L. Cavaleri, S. Langodan, P. Pezzutto, A. Benetazzo
We have explored the earliest stages of wind wave generation in the open sea, from the first initial wavelets appearing on an otherwise flat surface or low smooth undulations till the practically fully developed conditions for the very low range of wind speeds we have considered. We suggest the minimal wind speed for the appearance of the first wavelets to be close to 1.8 ms−1. The peculiar conditions associated to the development of coastal sea breezes allow us to consider the local waves as generated under time-limited conditions. The 2D spectra measured during these very early stages provide the first evidence of an active Phillips process generation in the field. After appearing in these very early stages, wavelets quickly disappear as soon as the developing wind waves take a leading role. We suggest that this process is due to the strong spatial gradients in the surface orbital velocity, which impedes the instability mechanism at the base of their formation, while at a later stage of development, these gradients decrease and wavelets reappear. In a decadal perspective, the progressive decrease of the intensity of the sea breezes in the northern Adriatic Sea, where we have carried out our measurements, is associated to the steadily milder winters, and therefore not sufficiently cold local sea temperatures in early summer.
{"title":"The earliest stages of wind wave generation in the open sea","authors":"L. Cavaleri, S. Langodan, P. Pezzutto, A. Benetazzo","doi":"10.1175/jpo-d-23-0217.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0217.1","url":null,"abstract":"\u0000We have explored the earliest stages of wind wave generation in the open sea, from the first initial wavelets appearing on an otherwise flat surface or low smooth undulations till the practically fully developed conditions for the very low range of wind speeds we have considered. We suggest the minimal wind speed for the appearance of the first wavelets to be close to 1.8 ms−1. The peculiar conditions associated to the development of coastal sea breezes allow us to consider the local waves as generated under time-limited conditions. The 2D spectra measured during these very early stages provide the first evidence of an active Phillips process generation in the field. After appearing in these very early stages, wavelets quickly disappear as soon as the developing wind waves take a leading role. We suggest that this process is due to the strong spatial gradients in the surface orbital velocity, which impedes the instability mechanism at the base of their formation, while at a later stage of development, these gradients decrease and wavelets reappear. In a decadal perspective, the progressive decrease of the intensity of the sea breezes in the northern Adriatic Sea, where we have carried out our measurements, is associated to the steadily milder winters, and therefore not sufficiently cold local sea temperatures in early summer.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139621310","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}
Carlos Conejero, Lionel Renault, Fabien Desbiolles, J. McWilliams, Hervé Giordani
Current Feedback (CFB) and Thermal Feedback (TFB) have been shown to strongly influence both atmospheric and oceanic dynamics at the oceanic mesoscale (10-250 km). At smaller scales, oceanic submesoscale currents (SMCs, 0.1-10 km) have a major influence on the ocean’s energy budget, variability, and ecosystems. However, submesoscale air-sea interactions are not well understood due to observational and modeling limitations related to their scales. Here, we use realistic submesoscale-permitting coupled oceanic and atmospheric model to quantify the spatiotemporal variability of TFB and CFB coupling in the Northwest Tropical Atlantic. While CFB still acts as a submesoscale eddy killer by inducing an energy sink from the SMCs to the atmosphere, it appears to be more efficient at the submesoscale by approximately 30% than at the mesoscale. Submesoscale CFB affects the surface stress, however, the finite timescale of SMCs for adjusting the atmospheric boundary layer results in a diminished low-level wind response, weakening partial ocean re-energization by about 70%. Unlike at the mesoscale, submesoscale CFB induces stress/wind convergence/divergence, influencing the atmospheric boundary layer through vertical motions. The linear relationship between the surface stress (wind) derivative fields and sea surface temperature gradients, widespread at the mesoscale, decreases by approximately 35% ±7% (77% ±10%) at the submesoscale. Additionally, submesoscale TFB induces turbulent heat fluxes comparable to those at the mesoscale. Seasonal variability in meso- and submesoscale CFB and TFB coupling is mostly related to background wind speed. Finally, disentangling submesoscale CFB and TFB is challenging because they can reinforce or counteract each other.
