Mesoscale eddies are ubiquitous features of the global ocean circulation. Traditionally, anticyclonic eddies are thought to be associated with positive temperature anomalies while cyclonic eddies are associated with negative temperature anomalies. However, our recent study found that about one-fifth of the eddies identified from global satellite observations are cold-core anticyclonic eddies (CAEs) and warm-core cyclonic eddies (WCEs). Here we show that in the tropical oceans where the probabilities of CAEs and WCEs are high, there are significantly more CAEs and WCEs in summer than in winter. We conduct a suite of idealized numerical model experiments initialized with composite eddy structures obtained from Argo profiles as well as a heat budget analysis. The results highlight the key role of relative wind-stress-induced Ekman pumping, surface mixed layer depth, and vertical entrainment in the formation and seasonal cycle of these unconventional eddies. The relative wind stress is found to be particularly effective in converting conventional eddies into CAEs or WCEs when the surface mixed layer is shallow. The abundance of CAEs and WCEs in the global ocean calls for further research on this topic.
{"title":"Generation of Cold Anticyclonic Eddies and Warm Cyclonic Eddies in the Tropical Oceans","authors":"Qinbiao Ni, Xiaoming Zhai, Zhibin Yang, Dake Chen","doi":"10.1175/jpo-d-22-0197.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0197.1","url":null,"abstract":"\u0000Mesoscale eddies are ubiquitous features of the global ocean circulation. Traditionally, anticyclonic eddies are thought to be associated with positive temperature anomalies while cyclonic eddies are associated with negative temperature anomalies. However, our recent study found that about one-fifth of the eddies identified from global satellite observations are cold-core anticyclonic eddies (CAEs) and warm-core cyclonic eddies (WCEs). Here we show that in the tropical oceans where the probabilities of CAEs and WCEs are high, there are significantly more CAEs and WCEs in summer than in winter. We conduct a suite of idealized numerical model experiments initialized with composite eddy structures obtained from Argo profiles as well as a heat budget analysis. The results highlight the key role of relative wind-stress-induced Ekman pumping, surface mixed layer depth, and vertical entrainment in the formation and seasonal cycle of these unconventional eddies. The relative wind stress is found to be particularly effective in converting conventional eddies into CAEs or WCEs when the surface mixed layer is shallow. The abundance of CAEs and WCEs in the global ocean calls for further research on this topic.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48271925","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}
Large-scale distribution and variations in active salt-finger (SF) in the western North Pacific were examined by detecting the active SF with a vertical density ratio Rρ = 1 − 2 at depths of 10-300m using a monthly gridded hydrographic dataset from 2001 to 2016. The active SF is distributed mostly in March along 40°N around the Subarctic Boundary (SAB), where the mixed layer deepens northward and corresponds to the Central Mode Water formation site with a density of +0.02σθ to +0.2σθ of surface density and mainly in 26.1-26.4σθ. This active SF along 40°N underwent seasonal variation and decayed rapidly from March to August from the shallower and less dense parts of the active SF with increasing mean density. The features of the active SF in March are consistent with the hypothesis that surface water with a horizontal density ratio RL = 1 − 2 is subducted and vertically superposed, resulting in an active SF. The mean density of the active SF in March is well correlated with the surface density with RL = 1 − 2, and both mean densities showed a decreasing trend from 2001 to 2016, following the surface warming trend (~0.057°C/yr) in the surface water with RL = 1 − 2. Large year-to-year variations in the active SF in March are explained by both horizontal and vertical extensions, and can be reproduced by four conditions: 1) from 1°N to 3°S of SAB, 2) RL=1-2, and 3) northward deepening of the mixed layer depth, and 4) the part with a density of +0.02σθ to +0.2σθ of surface density.
