Abstract

The North Atlantic Tracer Release Experiment (NATRE) was performed in an area moderately favorable to salt fingers. However, the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent. This is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems. Despite the lack of a staircase, optical shadowgraph profiles revealed that small-scale tilted laminae, previously observed in a salt-finger staircase, were abundant at the NATRE site. Using microstructure observations, the strength of salt-finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy (Γ, “the dissipation ratio”). By examining the dissipation ratio in a parameter space of density ratio (R _ρ ) and Richardson number (Ri), the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred. While the model for turbulence describes most dissipation occurring when Ri < 1, dissipation at larger Ri is better described by the salt-finger model. Based on the results of the parameter space analysis, a method is proposed for estimating the salt-finger enhancement of the diapycnal haline diffusivity (k _s ) over the thermal diffusivity (k _θ ). During April 1992 at the NATRE site, it was found that k _θ = (0.08 ± 0.01) cm² s⁻¹ and k _s = (0.13 ± 0.01) cm² s⁻¹ for the neutral density surface local to the tracer release isopycnal (σ _θ ∼ 26.75 kg m⁻³, z ∼ 300 m). The flux divergence of buoyancy was also computed, giving the diapycnal advection w∗ = −(1.7 ± 1.2) m yr⁻¹. Moreover, divergence of vertical buoyancy flux was dominated by the haline component. For comparison, the tracer release method gave a diffusivity of k _s = (0.12 ± 0.02) cm² s⁻¹ (May–November 1992) and a diapycnal velocity of w∗ = −(3 ± 1) m yr⁻¹ (May 1992–November 1994) at this site. The above numbers are contrasted to diffusivity estimates derived from turbulence theory alone. Best agreement between tracer-inferred mixing rates and microstructure based estimates is achieved when the salt-finger enhancement of k _s is taken into account.

Abstract

Internal wave theory is used to examine the generation, radiation, and energy dissipation of internal tides in the deep ocean. Estimates of vertical energy flux based on a previously developed model are adjusted to account for the influence of finite depth, varying stratification, and two-dimensional topography. Specific estimates of energy flux are made for midocean ridge topography. Weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory. Most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km. The Richardson number of the radiated internal tide typically exceeds unity for these motions, and so direct shear instability of the generated waves is not the dominant energy transfer mechanism. It also seems that wave–wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux. Instead, it appears that most of the internal tide energy is radiated over O(1000 km) distances. A small fraction of energy flux, less than 30%, is generated at smaller spatial scales, and this energy flux may dissipate locally. Estimates along the Mid-Atlantic Ridge in the South Atlantic suggest that the vertical energy flux of M ₂ internal tides is 3–5 mW m⁻², with 1–2 mW m⁻² likely contributing to local mixing. Along the East Pacific Rise, bathymetry is more smooth and tides are weaker, and estimates suggest internal tide energy flux is negligible. Radiated low modes are likely influenced by topographic scattering, though general topography scatters less than 10% of the low-mode energy to higher wavenumbers. Thus, low-mode internal tides may contribute to mixing at locations far away from their generation sites.

Abstract

Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04–0.10-Sv (1 Sv ≡ 10⁶ m³ s⁻¹) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy ε increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides. During spring to neap tides, ε varies from O(10⁻⁹) to O(10⁻¹⁰) W kg⁻¹ below 3500 m around the 2-layer interface. The detection of temperature overturns during tidal flow reversal, which almost fully opposes the deep up-canyon mean flow, confirms the canyon middepth enhancement of ε. The internal tide energy flux, particularly enhanced at the sill, compares with the lower-layer energy loss across the sill. Throughout the canyon away from the sill, near-inertial waves with downward-propagating energy dominate the internal wave field. The present study underlines the intricate pattern of the deep turbulent mixing affected by the mean flow, internal tides, and near-inertial waves.

