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Abstract
The wind drag on the sea surface is characterized by the aerodynamic roughness of the sea surface, z 0, which is regulated by surface wind waves. Many studies have related the dimensionless form of z 0 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 z 0 when swells dominate is indicated. Instead, a consistent parametric description of z 0 can be obtained by using the wave age estimated from the mean wave period, and normalizing z 0 by the mean wavelength. Normalizing z 0 by the significant wave height introduces a spurious residual dependence of z 0 on the wave steepness. A parametric relationship is developed between the dimensionless z 0 (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.
Significance Statement
In this paper, a consistent parametric description of the wave age dependence of the surface aerodynamic roughness is presented, with a wide range of sea states from dominant wind-driven seas to mixed seas in which the swells are dominant.
Abstract
The wind drag on the sea surface is characterized by the aerodynamic roughness of the sea surface, z 0, which is regulated by surface wind waves. Many studies have related the dimensionless form of z 0 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 z 0 when swells dominate is indicated. Instead, a consistent parametric description of z 0 can be obtained by using the wave age estimated from the mean wave period, and normalizing z 0 by the mean wavelength. Normalizing z 0 by the significant wave height introduces a spurious residual dependence of z 0 on the wave steepness. A parametric relationship is developed between the dimensionless z 0 (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.
Significance Statement
In this paper, a consistent parametric description of the wave age dependence of the surface aerodynamic roughness is presented, with a wide range of sea states from dominant wind-driven seas to mixed seas in which the swells are dominant.
Abstract
Mesoscale eddies can alter the propagation of wind-generated near-inertial waves (NIWs). Different from previous studies, the subsurface mooring observed NIWs are generated outside an anticyclonic eddy (ACE) and then interact with the arriving ACE. It is found that with the arrival of the ACE, the NIWs accelerate to propagate downward and the maximum vertical wavelength and group velocity of NIWs reach ∼500 m and ∼35 m day−1, respectively. When entering the core of the ACE, the near-inertial energy is trapped and finally stalls at a critical depth, which basically corresponds to the base of the ACE located at around 750-m depth. Through a ray-tracing model and dynamic analyses, this critical depth is much deeper than that of NIWs generated directly inside an ACE. By using depth–time varying stratification and relative vorticity, ray-tracing experiments further demonstrate that NIWs generated outside and passed over by an ACE can propagate to deep depths. Furthermore, energy budget analyses indicate that the net energy transfer from the ACE to NIWs plays an important role in the enhancement of downward-propagating near-inertial energy and its long-term persistence (∼45 days) in the critical layer. Within the critical layer, the enhancement of shear instability and nonlinear interactions among internal waves account for the loss of the trapped near-inertial energy and provide energy for furnishing deep ocean mixing.
Significance Statement
The interactions between near-inertial waves and a westward-moving anticyclonic eddy are investigated in this study. Knowledge about the propagation of near-inertial waves continues to be a topic of interest because near-inertial waves transfer energy from the mixed layer to the interior ocean, which is an important source of turbulent mixing. While much is known about how near-inertial energy propagates inside an anticyclonic eddy, few studies have examined how near-inertial energy propagates when it is generated outside an anticyclonic eddy and then enters the arriving anticyclonic eddy. In this study, the deep propagation and trapping of near-inertial energy by a westward-moving anticyclonic eddy is observed, which contributes greatly to the energy budget and the deep-ocean mixing.
Abstract
Mesoscale eddies can alter the propagation of wind-generated near-inertial waves (NIWs). Different from previous studies, the subsurface mooring observed NIWs are generated outside an anticyclonic eddy (ACE) and then interact with the arriving ACE. It is found that with the arrival of the ACE, the NIWs accelerate to propagate downward and the maximum vertical wavelength and group velocity of NIWs reach ∼500 m and ∼35 m day−1, respectively. When entering the core of the ACE, the near-inertial energy is trapped and finally stalls at a critical depth, which basically corresponds to the base of the ACE located at around 750-m depth. Through a ray-tracing model and dynamic analyses, this critical depth is much deeper than that of NIWs generated directly inside an ACE. By using depth–time varying stratification and relative vorticity, ray-tracing experiments further demonstrate that NIWs generated outside and passed over by an ACE can propagate to deep depths. Furthermore, energy budget analyses indicate that the net energy transfer from the ACE to NIWs plays an important role in the enhancement of downward-propagating near-inertial energy and its long-term persistence (∼45 days) in the critical layer. Within the critical layer, the enhancement of shear instability and nonlinear interactions among internal waves account for the loss of the trapped near-inertial energy and provide energy for furnishing deep ocean mixing.
Significance Statement
The interactions between near-inertial waves and a westward-moving anticyclonic eddy are investigated in this study. Knowledge about the propagation of near-inertial waves continues to be a topic of interest because near-inertial waves transfer energy from the mixed layer to the interior ocean, which is an important source of turbulent mixing. While much is known about how near-inertial energy propagates inside an anticyclonic eddy, few studies have examined how near-inertial energy propagates when it is generated outside an anticyclonic eddy and then enters the arriving anticyclonic eddy. In this study, the deep propagation and trapping of near-inertial energy by a westward-moving anticyclonic eddy is observed, which contributes greatly to the energy budget and the deep-ocean mixing.
Abstract
Large-scale distribution and variations in active salt fingers (SF) in the western North Pacific were examined by detecting the active SF with a vertical density ratio Rρ = 1–2 at depths of 10–300 m using a monthly gridded hydrographic dataset from 2001 to 2016. The active SF is distributed most frequently 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 from +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−1) 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 from +0.02σθ to +0.2σθ of surface density.
