This study examines mixed Rossby gravity (MRG) waves at mid-depths (500-2000 m) of the Indian Ocean, using multi-year velocity time series obtained from current meter moorings at 77°, 83°, and 93°E along the equator over the period 2000-2019. These data are analyzed in combination with a high-resolution wind-forced ocean general circulation model. The spectrum of observed meridional velocity showed elevated energy over a wide range of periods from about 10 to 100 days with the spectral peak at a period of about 30 days. The model was able to simulate the characteristics of the observed spectrum. Further diagnostics determined that the detected variability is generally consistent with theoretical MRG waves in a resting ocean. Statistical analysis and a model sensitivity experiment identified distinct variations at three periods, where meridional velocity has sizable energy. The 14-day variability is wind-driven and has a long zonal (∼3300 km) and vertical wavelength (∼4200 m). The 28-day variability is excited by the dynamical instability of the background flow in the equatorial western Indian Ocean near the surface and propagates to the study area. It is characterized by a shorter zonal (∼1100 km) and vertical wavelength (∼2800 m) compared to 14-day variability. The 43-day variability has a zonal wavelength (∼820 km) comparable to the 28-day variability, but does not show the tendency of propagation and is likely generated in situ through nonlinear interactions. These results show that various processes contribute to the excitation of MRG waves at mid-depths of the Indian Ocean.
{"title":"Mixed Rossby gravity waves at mid-depths of the equatorial Indian Ocean","authors":"M. Nagura, M. Mcphaden","doi":"10.1175/jpo-d-23-0254.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0254.1","url":null,"abstract":"\u0000This study examines mixed Rossby gravity (MRG) waves at mid-depths (500-2000 m) of the Indian Ocean, using multi-year velocity time series obtained from current meter moorings at 77°, 83°, and 93°E along the equator over the period 2000-2019. These data are analyzed in combination with a high-resolution wind-forced ocean general circulation model. The spectrum of observed meridional velocity showed elevated energy over a wide range of periods from about 10 to 100 days with the spectral peak at a period of about 30 days. The model was able to simulate the characteristics of the observed spectrum. Further diagnostics determined that the detected variability is generally consistent with theoretical MRG waves in a resting ocean. Statistical analysis and a model sensitivity experiment identified distinct variations at three periods, where meridional velocity has sizable energy. The 14-day variability is wind-driven and has a long zonal (∼3300 km) and vertical wavelength (∼4200 m). The 28-day variability is excited by the dynamical instability of the background flow in the equatorial western Indian Ocean near the surface and propagates to the study area. It is characterized by a shorter zonal (∼1100 km) and vertical wavelength (∼2800 m) compared to 14-day variability. The 43-day variability has a zonal wavelength (∼820 km) comparable to the 28-day variability, but does not show the tendency of propagation and is likely generated in situ through nonlinear interactions. These results show that various processes contribute to the excitation of MRG waves at mid-depths of the Indian Ocean.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141270214","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}
Growing evidence is found in observations and numerical modelling of the importance of steep seafloor topography for turbulent diapycnal mixing leading to redistribution of suspended matter and nutrients, especially in waters with abundant internal tides. One of the remaining questions is the extent of turbulent mixing away from and above nearly flat topography, which is addressed in this paper. Evaluated are observations from an opportunistic, week-long mooring of high-resolution temperature sensors above a small seafloor slope in about 1200 m water depth of the Eastern Mediterranean. The environment has weak tides, so that near-inertial motions and -shear dominate internal waves. Vertical displacement shapes suggest instabilities to represent locally generated turbulent overturns, rather than partial salinity-compensated intrusions dispersed isopycnally from turbulence near the slope. This conclusion is supported by the duration of instabilities, as all individual overturns last shorter than the mean buoyancy period and sequences of overturns last shorter than the local inertial period. The displacement shapes are more erratic than observed in stronger stratified waters in which shear drives turbulence, and better correspond with predominantly buoyancy-driven convection-turbulence. This convection-turbulence is confirmed from spectral information, generally occurring dominant close to the seafloor and only in weakly stratified layers well above it. Mean turbulence values are 10-100 times smaller than found above steep ocean topography, but 10 times larger than found in the open-ocean interior.
