Multi-decadal variability on time-scales between 20 and 70 years have been observed in the time-series of North Atlantic SST. Many mechanisms have been proposed to explain multidecadal variabilities in Atlantic. Generally, it is the interaction between the MOC and North Atlantic surface buoyancy distribution that sustains this variability, with buoyancy anomalies either due to ocean-only processes or to air-sea interactions. In this context, the role of the Arctic Ocean, especially its freshwater flux into the North Atlantic, has been under-appreciated. Bering Strait, the only oceanic pathway between the Pacific Ocean and the Arctic Ocean, has been found important in Arctic Ocean freshwater budget and in modulating the time-averaged state and long-term response of the MOC to high-latitude buoyancy forcing anomalies, via freshwater transport between the Pacific and Atlantic Oceans. In this paper, we use idealized configurations that include a Pacific-like wide basin and an Atlantic-like narrow basin. The two basins are connected both in the south and north to longitudinally periodic channels, representing the Southern Ocean and the Arctic Ocean respectively. The Pacific-like basin is opened to the north only through a shallow and narrow strait, while the Atlantic-like basin is fully open to the north. With the goal of studying the role of Bering Strait in the multi-decadal variability, we find that the freshwater transport from the Bering Strait forms a tongue-structure along the western boundary of the narrow basin, which enhances the local horizontal density gradient. The western boundary region becomes unstable to large-scale baroclinic anomalies, giving rise to multi-decadal variability.
{"title":"The Potential Role of Bering Strait in the Dynamics of Multi-decadal Variability in the North Atlantic: an idealized model study","authors":"Xiaoting Yang, P. Cessi","doi":"10.1175/jpo-d-23-0010.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0010.1","url":null,"abstract":"\u0000Multi-decadal variability on time-scales between 20 and 70 years have been observed in the time-series of North Atlantic SST. Many mechanisms have been proposed to explain multidecadal variabilities in Atlantic. Generally, it is the interaction between the MOC and North Atlantic surface buoyancy distribution that sustains this variability, with buoyancy anomalies either due to ocean-only processes or to air-sea interactions. In this context, the role of the Arctic Ocean, especially its freshwater flux into the North Atlantic, has been under-appreciated. Bering Strait, the only oceanic pathway between the Pacific Ocean and the Arctic Ocean, has been found important in Arctic Ocean freshwater budget and in modulating the time-averaged state and long-term response of the MOC to high-latitude buoyancy forcing anomalies, via freshwater transport between the Pacific and Atlantic Oceans. In this paper, we use idealized configurations that include a Pacific-like wide basin and an Atlantic-like narrow basin. The two basins are connected both in the south and north to longitudinally periodic channels, representing the Southern Ocean and the Arctic Ocean respectively. The Pacific-like basin is opened to the north only through a shallow and narrow strait, while the Atlantic-like basin is fully open to the north. With the goal of studying the role of Bering Strait in the multi-decadal variability, we find that the freshwater transport from the Bering Strait forms a tongue-structure along the western boundary of the narrow basin, which enhances the local horizontal density gradient. The western boundary region becomes unstable to large-scale baroclinic anomalies, giving rise to multi-decadal variability.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48972845","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}
Min Wang, X. Zhu, Hua Zheng, Juntian Chen, Zhao‐Jun Liu, Qiang Ren, Yansong Liu, Feng Nan, F. Yu, Qiang Li
Using a large-scale observation array of 27 simultaneous pressure-recording inverted echo sounders (PIESs), the standing wave features of the mode-1 M2 internal tide west of the Luzon Strait (LS) were identified. These features exhibited non-monotonic spatial phase shifts and half-wavelength amplitude modulation, resulting in spatially varying amplitudes under PIES observations, which have not been previously observed in field observations west of the LS. Satellite altimeter measurements also identified standing wave patterns consistent with the PIES observations. These patterns emanated from interference between the northwestward and southeastward beams from the LS and the slope of the southern Taiwan Strait, respectively. Near the LS, the two beams superimposed into partial standing waves, whereas the superimposed waves tended to become perfect standing waves near the slope of the southern Taiwan Strait. The nodes and antinodes of the wave shifted under the influence of an anticyclonic eddy. The eddy-induced background current modified the phase speed of the internal tides, and the superimposed standing wave nodes and antinodes deflected clockwise. The node shifted during three anticyclonic eddy events, and two stations on two sides of the wave node showed opposite variations in amplitude.