{"title":"Near-Surface Atmospheric Response to Meso- and Submesoscale Current and Thermal Feedbacks","authors":"Carlos Conejero, Lionel Renault, Fabien Desbiolles, J. McWilliams, Hervé Giordani","doi":"10.1175/jpo-d-23-0211.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0211.1","url":null,"abstract":"\u0000Current Feedback (CFB) and Thermal Feedback (TFB) have been shown to strongly influence both atmospheric and oceanic dynamics at the oceanic mesoscale (10-250 km). At smaller scales, oceanic submesoscale currents (SMCs, 0.1-10 km) have a major influence on the ocean’s energy budget, variability, and ecosystems. However, submesoscale air-sea interactions are not well understood due to observational and modeling limitations related to their scales. Here, we use realistic submesoscale-permitting coupled oceanic and atmospheric model to quantify the spatiotemporal variability of TFB and CFB coupling in the Northwest Tropical Atlantic. While CFB still acts as a submesoscale eddy killer by inducing an energy sink from the SMCs to the atmosphere, it appears to be more efficient at the submesoscale by approximately 30% than at the mesoscale. Submesoscale CFB affects the surface stress, however, the finite timescale of SMCs for adjusting the atmospheric boundary layer results in a diminished low-level wind response, weakening partial ocean re-energization by about 70%. Unlike at the mesoscale, submesoscale CFB induces stress/wind convergence/divergence, influencing the atmospheric boundary layer through vertical motions. The linear relationship between the surface stress (wind) derivative fields and sea surface temperature gradients, widespread at the mesoscale, decreases by approximately 35% ±7% (77% ±10%) at the submesoscale. Additionally, submesoscale TFB induces turbulent heat fluxes comparable to those at the mesoscale. Seasonal variability in meso- and submesoscale CFB and TFB coupling is mostly related to background wind speed. Finally, disentangling submesoscale CFB and TFB is challenging because they can reinforce or counteract each other.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139531882","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}
Observations from a tidal estuary show that tidal intrusion fronts occur regularly during flood tides near topographic features including constrictions and bends. A realistic model is used to study the generation of these fronts and their influence on stratification and mixing in the estuary. At the constriction, flow separation occurs on both sides of the jet flow downstream of the narrow opening, leading to sharp lateral salinity gradients and baroclinic secondary circulation. A tidal intrusion front, with a V-shaped convergence zone on the surface, is generated by the interaction between secondary circulation and the jet flow. Stratification is created at the front due to the straining of lateral salinity gradients by secondary circulation. Though stratification is expected to suppress turbulence, strong turbulent mixing is found near the surface front. The intense mixing is attributed to enhanced vertical shear due to both frontal baroclinicity and the twisting of lateral shear by secondary circulation. In the bend, flow separation occurs along the inner bank, resulting in lateral salinity gradients, secondary circulation, frontogenesis, and enhanced mixing near the front. In contrast to the V-shaped front at the constriction, an oblique linear surface convergence front occurs in the bend, which resembles a one-sided tidal intrusion front. Moreover, in addition to baroclinicity, channel curvature also affects secondary circulation, frontogenesis, and mixing in the bend. Overall in the estuary, the near-surface mixing associated with tidal intrusion fronts during flood tides is similar in magnitude to bottom boundary layer mixing that occurs primarily during ebbs.