{"title":"Large-scale distribution and variations of active salt-finger double-diffusion in the western North Pacific","authors":"Ryosuke Oyabu, I. Yasuda, Yusuke Sasaki","doi":"10.1175/jpo-d-22-0244.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0244.1","url":null,"abstract":"\u0000Large-scale distribution and variations in active salt-finger (SF) in the western North Pacific were examined by detecting the active SF with a vertical density ratio Rρ = 1 − 2 at depths of 10-300m using a monthly gridded hydrographic dataset from 2001 to 2016. The active SF is distributed mostly in March along 40°N around the Subarctic Boundary (SAB), where the mixed layer deepens northward and corresponds to the Central Mode Water formation site with a density of +0.02σθ to +0.2σθ of surface density and mainly in 26.1-26.4σθ. This active SF along 40°N underwent seasonal variation and decayed rapidly from March to August from the shallower and less dense parts of the active SF with increasing mean density. The features of the active SF in March are consistent with the hypothesis that surface water with a horizontal density ratio RL = 1 − 2 is subducted and vertically superposed, resulting in an active SF. The mean density of the active SF in March is well correlated with the surface density with RL = 1 − 2, and both mean densities showed a decreasing trend from 2001 to 2016, following the surface warming trend (~0.057°C/yr) in the surface water with RL = 1 − 2. Large year-to-year variations in the active SF in March are explained by both horizontal and vertical extensions, and can be reproduced by four conditions: 1) from 1°N to 3°S of SAB, 2) RL=1-2, and 3) northward deepening of the mixed layer depth, and 4) the part with a density of +0.02σθ to +0.2σθ of surface density.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41766663","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}
L. Johnson, B. Fox‐Kemper, Qing Li, H. Pham, S. Sarkar
This work evaluates the fidelity of various upper ocean turbulence parameterizations subject to realistic monsoon forcing and presents a finite-time ensemble vector (EV) method to better manage the design and numerical principles of these parameterizations. The EV method emphasizes the dynamics of a turbulence closure multi-model ensemble and is applied to evaluate ten different ocean surface boundary layer (OSBL) parameterizations within a single column (SC) model against two boundary layer large eddy simulations (LES). Both LES include realistic surface forcing, but one includes wind-driven shear turbulence only, while the other includes additional Stokes forcing through the wave-average equations that generates Langmuir turbulence. The finite-time EV framework focuses on what constitutes the local behavior of the mixed layer dynamical system and isolates the forcing and ocean state conditions where turbulence parameterizations most disagree. Identifying disagreement provides the potential to evaluate SC models comparatively against the LES. Observations collected during the 2018 Monsoon onset in the Bay of Bengal provide a case study to evaluate models under realistic and variable forcing conditions. The case study results highlight two regimes where models disagree a) during wind-driven deepening of the mixed layer and b) under strong diurnal forcing.
{"title":"A Finite-Time Ensemble Method for Mixed Layer Model Comparison","authors":"L. Johnson, B. Fox‐Kemper, Qing Li, H. Pham, S. Sarkar","doi":"10.1175/jpo-d-22-0107.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0107.1","url":null,"abstract":"\u0000This work evaluates the fidelity of various upper ocean turbulence parameterizations subject to realistic monsoon forcing and presents a finite-time ensemble vector (EV) method to better manage the design and numerical principles of these parameterizations. The EV method emphasizes the dynamics of a turbulence closure multi-model ensemble and is applied to evaluate ten different ocean surface boundary layer (OSBL) parameterizations within a single column (SC) model against two boundary layer large eddy simulations (LES). Both LES include realistic surface forcing, but one includes wind-driven shear turbulence only, while the other includes additional Stokes forcing through the wave-average equations that generates Langmuir turbulence. The finite-time EV framework focuses on what constitutes the local behavior of the mixed layer dynamical system and isolates the forcing and ocean state conditions where turbulence parameterizations most disagree. Identifying disagreement provides the potential to evaluate SC models comparatively against the LES. Observations collected during the 2018 Monsoon onset in the Bay of Bengal provide a case study to evaluate models under realistic and variable forcing conditions. The case study results highlight two regimes where models disagree a) during wind-driven deepening of the mixed layer and b) under strong diurnal forcing.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42216472","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}
We characterize the internal wave field at a standing meander of the Antarctic Circumpolar Current (ACC) where strong winds, bathymetry, and a strong eddy field combine to form a dynamic environment for the generation and dissipation of internal waves. We use Electromagnetic Autonomous Profiling Explorer float data spanning 0−1600 m depth collected from a meander near the Macquarie Ridge, south of Australia. Of the 112 internal waves identified, 69% are associated with upward energy propagation. Most of the upward propagating waves (35%) are found near the Polar Front and are likely generated by mean flow-topography interactions. Generation by wind forcing at the sea surface is likely responsible for more than 40% of the downward propagating waves. Our results highlight advection of the waves and wave-mean flow interactions within the ACC as the dominant processes affecting the wave dynamics. The larger dissipation timescales of the waves compared to advection suggests they are likely to dissipate away from the generation site. We find that about 79% (66%) of the waves in cyclonic eddies (the Subantarctic Front) are influenced by horizontal strain, whereas 92% of the waves in the slower Polar Front are influenced by the relative vorticity of the background flow. There is energy exchange between internal waves and the mean flow, in both directions. The mean energy transfer (1.4±1.0×10−11 m2 s−3) is from the mean flow to the waves in all dynamic regions except in anticyclonic eddies. The strongest energy exchange (5.0±3.7×10−11 m2 s−3) is associated with waves in cyclonic eddies.
