N. Fraser, Alan Fox, Stuart A. Cunningham, Willi Rath, F. Schwarzkopf, Arne Biastoch
The Atlantic meridional overturning circulation (MOC) is traditionally monitored in terms of zonally-integrated transport either in depth space or 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 characterised 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 (1) of 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 inter-gyre region. These results provide insights into the coupling between density- and depth-space overturning circulations.
大西洋经向翻转环流（MOC）传统上是通过深度空间或密度空间的分区综合传输来监测的。虽然这种观点具有简单的优点，但它掩盖了丰富而复杂的三维结构，因此人们对下沉和上涌分支的确切物理现象仍然知之甚少。在亚热带，深度空间和密度空间的 MOC 几乎相等，这表明垂直和平流体积传输是紧密耦合的，而这两个指标在高纬度地区的差异表明，任何这种耦合既不是瞬时的，也不是局部的。以前的研究已经描述了驱动近缘体积传输的表面浮力强迫和混合过程。在此，我们根据涡度预算对垂直体积传输进行了新的分析分解。我们的研究表明，大多数项都可以通过观测结果估算出来，并通过对北大西洋的高分辨率数值模拟提供了更多启示。我们的分析强调了（1）相对涡度平流对北大西洋北部副极地边界溢流水下沉的作用，以及（2）地层间区域浓密水下沉的地营β效应。这些结果提供了密度空间和深度空间翻转环流之间耦合的见解。
{"title":"Vertical Velocity Dynamics in the North Atlantic and Implications for AMOC","authors":"N. Fraser, Alan Fox, Stuart A. Cunningham, Willi Rath, F. Schwarzkopf, Arne Biastoch","doi":"10.1175/jpo-d-23-0229.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0229.1","url":null,"abstract":"\u0000The Atlantic meridional overturning circulation (MOC) is traditionally monitored in terms of zonally-integrated transport either in depth space or 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 characterised 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 (1) of 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 inter-gyre region. These results provide insights into the coupling between density- and depth-space overturning circulations.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"29 8","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141649343","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The existence of multiple equilibria (ice-covered and ice-free states) is explored using a set of coupled, nondimensional equations that describe the heat and salt balances in basins, such as the Arctic Ocean, that are subject to atmospheric forcing and two distinct water mass sources. Six nondimensional numbers describe the influences of: atmospheric cooling; evaporation minus precipitation; solar radiation; atmospheric temperature, diapycnal mixing, and the temperature contrast between the two water masses. It is shown that multiple equilibria resulting from the dependence of albedo on ice cover exists over a wide range of parameter space, especially so in the weak mixing limit. Multiple equilibria can also occur if diapycnal mixing increases to O(10−4 m2 s−1) or larger under ice-free conditions due to enhanced upward mixing of warm, salty water from below. Sensitivities to various forcing parameter are discussed.
{"title":"A simple model for multiple equilibria in ice-covered oceans","authors":"M. Spall","doi":"10.1175/jpo-d-24-0022.1","DOIUrl":"https://doi.org/10.1175/jpo-d-24-0022.1","url":null,"abstract":"\u0000The existence of multiple equilibria (ice-covered and ice-free states) is explored using a set of coupled, nondimensional equations that describe the heat and salt balances in basins, such as the Arctic Ocean, that are subject to atmospheric forcing and two distinct water mass sources. Six nondimensional numbers describe the influences of: atmospheric cooling; evaporation minus precipitation; solar radiation; atmospheric temperature, diapycnal mixing, and the temperature contrast between the two water masses. It is shown that multiple equilibria resulting from the dependence of albedo on ice cover exists over a wide range of parameter space, especially so in the weak mixing limit. Multiple equilibria can also occur if diapycnal mixing increases to O(10−4 m2 s−1) or larger under ice-free conditions due to enhanced upward mixing of warm, salty water from below. Sensitivities to various forcing parameter are discussed.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"21 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141646648","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Bubble plumes play a significant role in the air-sea interface by influencing processes such as air-sea gas exchange, aerosol production, modulation of oceanic carbon and nutrient cycles, and the vertical structure of the upper ocean. Using Acoustic Doppler Current Profiler (ADCP) data collected off the west coast of Ireland, we investigate the dynamics of bubble plumes and their relationship with sea state variables. In particular, we describe the patterns of bubble plumes vertical extension, duration and periodicity. We establish a power-law relationship between the average bubble penetration depth and wind speed, consistent with previous findings. Additionally, the study reveals a significant association between whitecapping coverage and observed acoustic volume backscatter intensity, underscoring the role of wave breaking in bubble plume generation. The shape of the probability distribution of bubble plume depths reveals a transition towards stronger and more organised bubble entrainment events during higher wind speeds. Furthermore, we show that deeper bubble plumes are associated with turbulent Langmuir number Lat∼ 0.3, highlighting the potential role of Langmuir circulation on the transport and deepening of bubble plumes. These results contribute to a better understanding of the complex interactions between ocean waves, wind, and bubble plumes, providing valuable insights for improving predictive models and enhancing our understanding of air-sea interactions.