潮汐河口的观测结果表明，潮汐入侵前沿经常在洪潮期间出现在地形特征（包括收缩和弯曲）附近。我们采用了一个逼真的模型来研究这些前沿的产生及其对河口分层和混合的影响。在收缩处，狭窄开口下游的喷射流两侧会发生水流分离，从而导致急剧的侧向盐度梯度和巴氏次级环流。在次级环流和喷射流的相互作用下，潮汐侵入前沿在海面上形成一个 V 形汇聚区。由于次级环流对侧向盐度梯度的挤压，在前沿形成了分层。虽然预计分层会抑制湍流，但在表层前沿附近发现了强烈的湍流混合。这种强烈的混合是由于锋面气压和次级环流对侧向剪切力的扭曲造成的垂直剪切力增强所致。在弯曲处，流体沿内岸发生分离，导致横向盐度梯度、次级环流、锋面生成和锋面附近混合增强。与收缩处的 V 形锋面不同，弯曲处出现了斜线形表面辐合锋面，类似于单侧潮汐入侵锋面。此外，除了气压线性，河道曲率也会影响弯道内的次级环流、锋面生成和混合。总体而言，在河口，洪潮期间与潮汐入侵前沿相关的近表面混合在程度上与主要发生在退潮期间的底部边界层混合相似。
{"title":"Tidal intrusion fronts, surface convergence, and mixing in an estuary with complex topography","authors":"Tong Bo, David K. Ralston, A. Garcia, W. Geyer","doi":"10.1175/jpo-d-23-0131.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0131.1","url":null,"abstract":"\u0000Observations from a tidal estuary show that tidal intrusion fronts occur regularly during flood tides near topographic features including constrictions and bends. A realistic model is used to study the generation of these fronts and their influence on stratification and mixing in the estuary. At the constriction, flow separation occurs on both sides of the jet flow downstream of the narrow opening, leading to sharp lateral salinity gradients and baroclinic secondary circulation. A tidal intrusion front, with a V-shaped convergence zone on the surface, is generated by the interaction between secondary circulation and the jet flow. Stratification is created at the front due to the straining of lateral salinity gradients by secondary circulation. Though stratification is expected to suppress turbulence, strong turbulent mixing is found near the surface front. The intense mixing is attributed to enhanced vertical shear due to both frontal baroclinicity and the twisting of lateral shear by secondary circulation. In the bend, flow separation occurs along the inner bank, resulting in lateral salinity gradients, secondary circulation, frontogenesis, and enhanced mixing near the front. In contrast to the V-shaped front at the constriction, an oblique linear surface convergence front occurs in the bend, which resembles a one-sided tidal intrusion front. Moreover, in addition to baroclinicity, channel curvature also affects secondary circulation, frontogenesis, and mixing in the bend. Overall in the estuary, the near-surface mixing associated with tidal intrusion fronts during flood tides is similar in magnitude to bottom boundary layer mixing that occurs primarily during ebbs.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139533559","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}
Yifan Wang, S. Guan, Zhiwei Zhang, Chun Zhou, Xing Xu, Chuncheng Guo, Wei Zhao, Jiwei Tian
Based on year-long observations from three moorings at 12°N, 14°N, and 16°N in the northwest Pacific, this study presents observational evidence for the occurrence and behavior of parametric subharmonic instability (PSI) of diurnal internal tides (ITs) both in the upper and abyssal ocean around the critical latitudes (O1 IT: 13.44°N; K1 IT: 14.52°N), which is relatively less explored compared with PSI of M2 ITs. At 14°N, near-inertial waves (NIWs) feature a “checkerboard” pattern with comparable upward- and downward-propagating components, while the diurnal ITs mainly feature a low-mode structure. The near-inertial kinetic energy (NIKE) at 14°N, correlated fairly well with the diurnal KE, is the largest among three moorings. The bicoherence analysis, and a causality analysis method newly introduced here, both show statistically significant phase locking between PSI triads at 14°N, while no significant signals emerge at 12°N and 16°N. The estimated PSI energy transfer rate shows a net energy transfer from diurnal ITs to NIWs with an annual-mean value of 1.5 × 10−10 W kg−1. The highly-sheared NIWs generated by PSI result in a 2–6 times larger probability of shear instability events at 14°N than 12°N and 16°N. Through swinging the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, the passages of anticyclonic and cyclonic eddies both result in elevated NIWs and shear instability events by enhancing PSI efficiency. Particularly, different from the general understanding that cyclonic eddies usually expel NIWs, enhanced NIWs and instability are observed within cyclonic eddies whose relative vorticity can modify PSI efficiency.