{"title":"Observations of internal wave interactions in a Southern Ocean standing meander","authors":"A. Cyriac, A. Meyer, H. Phillips, N. Bindoff","doi":"10.1175/jpo-d-22-0157.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0157.1","url":null,"abstract":"\u0000We characterize the internal wave field at a standing meander of the Antarctic Circumpolar Current (ACC) where strong winds, bathymetry, and a strong eddy field combine to form a dynamic environment for the generation and dissipation of internal waves. We use Electromagnetic Autonomous Profiling Explorer float data spanning 0−1600 m depth collected from a meander near the Macquarie Ridge, south of Australia. Of the 112 internal waves identified, 69% are associated with upward energy propagation. Most of the upward propagating waves (35%) are found near the Polar Front and are likely generated by mean flow-topography interactions. Generation by wind forcing at the sea surface is likely responsible for more than 40% of the downward propagating waves. Our results highlight advection of the waves and wave-mean flow interactions within the ACC as the dominant processes affecting the wave dynamics. The larger dissipation timescales of the waves compared to advection suggests they are likely to dissipate away from the generation site. We find that about 79% (66%) of the waves in cyclonic eddies (the Subantarctic Front) are influenced by horizontal strain, whereas 92% of the waves in the slower Polar Front are influenced by the relative vorticity of the background flow. There is energy exchange between internal waves and the mean flow, in both directions. The mean energy transfer (1.4±1.0×10−11 m2 s−3) is from the mean flow to the waves in all dynamic regions except in anticyclonic eddies. The strongest energy exchange (5.0±3.7×10−11 m2 s−3) is associated with waves in cyclonic eddies.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43487180","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}
Carly Schmidgall, Yidongfang Si, A. Stewart, A. Thompson, A. Hogg
The export of Antarctic Bottom Water (AABW) supplies the bottom cell of the global overturning circulation and plays a key role in regulating climate. This AABW outflow must cross, and is therefore mediated by, the Antarctic Circumpolar Current (ACC). Previous studies present widely-varying conceptions of the role of the ACC in directing AABW across the Southern Ocean, suggesting either that AABW may be zonally recirculated by the ACC, or that AABW may flow northward within deep western boundary currents (DWBC) against bathymetry. In this study the authors investigate how the forcing and geometry of the ACC influences the transport and transformation of AABW using a suite of process-oriented model simulations. The model exhibits a strong dependence on the elevation of bathymetry relative to AABW layer thickness: higher meridional ridges suppress zonal AABW exchange, increase the strength of flow in the DWBC, and reduce the meridional variation in AABW density across the ACC. Furthermore, the transport and transformation vary with density within the AABW layer, with denser varieties of AABW being less efficiently transported between basins. These findings indicate that changes in the thickness of the AABW layer, for example due to changes in Antarctic shelf processes, and tectonic changes in the sea floor shape may alter the pathways and transformation of AABW across the ACC.