{"title":"Dynamics of bubble plumes produced by breaking waves","authors":"D. Pelaez-Zapata, V. Pakrashi, Frédéric Dias","doi":"10.1175/jpo-d-23-0261.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0261.1","url":null,"abstract":"\u0000Bubble plumes play a significant role in the air-sea interface by influencing processes such as air-sea gas exchange, aerosol production, modulation of oceanic carbon and nutrient cycles, and the vertical structure of the upper ocean. Using Acoustic Doppler Current Profiler (ADCP) data collected off the west coast of Ireland, we investigate the dynamics of bubble plumes and their relationship with sea state variables. In particular, we describe the patterns of bubble plumes vertical extension, duration and periodicity. We establish a power-law relationship between the average bubble penetration depth and wind speed, consistent with previous findings. Additionally, the study reveals a significant association between whitecapping coverage and observed acoustic volume backscatter intensity, underscoring the role of wave breaking in bubble plume generation. The shape of the probability distribution of bubble plume depths reveals a transition towards stronger and more organised bubble entrainment events during higher wind speeds. Furthermore, we show that deeper bubble plumes are associated with turbulent Langmuir number Lat∼ 0.3, highlighting the potential role of Langmuir circulation on the transport and deepening of bubble plumes. These results contribute to a better understanding of the complex interactions between ocean waves, wind, and bubble plumes, providing valuable insights for improving predictive models and enhancing our understanding of air-sea interactions.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"331 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141686531","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Accurate estimation of the wind stress under extreme conditions is crucial for modeling storm surges and storm waves, which is important to the development of a warning system for coastal disaster prevention. The problem, however, is highly challenging owing to the presence of complex ocean surface processes under the action of unusually strong wind. In this study, the existing atmospheric wave boundary layer model is significantly enhanced by including various effects of wave breaking. Both the effect of wave breaking on the dissipation of energy and its effect on the transfer of momentum within the atmospheric boundary layer are carefully formulated. The wind stress coefficients obtained with the enhanced model are shown to be in good agreement with the measurements in not only deep but also shallow waters. The enhanced atmospheric wave boundary layer model is coupled with ocean wave as well as circulation models to simulate typhoon-induced storm surges and storm waves in the Pearl River Delta region. The computational results show that the coupled model with improved evaluation of the wind stress is substantially advantageous when compared with existing approaches.