{"title":"Direct evidence of standing internal tide west of the Luzon Strait observed by a large-scale observation array","authors":"Min Wang, X. Zhu, Hua Zheng, Juntian Chen, Zhao‐Jun Liu, Qiang Ren, Yansong Liu, Feng Nan, F. Yu, Qiang Li","doi":"10.1175/jpo-d-23-0043.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0043.1","url":null,"abstract":"\u0000Using a large-scale observation array of 27 simultaneous pressure-recording inverted echo sounders (PIESs), the standing wave features of the mode-1 M2 internal tide west of the Luzon Strait (LS) were identified. These features exhibited non-monotonic spatial phase shifts and half-wavelength amplitude modulation, resulting in spatially varying amplitudes under PIES observations, which have not been previously observed in field observations west of the LS. Satellite altimeter measurements also identified standing wave patterns consistent with the PIES observations. These patterns emanated from interference between the northwestward and southeastward beams from the LS and the slope of the southern Taiwan Strait, respectively. Near the LS, the two beams superimposed into partial standing waves, whereas the superimposed waves tended to become perfect standing waves near the slope of the southern Taiwan Strait. The nodes and antinodes of the wave shifted under the influence of an anticyclonic eddy. The eddy-induced background current modified the phase speed of the internal tides, and the superimposed standing wave nodes and antinodes deflected clockwise. The node shifted during three anticyclonic eddy events, and two stations on two sides of the wave node showed opposite variations in amplitude.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47364330","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, Ruixiang Zhao, Juntian Chen, Min Wang, Qiang Ren, Yansong Liu, Feng Nan, F. Yu, Jaehun Park
Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm/s. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ~3.9 and ~2.5 m/s north and south of the typhoon’s track, respectively, while those of mode 3 were ~2.1 and ~1.7 m/s, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode 2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ~300 m and lengthened the duration of energetic NIWs observed in the north.
{"title":"Near-Inertial Waves Reaching the Deep Basin in the South China Sea after Typhoon Mangkhut (2018)","authors":"Hua Zheng, X. Zhu, Ruixiang Zhao, Juntian Chen, Min Wang, Qiang Ren, Yansong Liu, Feng Nan, F. Yu, Jaehun Park","doi":"10.1175/jpo-d-22-0136.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0136.1","url":null,"abstract":"\u0000Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm/s. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ~3.9 and ~2.5 m/s north and south of the typhoon’s track, respectively, while those of mode 3 were ~2.1 and ~1.7 m/s, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode 2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ~300 m and lengthened the duration of energetic NIWs observed in the north.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49326600","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}
A perturbative solution of simplified primitive equations for nonlinear weakly stratified upwelling over a frictional slope is found that resolves the vertical structure of velocity fields and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The solution uses assumptions consistent with the model proposed by Lentz and Chapman (2004), including steady-state, constant cross-shore density gradient, no alongshore gradients, laterally inviscid, and consideration of cross-shore advection of alongshore momentum. The solution resolves the vertical structure of velocity fields (including subsurface maxima of compensational flow, not resolved by Lentz and Chapman) and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The dynamics are similar to Lentz and Chapman; bottom stress balances alongshore wind stress in a homogeneous density ocean, and is replaced by nonlinear cross-shore transport of alongshore momentum as the Burger number (S = αN / f , where α, N, and f are the bottom slope, buoyancy frequency, Coriolis frequency, respectively) increases. When the solution uses the empirical relation between cross-shore and vertical density gradients proposed by Lentz and Chapman, vorticity conservation is not satisfied and the nonlinear momentum transport estimated by the solution linearly increases with S, asymptotically matching Lentz and Chapman for S < 1. When the solution conserves interior potential vorticity, the momentum transport is proportional to S2 for S < 1 and is in better agreement with numerical simulations.