{"title":"Intrusions and turbulent mixing above a small Eastern Mediterranean seafloor-slope","authors":"H. van Haren","doi":"10.1175/jpo-d-23-0237.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0237.1","url":null,"abstract":"\u0000Growing evidence is found in observations and numerical modelling of the importance of steep seafloor topography for turbulent diapycnal mixing leading to redistribution of suspended matter and nutrients, especially in waters with abundant internal tides. One of the remaining questions is the extent of turbulent mixing away from and above nearly flat topography, which is addressed in this paper. Evaluated are observations from an opportunistic, week-long mooring of high-resolution temperature sensors above a small seafloor slope in about 1200 m water depth of the Eastern Mediterranean. The environment has weak tides, so that near-inertial motions and -shear dominate internal waves. Vertical displacement shapes suggest instabilities to represent locally generated turbulent overturns, rather than partial salinity-compensated intrusions dispersed isopycnally from turbulence near the slope. This conclusion is supported by the duration of instabilities, as all individual overturns last shorter than the mean buoyancy period and sequences of overturns last shorter than the local inertial period. The displacement shapes are more erratic than observed in stronger stratified waters in which shear drives turbulence, and better correspond with predominantly buoyancy-driven convection-turbulence. This convection-turbulence is confirmed from spectral information, generally occurring dominant close to the seafloor and only in weakly stratified layers well above it. Mean turbulence values are 10-100 times smaller than found above steep ocean topography, but 10 times larger than found in the open-ocean interior.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141269258","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}
Being situated in the estuary of the flood-dominated Hooghly River system, the macrotidal Indian Sundarban Delta (ISD) has become one of the most complex, dynamic and rapidly changing landforms on the earth’s surface. To study horizontal areal shifting of shoreline and its impact on mangrove-cover in the region, United State Geological Survey (USGS)-satellite data of 1980, 1990, 2000, 2010 and 2021 were used. Remote sensing and GIS techniques were employed in the investigation. Simultaneous prograding and retrograding shoreline shifting was distinguished almost in all the parts, though sediment-starved eastern and macrotidally more active southern lobes experienced dominantly retreating shift, and sediment-engorged western lobe demonstrated to be more dynamic. Net areal change over north-south tracks followed the trend of decreasing accretion to increasing erosion while going from west to east, whereas that over west-east tracks followed the trend of exponentially increasing erosion while going from north to south. Overall accretion of ∼91 sq. km in the ISD accounted for augmentation of sparse vegetation of ∼13 sq. km, whereas, ∼243 sq. km erosion called for depletion of sparse & moderate vegetation of ∼18 & ∼174 sq. km respectively over the 41-year period. Various oceanographic and riparian forces and actions, episodic natural events etc. vis-a-vis several anthropogenic interventions— all together contributed to such changes. The findings may help the coastal environmentalists, professionals, planners, decision-makers and implementers in formulating and taking up of suitable strategic measures for integrated and effective coastal zone management in this estuarine wetland-forest.
{"title":"Decadal changes (1980-2021) of shoreline and mangrove cover in Sundarban Delta, India using remote sensing and GIS","authors":"Sipra Biswas, Kallol Sarkar, Tapan Kumar Das","doi":"10.1175/jpo-d-23-0019.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0019.1","url":null,"abstract":"\u0000Being situated in the estuary of the flood-dominated Hooghly River system, the macrotidal Indian Sundarban Delta (ISD) has become one of the most complex, dynamic and rapidly changing landforms on the earth’s surface. To study horizontal areal shifting of shoreline and its impact on mangrove-cover in the region, United State Geological Survey (USGS)-satellite data of 1980, 1990, 2000, 2010 and 2021 were used. Remote sensing and GIS techniques were employed in the investigation. Simultaneous prograding and retrograding shoreline shifting was distinguished almost in all the parts, though sediment-starved eastern and macrotidally more active southern lobes experienced dominantly retreating shift, and sediment-engorged western lobe demonstrated to be more dynamic. Net areal change over north-south tracks followed the trend of decreasing accretion to increasing erosion while going from west to east, whereas that over west-east tracks followed the trend of exponentially increasing erosion while going from north to south. Overall accretion of ∼91 sq. km in the ISD accounted for augmentation of sparse vegetation of ∼13 sq. km, whereas, ∼243 sq. km erosion called for depletion of sparse & moderate vegetation of ∼18 & ∼174 sq. km respectively over the 41-year period. Various oceanographic and riparian forces and actions, episodic natural events etc. vis-a-vis several anthropogenic interventions— all together contributed to such changes. The findings may help the coastal environmentalists, professionals, planners, decision-makers and implementers in formulating and taking up of suitable strategic measures for integrated and effective coastal zone management in this estuarine wetland-forest.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141104080","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}
