferred to the eddy kinetic energy while the remainder is viscously dissipated or supports mixing. These predictions are validated by comparison with a suite of numerical simulations. The simulations further show that the wave-driven energisation of the eddies also accelerates the onset of hydrodynamical instabilities and the break down of the eddies, thereby increasing eddy kinetic energy, but reducing eddy lifetimes. Our estimates suggest that in regions of the ocean with both significant eddy fields and internal tides—such as parts of the Gulf Stream and Antarctic Circumpolar Current—the critical level effect could drive a ∼10% per month increase in the kinetic energy of a typical eddy. Our results provide a basis for parameterising internal tide-eddy interactions in global ocean models where they are currently unrepresented.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"6 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135592432","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract Submesoscale turbulence in the upper ocean consists of fronts, filaments, and vortices that have horizontal scales of order 100 m to 10 km. High-resolution numerical simulations have suggested that submesoscale turbulence is associated with strong vertical motion that could substantially enhance the vertical exchange between the thermocline and mixed layer, which may have an impact on marine ecosystems and climate. Theoretical, numerical, and observational work indicates that submesoscale turbulence is energized primarily by baroclinic instability in the mixed layer, which is most vigorous in winter. This study demonstrates how such mixed-layer baroclinic instabilities induce vertical exchange by drawing filaments of thermocline water into the mixed layer. A scaling law is proposed for the dependence of the exchange on environmental parameters. Linear stability analysis and nonlinear simulations indicate that the exchange, quantified by how much thermocline water is entrained into the mixed layer, is proportional to the mixed-layer depth, is inversely proportional to the Richardson number of the thermocline, and increases with increasing Richardson number of the mixed layer. The results imply that the tracer exchange between the thermocline and mixed layer is more efficient when the mixed layer is thicker, when the mixed-layer stratification is stronger, when the lateral buoyancy gradient is stronger, and when the thermocline stratification is weaker. The scaling suggests vigorous exchange between the permanent thermocline and deep mixed layers in winter, especially in mode water formation regions.
{"title":"Vertical Exchange Induced by Mixed-Layer Instabilities","authors":"Yangcheng Luo, Jörn Callies","doi":"10.1175/jpo-d-23-0059.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0059.1","url":null,"abstract":"Abstract Submesoscale turbulence in the upper ocean consists of fronts, filaments, and vortices that have horizontal scales of order 100 m to 10 km. High-resolution numerical simulations have suggested that submesoscale turbulence is associated with strong vertical motion that could substantially enhance the vertical exchange between the thermocline and mixed layer, which may have an impact on marine ecosystems and climate. Theoretical, numerical, and observational work indicates that submesoscale turbulence is energized primarily by baroclinic instability in the mixed layer, which is most vigorous in winter. This study demonstrates how such mixed-layer baroclinic instabilities induce vertical exchange by drawing filaments of thermocline water into the mixed layer. A scaling law is proposed for the dependence of the exchange on environmental parameters. Linear stability analysis and nonlinear simulations indicate that the exchange, quantified by how much thermocline water is entrained into the mixed layer, is proportional to the mixed-layer depth, is inversely proportional to the Richardson number of the thermocline, and increases with increasing Richardson number of the mixed layer. The results imply that the tracer exchange between the thermocline and mixed layer is more efficient when the mixed layer is thicker, when the mixed-layer stratification is stronger, when the lateral buoyancy gradient is stronger, and when the thermocline stratification is weaker. The scaling suggests vigorous exchange between the permanent thermocline and deep mixed layers in winter, especially in mode water formation regions.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"8 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135591473","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract Based on the Conditional Nonlinear Optimal Perturbation for boundary condition method and Regional Ocean Modeling System (ROMS), this study investigates the influence of wind stress uncertainty on predicting the short-term state transitions of the Kuroshio Extension (KE). The optimal time-dependent wind stress errors that lead to maximum prediction errors are obtained for two KE stable-to-unstable and two reverse transitions, which exhibit local multi-eddies structures with decreasing magnitude as the end time of prediction approaches. The optimal boundary errors initially induce small oceanic errors through Ekman pumping. Subsequently, these errors grow in magnitude as oceanic internal processes take effect, which exerts significant influences on the short-term prediction of the KE state transition process. Specifically, during stable-to-unstable (unstable-to-stable) transitions, the growing error induces an overestimation (underestimation) of the meridional sea surface height gradient across the KE axis, leading to the predicted KE state being more (less) stable. Furthermore, the dynamics mechanism analysis indicates that barotropic instability is crucial for the error growth in the prediction of both the stable-to-unstable and the reverse transition processes due to the horizontal shear of flow field. But work generated by wind stress error plays a more important role in the prediction of the unstable-to-stable transitions because of the synergistic effect of strong wind stress error and strong oceanic error. Eventually, the sensitive areas have been identified based on the optimal boundary errors. Reducing wind stress errors in sensitive areas can significantly improve prediction skills, offering theoretical guidance for devising observational strategies.
