Search

The Atlantic Meridional Overturning Circulation entails vigorous thermohaline transformations in the subpolar North Atlantic Ocean (SPNA). There, warm and saline waters originating in the (sub)tropics are converted into cooler and fresher waters by a combination of surface fluxes and sub‐surface mixing. Using microstructure measurements and a small‐scale variance conservation framework, we quantify the diapycnal and isopycnal contributions –by microscale turbulence and mesoscale eddies, respectively– to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for the majority of thermal (65%) and haline (84%) variance dissipation in the upper 400 m of the eastern SPNA. A simple dimensional analysis suggests that isopycnal stirring could account for O $\mathcal{O}$(5–10) Sv of diahaline volume flux, suggesting an important role of such stirring in regional water‐mass transformations. Our mixing measurements are thus consistent with recent indirect estimates in highlighting the importance of isopycnal stirring for North Atlantic overturning. Plain Language Summary: The North Atlantic hosts an ocean circulation system called the Atlantic Meridional Overturning Circulation (AMOC). It is often likened to a giant conveyor belt in the ocean, moving warm, salty waters from south to north and transforming them into cold, fresh waters that flow back southward at greater depth. The AMOC is a crucial element of the Earth's climate, and if it were to slow down, it could lead to major climatic changes. For a long time, scientists thought that the AMOC was mainly driven by cooling in the North Atlantic. But recently, we have discovered that the mixing of different water masses is also important. In our study, we used small‐scale measurements of ocean properties to examine the processes behind this mixing. Our findings show that large swirling flows known as mesoscale eddies, which are tens to hundreds of kilometers wide and hundreds of meters deep, play a dominant role in mixing heat and salt in the North Atlantic. This discovery helps us to better understand the AMOC and its future behavior. [ABSTRACT FROM AUTHOR]

Mesoscale eddies are known to enhance diapycnal mixing in the ocean, yet direct observation of this effect remains a significant challenge, especially in the robust Antarctic Circumpolar Current (ACC). To quantify the diapycnal mixing induced by mesoscale eddies in the standing meander of the ACC, satellite altimeter and Argo profile data were combined to composite eddies, where the 1.6 m dynamic height contour was used for the first time instead of the climatological Northern Sub-Antarctic Front (SAFN) to define the northern boundary of the ACC to eliminate the influence of frontal shift. The 3D structures of the composite anticyclonic/cyclonic eddy (CAE/CCE) were obtained. Both the CAE and CCE were similar in shape to Taylor columns, from sea surface to the neutral surface of 28.085 kgm − 3 (1689 ± 66 dbar) for the CAE, and from sea surface to 28.01 kgm − 3 (1491 ± 202 dbar) for the CCE. On the same neutral surface, the diffusivity (κ) inside the CCE was one to two orders of magnitude higher than that inside the CAE. Vertically, the maximum influence depth of the CCE on κ reached 1200 dbar, while for the CAE, it reached 800 dbar, where κ exceeded O (10 − 4)   m 2 s − 1 , and κ gradually decreased from these depths downwards. [ABSTRACT FROM AUTHOR]

Lightening of bottom water is required to close the abyssal overturning circulation, believed to play an important role in the climate system. A tracer release experiment and turbulence measurement programs have revealed how bottom water is lightened, and illuminated the associated circulation in the deep Brazil Basin, a representative region of the global ocean. Tracer was released on an isopycnal surface about 4000 m deep, over one of the fracture zones emanating from the Mid-Atlantic Ridge (MAR). Tracer that mixed toward the bottom moved toward the MAR across isopycnal surfaces that bend down to intersect the bottom at a rate implying a near-bottom buoyancy flux of 1.5 × 10−9 m2 s−3, somewhat larger than inferred from dissipation measurements. The diffusivity at the level of the tracer release is estimated at 4.4 ± 1 × 10−4 m2 s−1, again larger than inferred from dissipation rates. The main patch moved southwest at about 2 cm s−1 while sinking due to the divergence of buoyancy flux above the bottom layer. The isopycnal eddy diffusivity was about 100 m2 s−1. Westward flow away from the MAR is the return flow balancing the eastward near-bottom upslope flow. The southward component of the flow is roughly consistent with conservation of potential vorticity. The circulation as well as the pattern of diapycnal flux are qualitatively the same as in St. Laurent et al. (2001) but are more robust. The results indicate that diapycnal diffusivity is about twice that invoked by Morris et al. (2001) in calculating the basinwide buoyancy budget. Significance Statement: Buoyancy flux into the abyssal waters is required to close the overturning circulation of those waters, an important part of the climate system. This contribution presents a robust view of the strength of that buoyancy flux and the associated circulation. [ABSTRACT FROM AUTHOR]