{"title":"A Major Improvement of Atmospheric Wave Boundary Layer Model for Storm Surge Modeling by Including Effect of Wave Breaking on Air-Sea Momentum Exchange","authors":"Anyifang Zhang, Xiping Yu","doi":"10.1175/jpo-d-23-0233.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0233.1","url":null,"abstract":"\u0000Accurate estimation of the wind stress under extreme conditions is crucial for modeling storm surges and storm waves, which is important to the development of a warning system for coastal disaster prevention. The problem, however, is highly challenging owing to the presence of complex ocean surface processes under the action of unusually strong wind. In this study, the existing atmospheric wave boundary layer model is significantly enhanced by including various effects of wave breaking. Both the effect of wave breaking on the dissipation of energy and its effect on the transfer of momentum within the atmospheric boundary layer are carefully formulated. The wind stress coefficients obtained with the enhanced model are shown to be in good agreement with the measurements in not only deep but also shallow waters. The enhanced atmospheric wave boundary layer model is coupled with ocean wave as well as circulation models to simulate typhoon-induced storm surges and storm waves in the Pearl River Delta region. The computational results show that the coupled model with improved evaluation of the wind stress is substantially advantageous when compared with existing approaches.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"11 14","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140227325","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Arctic Beaufort Gyre plays a critical role for climate and marine ecosystems. This study investigates the response of the liquid freshwater in the Beaufort Gyre to various wind perturbations using numerical simulations. A new diagnostic call ‘freshwater renewal’ is introduced, which quantifies the amount of freshwater that has entered the Beaufort Gyre since a specific point in time. The findings reveal that the process of freshwater renewal is persistently efficient in the Beaufort Gyre region, occurring irrespective of the gyre’s status. The spatial distribution of freshwater renewal varies, influenced by factors such as wind forcing and gyre circulation patterns. Cyclonic wind perturbation associated with a negative Beaufort High sea level pressure anomaly triggers freshwater release from the Beaufort Gyre, with freshwater export and renewal dependent on wind perturbation locations and timescales. While some released Beaufort Gyre freshwater exits the Arctic Ocean through Davis and Fram straits, a considerable portion could remain within the Arctic Ocean for many years under specific conditions. Wind perturbation associated with the positive Arctic Oscillation enhances Arctic export of Beaufort Gyre freshwater, mainly through Fram Strait. The Arctic export of total freshwater and the Arctic export of the portion originating from the Beaufort Gyre have different timescales and magnitudes. Hence, it is essential to collectively examine different freshwater components in order to assess the role of Arctic export in the climate system.
{"title":"On the release and renewal of freshwater in the Beaufort Gyre of the Arctic Ocean","authors":"Qiang Wang","doi":"10.1175/jpo-d-23-0184.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0184.1","url":null,"abstract":"\u0000The Arctic Beaufort Gyre plays a critical role for climate and marine ecosystems. This study investigates the response of the liquid freshwater in the Beaufort Gyre to various wind perturbations using numerical simulations. A new diagnostic call ‘freshwater renewal’ is introduced, which quantifies the amount of freshwater that has entered the Beaufort Gyre since a specific point in time. The findings reveal that the process of freshwater renewal is persistently efficient in the Beaufort Gyre region, occurring irrespective of the gyre’s status. The spatial distribution of freshwater renewal varies, influenced by factors such as wind forcing and gyre circulation patterns. Cyclonic wind perturbation associated with a negative Beaufort High sea level pressure anomaly triggers freshwater release from the Beaufort Gyre, with freshwater export and renewal dependent on wind perturbation locations and timescales. While some released Beaufort Gyre freshwater exits the Arctic Ocean through Davis and Fram straits, a considerable portion could remain within the Arctic Ocean for many years under specific conditions. Wind perturbation associated with the positive Arctic Oscillation enhances Arctic export of Beaufort Gyre freshwater, mainly through Fram Strait. The Arctic export of total freshwater and the Arctic export of the portion originating from the Beaufort Gyre have different timescales and magnitudes. Hence, it is essential to collectively examine different freshwater components in order to assess the role of Arctic export in the climate system.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"28 12","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140225991","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Lightening of bottom water is required to close the abyssal overturning circulation, believed to play an important role in the climate system. A tracer release experiment and turbulence measurement programs have revealed how bottom water is lightened, and illuminated the associated circulation in the deep Brazil Basin, a representative region of the global ocean. Tracer was released on an isopycnal surface about 4000 m deep, over one of the fracture zones emanating from the Mid-Atlantic Ridge (MAR). Tracer that mixed toward the bottom moved toward the MAR across isopycnal surfaces that bend down to intersect the bottom at a rate implying a near-bottom buoyancy flux of 1.5 × 10−9 m2/s3, somewhat larger than inferred from dissipation measurements. The diffusivity at the level of the tracer release is estimated at 4.4 ± 1 × 10−4 m2/s, again larger than inferred from dissipation rates. The main patch moved southwest at about 2 cm/s while sinking due to the divergence of buoyancy flux above the bottom layer. The isopycnal eddy diffusivity was about 100 m2/s. Westward flow away from the MAR is the return flow balancing the eastward near-bottom upslope flow. The southward component of the flow is roughly consistent with conservation of potential vorticity. The circulation as well as the pattern of diapycnal flux are qualitatively the same as in St. Laurent et al (2001) but are more robust. The results indicate that diapycnal diffusivity is about twice that invoked by Morris et al. (2001) in calculating the basin-wide buoyancy budget.