{"title":"Dynamical Controls on Bottom Water Transport and Transformation across the Antarctic Circumpolar Current","authors":"Carly Schmidgall, Yidongfang Si, A. Stewart, A. Thompson, A. Hogg","doi":"10.1175/jpo-d-22-0113.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0113.1","url":null,"abstract":"\u0000The export of Antarctic Bottom Water (AABW) supplies the bottom cell of the global overturning circulation and plays a key role in regulating climate. This AABW outflow must cross, and is therefore mediated by, the Antarctic Circumpolar Current (ACC). Previous studies present widely-varying conceptions of the role of the ACC in directing AABW across the Southern Ocean, suggesting either that AABW may be zonally recirculated by the ACC, or that AABW may flow northward within deep western boundary currents (DWBC) against bathymetry. In this study the authors investigate how the forcing and geometry of the ACC influences the transport and transformation of AABW using a suite of process-oriented model simulations. The model exhibits a strong dependence on the elevation of bathymetry relative to AABW layer thickness: higher meridional ridges suppress zonal AABW exchange, increase the strength of flow in the DWBC, and reduce the meridional variation in AABW density across the ACC. Furthermore, the transport and transformation vary with density within the AABW layer, with denser varieties of AABW being less efficiently transported between basins. These findings indicate that changes in the thickness of the AABW layer, for example due to changes in Antarctic shelf processes, and tectonic changes in the sea floor shape may alter the pathways and transformation of AABW across the ACC.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45416452","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}
L. Clement, E. Frajka‐Williams, N. von Oppeln-Bronikowski, I. Goszczko, B. de Young
By ventilating the deep ocean, deep convection in the Labrador Sea plays a crucial role in the climate system. Unfortunately, the mechanisms leading to the cessation of convection and, hence, the mechanisms by which a changing climate might affect deep convection remain unclear. In winter 2020, three autonomous underwater gliders sampled the convective region and both its spatial and temporal boundaries. Both boundaries are characterised by higher sub-daily mixed-layer depth variability sampled by the gliders than the convective region. At the convection boundaries, buoyant intrusions–including eddies and filaments–instead of atmospheric warming primarily trigger restratification by bringing buoyancy with a comparable contribution from either fresh or warm intrusions. At the edges of these intrusions, submesoscale instabilities, such as symmetric instabilities and mixed-layer baroclinic instabilities, seem to contribute to the decay of the intrusions. In winter, enhanced lateral buoyancy gradients are correlated with strong destabilising surface heat fluxes and along-front winds. Consequently, winter atmospheric conditions and buoyant intrusions participate in halting convection by triggering restratification while surface fluxes are still destratifying. This study reveals freshwater anomalies in a narrow area offshore of the Labrador Current and near the convective region; this area has received less attention than the more eddy-rich West Greenland Current, but is a potential source of freshwater in closer proximity to the region of deep convection. Freshwater fluxes from the Arctic and Greenland are expected to increase under a changing climate, and our findings suggest that they may play an active role in the restratification of deep convection.
{"title":"Cessation of Labrador Sea Convection triggered by distinct fresh and warm (Sub)mesoscale Flows","authors":"L. Clement, E. Frajka‐Williams, N. von Oppeln-Bronikowski, I. Goszczko, B. de Young","doi":"10.1175/jpo-d-22-0178.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0178.1","url":null,"abstract":"\u0000By ventilating the deep ocean, deep convection in the Labrador Sea plays a crucial role in the climate system. Unfortunately, the mechanisms leading to the cessation of convection and, hence, the mechanisms by which a changing climate might affect deep convection remain unclear. In winter 2020, three autonomous underwater gliders sampled the convective region and both its spatial and temporal boundaries. Both boundaries are characterised by higher sub-daily mixed-layer depth variability sampled by the gliders than the convective region. At the convection boundaries, buoyant intrusions–including eddies and filaments–instead of atmospheric warming primarily trigger restratification by bringing buoyancy with a comparable contribution from either fresh or warm intrusions. At the edges of these intrusions, submesoscale instabilities, such as symmetric instabilities and mixed-layer baroclinic instabilities, seem to contribute to the decay of the intrusions. In winter, enhanced lateral buoyancy gradients are correlated with strong destabilising surface heat fluxes and along-front winds. Consequently, winter atmospheric conditions and buoyant intrusions participate in halting convection by triggering restratification while surface fluxes are still destratifying. This study reveals freshwater anomalies in a narrow area offshore of the Labrador Current and near the convective region; this area has received less attention than the more eddy-rich West Greenland Current, but is a potential source of freshwater in closer proximity to the region of deep convection. Freshwater fluxes from the Arctic and Greenland are expected to increase under a changing climate, and our findings suggest that they may play an active role in the restratification of deep convection.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46204027","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}