{"title":"Effects of Wind Stress Uncertainty on Short-term Prediction of the Kuroshio Extension State Transition Process","authors":"Hui Zhang, Qiang Wang, Mu Mu, Kun Zhang, Yu Geng","doi":"10.1175/jpo-d-23-0047.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0047.1","url":null,"abstract":"Abstract Based on the Conditional Nonlinear Optimal Perturbation for boundary condition method and Regional Ocean Modeling System (ROMS), this study investigates the influence of wind stress uncertainty on predicting the short-term state transitions of the Kuroshio Extension (KE). The optimal time-dependent wind stress errors that lead to maximum prediction errors are obtained for two KE stable-to-unstable and two reverse transitions, which exhibit local multi-eddies structures with decreasing magnitude as the end time of prediction approaches. The optimal boundary errors initially induce small oceanic errors through Ekman pumping. Subsequently, these errors grow in magnitude as oceanic internal processes take effect, which exerts significant influences on the short-term prediction of the KE state transition process. Specifically, during stable-to-unstable (unstable-to-stable) transitions, the growing error induces an overestimation (underestimation) of the meridional sea surface height gradient across the KE axis, leading to the predicted KE state being more (less) stable. Furthermore, the dynamics mechanism analysis indicates that barotropic instability is crucial for the error growth in the prediction of both the stable-to-unstable and the reverse transition processes due to the horizontal shear of flow field. But work generated by wind stress error plays a more important role in the prediction of the unstable-to-stable transitions because of the synergistic effect of strong wind stress error and strong oceanic error. Eventually, the sensitive areas have been identified based on the optimal boundary errors. Reducing wind stress errors in sensitive areas can significantly improve prediction skills, offering theoretical guidance for devising observational strategies.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"8 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135592438","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract The dissipation of low-mode internal tides as they propagate through mesoscale baroclinic eddies is examined using a series of numerical simulations, complemented by three-dimensional ray tracing calculations. The incident mode-1 internal tide is refracted into convergent energy beams, resulting in a zone of reduced energy flux in the lee of the eddy. The dissipation of internal tides is significantly enhanced in the upper water column within strongly baroclinic (anticyclonic) eddies, exhibiting a spatially asymmetric pattern, due to trapped high-mode internal tides. Where the eddy velocity opposes the internal tide propagation velocity, high-mode waves can be trapped within the eddy, whereas high modes can freely propagate away from regions where eddy and internal wave velocities are in the same direction. The trapped high modes with large vertical shear are then dissipated, with the asymmetric distribution of trapping leading to the asymmetric distribution of dissipation. Three-dimensional ray tracing solutions further illustrate the importance of the baroclinic current for wave trapping. Similar enhancement of dissipation is also found for a baroclinic cyclonic eddy. However, a barotropic eddy is incapable of facilitating robust high modes and thus cannot generate significant dissipation of internal tides, despite its strong velocities. Both energy transfer from low to high modes in the baroclinic eddy structure and trapping of those high modes by the eddy velocity field are therefore necessary to produce internal wave dissipation, a conclusion confirmed by examining the sensitivity of the internal tide dissipation to eddy radius, vorticity, and vertical scale. Significance Statement The oceanic tides drive underwater waves at the tidal frequency known as internal tides. When these waves break, or dissipate, they can lead to mixing of oceanic heat and salt which impacts the ocean circulation and climate. Accurate climate predictions require computer models that correctly represent the distribution of this mixing. Here we explore how an oceanic eddy, a swirling vortex of order 100–400 km across, can locally enhance the dissipation of oceanic internal tides. We find that strong ocean eddies can be hotspots for internal tide dissipation, for both clockwise and anticlockwise rotating vortices, and surface-enhanced eddies are most effective at internal tide dissipation. These results can improve climate model representations of tidally driven mixing, leading to more credible future predictions.