Andrew L. Stewart, Yan Wang, A. Solodoch, Ru Chen, J. McWilliams
Eastern Boundary Upwelling Systems (EBUSs) host equatorward wind-driven near-surface currents overlying poleward subsurface undercurrents. Various previous theories for these undercurrents have emphasized the role of poleward alongshore pressure gradient forces (APF). Energetic mesoscale variability may also serve to accelerate undercurrents via mesoscale stirring of the potential vorticity gradient imposed by the continental slope. However, it remains unclear whether this eddy rectification mechanism contributes substantially to driving poleward undercurrents in EBUS. This study isolates the influence of eddy rectification on undercurrents via a suite of idealized simulations forced either by alongshore winds, with or without an APF, or by randomly-generated mesoscale eddies. It is found that the simulations develop undercurrents with strengths comparable to those found in nature in both wind-forced and randomly forced experiments. Analysis of the momentum budget reveals that the along-isobath undercurrent flow is accelerated by isopycnal advective eddy momentum fluxes and the APF, and retarded by frictional drag. The undercurrent acceleration may manifest as eddy momentum fluxes or as topographic form stress depending on the coordinate system used to compute the momentum budget, which reconciles these findings with previous work that linked eddy acceleration of the undercurrent to topographic form stress. The leading-order momentum balance motivates a scaling for the strength of the undercurrent that explains most of the variance across the simulations. These findings indicate that eddy rectification is of comparable importance to the APF in driving poleward undercurrents in EBUSs, and motivate further work to diagnose this effect in high-resolution models and observations, and to parameterize it in coarse-resolution ocean/climate models.
{"title":"Formation of eastern boundary undercurrents via mesoscale eddy rectification","authors":"Andrew L. Stewart, Yan Wang, A. Solodoch, Ru Chen, J. McWilliams","doi":"10.1175/jpo-d-23-0196.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0196.1","url":null,"abstract":"\u0000Eastern Boundary Upwelling Systems (EBUSs) host equatorward wind-driven near-surface currents overlying poleward subsurface undercurrents. Various previous theories for these undercurrents have emphasized the role of poleward alongshore pressure gradient forces (APF). Energetic mesoscale variability may also serve to accelerate undercurrents via mesoscale stirring of the potential vorticity gradient imposed by the continental slope. However, it remains unclear whether this eddy rectification mechanism contributes substantially to driving poleward undercurrents in EBUS. This study isolates the influence of eddy rectification on undercurrents via a suite of idealized simulations forced either by alongshore winds, with or without an APF, or by randomly-generated mesoscale eddies. It is found that the simulations develop undercurrents with strengths comparable to those found in nature in both wind-forced and randomly forced experiments. Analysis of the momentum budget reveals that the along-isobath undercurrent flow is accelerated by isopycnal advective eddy momentum fluxes and the APF, and retarded by frictional drag. The undercurrent acceleration may manifest as eddy momentum fluxes or as topographic form stress depending on the coordinate system used to compute the momentum budget, which reconciles these findings with previous work that linked eddy acceleration of the undercurrent to topographic form stress. The leading-order momentum balance motivates a scaling for the strength of the undercurrent that explains most of the variance across the simulations. These findings indicate that eddy rectification is of comparable importance to the APF in driving poleward undercurrents in EBUSs, and motivate further work to diagnose this effect in high-resolution models and observations, and to parameterize it in coarse-resolution ocean/climate models.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141107148","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}
Q. Jamet, Alexandre Berger, B. Deremble, T. Penduff
Air-sea fluxes are the main drivers of ocean circulation, yet their representation in ocean only models remains challenging. While a zeroth-order formulation accounting only for the state of the atmosphere is well adopted by the community, surface ocean feedback has gained attention over the last decades. In this paper, we focus on thermodynamical indirect feedback of surface ocean currents, which completes the ’eddy killing’ effect induced by the mechanical feedback. In this study, we quantify both the mechanical and thermodynamical contributions in the context of idealized, coupled Quasi-Geostrophic simulations through sensitivity experiments on wind stress formulation. As compared to eddy killing which impacts kinetic energy levels, the indirect thermodynamical feedback induces significant changes in potential energy levels. The thermodynamical feedback also enhances by +27% the potential-to-kinetic turbulent energy conversion induced by relative wind stress formulation, as well as significant changes in both forward and inverse cascades of Potential Energy (PE). That is, accounting for ocean surface currents in the computation of wind stress significantly changes transfers of PE from the mean to the turbulent flow. These changes are mostly controlled by a reduced upscale energy flux rather than a more vigorous downscale flux, a process in line with results obtained for kinetic energy fluxes associated with the eddy killing effect.