Abstract

Observations of turbulent dissipation above rough bathymetry in the abyssal Brazil Basin are presented. Relative to regions with smooth bathymetry, dissipation is markedly enhanced above rough topography of the Mid-Atlantic Ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors. Furthermore, mixing levels in rough areas are modulated by the spring–neap tidal cycle. Internal waves generated by barotropic tidal flow over topography are the likely mechanism for supplying the energy needed to support the observed turbulent dissipation.

A model of the spatial and temporal patterns in the turbulent dissipation rate is used to constrain the diapycnal advection in an inverse calculation for the circulation in an area of rough bathymetry. This inverse model uses both beta-spiral and integrated forms of the advective budgets for heat, mass, and vorticity, and contains sufficient information to resolve the full three-dimensional flow. The inverse model solution reveals the presence of a bouyancy forced circulation driven by mixing in abyssal canyons. On isopycnals above the level of fracture-zone crests near the Mid-Atlantic Ridge, the flow is westward and fluid is downwelled toward greater density. Along deeper isopycnals, fluid is carried eastward and upwelled in canyons. The divergence of diapycnal mass flux is a significant forcing mechanism for this circulation. These results suggest that mixing in abyssal canyons plays an important role in the circulation of abyssal waters.

Abstract

Direct measurements of oceanic turbulent parameters were taken upstream of and across Drake Passage, in the region of the Subantarctic and Polar Fronts. Values of turbulent kinetic energy dissipation rate ε estimated by microstructure are up to two orders of magnitude lower than previously published estimates in the upper 1000 m. Turbulence levels in Drake Passage are systematically higher than values upstream, regardless of season. The dissipation of thermal variance χ is enhanced at middepth throughout the surveys, with the highest values found in northern Drake Passage, where water mass variability is the most pronounced. Using the density ratio, evidence for double-diffusive instability is presented. Subject to double-diffusive physics, the estimates of diffusivity using the Osborn–Cox method are larger than ensemble statistics based on ε and the buoyancy frequency.

Abstract

One of the major objectives of the Deep Basin Experiment, a component of the World Ocean Circulation Experiment, was to quantify the intensity and spatial distribution of deep vertical mixing within the Brazil Basin. In this study, basin-averaged estimates of deep vertical mixing rates are calculated using two independent methodologies and datasets: 1) vertical fluxes are derived from large-scale temperature and density budgets using direct measurements of deep flow through passages connecting the Brazil Basin to surrounding basins and a comprehensive hydrographic dataset within the basin interior and 2) vertical mixing rates are estimated from finescale bathymetry and hydrographic data using a functional relationship between turbulent dissipation and bathymetric roughness, deduced from localized measurements of ocean microstructure obtained during the Deep Basin Experiment. The space–time mean estimates of vertical mixing diffusivities across representative surfaces within the Antarctic Bottom Water layer fell in the range κ ∼ 1–5(× 10⁻⁴ m² s⁻¹) and were indistinguishable from each other within the estimation uncertainties. The mixing rates inferred from potential temperature budgets update, and are consistent with, earlier estimates that were based on less data. Mixing rates inferred from budgets bounded by neutral surfaces are not significantly different from the former. This implies that lateral eddy fluxes along isopycnals are not important in the potential temperature budgets, at least within the large estimation uncertainties. Unresolved processes, such as cabbeling and low frequency variability, which complicate inference of mixing from large-scale budgets, have been considered. The agreement between diffusivity estimates based on a modeled relationship between bathymetric roughness and turbulent dissipation, with those inferred from large-scale budgets, provides independent confirmation that the mixing rates have been accurately quantified.

Abstract

Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale L _T and the Ozmidov length scale L _O . Historic evidence supporting L _T ~ L _O has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of L _T and L _O , using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that L _T /L _O increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting L _T ~ L _O are revisited, and conceptual regimes explaining the relationship between L _T /L _O and a nondimensional overturn size are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.