{"title":"The Brazil Basin Tracer Release Experiment: Observations","authors":"James R. Ledwell (deceased)","doi":"10.1175/jpo-d-22-0249.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0249.1","url":null,"abstract":"\u0000Lightening of bottom water is required to close the abyssal overturning circulation, believed to play an important role in the climate system. A tracer release experiment and turbulence measurement programs have revealed how bottom water is lightened, and illuminated the associated circulation in the deep Brazil Basin, a representative region of the global ocean. Tracer was released on an isopycnal surface about 4000 m deep, over one of the fracture zones emanating from the Mid-Atlantic Ridge (MAR). Tracer that mixed toward the bottom moved toward the MAR across isopycnal surfaces that bend down to intersect the bottom at a rate implying a near-bottom buoyancy flux of 1.5 × 10−9 m2/s3, somewhat larger than inferred from dissipation measurements. The diffusivity at the level of the tracer release is estimated at 4.4 ± 1 × 10−4 m2/s, again larger than inferred from dissipation rates.\u0000The main patch moved southwest at about 2 cm/s while sinking due to the divergence of buoyancy flux above the bottom layer. The isopycnal eddy diffusivity was about 100 m2/s. Westward flow away from the MAR is the return flow balancing the eastward near-bottom upslope flow. The southward component of the flow is roughly consistent with conservation of potential vorticity. The circulation as well as the pattern of diapycnal flux are qualitatively the same as in St. Laurent et al (2001) but are more robust. The results indicate that diapycnal diffusivity is about twice that invoked by Morris et al. (2001) in calculating the basin-wide buoyancy budget.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"122 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140238013","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We assess the representation of mesoscale stirring in a suite of models against an estimate derived from microstructure data collected during the North Atlantic Tracer Release Experiment (NATRE). We draw heavily from the approximate temperature variance budget framework of Ferrari and Polzin (2005, Journal of Physical Oceanography). This framework assumes two sources of temperature variance away from boundaries: first, the vertical stirring of the large-scale mean vertical gradient by small-scale turbulence; and second, the lateral stirring of large-scale mean along-isopycnal gradients by mesoscale eddies. Temperature variance so produced is transformed and on average transferred down scales for ultimate dissipation at the microscale at a rate χ estimated using microstructure observations. Ocean models represent these pathways by a vertical mixing parameterization, and an along-isopycnal lateral mixing parameterization (if needed). We assess the rate of variance production by the latter as a residual from the NATRE dataset, and compare against the parameterized representations in a suite of model simulations. We find that variance production due to lateral stirring in a POP2 1/10° simulation agrees well, to within the estimated error bars, with that inferred from the NATRE estimate. A POP2 1° simulation and the ECCOV4r4 simulation appear to dissipate an order of magnitude too much variance by applying a lateral diffusivity, when compared to the NATRE estimate, particularly below 1250 m. The ECCOV4r4 adjusted lateral diffusivities are elevated where the microstructure suggests elevated χ sourced from mesoscale stirring. Such elevated values are absent in other diffusivity estimates suggesting the possibility of compensating errors and caution in interpreting ECCOV4r4’s adjusted lateral diffusivities.