We assess the representation of mesoscale stirring in a suite of models against an estimate derived from microstructure data collected during the North Atlantic Tracer Release Experiment (NATRE). We draw heavily from the approximate temperature variance budget framework of Ferrari and Polzin. This framework assumes two sources of temperature variance away from boundaries: first, the vertical stirring of the large-scale mean vertical gradient by small-scale turbulence; and second, the lateral stirring of large-scale mean along-isopycnal gradients by mesoscale eddies. Temperature variance so produced is transformed and on average transferred down scales for ultimate dissipation at the microscale at a rate χ estimated using microstructure observations. Ocean models represent these pathways by a vertical mixing parameterization, and an along-isopycnal lateral mixing parameterization (if needed). We assess the rate of variance production by the latter as a residual from the NATRE dataset and compare against the parameterized representations in a suite of model simulations. We find that variance production due to lateral stirring in a Parallel Ocean Program version 2 (POP2) 1/10° simulation agrees well, to within the estimated error bars, with that inferred from the NATRE estimate. A POP2 1° simulation and the Estimating the Circulation and Climate of the Ocean Version 4 release 4 (ECCOV4r4) simulation appear to dissipate an order of magnitude too much variance by applying a lateral diffusivity, when compared to the NATRE estimate, particularly below 1250 m. The ECCOV4r4-adjusted lateral diffusivities are elevated where the microstructure suggests elevated χ sourced from mesoscale stirring. Such elevated values are absent in other diffusivity estimates suggesting the possibility of compensating errors and caution in interpreting ECCOV4r4's adjusted lateral diffusivities. Significance Statement: We look at whether microstructure turbulence observations can provide a useful metric for judging the fidelity of representation of mesoscale stirring in a suite of models. We focus on the region of the North Atlantic Tracer Release Experiment (NATRE), the site of a major ocean turbulence observation campaign, and use an approximate variance budget framework for the region with observational estimates from Ferrari and Polzin (2005). The approach provides a novel framework to evaluate the approximate representation of mesoscale stirring in a variety of models. [ABSTRACT FROM AUTHOR]

We investigate the effect of wave–eddy interaction and dissipation of internal gravity waves propagating in a coherent mesoscale eddy simulated using a novel numerical model called the Internal Wave Energy Model based on the six-dimensional radiative transfer equation. We use an idealized mean flow structure and stratification, motivated by observations of a coherent eddy in the Canary Current System. In a spindown simulation using the Garret–Munk model spectrum as initial conditions, we find that wave energy decreases at the eddy rim. Lateral shear leads to wave energy gain due to a developing horizontal anisotropy outside the eddy and at the rim, while vertical shear leads to wave energy loss which is enhanced at the eddy rim. Wave energy loss by wave dissipation due to vertical shear dominates over horizontal shear. Our results show similar behavior of the internal gravity wave in a cyclonic as well as an anticyclonic eddy. Wave dissipation by vertical wave refraction occurs predominantly at the eddy rim near the surface, where related vertical diffusivities range from O ( 10 − 7 ) to O ( 10 − 5 )   m 2   s − 1 . Significance Statement: Using a novel model and observations from the Canary Current System of a coherent eddy of 100-km diameter, we explore the interaction between a realistic internal gravity wave field and this eddy. We study wave refraction and energy transfers between the waves and the eddy induced by the eddy's lateral and vertical shear. Waves lose energy at the eddy rim by vertical shear and gain outside of the eddy rim by horizontal shear. We find large vertical wave refraction by vertical shear at the eddy rim, where waves break and mix density, which can have wide ranging effects on the ocean's circulation. These results are important for understanding the ocean's mixing and energy cycle and to develop eddy and wave parameterizations. [ABSTRACT FROM AUTHOR]