J. Moum, W. Smyth, K. Hughes, Deepak Cherian, S. Warner, B. Bourlès, P Brandt, M. Dengler
Several years of moored turbulence measurements from χpods at three sites in the equatorial cold tongues of Atlantic and Pacific Oceans yield new insights into proxy estimates of turbulence that specifically target the cold tongues. They also reveal previously unknown wind dependencies of diurnally-varying turbulence in the near-critical stratified shear layers beneath the mixed layer and above the core of the Equatorial Undercurrent that we have come to understand as deep cycle (DC) turbulence. Isolated by the mixed layer above, theDClayer is only indirectly linked to surface forcing. Yet it varies diurnally in concert with daily changes in heating/cooling. Diurnal composites computed from 10-minute averaged data at fixed χpod depths show that transitions from daytime to nighttime mixing regimes are increasingly delayed with weakening wind stress, τ. These transitions are also delayed with respect to depth such that they follow a descent rate of roughly 6 meters per hour, independent of τ. We hypothesize that this wind-dependent delay is a direct result of wind-dependent diurnal warm layer deepening, which acts as the trigger to DC layer instability by bringing shear from the surface downward but at rates much slower than 6 meters per hour. This delay in initiation of DC layer instability contributes to a reduction in daily averaged values of turbulence dissipation. Both the absence of descending turbulence in the sheared DC layer prior to arrival of the diurnal warm layer shear and the magnitude of the subsequent descent rate after arrival are roughly predicted by laboratory experiments on entrainment in stratified shear flows.
{"title":"Wind Dependencies of Deep Cycle Turbulence in the Equatorial Cold Tongues","authors":"J. Moum, W. Smyth, K. Hughes, Deepak Cherian, S. Warner, B. Bourlès, P Brandt, M. Dengler","doi":"10.1175/jpo-d-22-0203.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0203.1","url":null,"abstract":"\u0000Several years of moored turbulence measurements from χpods at three sites in the equatorial cold tongues of Atlantic and Pacific Oceans yield new insights into proxy estimates of turbulence that specifically target the cold tongues. They also reveal previously unknown wind dependencies of diurnally-varying turbulence in the near-critical stratified shear layers beneath the mixed layer and above the core of the Equatorial Undercurrent that we have come to understand as deep cycle (DC) turbulence. Isolated by the mixed layer above, theDClayer is only indirectly linked to surface forcing. Yet it varies diurnally in concert with daily changes in heating/cooling. Diurnal composites computed from 10-minute averaged data at fixed χpod depths show that transitions from daytime to nighttime mixing regimes are increasingly delayed with weakening wind stress, τ. These transitions are also delayed with respect to depth such that they follow a descent rate of roughly 6 meters per hour, independent of τ. We hypothesize that this wind-dependent delay is a direct result of wind-dependent diurnal warm layer deepening, which acts as the trigger to DC layer instability by bringing shear from the surface downward but at rates much slower than 6 meters per hour. This delay in initiation of DC layer instability contributes to a reduction in daily averaged values of turbulence dissipation. Both the absence of descending turbulence in the sheared DC layer prior to arrival of the diurnal warm layer shear and the magnitude of the subsequent descent rate after arrival are roughly predicted by laboratory experiments on entrainment in stratified shear flows.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45758819","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}
L. Thomas, E. Skyllingstad, L. Rainville, Verena Hormann, L. Centurioni, J. Moum, O. Asselin, Craig M. Lee
Along with boundary layer turbulence, downward radiation of near-inertial waves (NIWs) damps inertial oscillations (IOs) in the surface ocean, however the latter can also energize abyssal mixing. Here we present observations made from a dipole vortex in the Iceland Basin where, after the period of direct wind forcing, IOs lost over half their kinetic energy (KE) in two inertial periods to radiation of NIWs with minimal turbulent dissipation of KE. The dipole’s vorticity gradient led to a rapid reduction in the NIW’s lateral wavelength via ζ-refraction that was accompanied by isopycnal undulations below the surface mixed layer. Pressure anomalies associated with the undulations were correlated with the NIW’s velocity yielding an energy flux of 310 mW m−2 pointed antiparallel to the vorticity gradient and a downward flux of 1 mW m−2 capable of driving the observed drop in KE. The minimal role of turbulence in the energetics after the IOs had been generated by the winds was confirmed using a large eddy simulation driven by the observed winds.