{"title":"Thermodynamical effects of ocean current feedback in a quasi-geostrophic coupled model","authors":"Q. Jamet, Alexandre Berger, B. Deremble, T. Penduff","doi":"10.1175/jpo-d-23-0159.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0159.1","url":null,"abstract":"\u0000Air-sea fluxes are the main drivers of ocean circulation, yet their representation in ocean only models remains challenging. While a zeroth-order formulation accounting only for the state of the atmosphere is well adopted by the community, surface ocean feedback has gained attention over the last decades. In this paper, we focus on thermodynamical indirect feedback of surface ocean currents, which completes the ’eddy killing’ effect induced by the mechanical feedback. In this study, we quantify both the mechanical and thermodynamical contributions in the context of idealized, coupled Quasi-Geostrophic simulations through sensitivity experiments on wind stress formulation. As compared to eddy killing which impacts kinetic energy levels, the indirect thermodynamical feedback induces significant changes in potential energy levels. The thermodynamical feedback also enhances by +27% the potential-to-kinetic turbulent energy conversion induced by relative wind stress formulation, as well as significant changes in both forward and inverse cascades of Potential Energy (PE). That is, accounting for ocean surface currents in the computation of wind stress significantly changes transfers of PE from the mean to the turbulent flow. These changes are mostly controlled by a reduced upscale energy flux rather than a more vigorous downscale flux, a process in line with results obtained for kinetic energy fluxes associated with the eddy killing effect.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141108338","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}
This paper provides a framework that unifies the characteristics of Langmuir turbulence, including the vortex force effect, velocity scalings, vertical flow structure, and crosswind spacing between surface streaks. The widely accepted CL2 mechanism is extended to explain the observed maximum alongwind velocity and downwelling velocity below the surface. Balancing the extended mechanism in the Craik-Leibovich equations, the scalings for the along-wind velocity u, cross-wind velocity v, and vertical velocity w are formulated as Here, Uf is the friction velocity, Us is the Stokes drift on the surface, and La = (Uf /Us)1/2 is the Langmuir number. Simulations using the Stratified Ocean Model with Adaptive Refinement in Large Eddy Simulation mode (LES-SOMAR) validate the scalings and reveal physical similarity for velocity and crosswind spacing. The horizontally averaged velocity along the wind ū/U on the surface grows with time, whereas v/V and w/W are confined. The root mean square (rms) of w peaks at wrms/W ≈ 0.85 at a depth of 1.3Zs, where Zs is the e-folding scale of the Stokes drift. The crosswind spacing L grows linearly with time but is finally limited by the depth of the water H, with maximum L/H = 3.3. This framework agrees with measurement collected in six different field campaigns.