{"title":"A perturbative solution for nonlinear stratified upwelling over frictional slope","authors":"Jang-Geun Choi, J. Pringle, T. Lippmann","doi":"10.1175/jpo-d-22-0191.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0191.1","url":null,"abstract":"\u0000A perturbative solution of simplified primitive equations for nonlinear weakly stratified upwelling over a frictional slope is found that resolves the vertical structure of velocity fields and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The solution uses assumptions consistent with the model proposed by Lentz and Chapman (2004), including steady-state, constant cross-shore density gradient, no alongshore gradients, laterally inviscid, and consideration of cross-shore advection of alongshore momentum. The solution resolves the vertical structure of velocity fields (including subsurface maxima of compensational flow, not resolved by Lentz and Chapman) and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The dynamics are similar to Lentz and Chapman; bottom stress balances alongshore wind stress in a homogeneous density ocean, and is replaced by nonlinear cross-shore transport of alongshore momentum as the Burger number (S = αN / f , where α, N, and f are the bottom slope, buoyancy frequency, Coriolis frequency, respectively) increases. When the solution uses the empirical relation between cross-shore and vertical density gradients proposed by Lentz and Chapman, vorticity conservation is not satisfied and the nonlinear momentum transport estimated by the solution linearly increases with S, asymptotically matching Lentz and Chapman for S < 1. When the solution conserves interior potential vorticity, the momentum transport is proportional to S2 for S < 1 and is in better agreement with numerical simulations.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42262670","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 wind drag on the sea surface is characterized by the aerodynamic roughness of the sea surface, z0, which is regulated by surface wind waves. Many studies have related the dimensionless form of z0 to the wave age parameter estimated from spectral peak information. These parametric relationships have been well developed for the wind-driven sea but not for mixed seas. Based on an analysis using observations from a fixed platform in the northern South China Sea, the deficiency of the spectral peak information in the parametric description z0 when swells dominate is indicated. Instead, a consistent parametric description of z0 can be obtained by using the wave age estimated from the mean wave period, and normalizing z0 by the mean wavelength. Normalizing z0 by the significant wave height introduces a spurious residual dependence of z0 on the wave steepness. A parametric relationship is developed between the dimensionless z0 (normalized by the mean wavelength) and the wave age from the mean wave period. A comparison of this new relationship to the wind speed-only formulation in COARE 3.5 is provided.
{"title":"On the Consistent Parametric Description of the Wave Age Dependence of the Sea Surface Roughness","authors":"Shuiqing Li","doi":"10.1175/jpo-d-23-0021.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0021.1","url":null,"abstract":"\u0000The wind drag on the sea surface is characterized by the aerodynamic roughness of the sea surface, z0, which is regulated by surface wind waves. Many studies have related the dimensionless form of z0 to the wave age parameter estimated from spectral peak information. These parametric relationships have been well developed for the wind-driven sea but not for mixed seas. Based on an analysis using observations from a fixed platform in the northern South China Sea, the deficiency of the spectral peak information in the parametric description z0 when swells dominate is indicated. Instead, a consistent parametric description of z0 can be obtained by using the wave age estimated from the mean wave period, and normalizing z0 by the mean wavelength. Normalizing z0 by the significant wave height introduces a spurious residual dependence of z0 on the wave steepness. A parametric relationship is developed between the dimensionless z0 (normalized by the mean wavelength) and the wave age from the mean wave period. A comparison of this new relationship to the wind speed-only formulation in COARE 3.5 is provided.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42017559","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. Quan, Zhiqiang Liu, H. Xue, Jianyu Hu, Qiang Wang, Han Zhang, Xiaohui Liu, G. Jin, Y. Wang
Using observations and theoretical models, a substantial topographic modulation on the quasi-geostrophic (QG) dynamics, which results in a primary surface mode distinct from the classic first baroclinic (BC1) mode with a flat bottom, is revealed in the Northern South China Sea (NSCS). In contrast to open oceans, the surface-intensified modes decay downward more rapidly over the continental slope of the NSCS, with a mean e-folding scale of approximately 1/5 of water depth. The subinertial flow variability appears to be vertically incoherent, with planetary and topographic Rossby waves dominating in the upper and deep layers, respectively. With a larger deformation radius ( Rd ), the surface mode Rossby waves propagate at a phase speed ~1.5 times of that of the BC1 mode. Moreover, the modal structures can be substantially modified by seasonal NSCS circulation, which is significantly enhanced over continental slopes. Analysis of the triad interactions further implies that the short waves tend to transfer energy to larger scales via the inverse cascade and only those with wavelengths larger than Rd ≈ 70 km in the NSCS can persist because of a slower unstable growth rate but a higher fraction of upscale energy transfer. The present theory excludes the bottom-trapped mode, which is closely associated with topographic Rossby waves and is observed to be significant in the abyssal NSCS. Hence, a complete normal-mode basis for any QG state is required for a study that focuses on flow variability throughout the water column. Our findings provide an insight into the vertical partition of horizontal kinetic energy for QG motions, as well as the relevant oceanic variation in marginal seas.
