Binbin Guo, Y. Shu, Weiqiang Wang, G. He, Qianyong Liang, Dongsheng Zhang, Lusha Yu, Jun Wang, Xiguang Deng, Yong Yang, Qiang Xie, Yinan Deng, Danyi Su
Observations of currents and temperatures from four moorings deployed around the deep slope (~2500 m) of Caiwei Guyot in the Pacific Prime Crust Zone were utilized to investigate topographically trapped waves at low-latitude seamounts. Contrasting with commonly reported persistent diurnal seamount-trapped wave cases at mid- and high-latitudes, the subinertial variability in deep currents and temperatures at the slope of Caiwei Guyot was primarily characterized by two distinct lower frequency bands, i.e., 13–24 and 3.3–4.7 days. These subinertial variabilities are interpreted as intermittent seamount-trapped waves and topographic Rossby waves (TRWs). During certain time periods, the observations include key signatures of seamount-trapped waves, such as near-opposite phases of azimuthal velocity (and temperature) on opposite flanks of the seamount, and patterns of temporal current rotation consistent with counter-rotating cells of horizontal current propagating counterclockwise around the seamount. After comparing these observations to idealized seamount-trapped wave solutions, we conclude that the 13–24-day (3.3–4.7-day) energy is mainly due to radial-vertical mode 5 (3) for azimuthal wavenumber 1 (3). Sometimes the subinertial energy remained pronounced at only one flank of the seamount, primarily explained as TRWs with 192–379 m vertical trapping scale and 14–28 km wavelength. Upper-layer mesoscale perturbations might provide energy for deep seamount-trapped waves and TRWs. This study highlights the role of topographically trapped waves in modulating the deep circulation at low-latitude seamounts.
{"title":"Observations of Intermittent Seamount-Trapped Waves and Topographic Rossby Waves around Slope of a Low-latitude Deep Seamount","authors":"Binbin Guo, Y. Shu, Weiqiang Wang, G. He, Qianyong Liang, Dongsheng Zhang, Lusha Yu, Jun Wang, Xiguang Deng, Yong Yang, Qiang Xie, Yinan Deng, Danyi Su","doi":"10.1175/jpo-d-22-0121.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0121.1","url":null,"abstract":"Observations of currents and temperatures from four moorings deployed around the deep slope (~2500 m) of Caiwei Guyot in the Pacific Prime Crust Zone were utilized to investigate topographically trapped waves at low-latitude seamounts. Contrasting with commonly reported persistent diurnal seamount-trapped wave cases at mid- and high-latitudes, the subinertial variability in deep currents and temperatures at the slope of Caiwei Guyot was primarily characterized by two distinct lower frequency bands, i.e., 13–24 and 3.3–4.7 days. These subinertial variabilities are interpreted as intermittent seamount-trapped waves and topographic Rossby waves (TRWs). During certain time periods, the observations include key signatures of seamount-trapped waves, such as near-opposite phases of azimuthal velocity (and temperature) on opposite flanks of the seamount, and patterns of temporal current rotation consistent with counter-rotating cells of horizontal current propagating counterclockwise around the seamount. After comparing these observations to idealized seamount-trapped wave solutions, we conclude that the 13–24-day (3.3–4.7-day) energy is mainly due to radial-vertical mode 5 (3) for azimuthal wavenumber 1 (3). Sometimes the subinertial energy remained pronounced at only one flank of the seamount, primarily explained as TRWs with 192–379 m vertical trapping scale and 14–28 km wavelength. Upper-layer mesoscale perturbations might provide energy for deep seamount-trapped waves and TRWs. This study highlights the role of topographically trapped waves in modulating the deep circulation at low-latitude seamounts.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"32 1","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-11-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139244120","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}
Tidal bores form in funnel-shaped estuaries and bays initially and cause unique flow and turbulence dynamics when passing through meandering channels, such as in the Qiantang Estuary, Amazon River Estuary, and Seine River Estuary. To understand the lateral currents and turbulence processes of tidal bores in curved channels, velocity profiles and free-surface elevations are sampled for four semidiurnal tidal cycles in a curved channel located upstream of the Qiantang Estuary. During flood tides, the evolution of lateral currents experiences three distinct phases: first, there are inner-bank-toward lateral currents at the front of tidal bores, followed by two-layer helical currents in the middle of the flood tides, and finally, outer-bank- pointing lateral currents at the end of flood tides. Tidal bore breaking creates outbursts of turbulent kinetic energy. The enhanced turbulence emerges above the middle layers and persists for more than ten minutes after the breaking front. The lateral momentum balance indicates that the decreasing lateral barotropic pressure gradient (LBTPG) and the increasing summation of centrifugal and Coriolis acceleration give rise to the variation in lateral currents. The phase lead of bores near the outer bank induced by shoal-channel topography generates inner-bank pointing LBTPG at the bore front and then gradually weakens it. Significant turbulence following bore breaking may be induced via the wave-induced turbulence mechanism by violent secondary waves. This research shows that complicated lateral currents are an important component of tidal bores flowing through meandering channels and that secondary waves after bore breaking can continually feed turbulence.