{"title":"Observations of Parametric Subharmonic Instability of Diurnal Internal Tides in the Northwest Pacific","authors":"Yifan Wang, S. Guan, Zhiwei Zhang, Chun Zhou, Xing Xu, Chuncheng Guo, Wei Zhao, Jiwei Tian","doi":"10.1175/jpo-d-23-0055.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0055.1","url":null,"abstract":"\u0000Based on year-long observations from three moorings at 12°N, 14°N, and 16°N in the northwest Pacific, this study presents observational evidence for the occurrence and behavior of parametric subharmonic instability (PSI) of diurnal internal tides (ITs) both in the upper and abyssal ocean around the critical latitudes (O1 IT: 13.44°N; K1 IT: 14.52°N), which is relatively less explored compared with PSI of M2 ITs. At 14°N, near-inertial waves (NIWs) feature a “checkerboard” pattern with comparable upward- and downward-propagating components, while the diurnal ITs mainly feature a low-mode structure. The near-inertial kinetic energy (NIKE) at 14°N, correlated fairly well with the diurnal KE, is the largest among three moorings. The bicoherence analysis, and a causality analysis method newly introduced here, both show statistically significant phase locking between PSI triads at 14°N, while no significant signals emerge at 12°N and 16°N. The estimated PSI energy transfer rate shows a net energy transfer from diurnal ITs to NIWs with an annual-mean value of 1.5 × 10−10 W kg−1. The highly-sheared NIWs generated by PSI result in a 2–6 times larger probability of shear instability events at 14°N than 12°N and 16°N. Through swinging the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, the passages of anticyclonic and cyclonic eddies both result in elevated NIWs and shear instability events by enhancing PSI efficiency. Particularly, different from the general understanding that cyclonic eddies usually expel NIWs, enhanced NIWs and instability are observed within cyclonic eddies whose relative vorticity can modify PSI efficiency.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139443063","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}
Past work has shown that interannual California coastal sea level variability is mostly of equatorial origin, and decades of satellite sea surface height (SSH) and in situ dynamic height observations indicate that this interannual signal propagates westward from the California coast as nondispersive Rossby waves (RWs). These observations agree with standard linear vertical mode theory except that even when mean flow and bottom topography are considered, the fastest baroclinic vertical mode RW in each case is always much slower (1.6 – 2.3 cm/s) than the observed 4.2 cm/s. This order one disagreement is only resolved if the standard bottom boundary condition that the vertical velocity w′ = 0 is replaced by perturbation pressure p′ = 0. Zero p′ is an appropriate bottom boundary condition because south of San Francisco the northeastern Pacific Ocean boundary acts approximately like an impermeable vertical wall to the interannual equatorial wave signal, and therefore equatorial quasi-geostrophic p′ is horizontally constant along the boundary. Thus if equatorial p′ = 0 at the bottom, then this condition also applies off California. The large-scale equatorial ocean boundary signal is due to wind-forced eastward group velocity equatorial Kelvin waves, which at interannual and lower frequencies propagate at such a shallow angle to the horizontal that none of the baroclinic equatorial Kelvin wave signal reaches the ocean floor before striking the eastern Pacific boundary. Off California this signal can thus be approximated by a first baroclinic mode with p′ = 0 at the bottom, and hence the long RW speed there agrees with that observed (both approximately 4.2 cm/s).
{"title":"Why is the Westward Rossby Wave Propagation from the California Coast “Too Fast”?","authors":"A. J. Clarke, Sean Buchanan","doi":"10.1175/jpo-d-23-0024.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0024.1","url":null,"abstract":"\u0000Past work has shown that interannual California coastal sea level variability is mostly of equatorial origin, and decades of satellite sea surface height (SSH) and in situ dynamic height observations indicate that this interannual signal propagates westward from the California coast as nondispersive Rossby waves (RWs). These observations agree with standard linear vertical mode theory except that even when mean flow and bottom topography are considered, the fastest baroclinic vertical mode RW in each case is always much slower (1.6 – 2.3 cm/s) than the observed 4.2 cm/s. This order one disagreement is only resolved if the standard bottom boundary condition that the vertical velocity w′ = 0 is replaced by perturbation pressure p′ = 0. Zero p′ is an appropriate bottom boundary condition because south of San Francisco the northeastern Pacific Ocean boundary acts approximately like an impermeable vertical wall to the interannual equatorial wave signal, and therefore equatorial quasi-geostrophic p′ is horizontally constant along the boundary. Thus if equatorial p′ = 0 at the bottom, then this condition also applies off California. The large-scale equatorial ocean boundary signal is due to wind-forced eastward group velocity equatorial Kelvin waves, which at interannual and lower frequencies propagate at such a shallow angle to the horizontal that none of the baroclinic equatorial Kelvin wave signal reaches the ocean floor before striking the eastern Pacific boundary. Off California this signal can thus be approximated by a first baroclinic mode with p′ = 0 at the bottom, and hence the long RW speed there agrees with that observed (both approximately 4.2 cm/s).","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139442863","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}
T. Uchida, Q. Jamet, W. Dewar, B. Deremble, A. Poje, Luolin Sun
We examine the ocean energy cycle where the eddies are defined about the ensemble mean of a partially air-sea coupled, eddy-rich ensemble simulation of the North Atlantic. The decomposition about the ensemble mean leads to a parameter-free definition of eddies, which is interpreted as the expression of oceanic chaos. Using the ensemble framework, we define the reservoirs of mean and eddy kinetic energy (MKE and EKE respectively) and mean total dynamic enthalpy (MTDE). We opt for the usage of dynamic enthalpy (DE) as a proxy for potential energy due to its dynamically consistent relation to hydrostatic pressure in Boussinesq fluids and non-reliance on any reference stratification. The curious result that emerges is that the potential energy reservoir cannot be decomposed into its mean and eddy components, and the eddy flux of DE can be absorbed into the EKE budget as pressure work. We find from the energy cycle that while baroclinic instability, associated with a positive vertical eddy buoyancy flux, tends to peak around February, EKE takes its maximum around September in the wind-driven gyre. Interestingly, the energy input from MKE to EKE, a process sometimes associated with barotropic processes, becomes larger than the vertical eddy buoyancy flux during the summer and autumn. Our results question the common notion that the inverse energy cascade of winter-time EKE energized by baroclinic instability within the mixed layer is solely responsible for the summer-to-autumn peak in EKE, and suggest that both the eddy transport of DE and transfer of energy from MKE to EKE contribute to the seasonal EKE maxima.