{"title":"Enhanced Dissipation of Internal Tides in a Mesoscale Baroclinic Eddy","authors":"Yang Wang, Sonya Legg","doi":"10.1175/jpo-d-23-0045.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0045.1","url":null,"abstract":"Abstract The dissipation of low-mode internal tides as they propagate through mesoscale baroclinic eddies is examined using a series of numerical simulations, complemented by three-dimensional ray tracing calculations. The incident mode-1 internal tide is refracted into convergent energy beams, resulting in a zone of reduced energy flux in the lee of the eddy. The dissipation of internal tides is significantly enhanced in the upper water column within strongly baroclinic (anticyclonic) eddies, exhibiting a spatially asymmetric pattern, due to trapped high-mode internal tides. Where the eddy velocity opposes the internal tide propagation velocity, high-mode waves can be trapped within the eddy, whereas high modes can freely propagate away from regions where eddy and internal wave velocities are in the same direction. The trapped high modes with large vertical shear are then dissipated, with the asymmetric distribution of trapping leading to the asymmetric distribution of dissipation. Three-dimensional ray tracing solutions further illustrate the importance of the baroclinic current for wave trapping. Similar enhancement of dissipation is also found for a baroclinic cyclonic eddy. However, a barotropic eddy is incapable of facilitating robust high modes and thus cannot generate significant dissipation of internal tides, despite its strong velocities. Both energy transfer from low to high modes in the baroclinic eddy structure and trapping of those high modes by the eddy velocity field are therefore necessary to produce internal wave dissipation, a conclusion confirmed by examining the sensitivity of the internal tide dissipation to eddy radius, vorticity, and vertical scale. Significance Statement The oceanic tides drive underwater waves at the tidal frequency known as internal tides. When these waves break, or dissipate, they can lead to mixing of oceanic heat and salt which impacts the ocean circulation and climate. Accurate climate predictions require computer models that correctly represent the distribution of this mixing. Here we explore how an oceanic eddy, a swirling vortex of order 100–400 km across, can locally enhance the dissipation of oceanic internal tides. We find that strong ocean eddies can be hotspots for internal tide dissipation, for both clockwise and anticlockwise rotating vortices, and surface-enhanced eddies are most effective at internal tide dissipation. These results can improve climate model representations of tidally driven mixing, leading to more credible future predictions.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"35 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134976791","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract Salt mixing enables the transport of water between the inflow and outflow layers of estuarine circulation and therefore closes the circulation by driving a diahaline exchange flow. A recently derived universal law links the salt mixing inside an estuarine volume bounded by an isohaline surface to freshwater discharge: it states that on long-term average, the area-integrated mixing across the bounding isohaline is directly proportional to the freshwater discharge entering the estuary. However, even though numerous studies predict that periods of extreme discharge will become more frequent with climate change, the direct impact of such periods on estuarine mixing and circulation has yet to be investigated. Therefore, this numerical modeling study focuses on salinity mixing and diahaline exchange flows during a low-discharge and an extreme high-discharge period. To this end, we apply a realistic numerical setup of the Elbe estuary in northern Germany, using curvilinear coordinates that follow the navigational channel. This is the first time the direct relationship between diahaline exchange flow and salt mixing as well as the spatial distribution of the diahaline exchange flow are shown in a realistic tidal setup. The spatial distribution is highly correlated with the local mixing gradient for salinity, such that inflow occurs near the bottom at the upstream end of the isohaline. Meanwhile, outflow occurs near the surface at its downstream end. Lastly, increased vertical stratification occurs within the estuary during the high-discharge period, while estuarine-wide mixing strongly converges to the universal law for averaging periods of the discharge event time scale.