本文提供了一个统一朗缪尔湍流特征的框架，包括涡力效应、速度标度、垂直流结构和表面条纹之间的横风间距。对广为接受的 CL2 机制进行了扩展，以解释观测到的最大顺风速度和表面以下的下沉速度。在克雷克-莱博维奇方程中平衡扩展机制，顺风速度 u、横风速度 v 和垂直速度 w 的标度公式为 Uf 为摩擦速度，Us 为表面斯托克斯漂移，La = (Uf /Us)1/2 为朗缪尔数。利用大涡模拟模式下的自适应细化分层海洋模式（LES-SOMAR）进行的模拟验证了这些标度，并揭示了速度和横风间距的物理相似性。沿海面风向的水平平均速度 ū/U 随时间增长，而 v/V 和 w/W 则受到限制。在深度为 1.3Zs 时，w 的均方根（rms）达到峰值，即 wrms/W ≈ 0.85，其中 Zs 是斯托克斯漂移的电子折叠尺度。横风间距 L 随时间线性增长，但最终受到水深 H 的限制，最大 L/H = 3.3。这一框架与在六次不同的实地测量中收集到的数据相吻合。
{"title":"Scaling and flow structure of Langmuir turbulence in inertial frames","authors":"Yun Chang, Alberto Scotti","doi":"10.1175/jpo-d-23-0258.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0258.1","url":null,"abstract":"\u0000This paper provides a framework that unifies the characteristics of Langmuir turbulence, including the vortex force effect, velocity scalings, vertical flow structure, and crosswind spacing between surface streaks. The widely accepted CL2 mechanism is extended to explain the observed maximum alongwind velocity and downwelling velocity below the surface. Balancing the extended mechanism in the Craik-Leibovich equations, the scalings for the along-wind velocity u, cross-wind velocity v, and vertical velocity w are formulated as Here, Uf is the friction velocity, Us is the Stokes drift on the surface, and La = (Uf /Us)1/2 is the Langmuir number. Simulations using the Stratified Ocean Model with Adaptive Refinement in Large Eddy Simulation mode (LES-SOMAR) validate the scalings and reveal physical similarity for velocity and crosswind spacing. The horizontally averaged velocity along the wind ū/U on the surface grows with time, whereas v/V and w/W are confined. The root mean square (rms) of w peaks at wrms/W ≈ 0.85 at a depth of 1.3Zs, where Zs is the e-folding scale of the Stokes drift. The crosswind spacing L grows linearly with time but is finally limited by the depth of the water H, with maximum L/H = 3.3. This framework agrees with measurement collected in six different field campaigns.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141121217","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}
Xihan Zhang, M. Nikurashin, Beatriz Peña‐Molino, Stephen R. Rintoul, Edward Doddridge
The vertically integrated zonal momentum balance of the Antarctic Circumpolar Current (ACC) is dominated by wind stress at the surface and topographic form stress (TFS) at the bottom. It has been argued that wind stress is transferred from the surface to the bottom by transient baroclinic eddies, via interfacial form stress, to establish the balance between wind stress and TFS. However, ocean models indicate TFS responds rapidly to changes in wind stress, suggesting that barotropic processes play a role in this balance. We investigate the dynamics governing the wind-TFS balance of the ACC and its response to wind using an idealized, wind- and buoyancy-driven channel model. We show that the balance is established and maintained at equilibrium by the barotropic dynamics. The balance results from continuity of the flow, in which the Ekman transport at the surface, balanced by wind stress, is compensated by a return flow at depth, balanced by TFS. This leads to a match between wind stress and TFS which is independent of momentum stresses in the interior. Transient baroclinic eddies oppose the wind-driven isopycnal steepening via eddy buoyancy fluxes, which act to flatten the isopycnals. The eddy-driven isopycnal flattening corresponds to a reduction in the zonal geostrophic shear and thus a redistribution of the zonal momentum in the interior via eddy momentum stresses. The maintenance of the vertically integrated ACC momentum balance by the barotropic dynamics explains the fast response of the wind-TFS balance to changes in wind forcing.