{"title":"Assessing modelled mesoscale stirring using microscale observations","authors":"D. A. Cherian, Y. Guo, F. O. Bryan","doi":"10.1175/jpo-d-23-0135.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0135.1","url":null,"abstract":"\u0000We assess the representation of mesoscale stirring in a suite of models against an estimate derived from microstructure data collected during the North Atlantic Tracer Release Experiment (NATRE). We draw heavily from the approximate temperature variance budget framework of Ferrari and Polzin (2005, Journal of Physical Oceanography). This framework assumes two sources of temperature variance away from boundaries: first, the vertical stirring of the large-scale mean vertical gradient by small-scale turbulence; and second, the lateral stirring of large-scale mean along-isopycnal gradients by mesoscale eddies. Temperature variance so produced is transformed and on average transferred down scales for ultimate dissipation at the microscale at a rate χ estimated using microstructure observations. Ocean models represent these pathways by a vertical mixing parameterization, and an along-isopycnal lateral mixing parameterization (if needed). We assess the rate of variance production by the latter as a residual from the NATRE dataset, and compare against the parameterized representations in a suite of model simulations. We find that variance production due to lateral stirring in a POP2 1/10° simulation agrees well, to within the estimated error bars, with that inferred from the NATRE estimate. A POP2 1° simulation and the ECCOV4r4 simulation appear to dissipate an order of magnitude too much variance by applying a lateral diffusivity, when compared to the NATRE estimate, particularly below 1250 m. The ECCOV4r4 adjusted lateral diffusivities are elevated where the microstructure suggests elevated χ sourced from mesoscale stirring. Such elevated values are absent in other diffusivity estimates suggesting the possibility of compensating errors and caution in interpreting ECCOV4r4’s adjusted lateral diffusivities.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"7 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140244983","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The interactions between oceanic mesoscale eddies, submesoscale currents, and internal gravity waves (IWs) are investigated in submesoscale resolving realistic simulations in the North Atlantic Ocean. Using a novel analysis framework that couples the coarse-graining method in space with temporal filtering and a Helmholtz decomposition, we quantify the effects of the interactions on the cross-scale kinetic energy (KE) and enstrophy fluxes. By systematically comparing solutions with and without IW forcing we show that externally-forced IWs stimulate a reduction in the KE inverse cascade associated with mesoscale rotational motions and an enhancement in the KE forward cascade associated with divergent submesoscale currents – i.e., a stimulated cascade process. The corresponding IW effects on the enstrophy fluxes are seasonally dependent, with a stimulated reduction (enhancement) in the forward enstrophy cascade during summer (winter). Direct KE and enstrophy transfers from currents to IWs are also found, albeit with weaker magnitudes compared with the stimulated cascades. We further find that the forward KE and enstrophy fluxes associated with IW motions are almost entirely driven by scattering of the waves by the rotational eddy field, rather than by wave-wave interactions. This process is investigated in detail in a companion manuscript. Finally, we demonstrate that the stimulated cascades are spatially localized in coherent structures. Specifically, the magnitude and direction of the bi-directional KE fluxes at submesoscales are highly correlated with, and inversely proportional to, divergence-dominated circulations, and the inverse KE fluxes at mesoscales are highly correlated with strain dominated circulations. The predominantly forward enstrophy fluxes in both seasons are also correlated with strain dominated flow structures.