{"title":"Local mixing determines spatial structure of diahaline exchange flow in a mesotidal estuary – a study of extreme runoff conditions","authors":"Lloyd Reese, Ulf Gräwe, Knut Klingbeil, Xiangyu Li, Marvin Lorenz, Hans Burchard","doi":"10.1175/jpo-d-23-0052.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0052.1","url":null,"abstract":"Abstract Salt mixing enables the transport of water between the inflow and outflow layers of estuarine circulation and therefore closes the circulation by driving a diahaline exchange flow. A recently derived universal law links the salt mixing inside an estuarine volume bounded by an isohaline surface to freshwater discharge: it states that on long-term average, the area-integrated mixing across the bounding isohaline is directly proportional to the freshwater discharge entering the estuary. However, even though numerous studies predict that periods of extreme discharge will become more frequent with climate change, the direct impact of such periods on estuarine mixing and circulation has yet to be investigated. Therefore, this numerical modeling study focuses on salinity mixing and diahaline exchange flows during a low-discharge and an extreme high-discharge period. To this end, we apply a realistic numerical setup of the Elbe estuary in northern Germany, using curvilinear coordinates that follow the navigational channel. This is the first time the direct relationship between diahaline exchange flow and salt mixing as well as the spatial distribution of the diahaline exchange flow are shown in a realistic tidal setup. The spatial distribution is highly correlated with the local mixing gradient for salinity, such that inflow occurs near the bottom at the upstream end of the isohaline. Meanwhile, outflow occurs near the surface at its downstream end. Lastly, increased vertical stratification occurs within the estuary during the high-discharge period, while estuarine-wide mixing strongly converges to the universal law for averaging periods of the discharge event time scale.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"2015 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135200012","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract Ocean surface currents introduce variations into the surface wind-stress that can change the component of the stress aligned with the thermal wind shear at fronts. This modifies the Ekman buoyancy flux, such that the current feedback on the stress tends to generate an effective flux of buoyancy and potential vorticity to the mixed-layer. Scaling arguments and idealized simulations resolving both mesoscale and submesoscale turbulence suggest that this pathway for air-sea interaction can be important both locally at individual submesoscale fronts with strong surface currents—where it can introduce equivalent advective heat fluxes exceeding several hundredWm −2 —and in the spatial mean where it reduces the integrated Ekman buoyancy flux by approximately 50%. The accompanying source of surface potential vorticity injection suggests that at some fronts the current feedback modification of the Ekman buoyancy flux may be significant in terms of both submesoscale dynamics and boundary layer energetics, with an implied modification of symmetric instability growth rates and dissipation that scales similarly to the energy lost through the negative wind work generated by the current feedback. This provides an example of how the shift of dynamical regimes into the submesoscale may promote the importance of air-sea interaction mechanisms that differ from those most active at larger scale.
{"title":"The current feedback on stress modifies the Ekman buoyancy flux at fronts","authors":"Jacob O. Wenegrat","doi":"10.1175/jpo-d-23-0005.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0005.1","url":null,"abstract":"Abstract Ocean surface currents introduce variations into the surface wind-stress that can change the component of the stress aligned with the thermal wind shear at fronts. This modifies the Ekman buoyancy flux, such that the current feedback on the stress tends to generate an effective flux of buoyancy and potential vorticity to the mixed-layer. Scaling arguments and idealized simulations resolving both mesoscale and submesoscale turbulence suggest that this pathway for air-sea interaction can be important both locally at individual submesoscale fronts with strong surface currents—where it can introduce equivalent advective heat fluxes exceeding several hundredWm −2 —and in the spatial mean where it reduces the integrated Ekman buoyancy flux by approximately 50%. The accompanying source of surface potential vorticity injection suggests that at some fronts the current feedback modification of the Ekman buoyancy flux may be significant in terms of both submesoscale dynamics and boundary layer energetics, with an implied modification of symmetric instability growth rates and dissipation that scales similarly to the energy lost through the negative wind work generated by the current feedback. This provides an example of how the shift of dynamical regimes into the submesoscale may promote the importance of air-sea interaction mechanisms that differ from those most active at larger scale.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"82 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135536479","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract In this paper we present the analytical derivation of a local Water Mass Transformation (WMT) framework for an individual water column. We exactly formulate the mapping of the governing equations from geopotential coordinates to an arbitrary tracer space. Unique definitions for the local effective vertical dia-surface fluxes are given. In tracer space we derive new relations between the local dia-tracer fluxes and the mixing per tracer class. The key relation between the effective vertical dia-tracer velocity and the mixing per tracer class directly formulates how the overturning circulation is linked to local tracer variance dissipation. Horizontal integration of the governing equations in tracer space and the relations between the dia-tracer quantities finally recovers the well-known integral WMT formulations.