{"title":"Observed Oceanic Surface Modes in the Northern South China Sea","authors":"Q. Quan, Zhiqiang Liu, H. Xue, Jianyu Hu, Qiang Wang, Han Zhang, Xiaohui Liu, G. Jin, Y. Wang","doi":"10.1175/jpo-d-22-0250.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0250.1","url":null,"abstract":"\u0000Using observations and theoretical models, a substantial topographic modulation on the quasi-geostrophic (QG) dynamics, which results in a primary surface mode distinct from the classic first baroclinic (BC1) mode with a flat bottom, is revealed in the Northern South China Sea (NSCS). In contrast to open oceans, the surface-intensified modes decay downward more rapidly over the continental slope of the NSCS, with a mean e-folding scale of approximately 1/5 of water depth. The subinertial flow variability appears to be vertically incoherent, with planetary and topographic Rossby waves dominating in the upper and deep layers, respectively. With a larger deformation radius ( Rd ), the surface mode Rossby waves propagate at a phase speed ~1.5 times of that of the BC1 mode. Moreover, the modal structures can be substantially modified by seasonal NSCS circulation, which is significantly enhanced over continental slopes. Analysis of the triad interactions further implies that the short waves tend to transfer energy to larger scales via the inverse cascade and only those with wavelengths larger than Rd ≈ 70 km in the NSCS can persist because of a slower unstable growth rate but a higher fraction of upscale energy transfer. The present theory excludes the bottom-trapped mode, which is closely associated with topographic Rossby waves and is observed to be significant in the abyssal NSCS. Hence, a complete normal-mode basis for any QG state is required for a study that focuses on flow variability throughout the water column. Our findings provide an insight into the vertical partition of horizontal kinetic energy for QG motions, as well as the relevant oceanic variation in marginal seas.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41606671","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 continuous, moored observation revealed significant variability in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Cause of such AMOC variability is an extensively studied subject. This study focuses on the short-term variability, which ranges up to seasonal and interannual timescales. A mechanism is proposed from the perspective of ocean water redistribution by layers. By offering explanations for four phenomena of AMOC variability in the subtropical and tropical oceans (seasonality, meridional coherence, layered-transport compensation as observed at 26.5°N, and the 2009/2010 downturn occurred at 26.5°N), this mechanism suggests that the short-term AMOC variabilities in the entire subtropical and tropical regions are governed by a basin-wide adiabatic water redistribution process or the so-called sloshing dynamics rather than diapycnal processes.
{"title":"Mechanism on the short-term variability of the Atlantic meridional overturning circulation in the subtropical and tropical regions","authors":"Lei Han","doi":"10.1175/jpo-d-23-0027.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0027.1","url":null,"abstract":"\u0000The continuous, moored observation revealed significant variability in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Cause of such AMOC variability is an extensively studied subject. This study focuses on the short-term variability, which ranges up to seasonal and interannual timescales. A mechanism is proposed from the perspective of ocean water redistribution by layers. By offering explanations for four phenomena of AMOC variability in the subtropical and tropical oceans (seasonality, meridional coherence, layered-transport compensation as observed at 26.5°N, and the 2009/2010 downturn occurred at 26.5°N), this mechanism suggests that the short-term AMOC variabilities in the entire subtropical and tropical regions are governed by a basin-wide adiabatic water redistribution process or the so-called sloshing dynamics rather than diapycnal processes.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43265729","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}
A. Caceres-Euse, Verónica Morales‐Márquez, A. Molcard
This study analyzes horizontal and vertical wind-driven circulation responses in small semi-enclosed bays, the associated offshore dynamic conditions and the relative importance of each term in the momentum balance equations using a multiplatform observational system. The observational platform consists of 3 ADCPs and a land-based radar monitoring the velocity field within the bay and in the contiguous offshore area. The wind-driven patterns in the bay can switch from a barotropic cyclonic or anti-cyclonic circulation to a 2-layers baroclinic mode response as a function of the wind regime (its direction and magnitude). For the baroclinic mode, the vertical location of the inflection point in the velocity profile can vary according to the proximity of the boundary current to the entrance of the bay. The influence of offshore meteo-marine conditions in the inner-bay dynamics is evidenced under strong to mid wind conditions, and is almost inexistent under negligible wind. The momentum balance analysis as well as the non-dimensional numbers evidence the impact of wind stress, coastline shape, stratification, and the nonlinear advective terms. Advection can be at the same order of magnitude as pressure gradient, Coriolis or wind stress terms, and greater than the bottom stress terms. The nonlinear terms in the momentum equations are frequently neglected when analyzing wind-driven circulation by means of in-situ data or analytical models.