Significance Statement
It has been recognized that salt-finger (SF) double-diffusive convection is not active in the western North Pacific Ocean. This study demonstrated the distribution and seasonal/interannual variations of active SF in the western North Pacific for the first time: the formation of active SF along 40°N in 140°E–180° around the Subarctic Boundary in March and rapid decay until August, and large year-to-year variations of vertical and horizontal extensions with density decreasing trend. This study also proposed a formation mechanism that is relevant to the active SF density decrease and warming trend in the western North Pacific.
Abstract
Large-scale distribution and variations in active salt fingers (SF) in the western North Pacific were examined by detecting the active SF with a vertical density ratio Rρ = 1–2 at depths of 10–300 m using a monthly gridded hydrographic dataset from 2001 to 2016. The active SF is distributed most frequently 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 from +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−1) 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 from +0.02σθ to +0.2σθ of surface density.
Significance Statement
It has been recognized that salt-finger (SF) double-diffusive convection is not active in the western North Pacific Ocean. This study demonstrated the distribution and seasonal/interannual variations of active SF in the western North Pacific for the first time: the formation of active SF along 40°N in 140°E–180° around the Subarctic Boundary in March and rapid decay until August, and large year-to-year variations of vertical and horizontal extensions with density decreasing trend. This study also proposed a formation mechanism that is relevant to the active SF density decrease and warming trend in the western North Pacific.
Abstract
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 time scales 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.
Abstract
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 time scales 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.
Abstract
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 characterized by higher subdaily 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 destabilizing surface heat fluxes and alongfront 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.
Abstract
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 characterized by higher subdaily 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 destabilizing surface heat fluxes and alongfront 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.
Abstract
The seasonal warming of Antarctic Winter Water (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 (r 2 = 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.
Significance Statement
Around Antarctica, the summer warming of the subsurface cold Antarctic Winter Water feeds the upper layer of the overturning circulation. This study aims to quantify the mechanisms that mediate the warming of Antarctic Winter Water. Our results reveal that the observed warming of this layer can be explained by both surface wind-driven mixing processes as well as double-diffusive convection occurring beneath the Winter Water layer. Understanding the role of these mechanisms is important for understanding the regions upper-ocean heat distribution, the rates of water mass transformation and how they might respond to changes in sea ice, stratification, and the overlying large-scale winds.
Abstract
The seasonal warming of Antarctic Winter Water (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 (r 2 = 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.
Significance Statement
Around Antarctica, the summer warming of the subsurface cold Antarctic Winter Water feeds the upper layer of the overturning circulation. This study aims to quantify the mechanisms that mediate the warming of Antarctic Winter Water. Our results reveal that the observed warming of this layer can be explained by both surface wind-driven mixing processes as well as double-diffusive convection occurring beneath the Winter Water layer. Understanding the role of these mechanisms is important for understanding the regions upper-ocean heat distribution, the rates of water mass transformation and how they might respond to changes in sea ice, stratification, and the overlying large-scale winds.
Abstract
The observed development of deep mixed layers and the dependence of intense, deep-mixing events on wind and wave conditions are studied using an ocean LES model with and without an imposed Stokes-drift wave forcing. Model results are compared to glider measurements of the ocean vertical temperature, salinity, and turbulence kinetic energy (TKE) dissipation rate structure collected in the Icelandic Basin. Observed wind stress reached 0.8 N m−2 with significant wave height of 4–6 m, while boundary layer depths reached 180 m. We find that wave forcing, via the commonly used Stokes drift vortex force parameterization, is crucial for accurate prediction of boundary layer depth as characterized by measured and predicted TKE dissipation rate profiles. Analysis of the boundary layer kinetic energy (KE) budget using a modified total Lagrangian-mean energy equation, derived for the wave-averaged Boussinesq equations by requiring that the rotational inertial terms vanish identically as in the standard energy budget without Stokes forcing, suggests that wind work should be calculated using both the surface current and surface Stokes drift. A large percentage of total wind energy is transferred to model TKE via regular and Stokes drift shear production and dissipated. However, resonance by clockwise rotation of the winds can greatly enhance the generation of inertial current mean KE (MKE). Without resonance, TKE production is about 5 times greater than MKE generation, whereas with resonance this ratio decreases to roughly 2. The results have implications for the problem of estimating the global kinetic energy budget of the ocean.
Abstract
The observed development of deep mixed layers and the dependence of intense, deep-mixing events on wind and wave conditions are studied using an ocean LES model with and without an imposed Stokes-drift wave forcing. Model results are compared to glider measurements of the ocean vertical temperature, salinity, and turbulence kinetic energy (TKE) dissipation rate structure collected in the Icelandic Basin. Observed wind stress reached 0.8 N m−2 with significant wave height of 4–6 m, while boundary layer depths reached 180 m. We find that wave forcing, via the commonly used Stokes drift vortex force parameterization, is crucial for accurate prediction of boundary layer depth as characterized by measured and predicted TKE dissipation rate profiles. Analysis of the boundary layer kinetic energy (KE) budget using a modified total Lagrangian-mean energy equation, derived for the wave-averaged Boussinesq equations by requiring that the rotational inertial terms vanish identically as in the standard energy budget without Stokes forcing, suggests that wind work should be calculated using both the surface current and surface Stokes drift. A large percentage of total wind energy is transferred to model TKE via regular and Stokes drift shear production and dissipated. However, resonance by clockwise rotation of the winds can greatly enhance the generation of inertial current mean KE (MKE). Without resonance, TKE production is about 5 times greater than MKE generation, whereas with resonance this ratio decreases to roughly 2. The results have implications for the problem of estimating the global kinetic energy budget of the ocean.