在北大西洋次中尺度解析模拟中研究了海洋中尺度漩涡、次中尺度海流和内重力波（IWs）之间的相互作用。我们采用一种新的分析框架，将空间粗粒度方法与时间滤波和亥姆霍兹分解相结合，量化了相互作用对跨尺度动能（KE）和熵通量的影响。通过系统地比较有 IW 强迫和无 IW 强迫的解，我们发现外力强迫的 IW 会减少与中尺度旋转运动相关的 KE 逆级联，增强与发散的次中尺度海流相关的 KE 正级联--即一个受刺激的级联过程。相应的 IW 对营养盐通量的影响与季节有关，夏季（冬季）受刺激的营养盐前向级联减少（增强）。我们还发现了从海流到 IW 的直接 KE 和营养富集转移，尽管与受激级联相比，转移幅度较小。我们还发现，与 IW 运动相关的前向 KE 和营养盐通量几乎完全是由旋转涡场对波浪的散射驱动的，而不是由波浪相互作用驱动的。我们将在另一篇手稿中详细研究这一过程。最后，我们证明了受激级联在相干结构中的空间定位。具体来说，亚中尺度的双向 KE 通量的大小和方向与发散主导的环流高度相关，并且成反比；中尺度的反向 KE 通量与应变主导的环流高度相关。两个季节中主要的正向能量通量也与以应变为主的气流结构相关。
{"title":"Eddy - Internal Wave Interactions: Stimulated Cascades in Cross-scale Kinetic Energy and Enstrophy Fluxes","authors":"R. Barkan, K. Srinivasan, J. McWilliams","doi":"10.1175/jpo-d-23-0191.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0191.1","url":null,"abstract":"\u0000The interactions between oceanic mesoscale eddies, submesoscale currents, and internal gravity waves (IWs) are investigated in submesoscale resolving realistic simulations in the North Atlantic Ocean. Using a novel analysis framework that couples the coarse-graining method in space with temporal filtering and a Helmholtz decomposition, we quantify the effects of the interactions on the cross-scale kinetic energy (KE) and enstrophy fluxes. By systematically comparing solutions with and without IW forcing we show that externally-forced IWs stimulate a reduction in the KE inverse cascade associated with mesoscale rotational motions and an enhancement in the KE forward cascade associated with divergent submesoscale currents – i.e., a stimulated cascade process. The corresponding IW effects on the enstrophy fluxes are seasonally dependent, with a stimulated reduction (enhancement) in the forward enstrophy cascade during summer (winter). Direct KE and enstrophy transfers from currents to IWs are also found, albeit with weaker magnitudes compared with the stimulated cascades. We further find that the forward KE and enstrophy fluxes associated with IW motions are almost entirely driven by scattering of the waves by the rotational eddy field, rather than by wave-wave interactions. This process is investigated in detail in a companion manuscript. Finally, we demonstrate that the stimulated cascades are spatially localized in coherent structures. Specifically, the magnitude and direction of the bi-directional KE fluxes at submesoscales are highly correlated with, and inversely proportional to, divergence-dominated circulations, and the inverse KE fluxes at mesoscales are highly correlated with strain dominated circulations. The predominantly forward enstrophy fluxes in both seasons are also correlated with strain dominated flow structures.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"12 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140249037","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Wenda Zhang, S. Griffies, R. Hallberg, Yi-Hung Kuo, C. L. Wolfe
The vertical structure of ocean eddies is generally surface-intensified, commonly attributed to the dominant baroclinic modes arising from the boundary conditions (BCs). Conventional BC considerations mostly focus on either flat- or rough-bottom conditions. The impact of surface buoyancy anomalies—often represented by surface potential vorticity (PV) anomalies—has not been fully explored. Here, we study the role of the surface PV in setting the vertical distribution of eddy kinetic energy (EKE) in an idealized adiabatic ocean model driven by wind stress. The simulated EKE profile in the extra-tropical ocean tends to peak at the surface and have an e-folding depth typically smaller than half of the ocean depth. This vertical structure can be reasonably represented by a single surface quasi-geostrophic (SQG) mode at the energy-containing scale resulting from the large-scale PV structure. Due to isopycnal outcropping and interior PV homogenization, the surface meridional PV gradient is substantially stronger than the interior PV gradient, yielding surface-trapped baroclinically unstable modes with horizontal scales comparable to or smaller than the deformation radius. These surface-trapped eddies then grow in size both horizontally and vertically through an inverse energy cascade up to the energy-containing scale, which dominates the vertical distribution of EKE. As for smaller horizontal scales, the EKE distribution decays faster with depth. Guided by this interpretation, an SQG-based scale-aware parameterization of the EKE profile is proposed. Preliminary offline diagnosis of a high-resolution simulation shows the proposed scheme successfully reproducing the dependence of the vertical structure of EKE on the horizontal grid resolution.