{"title":"Damping of inertial motions through the radiation of near-inertial waves in a dipole vortex in the Iceland Basin","authors":"L. Thomas, E. Skyllingstad, L. Rainville, Verena Hormann, L. Centurioni, J. Moum, O. Asselin, Craig M. Lee","doi":"10.1175/jpo-d-22-0202.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0202.1","url":null,"abstract":"\u0000Along with boundary layer turbulence, downward radiation of near-inertial waves (NIWs) damps inertial oscillations (IOs) in the surface ocean, however the latter can also energize abyssal mixing. Here we present observations made from a dipole vortex in the Iceland Basin where, after the period of direct wind forcing, IOs lost over half their kinetic energy (KE) in two inertial periods to radiation of NIWs with minimal turbulent dissipation of KE. The dipole’s vorticity gradient led to a rapid reduction in the NIW’s lateral wavelength via ζ-refraction that was accompanied by isopycnal undulations below the surface mixed layer. Pressure anomalies associated with the undulations were correlated with the NIW’s velocity yielding an energy flux of 310 mW m−2 pointed antiparallel to the vorticity gradient and a downward flux of 1 mW m−2 capable of driving the observed drop in KE. The minimal role of turbulence in the energetics after the IOs had been generated by the winds was confirmed using a large eddy simulation driven by the observed winds.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47786504","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}
Xianxian Han, A. Stewart, Dake Chen, Xiaohui Liu, Tao Lian
Antarctic Bottom Water is primarily formed via overflows of dense shelf water (DSW) around the Antarctic continental margins. The dynamics of these overflows therefore influence the global abyssal stratification and circulation. Previous studies indicate that dense overflows can be unstable, energizing Topographic Rossby Waves (TRW) over the continental slope. However, it remains unclear how the wavelength and frequency of the TRWs are related to the properties of the overflowing DSW and other environmental conditions, and how the TRW properties influence the downslope transport of DSW. This study uses idealized high-resolution numerical simulations to investigate the dynamics of overflow-forced TRWs and the associated downslope transport of DSW. It is shown that the propagation of TRWs is constrained by the geostrophic along-slope flow speed of the DSW and by the dynamics of linear plane waves, allowing the wavelength and frequency of the waves to be predicted a priori. The rate of downslope DSW transport depends nonmonotonically on the slope steepness: steep slopes approximately suppress TRW formation, resulting in steady, frictionally-dominated DSW descent. For slopes of intermediate steepness, the overflow becomes unstable and generates TRWs, accompanied by interfacial form stresses that drive DSW downslope relatively rapidly. For gentle slopes, the TRWs lead to the formation of coherent eddies that inhibit downslope DSW transport. These findings may explain the variable properties of TRWs observed in oceanic overflows, and imply that the rate at which DSW descends to the abyssal ocean depends sensitively on the manifestation of TRWs and/or nonlinear eddies over the continental slope.