{"title":"Structures of Lateral flow and turbulence in a breaking tidal bore rushing through a curved channel of the Qiantang Estuary","authors":"Qianjiang Zhang, Cunhong Pan, Weifang Gu, Feng Zhou","doi":"10.1175/jpo-d-23-0044.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0044.1","url":null,"abstract":"Tidal bores form in funnel-shaped estuaries and bays initially and cause unique flow and turbulence dynamics when passing through meandering channels, such as in the Qiantang Estuary, Amazon River Estuary, and Seine River Estuary. To understand the lateral currents and turbulence processes of tidal bores in curved channels, velocity profiles and free-surface elevations are sampled for four semidiurnal tidal cycles in a curved channel located upstream of the Qiantang Estuary. During flood tides, the evolution of lateral currents experiences three distinct phases: first, there are inner-bank-toward lateral currents at the front of tidal bores, followed by two-layer helical currents in the middle of the flood tides, and finally, outer-bank- pointing lateral currents at the end of flood tides. Tidal bore breaking creates outbursts of turbulent kinetic energy. The enhanced turbulence emerges above the middle layers and persists for more than ten minutes after the breaking front. The lateral momentum balance indicates that the decreasing lateral barotropic pressure gradient (LBTPG) and the increasing summation of centrifugal and Coriolis acceleration give rise to the variation in lateral currents. The phase lead of bores near the outer bank induced by shoal-channel topography generates inner-bank pointing LBTPG at the bore front and then gradually weakens it. Significant turbulence following bore breaking may be induced via the wave-induced turbulence mechanism by violent secondary waves. This research shows that complicated lateral currents are an important component of tidal bores flowing through meandering channels and that secondary waves after bore breaking can continually feed turbulence.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"65 2","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139251898","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 simple scaling relation for internal-tide generation proposed by Jayne and St. Laurent is widely used for parameterizing turbulent mixing induced by breaking of internal tides. Based on the internal-tide generation derived from a 0.1° ocean general circulation model, we show that depending on which stratification is used, this relation produces different vertical distributions of internal-tide generation. When using the buoyancy frequency at the seafloor, which is a common practice, the scaling relation produces, relative to the model, too strong internal-tide generation in the upper 2000 m and too weak internal-tide generation in the lower 2000 m. Moreover, the different vertical distributions in the different ocean basins, characterized by a generally decreasing internal tide generation with increasing depth in the Indo-Pacific but not-decreasing or even increasing internal tide generation with increasing depth in the upper 3000 m of the Atlantic, cannot be captured when using bottom stratification. These unsatisfactory features can be easily removed by replacing the buoyancy frequency at the seafloor by a buoyancy frequency averaged over a large part of the water column. To our knowledge, this sensitivity to stratification has not been explicitly quantified for the global ocean. Because of this sensitivity, the scaling relation of Jayne and St. Laurent should be used with an averaged stratification to ensure a more adequate representation of turbulent diffusivity due to tidal mixing and water mass transformation in the deep oceans.