{"title":"Maintenance of the zonal momentum balance of the Antarctic Circumpolar Current by barotropic dynamics","authors":"Xihan Zhang, M. Nikurashin, Beatriz Peña‐Molino, Stephen R. Rintoul, Edward Doddridge","doi":"10.1175/jpo-d-23-0042.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0042.1","url":null,"abstract":"\u0000The vertically integrated zonal momentum balance of the Antarctic Circumpolar Current (ACC) is dominated by wind stress at the surface and topographic form stress (TFS) at the bottom. It has been argued that wind stress is transferred from the surface to the bottom by transient baroclinic eddies, via interfacial form stress, to establish the balance between wind stress and TFS. However, ocean models indicate TFS responds rapidly to changes in wind stress, suggesting that barotropic processes play a role in this balance. We investigate the dynamics governing the wind-TFS balance of the ACC and its response to wind using an idealized, wind- and buoyancy-driven channel model. We show that the balance is established and maintained at equilibrium by the barotropic dynamics. The balance results from continuity of the flow, in which the Ekman transport at the surface, balanced by wind stress, is compensated by a return flow at depth, balanced by TFS. This leads to a match between wind stress and TFS which is independent of momentum stresses in the interior. Transient baroclinic eddies oppose the wind-driven isopycnal steepening via eddy buoyancy fluxes, which act to flatten the isopycnals. The eddy-driven isopycnal flattening corresponds to a reduction in the zonal geostrophic shear and thus a redistribution of the zonal momentum in the interior via eddy momentum stresses. The maintenance of the vertically integrated ACC momentum balance by the barotropic dynamics explains the fast response of the wind-TFS balance to changes in wind forcing.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140976181","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}
Alexander Andriatis, L. Lenain, Matthew H. Alford, Nathaniel Winstead, Joseph Geiman
We report novel observations of the onset and growth of Langmuir circulations (LCs) from simultaneous airborne and subsurface in-situ measurements. Under weak, fetch-limited wind wave forcing with stabilizing buoyancy forcing, the onset of LCs is observed for wind speeds greater than about 1 m s−1. LCs appear non-uniformly in space, consistent with previous laboratory experiments and suggestive of coupled wave-turbulence interaction. Following an increase in wind speed from < 1 m s−1 to sustained 3 m s−1 winds, a shallow (< 0.7 m) diurnal warm layer is observed to deepen at 1 m hr−1, while the cross-cell scales of LCs grow at 2 m hr−1, as observed in sea surface temperature collected from a research aircraft. Subsurface temperature structures show temperature intrusions into the base of the diurnal warm layer of the same scale as bubble entrainment depth during the deepening period, and are comparable to temperature structures observed during strong wind forcing with a deep mixed layer that is representative of previous LC studies. We show that an LES run with observed initial conditions and forcing is able to reproduce the onset and rate of boundary layer deepening. The surface temperature expression however is significantly different from observations, and the model exhibits large sensitivity to the numerical representation of surface radiative heating. These novel observations of Langmuir circulations offer a benchmark for further improvement of numerical models.
{"title":"Observations and Numerical Simulations of the Onset and Growth of Langmuir Circulations","authors":"Alexander Andriatis, L. Lenain, Matthew H. Alford, Nathaniel Winstead, Joseph Geiman","doi":"10.1175/jpo-d-24-0004.1","DOIUrl":"https://doi.org/10.1175/jpo-d-24-0004.1","url":null,"abstract":"\u0000We report novel observations of the onset and growth of Langmuir circulations (LCs) from simultaneous airborne and subsurface in-situ measurements. Under weak, fetch-limited wind wave forcing with stabilizing buoyancy forcing, the onset of LCs is observed for wind speeds greater than about 1 m s−1. LCs appear non-uniformly in space, consistent with previous laboratory experiments and suggestive of coupled wave-turbulence interaction. Following an increase in wind speed from < 1 m s−1 to sustained 3 m s−1 winds, a shallow (< 0.7 m) diurnal warm layer is observed to deepen at 1 m hr−1, while the cross-cell scales of LCs grow at 2 m hr−1, as observed in sea surface temperature collected from a research aircraft. Subsurface temperature structures show temperature intrusions into the base of the diurnal warm layer of the same scale as bubble entrainment depth during the deepening period, and are comparable to temperature structures observed during strong wind forcing with a deep mixed layer that is representative of previous LC studies. We show that an LES run with observed initial conditions and forcing is able to reproduce the onset and rate of boundary layer deepening. The surface temperature expression however is significantly different from observations, and the model exhibits large sensitivity to the numerical representation of surface radiative heating. These novel observations of Langmuir circulations offer a benchmark for further improvement of numerical models.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140981912","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}
D. Chaudhuri, D. Sengupta, E. D’Asaro, J. Farrar, Manikandan Mathur, Sundar Ranganathan
We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using six years of hourly observations from four moorings. The mean annual energy input from surface winds to near-inertial mixed-layer currents is 10–20 kJ/m2, occurring mainly in distinct synoptic “events” from April to September. A total of fifteen events are analyzed: Seven when the ocean is capped by a thin layer of low-salinity river water (fresh) and eight when it is not (salty). The average near-inertial energy input from winds is 40% higher in the fresh cases than in the salty cases. During the fresh events, (A) mixed layer near-inertial motions decay about two times faster, and (B) near-inertial kinetic energy below the mixed layer is reduced by at least a factor of three relative to the salty cases. The near-inertial horizontal wavelength was measured for one fresh and one salty event; the fresh was about three times shorter initially. A linear model of near-inertial wave propagation tuned to these data reproduces (B); the thin (10 m) mixed layers during the fresh events excite high modes, which propagate more slowly than the low modes excited by the thicker (40 m) mixed layers in the salty events. The model does not reproduce (A); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible.