{"title":"The role of surface potential vorticity in the vertical structure of mesoscale eddies in wind-driven ocean circulations","authors":"Wenda Zhang, S. Griffies, R. Hallberg, Yi-Hung Kuo, C. L. Wolfe","doi":"10.1175/jpo-d-23-0203.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0203.1","url":null,"abstract":"\u0000The vertical structure of ocean eddies is generally surface-intensified, commonly attributed to the dominant baroclinic modes arising from the boundary conditions (BCs). Conventional BC considerations mostly focus on either flat- or rough-bottom conditions. The impact of surface buoyancy anomalies—often represented by surface potential vorticity (PV) anomalies—has not been fully explored. Here, we study the role of the surface PV in setting the vertical distribution of eddy kinetic energy (EKE) in an idealized adiabatic ocean model driven by wind stress. The simulated EKE profile in the extra-tropical ocean tends to peak at the surface and have an e-folding depth typically smaller than half of the ocean depth. This vertical structure can be reasonably represented by a single surface quasi-geostrophic (SQG) mode at the energy-containing scale resulting from the large-scale PV structure. Due to isopycnal outcropping and interior PV homogenization, the surface meridional PV gradient is substantially stronger than the interior PV gradient, yielding surface-trapped baroclinically unstable modes with horizontal scales comparable to or smaller than the deformation radius. These surface-trapped eddies then grow in size both horizontally and vertically through an inverse energy cascade up to the energy-containing scale, which dominates the vertical distribution of EKE. As for smaller horizontal scales, the EKE distribution decays faster with depth. Guided by this interpretation, an SQG-based scale-aware parameterization of the EKE profile is proposed. Preliminary offline diagnosis of a high-resolution simulation shows the proposed scheme successfully reproducing the dependence of the vertical structure of EKE on the horizontal grid resolution.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"8 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140253969","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The vertical buoyancy flux (Bf) under the turbulent thermal wind (TTW) balance, BfTTW, plays an important role in restratifying the surface mixed layer in winter. So far most of the global ocean models are too coarse to resolve this process. In this paper, a scale-aware parameterization is proposed for BfTTW and implemented in a hierarchy of regional ocean simulations over the winter Kuroshio extension with horizontal resolutions ranging from 27 km to 1 km. The parameterization depends on the Coriolis parameter, model-simulated turbulent vertical viscosity, horizontal density gradient and a scaling relationship to adjust for the effects of model horizontal resolution on the simulated horizontal density gradient. It shows good skills in reconciling the difference between BfTTW in the coarse-resolution simulations (27 km, 9km and 3 km) and in the 1-km simulation where BfTTW is well resolved. Furthermore, implementation of the parameterization improves the simulated stratification in the surface mixed layer in coarse-resolution simulations.
{"title":"A Scale-aware Parameterization of Restratification Effect of Turbulent Thermal Wind Balance","authors":"P. Yang, Z. Jing, Haiyuan Yang, Lixin Wu","doi":"10.1175/jpo-d-23-0169.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0169.1","url":null,"abstract":"\u0000The vertical buoyancy flux (Bf) under the turbulent thermal wind (TTW) balance, BfTTW, plays an important role in restratifying the surface mixed layer in winter. So far most of the global ocean models are too coarse to resolve this process. In this paper, a scale-aware parameterization is proposed for BfTTW and implemented in a hierarchy of regional ocean simulations over the winter Kuroshio extension with horizontal resolutions ranging from 27 km to 1 km. The parameterization depends on the Coriolis parameter, model-simulated turbulent vertical viscosity, horizontal density gradient and a scaling relationship to adjust for the effects of model horizontal resolution on the simulated horizontal density gradient. It shows good skills in reconciling the difference between BfTTW in the coarse-resolution simulations (27 km, 9km and 3 km) and in the 1-km simulation where BfTTW is well resolved. Furthermore, implementation of the parameterization improves the simulated stratification in the surface mixed layer in coarse-resolution simulations.","PeriodicalId":506940,"journal":{"name":"Journal of Physical Oceanography","volume":"13 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140254015","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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