Jayne 和 St. Laurent 提出的内潮生成的简单比例关系被广泛用于参数化内潮破裂引起的湍流混合。根据 0.1° 海洋全环流模式得出的内潮生成量，我们发现，根据使用的分层情况，该比例关系会产生不同的内潮生成量垂直分布。此外，不同大洋盆地的垂直分布也不尽相同，印度洋-太平洋地区的内潮生成量随着深度的增加而普遍减少，而大西洋上层 3000 米地区的内潮生成量却没有减少，甚至还在增加。用水柱大部分区域的平均浮力频率来代替海底浮力频率，可以很容易地消除这些不理想的特征。据我们所知，全球海洋对分层的这种敏感性尚未明确量化。由于这种敏感性，Jayne 和 St. Laurent 的比例关系应与平均分层一起使用，以确保更充分地反映深海中潮汐混合和水团转换引起的湍流扩散性。
{"title":"Sensitivity of internal-tide generation to stratification and its implication for deep overturning circulations","authors":"Veit Lüschow, Jin-Song von Storch","doi":"10.1175/jpo-d-23-0058.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0058.1","url":null,"abstract":"The simple scaling relation for internal-tide generation proposed by Jayne and St. Laurent is widely used for parameterizing turbulent mixing induced by breaking of internal tides. Based on the internal-tide generation derived from a 0.1° ocean general circulation model, we show that depending on which stratification is used, this relation produces different vertical distributions of internal-tide generation. When using the buoyancy frequency at the seafloor, which is a common practice, the scaling relation produces, relative to the model, too strong internal-tide generation in the upper 2000 m and too weak internal-tide generation in the lower 2000 m. Moreover, the different vertical distributions in the different ocean basins, characterized by a generally decreasing internal tide generation with increasing depth in the Indo-Pacific but not-decreasing or even increasing internal tide generation with increasing depth in the upper 3000 m of the Atlantic, cannot be captured when using bottom stratification. These unsatisfactory features can be easily removed by replacing the buoyancy frequency at the seafloor by a buoyancy frequency averaged over a large part of the water column. To our knowledge, this sensitivity to stratification has not been explicitly quantified for the global ocean. Because of this sensitivity, the scaling relation of Jayne and St. Laurent should be used with an averaged stratification to ensure a more adequate representation of turbulent diffusivity due to tidal mixing and water mass transformation in the deep oceans.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"93 1","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139259091","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}
Hua Zheng, X. Zhu, Min Wang, Juntian Chen, Feng Nan, F. Yu
Abyssal vorticity balance in the northeast South China Sea was assessed for over a year based on observations from 28 current- and pressure-recording inverted echo sounders distributed west of the Luzon Strait. The regional first-order balance was dominated by the planetary vorticity flux and bottom pressure torque, which reflect the external and internal dynamics of abyssal circulation. Vertical motion considerably contributed to the planetary vorticity flux, whereas the contribution of horizontal motion was negligible. Positive and negative planetary vorticity fluxes dominate the areas along the eastern and western boundaries, indicating upward and downward vertical transport, respectively. The opposite planetary vorticity fluxes in the different areas were accompanied by different current patterns; regional anticyclonic and cyclonic characteristics appeared near the western and eastern boundaries, respectively, owing to the deep topography as the abyssal current followed the boundary. The planetary vorticity flux near the eastern boundary was substantial in spring and autumn; in contrast, along the western boundary it was enhanced only in spring. Deep eddies played important roles in planetary vorticity flux and regional vorticity balance. The results of this study reveal the formation dynamics of abyssal circulation in the South China Sea as well as its spatiotemporal distributions, providing a more detailed description of abyssal circulation.