Based on yearlong observations from three moorings at 12°, 14°, and 16°N in the northwest Pacific, this study presents observational evidence for the occurrence and behavior of parametric subharmonic instability (PSI) of diurnal internal tides (ITs) both in the upper and abyssal ocean around the critical latitudes (O1 IT: 13.44°N; K1 IT: 14.52°N), which is relatively less explored in comparison with PSI of M2 ITs. At 14°N, near-inertial waves (NIWs) feature a "checkerboard" pattern with comparable upward- and downward-propagating components, while the diurnal ITs mainly feature a low-mode structure. The near-inertial kinetic energy at 14°N, correlated fairly well with the diurnal KE, is the largest among three moorings. The bicoherence analysis, and a causality analysis method newly introduced here, both show statistically significant phase locking between PSI triads at 14°N, while no significant signals emerge at 12° and 16°N. The estimated PSI energy transfer rate shows a net energy transfer from diurnal ITs to NIWs with an annual-mean value of 1.5 × 10−10 W kg−1. The highly sheared NIWs generated by PSI result in a 2–6 times larger probability of shear instability events at 14°N than 12° and 16°N. Through swinging the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, the passages of anticyclonic and cyclonic eddies both result in elevated NIWs and shear instability events by enhancing PSI efficiency. Particularly, different from the general understanding that cyclonic eddies usually expel NIWs, enhanced NIWs and instability are observed within cyclonic eddies whose relative vorticity can modify PSI efficiency. Significance Statement: Parametric subharmonic instability (PSI) effectively transfers energy from low-mode internal tides (ITs) to high-mode near-inertial waves (NIWs), triggering elevated mixing around critical latitudes. This study provides observational evidence for the occurrence of PSI of diurnal ITs in the northwest Pacific and its role in enhancing shear instability. Generally, anticyclonic eddies act to trap NIWs while cyclonic eddies tend to expel NIWs. Here we document elevated NIWs and shear instability within both anticyclonic and cyclonic eddies, which shift the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, thereby enhancing PSI efficiency. Processes associated with PSI and the modulation of PSI efficiency by mesoscale eddies have significant implications for improving mixing parameterizations in ocean circulation and climate models. [ABSTRACT FROM AUTHOR]

Through an expansive series of simulations, we investigate the effects of spatially uniform shear on the transport, structure, and dynamics of salt fingers. The simulations reveal that shear adversely affects the heat and salt fluxes of the system, reducing them by up to an order of magnitude. We characterize this in detail across a broad range of Richardson numbers and density ratios. We demonstrate that the density ratio is strongly related to the amount of shear required to disrupt fingers, with larger density ratio systems being more susceptible to disruption. An empirical relationship is proposed that captures this behavior that could be implemented into global ocean models. The results of these simulations accurately reproduce the microstructure measurements from North Atlantic Tracer Release Experiment (NATRE) observations. This work suggests that typical salt finger fluxes in the ocean will likely be a factor of 2–3 less than predicted by models not taking the effects of shear on double-diffusive systems into account. Significance Statement: The purpose of this work is to measure how large-scale (>1 m) motion affects specific small-scale (∼1 cm) mixing in the ocean. We approach this topic using simulations of meter-scale boxes of fluid containing both temperature and salt concentration with an applied background flow. We measure how this background flow changes the mixing of temperature and salt as the strength of the applied flow increases. We find that current estimates may overpredict mixing at this scale throughout the World Ocean by about a factor of 2–3, on average. This could have consequences for estimates of climate in addition to heat, salt, nutrient, and pollutant transport. [ABSTRACT FROM AUTHOR]

Enhanced diapycnal mixing induced by the near-bottom breaking of internal waves is an essential component of the lower meridional overturning circulation. Despite its crucial role in the ocean circulation, tidally driven internal wave breaking is challenging to observe due to its inherently short spatial and temporal scales. We present detailed moored and shipboard observations that resolve the spatiotemporal variability of the tidal response over a small-scale bump embedded in the continental slope of Tasmania. Cross-shore tidal currents drive a nonlinear trapped response over the steep bottom around the bump. The observations are roughly consistent with two-dimensional high-mode tidal lee-wave theory. However, the alongshore tidal velocities are large, suggesting that the alongshore bathymetric variability modulates the tidal response driven by the cross-shore tidal flow. The semidiurnal tide and energy dissipation rate are correlated at subtidal time scales, but with complex temporal variability. Energy dissipation from a simple scattering model shows that the elevated near-bottom turbulence can be sustained by the impinging mode-1 internal tide, where the dissipation over the bump is O(1%) of the incident depth-integrated energy flux. Despite this small fraction, tidal dissipation is enhanced over the bump due to steep topography at a horizontal scale of O(1) km and may locally drive significant diapycnal mixing. Significance Statement: Near-bottom turbulent mixing is a key element of the global abyssal circulation. We present observations of the spatiotemporal variability of tidally driven turbulent processes over a small-scale topographic bump off Tasmania. The semidiurnal tide generates large-amplitude transient lee waves and hydraulic jumps that are unstable and dissipate the tidal energy. These processes are consistent with the scattering of the incident low-mode internal tide on the continental slope of Tasmania. Despite elevated turbulence over the bump, near-bottom energy dissipation is small relative to the incident wave energy flux. [ABSTRACT FROM AUTHOR]