{"title":"Imprint of chaos on the ocean energy cycle from an eddying North Atlantic ensemble","authors":"T. Uchida, Q. Jamet, W. Dewar, B. Deremble, A. Poje, Luolin Sun","doi":"10.1175/jpo-d-23-0176.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0176.1","url":null,"abstract":"\u0000We examine the ocean energy cycle where the eddies are defined about the ensemble mean of a partially air-sea coupled, eddy-rich ensemble simulation of the North Atlantic. The decomposition about the ensemble mean leads to a parameter-free definition of eddies, which is interpreted as the expression of oceanic chaos. Using the ensemble framework, we define the reservoirs of mean and eddy kinetic energy (MKE and EKE respectively) and mean total dynamic enthalpy (MTDE). We opt for the usage of dynamic enthalpy (DE) as a proxy for potential energy due to its dynamically consistent relation to hydrostatic pressure in Boussinesq fluids and non-reliance on any reference stratification. The curious result that emerges is that the potential energy reservoir cannot be decomposed into its mean and eddy components, and the eddy flux of DE can be absorbed into the EKE budget as pressure work. We find from the energy cycle that while baroclinic instability, associated with a positive vertical eddy buoyancy flux, tends to peak around February, EKE takes its maximum around September in the wind-driven gyre. Interestingly, the energy input from MKE to EKE, a process sometimes associated with barotropic processes, becomes larger than the vertical eddy buoyancy flux during the summer and autumn. Our results question the common notion that the inverse energy cascade of winter-time EKE energized by baroclinic instability within the mixed layer is solely responsible for the summer-to-autumn peak in EKE, and suggest that both the eddy transport of DE and transfer of energy from MKE to EKE contribute to the seasonal EKE maxima.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139444658","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}
Wind-driven upwelling of cold, nutrient-rich water is a key feature near the eastern boundaries of major ocean basins, with significant implications for the local physical environment and marine ecosystems. Despite the traditional two-dimensional description of upwelling as a passive response to surface offshore Ekman transport, recent observations have revealed spatial variability in the circulation structures across different upwelling locations. Yet, a systematic understanding of the factors governing the spatial patterns of coastal upwelling remains elusive. Here, we demonstrate that coastal upwelling pathways are influenced by two pairs of competing factors. The first competition occurs between wind forcing and eddy momentum flux, which shapes the Eulerian-mean circulation; the second competition arises between the Eulerian-mean and eddy-induced circulation. The importance of nonlinear eddy momentum flux over sloping topography can be described by the local slope Burger number, S = αN/f, where α is the topographic slope angle and N and f are the buoyancy and Coriolis frequencies. When S is small, the classic coastal upwelling structure emerges in the residual circulation, where water upwells along the sloping bottom. However, this comes with the added complexity that mesoscale eddies may drive a subduction route back into the ocean interior. As S increases, the upwelling branch is increasingly suppressed, unable to reach the surface and instead directed offshore by the eddy-induced circulation. The sensitivity of upwelling structures to variable wind stress and surface buoyancy forcing is further explored. The diagnostics may help to improve our understanding of coastal upwelling systems and yield a more physical representation of coastal upwelling in coarse-resolution numerical models.