{"title":"Regional Abyssal Vorticity Balance in the Northeast South China Sea: External and Internal Dynamics of Abyssal Circulation","authors":"Hua Zheng, X. Zhu, Min Wang, Juntian Chen, Feng Nan, F. Yu","doi":"10.1175/jpo-d-23-0060.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0060.1","url":null,"abstract":"Abyssal vorticity balance in the northeast South China Sea was assessed for over a year based on observations from 28 current- and pressure-recording inverted echo sounders distributed west of the Luzon Strait. The regional first-order balance was dominated by the planetary vorticity flux and bottom pressure torque, which reflect the external and internal dynamics of abyssal circulation. Vertical motion considerably contributed to the planetary vorticity flux, whereas the contribution of horizontal motion was negligible. Positive and negative planetary vorticity fluxes dominate the areas along the eastern and western boundaries, indicating upward and downward vertical transport, respectively. The opposite planetary vorticity fluxes in the different areas were accompanied by different current patterns; regional anticyclonic and cyclonic characteristics appeared near the western and eastern boundaries, respectively, owing to the deep topography as the abyssal current followed the boundary. The planetary vorticity flux near the eastern boundary was substantial in spring and autumn; in contrast, along the western boundary it was enhanced only in spring. Deep eddies played important roles in planetary vorticity flux and regional vorticity balance. The results of this study reveal the formation dynamics of abyssal circulation in the South China Sea as well as its spatiotemporal distributions, providing a more detailed description of abyssal circulation.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"5 3","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-11-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139275017","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}
P. F. Tedesco, L. E. Baker, A. C. Naveira Garabato, M. R. Mazloff, S. T. Gille, C. P. Caulfield, A. Mashayek
Abstract Submesoscale currents and internal gravity waves achieve an intense turbulent cascade near the ocean surface (0 m – O (100) m depth), which is thought to give rise to significant energy sources and sinks for mesoscale eddies. Here, we characterise the contributions of Non-Wave Currents (NWCs; including eddies and fronts) and Internal Gravity Waves (IGWs; including near-inertial motions, lee waves and the internal wave continuum) to near-surface submesoscale turbulence in the Drake Passage. Using a numerical simulation, we combine Lagrangian filtering and a Helmholtz decomposition to identify NWCs and IGWs and to characterise their dynamics (rotational vs. divergent). We show that NWCs and IGWs contribute in different proportions to the inverse and forward turbulent kinetic energy cascades, based on their dynamics and spatiotemporal scales. Purely rotational NWCs cause most of the inverse cascade, while coupled rotational– divergent components of NWCs and coupled NWC–IGWs cause the forward cascade. The cascade changes direction at a spatial scale at which motions become increasingly divergent. However, the forward cascade is ultimately limited by the motions’ spatiotemporal scales. The bulk of the forward cascade (80 – 95%) is caused by NWCs and IGWs of small spatiotemporal scales ( L <10 km; T <6 hours), which are primarily rotational: submesoscale eddies, fronts, and the internal wave continuum. These motions also cause a significant part of the inverse cascade (30%). Our results highlight the requirement for high spatiotemporal resolutions to diagnose the properties and large-scale impacts of near-surface submesoscale turbulence accurately, with significant implications for ocean energy cycle study strategies.
{"title":"Spatiotemporal characteristics of the near-surface turbulent cascade at the submesoscale in the Drake Passage","authors":"P. F. Tedesco, L. E. Baker, A. C. Naveira Garabato, M. R. Mazloff, S. T. Gille, C. P. Caulfield, A. Mashayek","doi":"10.1175/jpo-d-23-0108.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0108.1","url":null,"abstract":"Abstract Submesoscale currents and internal gravity waves achieve an intense turbulent cascade near the ocean surface (0 m – O (100) m depth), which is thought to give rise to significant energy sources and sinks for mesoscale eddies. Here, we characterise the contributions of Non-Wave Currents (NWCs; including eddies and fronts) and Internal Gravity Waves (IGWs; including near-inertial motions, lee waves and the internal wave continuum) to near-surface submesoscale turbulence in the Drake Passage. Using a numerical simulation, we combine Lagrangian filtering and a Helmholtz decomposition to identify NWCs and IGWs and to characterise their dynamics (rotational vs. divergent). We show that NWCs and IGWs contribute in different proportions to the inverse and forward turbulent kinetic energy cascades, based on their dynamics and spatiotemporal scales. Purely rotational NWCs cause most of the inverse cascade, while coupled rotational– divergent components of NWCs and coupled NWC–IGWs cause the forward cascade. The cascade changes direction at a spatial scale at which motions become increasingly divergent. However, the forward cascade is ultimately limited by the motions’ spatiotemporal scales. The bulk of the forward cascade (80 – 95%) is caused by NWCs and IGWs of small spatiotemporal scales ( L <10 km; T <6 hours), which are primarily rotational: submesoscale eddies, fronts, and the internal wave continuum. These motions also cause a significant part of the inverse cascade (30%). Our results highlight the requirement for high spatiotemporal resolutions to diagnose the properties and large-scale impacts of near-surface submesoscale turbulence accurately, with significant implications for ocean energy cycle study strategies.