{"title":"A rigorous derivation of the Water Mass Transformation framework, the relation between mixing and dia-surface exchange flow, and links to recent theories in estuarine research","authors":"Knut Klingbeil, Erika Henella","doi":"10.1175/jpo-d-23-0130.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0130.1","url":null,"abstract":"Abstract In this paper we present the analytical derivation of a local Water Mass Transformation (WMT) framework for an individual water column. We exactly formulate the mapping of the governing equations from geopotential coordinates to an arbitrary tracer space. Unique definitions for the local effective vertical dia-surface fluxes are given. In tracer space we derive new relations between the local dia-tracer fluxes and the mixing per tracer class. The key relation between the effective vertical dia-tracer velocity and the mixing per tracer class directly formulates how the overturning circulation is linked to local tracer variance dissipation. Horizontal integration of the governing equations in tracer space and the relations between the dia-tracer quantities finally recovers the well-known integral WMT formulations.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"116 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135815969","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract It is well known that strong low-mode internal tides generated in Luzon Strait propagate westward to impinge continental slopes in the northeastern South China Sea (SCS). The reflection and scattering of these internal tides including diurnal and semidiurnal components on the slopes are quantitatively investigated using two sets of mooring data and a linear internal tide model with realistic topography and stratification. Flux reflections computed from mooring data collected on the continental slopes are consistent with the linear model. Based on the results of the observations and simulations, a map of low-mode internal tide reflection and scattering coefficients along the continental margin in the northeastern SCS is revealed. On average, diurnal internal tides lose 38% of their energy to high modes (≥ mode-4) that are assumed to dissipate on the slopes, transmit 28% onto the continental shelf, and reflect 31% back to the deep ocean. On the contrary, most of the semidiurnal energy (89%) transmit onto the continental shelf, and only 11% is scattered to high modes (7%), and reflected back to the deep ocean (4%). For diurnal internal tides, a large fraction of energy that is scattered to high modes and reflected back to the deep sea can be attributed to the critical-supercritical slopes, while the weak reflection for the semidiurnal energy is due to the subcritical slopes. These quantitative descriptions for evolutions of low-mode internal tides incident to the slopes provide an energy budget map on the continental slopes in the northeastern SCS.
{"title":"Reflection and Scattering of Low-mode Internal Tides on the Continental Slope of the South China Sea","authors":"Wei Li, Xiaohui Xie","doi":"10.1175/jpo-d-23-0087.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0087.1","url":null,"abstract":"Abstract It is well known that strong low-mode internal tides generated in Luzon Strait propagate westward to impinge continental slopes in the northeastern South China Sea (SCS). The reflection and scattering of these internal tides including diurnal and semidiurnal components on the slopes are quantitatively investigated using two sets of mooring data and a linear internal tide model with realistic topography and stratification. Flux reflections computed from mooring data collected on the continental slopes are consistent with the linear model. Based on the results of the observations and simulations, a map of low-mode internal tide reflection and scattering coefficients along the continental margin in the northeastern SCS is revealed. On average, diurnal internal tides lose 38% of their energy to high modes (≥ mode-4) that are assumed to dissipate on the slopes, transmit 28% onto the continental shelf, and reflect 31% back to the deep ocean. On the contrary, most of the semidiurnal energy (89%) transmit onto the continental shelf, and only 11% is scattered to high modes (7%), and reflected back to the deep ocean (4%). For diurnal internal tides, a large fraction of energy that is scattered to high modes and reflected back to the deep sea can be attributed to the critical-supercritical slopes, while the weak reflection for the semidiurnal energy is due to the subcritical slopes. These quantitative descriptions for evolutions of low-mode internal tides incident to the slopes provide an energy budget map on the continental slopes in the northeastern SCS.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"133 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136313135","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract Energetic internal tides (ITs) are generated from the Luzon Strait (LS) and propagate westward into the South China Sea (SCS). Owing to the lack of large-scale synchronous measurements, the propagation features and seasonal variations of diurnal ITs remain unclear. From 2018 to 2019, mode-1 diurnal ITs west of the LS were continuously observed using a large-scale moored array of 27 pressure inverted echo sounders (PIESs) and a thermistor chain. Measurements confirmed that diurnal ITs radiate from the LS with a north–south asymmetrical pattern, with the most energetic channel located in the middle and south of the LS. The total energy radiated into the SCS across 120°E is 2.67 GW for the K 1 and 1.54 GW for O 1 ITs, approximately two times larger than those inferred from satellite observations. K 1 dominates among the diurnal ITs, with its maximum isopycnal displacement (amplitude) and energy input to the SCS being the strongest in summer (i.e., 16.3 m and 2.81 GW, respectively). The propagation speed of K 1 is higher in summer and autumn along the main channel (i.e., 4.33 and 4.36 m s −1 , respectively). Seasonal stratification and circulation play important roles in the seasonal variation of amplitude and propagation speed of the K 1 ITs. The seasonal variability of diurnal-band ITs, which includes all diurnal constituents, is location-dependent and primarily results from the superposition of the K 1 and P 1 ITs. In particular, vertical displacement is strong in summer and winter along the main channel of the K 1 and P 1 ITs. The seasonal amplitude of K 1 can modulate this seasonal feature.