由风驱动的富含营养物质的冷水上升流是主要大洋盆地东部边界附近的一个主要特征，对当地物理环境和海洋生态系统具有重要影响。尽管传统上将上升流描述为对海面离岸埃克曼输运的被动响应，但最近的观测发现，不同上升流位置的环流结构存在空间差异。然而，对支配沿岸上升流空间模式的因素仍缺乏系统的了解。在这里，我们证明沿岸上升流的路径受到两对竞争因素的影响。第一对竞争发生在风力和涡动量通量之间，风力和涡动量通量塑造了欧拉平均环流；第二对竞争发生在欧拉平均环流和由涡引起的环流之间。在倾斜地形上，非线性涡动量的重要性可以用局部坡度伯格数 S = αN/f 来描述，其中 α 是地形坡角，N 和 f 是浮力频率和科里奥利频率。当 S 较小时，残余环流中会出现典型的沿岸上升流结构，即海水沿坡底上升。然而，这也增加了复杂性，即中尺度漩涡可能会推动潜流返回海洋内部。随着 S 的增加，上升流分支越来越受到抑制，无法到达海面，而是被漩涡引起的环流引向近海。进一步探讨了上升流结构对可变风应力和表面浮力强迫的敏感性。这种诊断方法有助于提高我们对沿岸上升流系统的认识，并能在粗分辨率数值模式中对沿岸 上升流进行更实际的描述。
{"title":"On the Pathways of Wind-Driven Coastal Upwelling: Nonlinear Momentum Flux and Baroclinic Instability","authors":"Dou Li, X. Ruan","doi":"10.1175/jpo-d-23-0098.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0098.1","url":null,"abstract":"Wind-driven upwelling of cold, nutrient-rich water is a key feature near the eastern boundaries of major ocean basins, with significant implications for the local physical environment and marine ecosystems. Despite the traditional two-dimensional description of upwelling as a passive response to surface offshore Ekman transport, recent observations have revealed spatial variability in the circulation structures across different upwelling locations. Yet, a systematic understanding of the factors governing the spatial patterns of coastal upwelling remains elusive. Here, we demonstrate that coastal upwelling pathways are influenced by two pairs of competing factors. The first competition occurs between wind forcing and eddy momentum flux, which shapes the Eulerian-mean circulation; the second competition arises between the Eulerian-mean and eddy-induced circulation. The importance of nonlinear eddy momentum flux over sloping topography can be described by the local slope Burger number, S = αN/f, where α is the topographic slope angle and N and f are the buoyancy and Coriolis frequencies. When S is small, the classic coastal upwelling structure emerges in the residual circulation, where water upwells along the sloping bottom. However, this comes with the added complexity that mesoscale eddies may drive a subduction route back into the ocean interior. As S increases, the upwelling branch is increasingly suppressed, unable to reach the surface and instead directed offshore by the eddy-induced circulation. The sensitivity of upwelling structures to variable wind stress and surface buoyancy forcing is further explored. The diagnostics may help to improve our understanding of coastal upwelling systems and yield a more physical representation of coastal upwelling in coarse-resolution numerical models.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139129314","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}
Idealized numerical simulations of the Kuroshio western boundary current flowing over the Hirase seamount were conducted to examine the mechanisms of phenomena observed by shipboard and mooring measurements. Along the Kuroshio, enhanced mixing [vertical diffusivity, Kρ = O(10−2) m2 s−1] was observed in a low-stratification layer between high-shear layers around low tide, and a V-shaped band of the negative vertical component of relative vorticity (ζz) was also observed. Those features were reproduced in simulations of the Kuroshio that included the D2 tide. In the simulation, a streak of negative ζz detached from the Hirase turned into vertically tilted 10-km-scale vortices. The buoyancy frequency squared (N2) budget at the mooring position showed that the low stratification was caused by vertical and horizontal advection and horizontal tilting. The Kρ tended to increase when the Ertel potential vorticity (PV) < 0, as expected given the inertial instability. However, the magnitude of Kρ also depended on the tidal phase near Hirase, and Kρ was increased in the high vertical shear zones at the periphery of vortices where a strain motion is large. These results indicate that not only inertial instability but also tidal and vertical shear effects are important for driving turbulent mixing. A basin-scale distribution of wind stress drives a strong surface-intensified current in the western part of each ocean basin, such as the Gulf Stream and the Kuroshio. This western boundary current is regarded as a place where the kinetic energy and vorticity generated by winds are dissipated, allowing the basin-scale circulation to keep a steady state, but its dissipation mechanisms are not well understood. To understand the mechanisms, we conducted idealized numerical simulations that isolate the interactions between a seamount and the current as well as tidal currents, and compared results with observations. Our findings provide insights into how the current transfers kinetic energy to smaller scales when it flows over a seamount.