Abstract

The ocean surface boundary layer is a gateway of energy transfer into the ocean. Wind-driven shear and meteorologically forced convection inject turbulent kinetic energy into the surface boundary layer, mixing the upper ocean and transforming its density structure. In the absence of direct observations or the capability to resolve subgrid-scale 3D turbulence in operational ocean models, the oceanography community relies on surface boundary layer similarity scalings (BLS) of shear and convective turbulence to represent this mixing. Despite their importance, near-surface mixing processes (and ubiquitous BLS representations of these processes) have been undersampled in high-energy forcing regimes such as the Southern Ocean. With the maturing of autonomous sampling platforms, there is now an opportunity to collect high-resolution spatial and temporal measurements in the full range of forcing conditions. Here, we characterize near-surface turbulence under strong wind forcing using the first long-duration glider microstructure survey of the Southern Ocean. We leverage these data to show that the measured turbulence is significantly higher than standard shear-convective BLS in the shallower parts of the surface boundary layer and lower than standard shear-convective BLS in the deeper parts of the surface boundary layer; the latter of which is not easily explained by present wave-effect literature. Consistent with the CBLAST (Coupled Boundary Layers and Air Sea Transfer) low winds experiment, this bias has the largest magnitude and spread in the shallowest 10% of the actively mixing layer under low-wind and breaking wave conditions, when relatively low levels of turbulent kinetic energy (TKE) in surface regime are easily biased by wave events.

Abstract

The Kuroshio occasionally carries warm and salty North Pacific Water into fresher waters of the South China Sea, forming a front with a complex temperature–salinity (T–S) structure to the west of the Luzon Strait. In this study, we examine the T–S interleavings formed by alternating layers of North Pacific Water with South China Sea Water in a front formed during the winter monsoon season of 2014. Using observations from a glider array following a free-floating wave-powered vertical profiling float to calculate the fine-scale parameters Turner angle, Tu, and Richardson number, Ri, we identified areas favorable to double-diffusion convection and shear instability observed in a T–S interleaving. We evaluated the contribution of double-diffusion convection and shear instabilities to the thermal variance diffusivity, χ, using microstructure data and compared it with previous parameterization schemes based on fine-scale properties. We discover that turbulent mixing is not accurately parameterized when both Tu and Ri are within critical ranges (Tu > 60; Ri < ¼). In particular, χ associated with salt finger processes was an order of magnitude higher (6.7 × 10⁻⁷ K² s⁻¹) than in regions where only velocity shear was likely to drive mixing (8.7 × 10⁻⁸ K² s⁻¹).

Abstract

Data from three midlatitude, month-long surveys are examined for evidence of enhanced vertical mixing associated with the transition layer (TL), here defined as the strongly stratified layer that exists between the well mixed layer and the thermocline below. In each survey, microstructure estimates of turbulent dissipation were collected concurrently with fine-structure stratification and shear. Survey-wide averages are formed in a “TL coordinate” z _TL, which is referenced around the depth of maximum stratification for each profile. Averaged profiles show characteristic TL structures such as peaks in stratification N ² and shear variance S ², which fall off steeply above z _TL = 0 and more gradually below. Turbulent dissipation rates ɛ are 5–10 times larger than those found in the upper thermocline (TC). The gradient Richardson number Ri = N ²/S ² becomes unstable (Ri < 0.25) within ~10 m of the TL upper boundary, suggesting that shear instability is active in the TL for z _TL > 0. Ri is stable for z _TL ≤ 0. Turbulent dissipation is found to scale exponentially with depth for z _TL ≤ 0, but the decay scales are different for the TL and upper TC: ɛ scales well with either N ² or S ². Owing to the strong correlation between S ² and N ², existing TC scalings of the form ɛ ~ |S| ^p |N| ^q overpredict variations in ɛ. The scale dependence of shear variance is not found to significantly affect the scalings of ɛ versus N ² and S ² for z _TL ≤ 0. However, the onset of unstable Ri at the top of the TL is sensitively dependent to the resolution of the shears.