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"131 3","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135342435","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 mechanism of initial and transient perturbations of symmetric instability (SI) in a hydrostatic flow with lateral shear is analyzed by applying the generalized stability analysis. It is well known that the SI’s most rapidly growing motion is along isopycnals, and the growth rates consist of growing, neutral, and decaying modes. The eigenvectors of these 3 modes are not orthogonal to each other, hence the initial and transient perturbations bear little resemblance to the normal mode. Our findings indicate that the emergence of normal modes occurs within a time span of 1-3 inertial periods, which we refer to as the transient state. The overall growth of perturbation energy is divided into three components: geostrophic shear production (GSP), lateral shear production (LSP), and meridional buoyancy flux (MB). During the transient state, the perturbation energy is partly driven by MB, contrary to the normal mode which has zero MB. The relative energy contribution is evaluated through the ratio to GSP. While the MB to GSP ratio of the initial mode is higher than that of the normal mode, the LSP to GSP ratio remains constant. In the absence of the fastest-growing normal mode, MB can serve as the predominant initial energy source. The precise transition in the energy regime is contingent upon the geostrophic Richardson number and Rossby number.
{"title":"Initial and transient growth of symmetric instability","authors":"Satoshi Kimura","doi":"10.1175/jpo-d-23-0048.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0048.1","url":null,"abstract":"Abstract The mechanism of initial and transient perturbations of symmetric instability (SI) in a hydrostatic flow with lateral shear is analyzed by applying the generalized stability analysis. It is well known that the SI’s most rapidly growing motion is along isopycnals, and the growth rates consist of growing, neutral, and decaying modes. The eigenvectors of these 3 modes are not orthogonal to each other, hence the initial and transient perturbations bear little resemblance to the normal mode. Our findings indicate that the emergence of normal modes occurs within a time span of 1-3 inertial periods, which we refer to as the transient state. The overall growth of perturbation energy is divided into three components: geostrophic shear production (GSP), lateral shear production (LSP), and meridional buoyancy flux (MB). During the transient state, the perturbation energy is partly driven by MB, contrary to the normal mode which has zero MB. The relative energy contribution is evaluated through the ratio to GSP. While the MB to GSP ratio of the initial mode is higher than that of the normal mode, the LSP to GSP ratio remains constant. In the absence of the fastest-growing normal mode, MB can serve as the predominant initial energy source. The precise transition in the energy regime is contingent upon the geostrophic Richardson number and Rossby number.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"27 13","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135391934","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}
Trygve Halsne, Alvise Benetazzo, Francesco Barbariol, Kai Håkon Christensen, Ana Carrasco, Øyvind Breivik
Abstract Accurate estimates of extreme waves are central for maritime activities, and stochastic wave models are the best option available for practical applications. However, the way currents influence the statistics of space-time extremes in spectral wave models has not been properly assessed. Here we demonstrate impacts of the wave modulation caused by one of the world’s strongest open ocean tidal currents, which reaches speeds of at least 3 m s −1 . For a bimodal swell and wind sea state, we find that most intense interactions occur when the wind sea opposes the tidal current, with an increase in significant wave height and spectral steepness up to 45 % and 167 %, respectively. The steepness modulation strengthen the second-order Stokes contribution for the normalized extreme crests, which increases between 5–14 % during opposing wind sea and current. The normalized extreme wave heights have a strong dependence on the narrow-bandedness parameter, which is sensitive to the variance distribution in the bimodal spectrum, and we find an increase up to 12 % with currents opposing the wind sea. In another case of swell opposing a tidal jet, we find the spectral steepness to exceed the increase predicted by a simplified modulation model. We find support in single-point observations that using tidal currents as forcing in wave models improves the representation of the expected maximum waves, but that action must be taken to close the gap of measurements in strong currents.