{"title":"Numerical Simulation of the Kuroshio Flowing over the Hirase Seamount in the Tokara Strait in Autumn: Tidal Vortex Shedding in a Baroclinic Jet","authors":"R. Inoue, E. Tsutsumi, Hirohiko Nakamura","doi":"10.1175/jpo-d-23-0050.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0050.1","url":null,"abstract":"Idealized numerical simulations of the Kuroshio western boundary current flowing over the Hirase seamount were conducted to examine the mechanisms of phenomena observed by shipboard and mooring measurements. Along the Kuroshio, enhanced mixing [vertical diffusivity, Kρ = O(10−2) m2 s−1] was observed in a low-stratification layer between high-shear layers around low tide, and a V-shaped band of the negative vertical component of relative vorticity (ζz) was also observed. Those features were reproduced in simulations of the Kuroshio that included the D2 tide. In the simulation, a streak of negative ζz detached from the Hirase turned into vertically tilted 10-km-scale vortices. The buoyancy frequency squared (N2) budget at the mooring position showed that the low stratification was caused by vertical and horizontal advection and horizontal tilting. The Kρ tended to increase when the Ertel potential vorticity (PV) < 0, as expected given the inertial instability. However, the magnitude of Kρ also depended on the tidal phase near Hirase, and Kρ was increased in the high vertical shear zones at the periphery of vortices where a strain motion is large. These results indicate that not only inertial instability but also tidal and vertical shear effects are important for driving turbulent mixing. A basin-scale distribution of wind stress drives a strong surface-intensified current in the western part of each ocean basin, such as the Gulf Stream and the Kuroshio. This western boundary current is regarded as a place where the kinetic energy and vorticity generated by winds are dissipated, allowing the basin-scale circulation to keep a steady state, but its dissipation mechanisms are not well understood. To understand the mechanisms, we conducted idealized numerical simulations that isolate the interactions between a seamount and the current as well as tidal currents, and compared results with observations. Our findings provide insights into how the current transfers kinetic energy to smaller scales when it flows over a seamount.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139129771","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}
The present study adopts an energy-based approach to interpret the negative phase of Indian Ocean dipole (IOD) events. This is accomplished by diagnosing the output of hindcast experiments from 1958 to 2018 based on a linear ocean model. The authors have performed a composite analysis for a set of negative IOD (nIOD) events, distinguishing between independent nIOD events and concurrent nIOD events with El Niño–Southern Oscillation (ENSO). The focus is on investigating the mechanism of nIOD events in terms of wave energy transfer, employing a linear wave theory that considers the group velocity. The proposed diagnostic scheme offers a unified framework for studying the interaction between equatorial and off-equatorial waves. Both the first and third baroclinic modes exhibit interannual variations characterized by a distinct packet of eastward energy flux associated with equatorial Kelvin waves. During October–December, westerly wind anomalies induce the propagation of eastward-moving equatorial waves, leading to thermocline deepening in the central-eastern equatorial Indian Ocean, a feature absent during neutral IOD years. The development of wave energy demonstrates different patterns during nIOD events of various types. In concurrent nIOD–ENSO years, characterized by strong westerly winds, the intense eastward transfer of wave energy becomes prominent as early as October. This differs significantly from the situation manifested in independent nIOD years. The intensity of the energy-flux streamfunction/potential reaches its peak around November and then rapidly diminishes in December during both types of nIOD years. The present study provides an interpretation of wave energy transfer episodes in the upper ocean during the negative phase of the Indian Ocean dipole (IOD) based on the diagnosis of hindcast experiments. The results suggest that the reflection of Kelvin and Rossby waves at the eastern and western boundaries of the Indian Ocean (IO), respectively, accompanied by variations in thermocline depth, plays a crucial role in the development process of IOD events. Specifically, during the negative phase of the IOD, the tropical IO exhibits positive signals of energy-flux streamfunction in the Northern Hemisphere, along with positive signals of energy-flux potential associated with westerly wind anomalies occurring in October–December. These findings highlight the significance of these factors in shaping the characteristics of negative IOD events.