吕宋海峡(LS)产生了高能内潮(ITs)，并向西传播到南海(SCS)。由于缺乏大规模的同步测量，ITs的日传播特征和季节变化仍然不清楚。从2018年到2019年，利用27个压力反向回声测深仪(pess)和热敏电阻链组成的大型系泊阵列，连续观测了LS以西的1型日ITs。测量证实，ITs的日辐射具有南北不对称的模式，其中能量最大的通道位于LS的中部和南部。通过120°E辐射到南海的总能量为k1的2.67 GW和O 1 ITs的1.54 GW，大约是卫星观测推断的两倍。k1在ITs日变化中占主导地位，其最大等压位移(振幅)和向南海输入的能量在夏季最强(分别为16.3 m和2.81 GW)。k1在夏季和秋季沿主航道的传播速度较高(分别为4.33和4.36 m s - 1)。季节分层和环流在k1 ITs振幅和传播速度的季节变化中起重要作用。日波段ITs的季节变化主要是由k1和p1 ITs的叠加引起的，它包括所有日组分。特别是沿k1和p1 ITs主通道，夏季和冬季垂直位移强烈。k1的季节性振幅可以调节这种季节性特征。
{"title":"Propagation Features of Diurnal Internal Tides West of the Luzon Strait Revealed by a Large PIES Array","authors":"Min Wang, Xiao-Hua Zhu, Hua Zheng, Juntian Chen, Ruixiang Zhao, Zhao-Jun Liu, Qiang Ren, Yansong Liu, Feng Nan, Fei Yu, Jianfeng Wang, Qiang Li","doi":"10.1175/jpo-d-22-0206.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0206.1","url":null,"abstract":"Abstract Energetic internal tides (ITs) are generated from the Luzon Strait (LS) and propagate westward into the South China Sea (SCS). Owing to the lack of large-scale synchronous measurements, the propagation features and seasonal variations of diurnal ITs remain unclear. From 2018 to 2019, mode-1 diurnal ITs west of the LS were continuously observed using a large-scale moored array of 27 pressure inverted echo sounders (PIESs) and a thermistor chain. Measurements confirmed that diurnal ITs radiate from the LS with a north–south asymmetrical pattern, with the most energetic channel located in the middle and south of the LS. The total energy radiated into the SCS across 120°E is 2.67 GW for the K 1 and 1.54 GW for O 1 ITs, approximately two times larger than those inferred from satellite observations. K 1 dominates among the diurnal ITs, with its maximum isopycnal displacement (amplitude) and energy input to the SCS being the strongest in summer (i.e., 16.3 m and 2.81 GW, respectively). The propagation speed of K 1 is higher in summer and autumn along the main channel (i.e., 4.33 and 4.36 m s −1 , respectively). Seasonal stratification and circulation play important roles in the seasonal variation of amplitude and propagation speed of the K 1 ITs. The seasonal variability of diurnal-band ITs, which includes all diurnal constituents, is location-dependent and primarily results from the superposition of the K 1 and P 1 ITs. In particular, vertical displacement is strong in summer and winter along the main channel of the K 1 and P 1 ITs. The seasonal amplitude of K 1 can modulate this seasonal feature.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"213 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134913035","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hemantha W. Wijesekera, Conrad A. Luecke, David W. Wang, Ewa Jarosz, Sergio DeRada, William J. Teague, Kyung-Il Chang, Jae Hak Lee, Hong-Sik Min, SungHyun Nam
Abstract Small-scale processes at the southwestern boundary of the Ulleung Basin (UB) in the Japan/East Sea (JES) were examined using combined ship-based and moored observations along with model output. Model results show baroclinic semidiurnal tides are generated at the shelf break and corresponding slope connecting the Korea/Tsushima Strait with the UB, and propagate into the UB with large barotropic to baroclinic energy conversion over the slope. Observations show high-frequency internal-wave packets and indicate strong velocity shear and energetic turbulence associated with baroclinic tides in the stratified bottom-layer. Solitary-like waves with frequencies 0.2 N to 0.5 N (buoyancy frequency, N ) were found at the edge of the shelf break with supercritical flow. For subcritical flow, a hydraulic jump formed over the shelf break with weakly-dispersive internal lee waves with frequencies varying from 0.5 N to N . These high-frequency lee waves were trapped in the stratified bottom-layer, with wave stress similar to the turbulent stress near the bottom. The power loss due to turbulent bottom drag can be an important factor for energy loss associated with the hydraulic jump. Turbulent-kinetic-energy dissipation rates of ~10 −4 W kg −1 were found. Large downward heat and salt fluxes below the high-salinity core mix warm/salty Tsushima Current Water with cold/low-salinity JES intermediate water. Mixing over the shelf break could be very important to the JES circulation since the calculated diapycnal upwelling (1-6 m d −1 ) at the shelf break and slope is substantially greater than the basin-averaged estimate from chemical tracers and modeling studies.