{"title":"Wave modulation in a strong tidal current and its impact on extreme waves","authors":"Trygve Halsne, Alvise Benetazzo, Francesco Barbariol, Kai Håkon Christensen, Ana Carrasco, Øyvind Breivik","doi":"10.1175/jpo-d-23-0051.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0051.1","url":null,"abstract":"Abstract Accurate estimates of extreme waves are central for maritime activities, and stochastic wave models are the best option available for practical applications. However, the way currents influence the statistics of space-time extremes in spectral wave models has not been properly assessed. Here we demonstrate impacts of the wave modulation caused by one of the world’s strongest open ocean tidal currents, which reaches speeds of at least 3 m s −1 . For a bimodal swell and wind sea state, we find that most intense interactions occur when the wind sea opposes the tidal current, with an increase in significant wave height and spectral steepness up to 45 % and 167 %, respectively. The steepness modulation strengthen the second-order Stokes contribution for the normalized extreme crests, which increases between 5–14 % during opposing wind sea and current. The normalized extreme wave heights have a strong dependence on the narrow-bandedness parameter, which is sensitive to the variance distribution in the bimodal spectrum, and we find an increase up to 12 % with currents opposing the wind sea. In another case of swell opposing a tidal jet, we find the spectral steepness to exceed the increase predicted by a simplified modulation model. We find support in single-point observations that using tidal currents as forcing in wave models improves the representation of the expected maximum waves, but that action must be taken to close the gap of measurements in strong currents.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"4 6","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135390196","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 effect of climate warming in response to rising atmospheric CO2 on the ventilation of the ocean remains uncertain. Here we make theoretical advances in elucidating the relationship between ideal age and transit time distribution (TTD) in a time-dependent flow. Subsequently, we develop an offline tracer-transport model to characterize the ventilation patterns and timescales in the time-evolving circulation for the 1850-to-2300 period as simulated with the Community Earth System Model version 1 (CESMv1) under business-as-usual warming scenario. We found that by 2300 2.1% less water originates from the high-latitude deep water formation regions (both hemispheres) compared to 1850. In compensation, there is an increase in the water originating from the subantarctic. We also found that slowing meridional overturning circulation causes a gradual increase in mean age during the 1850 to 2300 period, with a globally averaged mean-age increase of ~110 years in 2300. Where and when the water will be re-exposed to the atmosphere depends on the post-2300 circulation. For example, if we assume that the circulation persists in its year-2300 state (scenario 1), the mean interior-to-surface transit time in year 1850 is ~1140 years. In contrast, if we assume that the circulation abruptly recovers to its year-1850 state (scenario 2), the mean interior-to-surface transit time in 1850 is only ~740 years. By 2300, these differences become even larger; in scenario 1, the mean interior-to-surface transit time increases by ~200 years, whereas scenario 2 decreases by ~80 years. The dependence of interior-to-surface transit time on the future ocean circulation produces an additional unavoidable uncertainty in the long-term durability of marine carbon dioxide removal strategies.