{"title":"Controls of topographic Rossby wave properties and downslope transport in dense overflows","authors":"Xianxian Han, A. Stewart, Dake Chen, Xiaohui Liu, Tao Lian","doi":"10.1175/jpo-d-22-0237.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0237.1","url":null,"abstract":"\u0000Antarctic Bottom Water is primarily formed via overflows of dense shelf water (DSW) around the Antarctic continental margins. The dynamics of these overflows therefore influence the global abyssal stratification and circulation. Previous studies indicate that dense overflows can be unstable, energizing Topographic Rossby Waves (TRW) over the continental slope. However, it remains unclear how the wavelength and frequency of the TRWs are related to the properties of the overflowing DSW and other environmental conditions, and how the TRW properties influence the downslope transport of DSW. This study uses idealized high-resolution numerical simulations to investigate the dynamics of overflow-forced TRWs and the associated downslope transport of DSW. It is shown that the propagation of TRWs is constrained by the geostrophic along-slope flow speed of the DSW and by the dynamics of linear plane waves, allowing the wavelength and frequency of the waves to be predicted a priori. The rate of downslope DSW transport depends nonmonotonically on the slope steepness: steep slopes approximately suppress TRW formation, resulting in steady, frictionally-dominated DSW descent. For slopes of intermediate steepness, the overflow becomes unstable and generates TRWs, accompanied by interfacial form stresses that drive DSW downslope relatively rapidly. For gentle slopes, the TRWs lead to the formation of coherent eddies that inhibit downslope DSW transport. These findings may explain the variable properties of TRWs observed in oceanic overflows, and imply that the rate at which DSW descends to the abyssal ocean depends sensitively on the manifestation of TRWs and/or nonlinear eddies over the continental slope.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":"1 1","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41474035","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 warming of AntarcticWinterWater (WW) is a key process that occurs along the path of deep water transformation to intermediate waters. These intermediate waters then enter the upper branch of the circumpolar overturning circulation. Despite its importance, the driving mechanisms that mediate the warming of Antarctic WW remain unknown, and their quantitative evaluation is lacking. Using 38 days of glider measurements of microstructure shear, we characterize the rate of turbulent dissipation and its drivers over a summer season in the northern Weddell Sea. Observed dissipation rates in the surface layer are mainly forced by winds, and explained by the stress scaling (r2=0.84). However, mixing to the base of the mixed layer during strong wind events is suppressed by vertical stratification from sea ice melt. Between the WW layer and the warm and saline circumpolar deep water, a subsurface layer of enhanced dissipation is maintained by double-diffusive convection (DDC). We develop a WW layer temperature budget and show that a warming trend (0.2°C over 28 days) is driven by a convergence of heat flux through mechanically-driven mixing at the base of the mixed layer and DDC at the base of the WW layer. Notably, excluding the contribution from DDC results in an underestimation of WW warming by 23%, highlighting the importance of adequately representing DDC in ocean models. These results further suggest that an increase in storm intensity and frequency during summer could increase the rate of warming of WW with implications for rates of upper ocean water mass transformation.
{"title":"Vertical convergence of turbulent and double-diffusive heat flux drives warming and erosion of Antarctic Winter Water in summer","authors":"I. Giddy, I. Fer, S. Swart, S. Nicholson","doi":"10.1175/jpo-d-22-0259.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0259.1","url":null,"abstract":"\u0000The seasonal warming of AntarcticWinterWater (WW) is a key process that occurs along the path of deep water transformation to intermediate waters. These intermediate waters then enter the upper branch of the circumpolar overturning circulation. Despite its importance, the driving mechanisms that mediate the warming of Antarctic WW remain unknown, and their quantitative evaluation is lacking. Using 38 days of glider measurements of microstructure shear, we characterize the rate of turbulent dissipation and its drivers over a summer season in the northern Weddell Sea. Observed dissipation rates in the surface layer are mainly forced by winds, and explained by the stress scaling (r2=0.84). However, mixing to the base of the mixed layer during strong wind events is suppressed by vertical stratification from sea ice melt. Between the WW layer and the warm and saline circumpolar deep water, a subsurface layer of enhanced dissipation is maintained by double-diffusive convection (DDC). We develop a WW layer temperature budget and show that a warming trend (0.2°C over 28 days) is driven by a convergence of heat flux through mechanically-driven mixing at the base of the mixed layer and DDC at the base of the WW layer. Notably, excluding the contribution from DDC results in an underestimation of WW warming by 23%, highlighting the importance of adequately representing DDC in ocean models. These results further suggest that an increase in storm intensity and frequency during summer could increase the rate of warming of WW with implications for rates of upper ocean water mass transformation.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42202429","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}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：