利用船舶观测和系泊观测结合模型输出，对日本/东海郁陵盆地(UB)西南边界的小尺度过程进行了研究。模式结果表明，斜压半日潮产生于连接朝鲜/对马海峡与UB的陆架断裂带和相应的斜坡处，并在斜坡上以较大的正斜压能量转换传播到UB。观测显示了高频内波包，并表明在分层的底层与斜压潮汐有关的强速度切变和高能湍流。在具有超临界流动的陆架断裂带边缘发现了频率为0.2 N ~ 0.5 N(浮力频率，N)的孤立波。对于亚临界流动，在大陆架断裂上形成一个水力跃变，其频率在0.5 N到N之间变化的弱色散内背风波。这些高频背风波被困在分层的底层，波浪应力与底部附近的湍流应力相似。湍流底阻力造成的功率损失可能是与液压跃变相关的能量损失的重要因素。湍流动能耗散率为~10−4 W kg−1。高盐度岩心下方的大向下热盐通量将暖/咸对马流与冷/低盐度JES中间水混合。大陆架断裂上的混合可能对JES环流非常重要，因为在大陆架断裂和斜坡处计算出的纵向上升流(1-6 m d - 1)大大大于化学示踪剂和模拟研究得出的盆地平均估计。
{"title":"Mixing processes at the southwestern entrance to the Japan/East Sea","authors":"Hemantha W. Wijesekera, Conrad A. Luecke, David W. Wang, Ewa Jarosz, Sergio DeRada, William J. Teague, Kyung-Il Chang, Jae Hak Lee, Hong-Sik Min, SungHyun Nam","doi":"10.1175/jpo-d-23-0061.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0061.1","url":null,"abstract":"Abstract Small-scale processes at the southwestern boundary of the Ulleung Basin (UB) in the Japan/East Sea (JES) were examined using combined ship-based and moored observations along with model output. Model results show baroclinic semidiurnal tides are generated at the shelf break and corresponding slope connecting the Korea/Tsushima Strait with the UB, and propagate into the UB with large barotropic to baroclinic energy conversion over the slope. Observations show high-frequency internal-wave packets and indicate strong velocity shear and energetic turbulence associated with baroclinic tides in the stratified bottom-layer. Solitary-like waves with frequencies 0.2 N to 0.5 N (buoyancy frequency, N ) were found at the edge of the shelf break with supercritical flow. For subcritical flow, a hydraulic jump formed over the shelf break with weakly-dispersive internal lee waves with frequencies varying from 0.5 N to N . These high-frequency lee waves were trapped in the stratified bottom-layer, with wave stress similar to the turbulent stress near the bottom. The power loss due to turbulent bottom drag can be an important factor for energy loss associated with the hydraulic jump. Turbulent-kinetic-energy dissipation rates of ~10 −4 W kg −1 were found. Large downward heat and salt fluxes below the high-salinity core mix warm/salty Tsushima Current Water with cold/low-salinity JES intermediate water. Mixing over the shelf break could be very important to the JES circulation since the calculated diapycnal upwelling (1-6 m d −1 ) at the shelf break and slope is substantially greater than the basin-averaged estimate from chemical tracers and modeling studies.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"8 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135740865","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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