{"title":"Surface-to-Interior Transport Timescales and Ventilation Patterns in a Time-Dependent Circulation Driven by Sustained Climate Warming","authors":"Y. Liu, F. Primeau","doi":"10.1175/jpo-d-23-0113.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0113.1","url":null,"abstract":"Abstract The effect of climate warming in response to rising atmospheric CO2 on the ventilation of the ocean remains uncertain. Here we make theoretical advances in elucidating the relationship between ideal age and transit time distribution (TTD) in a time-dependent flow. Subsequently, we develop an offline tracer-transport model to characterize the ventilation patterns and timescales in the time-evolving circulation for the 1850-to-2300 period as simulated with the Community Earth System Model version 1 (CESMv1) under business-as-usual warming scenario. We found that by 2300 2.1% less water originates from the high-latitude deep water formation regions (both hemispheres) compared to 1850. In compensation, there is an increase in the water originating from the subantarctic. We also found that slowing meridional overturning circulation causes a gradual increase in mean age during the 1850 to 2300 period, with a globally averaged mean-age increase of ~110 years in 2300. Where and when the water will be re-exposed to the atmosphere depends on the post-2300 circulation. For example, if we assume that the circulation persists in its year-2300 state (scenario 1), the mean interior-to-surface transit time in year 1850 is ~1140 years. In contrast, if we assume that the circulation abruptly recovers to its year-1850 state (scenario 2), the mean interior-to-surface transit time in 1850 is only ~740 years. By 2300, these differences become even larger; in scenario 1, the mean interior-to-surface transit time increases by ~200 years, whereas scenario 2 decreases by ~80 years. The dependence of interior-to-surface transit time on the future ocean circulation produces an additional unavoidable uncertainty in the long-term durability of marine carbon dioxide removal strategies.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"2010 27","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135636491","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 Among the global mapping of lee wave generation, a missing piece exists in the northwestern Pacific Ocean (NPO), which features complex topographies and energetic circulations. This study applies Bell’s theory to estimate and map internal lee waves generated by geostrophic flows in the NPO using Mercator Ocean reanalysis data and the full topographic spectra obtained from the latest synthetic bathymetry product. Unlike the dominant contributions from abyssal hills in the Southern Ocean, multiple topographies, including ridges, rises, and continental margins, result in an inhomogeneous lee wave generation with multiple hotspots in the NPO. The generation rate is generally higher in the Philippine basin and lower in the central Pacific seamounts. Over ridges, the rough topography creates a high potential for triggering lee waves. Over rises and continental margins, the stronger currents at the shallow depths are favorable for lee wave generation. In the Kuroshio extension region, the rough topography and strong currents cause the strongest lee wave generation, with an energy flux reaching 100 mW m −2 . By mean–eddy decomposition, it is found that the lee wave hotspots contributed by mean flow are concentrated in specific regions, while those by geostrophic eddies are widely distributed. Geostrophic eddies are the primary contributor to lee wave generation, which account for 74.6% of the total energy transferred from geostrophic flow to lee waves. This study also reveals that tides suppress the lee wave generation by 14%, and geostrophic flow can cause an asymmetric generation of internal tides.
{"title":"Internal Lee Wave Generation from Geostrophic Flow in the Northwestern Pacific Ocean","authors":"Ji Li, Zhenhua Xu, Zhanjiu Hao, Jia You, Peiwen Zhang, Baoshu Yin","doi":"10.1175/jpo-d-23-0035.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0035.1","url":null,"abstract":"Abstract Among the global mapping of lee wave generation, a missing piece exists in the northwestern Pacific Ocean (NPO), which features complex topographies and energetic circulations. This study applies Bell’s theory to estimate and map internal lee waves generated by geostrophic flows in the NPO using Mercator Ocean reanalysis data and the full topographic spectra obtained from the latest synthetic bathymetry product. Unlike the dominant contributions from abyssal hills in the Southern Ocean, multiple topographies, including ridges, rises, and continental margins, result in an inhomogeneous lee wave generation with multiple hotspots in the NPO. The generation rate is generally higher in the Philippine basin and lower in the central Pacific seamounts. Over ridges, the rough topography creates a high potential for triggering lee waves. Over rises and continental margins, the stronger currents at the shallow depths are favorable for lee wave generation. In the Kuroshio extension region, the rough topography and strong currents cause the strongest lee wave generation, with an energy flux reaching 100 mW m −2 . By mean–eddy decomposition, it is found that the lee wave hotspots contributed by mean flow are concentrated in specific regions, while those by geostrophic eddies are widely distributed. Geostrophic eddies are the primary contributor to lee wave generation, which account for 74.6% of the total energy transferred from geostrophic flow to lee waves. This study also reveals that tides suppress the lee wave generation by 14%, and geostrophic flow can cause an asymmetric generation of internal tides.