{"title":"On the observed wind-driven circulation response in small semi-enclosed bays","authors":"A. Caceres-Euse, Verónica Morales‐Márquez, A. Molcard","doi":"10.1175/jpo-d-22-0224.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0224.1","url":null,"abstract":"\u0000This study analyzes horizontal and vertical wind-driven circulation responses in small semi-enclosed bays, the associated offshore dynamic conditions and the relative importance of each term in the momentum balance equations using a multiplatform observational system. The observational platform consists of 3 ADCPs and a land-based radar monitoring the velocity field within the bay and in the contiguous offshore area. The wind-driven patterns in the bay can switch from a barotropic cyclonic or anti-cyclonic circulation to a 2-layers baroclinic mode response as a function of the wind regime (its direction and magnitude). For the baroclinic mode, the vertical location of the inflection point in the velocity profile can vary according to the proximity of the boundary current to the entrance of the bay. The influence of offshore meteo-marine conditions in the inner-bay dynamics is evidenced under strong to mid wind conditions, and is almost inexistent under negligible wind. The momentum balance analysis as well as the non-dimensional numbers evidence the impact of wind stress, coastline shape, stratification, and the nonlinear advective terms. Advection can be at the same order of magnitude as pressure gradient, Coriolis or wind stress terms, and greater than the bottom stress terms. The nonlinear terms in the momentum equations are frequently neglected when analyzing wind-driven circulation by means of in-situ data or analytical models.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49313158","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}
Caique D. Luko, Cauê Z. Lazaneo, Ilson C. A. da Silveira, Filipe Pereira, Amit Tandon
Abstract The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.
{"title":"Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders","authors":"Caique D. Luko, Cauê Z. Lazaneo, Ilson C. A. da Silveira, Filipe Pereira, Amit Tandon","doi":"10.1175/jpo-d-22-0122.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0122.1","url":null,"abstract":"Abstract The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"27 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136376066","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 seasonal variability of the eddy kinetic energy (EKE) along the Kuroshio Current (KC) is examined using outputs from an eddy-resolving (1/10°) ocean model. Using a theoretical framework for climatological monthly mean EKE, the mechanisms governing the seasonal cycle of upper-ocean EKE are investigated. East of Taiwan, the EKE shows two comparable peaks in spring and summer in the surface layer; only the spring one is evident in the subsurface layer. The seasonality is determined by mixed barotropic (BTI) and baroclinic (BCI) instabilities. Northeast of Taiwan, the EKE is also elevated during spring–summer but with a sole peak in summer, which is dominated by the meridional EKE advection by the KC. In the middle part of the KC in the East China Sea, the mesoscale (>150 km) EKE (EKEMS) is relatively strong during spring–summer, whereas the submesoscale (50–150 km) EKE (EKESM) is significantly enhanced during winter–spring. The seasonal cycles of EKEMS and EKESM are primarily controlled by the external forcing and BCI, respectively. In particular, the higher EKEMS level in summer is mainly due to the increased wind work. West of the Tokara Strait, the EKE exhibits a prominent peak in winter and has its minimum in summer, which is regulated by the BCI. As the submesoscale signals are partially resolved by the model, further studies with higher-resolution simulations and observations are needed for a better understanding of the EKESM seasonality and its contribution to the seasonally modulating EKEMS along the KC.
{"title":"Seasonal Variability of Eddy Kinetic Energy along the Kuroshio Current","authors":"Xiaomei Yan, Dujuan Kang, E. Curchitser, Xiaohui Liu, Chongguang Pang, Linlin Zhang","doi":"10.1175/jpo-d-22-0155.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0155.1","url":null,"abstract":"\u0000The seasonal variability of the eddy kinetic energy (EKE) along the Kuroshio Current (KC) is examined using outputs from an eddy-resolving (1/10°) ocean model. Using a theoretical framework for climatological monthly mean EKE, the mechanisms governing the seasonal cycle of upper-ocean EKE are investigated. East of Taiwan, the EKE shows two comparable peaks in spring and summer in the surface layer; only the spring one is evident in the subsurface layer. The seasonality is determined by mixed barotropic (BTI) and baroclinic (BCI) instabilities. Northeast of Taiwan, the EKE is also elevated during spring–summer but with a sole peak in summer, which is dominated by the meridional EKE advection by the KC. In the middle part of the KC in the East China Sea, the mesoscale (>150 km) EKE (EKEMS) is relatively strong during spring–summer, whereas the submesoscale (50–150 km) EKE (EKESM) is significantly enhanced during winter–spring. The seasonal cycles of EKEMS and EKESM are primarily controlled by the external forcing and BCI, respectively. In particular, the higher EKEMS level in summer is mainly due to the increased wind work. West of the Tokara Strait, the EKE exhibits a prominent peak in winter and has its minimum in summer, which is regulated by the BCI. As the submesoscale signals are partially resolved by the model, further studies with higher-resolution simulations and observations are needed for a better understanding of the EKESM seasonality and its contribution to the seasonally modulating EKEMS along the KC.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48789755","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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