{"title":"Interpreting Negative IOD Events Based on the Transfer Routes of Wave Energy in the Upper Ocean","authors":"Zimeng Li, H. Aiki","doi":"10.1175/jpo-d-22-0267.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0267.1","url":null,"abstract":"The present study adopts an energy-based approach to interpret the negative phase of Indian Ocean dipole (IOD) events. This is accomplished by diagnosing the output of hindcast experiments from 1958 to 2018 based on a linear ocean model. The authors have performed a composite analysis for a set of negative IOD (nIOD) events, distinguishing between independent nIOD events and concurrent nIOD events with El Niño–Southern Oscillation (ENSO). The focus is on investigating the mechanism of nIOD events in terms of wave energy transfer, employing a linear wave theory that considers the group velocity. The proposed diagnostic scheme offers a unified framework for studying the interaction between equatorial and off-equatorial waves. Both the first and third baroclinic modes exhibit interannual variations characterized by a distinct packet of eastward energy flux associated with equatorial Kelvin waves. During October–December, westerly wind anomalies induce the propagation of eastward-moving equatorial waves, leading to thermocline deepening in the central-eastern equatorial Indian Ocean, a feature absent during neutral IOD years. The development of wave energy demonstrates different patterns during nIOD events of various types. In concurrent nIOD–ENSO years, characterized by strong westerly winds, the intense eastward transfer of wave energy becomes prominent as early as October. This differs significantly from the situation manifested in independent nIOD years. The intensity of the energy-flux streamfunction/potential reaches its peak around November and then rapidly diminishes in December during both types of nIOD years. The present study provides an interpretation of wave energy transfer episodes in the upper ocean during the negative phase of the Indian Ocean dipole (IOD) based on the diagnosis of hindcast experiments. The results suggest that the reflection of Kelvin and Rossby waves at the eastern and western boundaries of the Indian Ocean (IO), respectively, accompanied by variations in thermocline depth, plays a crucial role in the development process of IOD events. Specifically, during the negative phase of the IOD, the tropical IO exhibits positive signals of energy-flux streamfunction in the Northern Hemisphere, along with positive signals of energy-flux potential associated with westerly wind anomalies occurring in October–December. These findings highlight the significance of these factors in shaping the characteristics of negative IOD events.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139128140","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}
Daehyuk Kim, Hong-Ryeol Shin, Cheol-Ho Kim, Joowan Kim, Naoki Hirose
The effects of external forcing variation on the intrinsic variability in the upper layer circulation occurring within the East Sea (Japan Sea) and its physical mechanism are analyzed using numerical experiments. In this study, the experiments were conducted with climatological annual/monthly mean forcings (constant/seasonal forcings). The intrinsic variability is mainly distributed in the meandering regions around the main current path with the comparatively large variability limited to the southern region. The reason of greater intrinsic variability mainly in the southern part of the East Sea than in the northern part is that more energy is required from external forcings to change the thicker upper layer formed in the northern part due to seasonal forcings (strong wind stress and surface heat flux). Although the experiments show slight differences, westward propagation of the Rossby wave appears in areas where the variability is large. The transport of the eddy momentum flux associated with the Rossby wave modulates the strength of the eastward jet and the north-south shift of its axis. Among the external forcings, the volume transport through the Korea/Tsushima Strait is the most important driver of intrinsic variability, and wind stress plays an important role in expanding and strengthening intrinsic variability.
{"title":"Intrinsic low-frequency variability in the upper layer circulation of the East Sea (Japan Sea)","authors":"Daehyuk Kim, Hong-Ryeol Shin, Cheol-Ho Kim, Joowan Kim, Naoki Hirose","doi":"10.1175/jpo-d-23-0030.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0030.1","url":null,"abstract":"The effects of external forcing variation on the intrinsic variability in the upper layer circulation occurring within the East Sea (Japan Sea) and its physical mechanism are analyzed using numerical experiments. In this study, the experiments were conducted with climatological annual/monthly mean forcings (constant/seasonal forcings). The intrinsic variability is mainly distributed in the meandering regions around the main current path with the comparatively large variability limited to the southern region. The reason of greater intrinsic variability mainly in the southern part of the East Sea than in the northern part is that more energy is required from external forcings to change the thicker upper layer formed in the northern part due to seasonal forcings (strong wind stress and surface heat flux). Although the experiments show slight differences, westward propagation of the Rossby wave appears in areas where the variability is large. The transport of the eddy momentum flux associated with the Rossby wave modulates the strength of the eastward jet and the north-south shift of its axis. Among the external forcings, the volume transport through the Korea/Tsushima Strait is the most important driver of intrinsic variability, and wind stress plays an important role in expanding and strengthening intrinsic variability.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139142289","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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