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"52 12","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135614661","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 global ocean circulation and climate models, bottom-enhanced turbulent mixing is often parameterized such that the vertical decay scale of the energy dissipation rate ζ is universally constant at 500 m. In this study, using a non-hydrostatic two-dimensional numerical model in the horizontal-vertical plane that incorporates a monochromatic sinusoidal seafloor topography and the Garrett-Munk (GM) background internal wave field, we find that ζ of the internal lee wave-driven bottom-enhanced mixing is actually variable depending on the magnitude of the steady flow U 0 , the horizontal wavenumber k H , and the height h T of the seafloor topography. When the steepness parameter ( Sp=Nh T /U 0 where N is the buoyancy frequency near the seafloor) is less than 0.3, internal lee waves propagate upward from the seafloor while interacting with the GM internal wave field to create a turbulent mixing region with ζ that extends further upward from the seafloor as U 0 increases, but is nearly independent of k H . In contrast, when Sp exceeds 0.3, inertial oscillations (IOs) not far above the seafloor are enhanced by the intermittent supply of internal lee wave energy Doppler-shifted to the near-inertial frequency, which occurs depending on the sign and magnitude of the background IO shear. The composite flow, consisting of the superposition of U 0 and the IOs, interacts with the seafloor topography to efficiently generate internal lee waves during the period centered on the time of the composite flow maximum, but their upward propagation is inhibited by the increased IO shear, creating a turbulent mixing region of small ζ .
在全球海洋环流和气候模式中，底部增强的湍流混合经常被参数化，使得能量耗散率ζ的垂直衰减尺度在500 m处普遍恒定。在这项研究中，使用一个包含单色正弦海底地形和Garrett-Munk (GM)背景内波场的水平-垂直平面非流体静力二维数值模型，我们发现内部背风波驱动的底部增强混合的ζ实际上是可变的，这取决于稳定流的大小U 0，水平波数k H和海底地形的高度H T。当陡度参数(Sp=Nh T / u0，其中N为海底附近的浮力频率)小于0.3时，内背风波从海底向上传播，同时与GM内波场相互作用，形成一个带有ζ的湍流混合区，随着u0的增加，ζ从海底向上延伸，但几乎与kh无关。相反，当Sp超过0.3时，内部背风波能量多普勒频移至近惯性频率的间歇性供应增强了海床上方不远的惯性振荡(IOs)，这取决于背景IO切变的符号和大小。由u0和IOs叠加组成的复合流与海底地形相互作用，在复合流最大时间为中心的时段内有效产生内背风波，但其向上传播受到IO切变增加的抑制，形成一个小ζ的湍流混合区。
{"title":"The vertical structure of internal lee wave-driven benthic mixing hotspots","authors":"Ying He, Toshiyuki Hibiya","doi":"10.1175/jpo-d-22-0268.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0268.1","url":null,"abstract":"Abstract In global ocean circulation and climate models, bottom-enhanced turbulent mixing is often parameterized such that the vertical decay scale of the energy dissipation rate ζ is universally constant at 500 m. In this study, using a non-hydrostatic two-dimensional numerical model in the horizontal-vertical plane that incorporates a monochromatic sinusoidal seafloor topography and the Garrett-Munk (GM) background internal wave field, we find that ζ of the internal lee wave-driven bottom-enhanced mixing is actually variable depending on the magnitude of the steady flow U 0 , the horizontal wavenumber k H , and the height h T of the seafloor topography. When the steepness parameter ( Sp=Nh T /U 0 where N is the buoyancy frequency near the seafloor) is less than 0.3, internal lee waves propagate upward from the seafloor while interacting with the GM internal wave field to create a turbulent mixing region with ζ that extends further upward from the seafloor as U 0 increases, but is nearly independent of k H . In contrast, when Sp exceeds 0.3, inertial oscillations (IOs) not far above the seafloor are enhanced by the intermittent supply of internal lee wave energy Doppler-shifted to the near-inertial frequency, which occurs depending on the sign and magnitude of the background IO shear. The composite flow, consisting of the superposition of U 0 and the IOs, interacts with the seafloor topography to efficiently generate internal lee waves during the period centered on the time of the composite flow maximum, but their upward propagation is inhibited by the increased IO shear, creating a turbulent mixing region of small ζ .","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"308 2","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135321519","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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