{"title":"Near-inertial response of a salinity-stratified ocean","authors":"D. Chaudhuri, D. Sengupta, E. D’Asaro, J. Farrar, Manikandan Mathur, Sundar Ranganathan","doi":"10.1175/jpo-d-23-0173.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0173.1","url":null,"abstract":"\u0000We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using six years of hourly observations from four moorings. The mean annual energy input from surface winds to near-inertial mixed-layer currents is 10–20 kJ/m2, occurring mainly in distinct synoptic “events” from April to September. A total of fifteen events are analyzed: Seven when the ocean is capped by a thin layer of low-salinity river water (fresh) and eight when it is not (salty). The average near-inertial energy input from winds is 40% higher in the fresh cases than in the salty cases. During the fresh events, (A) mixed layer near-inertial motions decay about two times faster, and (B) near-inertial kinetic energy below the mixed layer is reduced by at least a factor of three relative to the salty cases. The near-inertial horizontal wavelength was measured for one fresh and one salty event; the fresh was about three times shorter initially. A linear model of near-inertial wave propagation tuned to these data reproduces (B); the thin (10 m) mixed layers during the fresh events excite high modes, which propagate more slowly than the low modes excited by the thicker (40 m) mixed layers in the salty events. The model does not reproduce (A); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141007021","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}
Boer Zhang, Marianna Linz, Shantong Sun, Andrew F. Thompson
The age of seawater refers to the amount of the time that has elapsed since that water encountered the surface. This age measures the ventilation rate of the ocean, and the spatial distribution of age can be influenced by multiple processes, such as the overturning circulation, ocean mixing, and air-sea exchange. In this work, we aim to gain new quantitative insights about how the ocean’s age tracer distribution reflects the strength of the meridional overturning circulation and diapycnal diffusivity. We propose an integral constraint that relates the age tracer flow across an isopycnal surface to the geometry of the surface. With the integral constraint, a relationship between the globally-averaged effective diapycnal diffusivity and the meridional overturning strength at an arbitrary density level can be inferred from the age tracer concentration near that level. The theory is tested in a set of idealized single-basin simulations. A key insight from this study is that the age difference between regions of upwelling and downwelling, rather than any single absolute age value, is the best indicator of overturning strength. The framework has also been adapted to estimate the strength of abyssal overturning circulation in the modern North Pacific, and we demonstrate that the age field provides an estimate of the circulation strength consistent with previous studies. This framework could potentially constrain ocean circulation and mixing rates from age-like realistic tracers (e.g., radiocarbon) in both past and present climates.
{"title":"A framework for constraining ocean mixing rates and overturning circulation from age tracers","authors":"Boer Zhang, Marianna Linz, Shantong Sun, Andrew F. Thompson","doi":"10.1175/jpo-d-23-0162.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0162.1","url":null,"abstract":"\u0000The age of seawater refers to the amount of the time that has elapsed since that water encountered the surface. This age measures the ventilation rate of the ocean, and the spatial distribution of age can be influenced by multiple processes, such as the overturning circulation, ocean mixing, and air-sea exchange. In this work, we aim to gain new quantitative insights about how the ocean’s age tracer distribution reflects the strength of the meridional overturning circulation and diapycnal diffusivity. We propose an integral constraint that relates the age tracer flow across an isopycnal surface to the geometry of the surface. With the integral constraint, a relationship between the globally-averaged effective diapycnal diffusivity and the meridional overturning strength at an arbitrary density level can be inferred from the age tracer concentration near that level. The theory is tested in a set of idealized single-basin simulations. A key insight from this study is that the age difference between regions of upwelling and downwelling, rather than any single absolute age value, is the best indicator of overturning strength. The framework has also been adapted to estimate the strength of abyssal overturning circulation in the modern North Pacific, and we demonstrate that the age field provides an estimate of the circulation strength consistent with previous studies. This framework could potentially constrain ocean circulation and mixing rates from age-like realistic tracers (e.g., radiocarbon) in both past and present climates.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141019278","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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