In global ocean circulation and climate models, bottom-enhanced turbulent mixing is often parameterized such that the vertical decay scale of the energy dissipation rate ζ is universally constant at 500 m. In this study, using a nonhydrostatic two-dimensional numerical model in the horizontal–vertical plane that incorporates a monochromatic sinusoidal seafloor topography and the Garrett–Munk (GM) background internal wave field, we find that ζ of the internal lee-wave-driven bottom-enhanced mixing is actually variable depending on the magnitude of the steady flow U0, the horizontal wavenumber kH, and the height hT of the seafloor topography. When the steepness parameter (Sp = NhT/U0 where N is the buoyancy frequency near the seafloor) is less than 0.3, internal lee waves propagate upward from the seafloor while interacting with the GM internal wave field to create a turbulent mixing region with ζ that extends farther upward from the seafloor as U0 increases, but is nearly independent of kH. In contrast, when Sp exceeds 0.3, inertial oscillations (IOs) not far above the seafloor are enhanced by the intermittent supply of internal lee-wave energy Doppler-shifted to the near-inertial frequency, which occurs depending on the sign and magnitude of the background IO shear. The composite flow, consisting of the superposition of U0 and the IOs, interacts with the seafloor topography to efficiently generate internal lee waves during the period centered on the time of the composite flow maximum, but their upward propagation is inhibited by the increased IO shear, creating a turbulent mixing region of small ζ. [ABSTRACT FROM AUTHOR]

In this paper we present the analytical derivation of a local water mass transformation (WMT) framework for an individual water column. We exactly formulate the mapping of the governing equations from geopotential coordinates to an arbitrary tracer space. Unique definitions for the local effective vertical diasurface fluxes are given. In tracer space we derive new relations between the local diatracer fluxes and the mixing per tracer class. The key relation between the effective vertical diatracer velocity and the mixing per tracer class directly formulates how the overturning circulation is linked to local tracer variance dissipation. Horizontal integration of the governing equations in tracer space and the relations between the diatracer quantities finally recovers the well-known integral WMT formulations. [ABSTRACT FROM AUTHOR]

Polar and subpolar oceans play a particularly important role in the global climate and its temporal changes, yet these regions are less well sampled than the rest of the global ocean. To better understand the physical or biogeochemical properties and their variabilities in these regions, accurate data mapping is crucial. In this paper, we introduce a mapping methodology that includes a water column shrinking and stretching constraint (SSC) based on the principle of conservation of potential vorticity. To demonstrate the mapping scheme efficiency, we map the ocean temperature in the southern Sea of Okhotsk, where the bottom topography comprises a broad and shallow shelf, a sharp continental slope, and a deep oceanic basin. Such topographic features are typical of polar and subpolar marginal seas. Results reveal that the SSC integrated (SSCI) mapping strongly reduces the mapping error in the broad and shallow shelf compared with a recently introduced topographic constraint integrated (TCI) mapping procedure. We also tested our mapping scheme in the Southern Ocean, which has a comparatively slanted shelf, a wider and gentler slope, and a deep and broad oceanic basin. We found that the SSCI and TCI methods are practically equivalent there. The SSCI mapping is thus an effective method to map the ocean's properties in various topographic environments and should be adequate in all polar and subpolar regions. Importantly, we introduced a standardized procedure for determining the decorrelation length scales—a necessary step prior to implementing any mapping scheme—in any topographic conditions. [ABSTRACT FROM AUTHOR]

This article is devoted to an analysis of double diffusion processes responsible for diapycnal mixing in the zone of the westward Primorye Current in the northern part of the Sea of Japan. This article presents the results of processing and analysis of a long time series of regular measurements of vertical profiles of thermohaline characteristics and current velocity over the upper part of the continental slope between the depths of 60 and 420 m using the Aqualog mooring station from April to October 2015. Efficient heat and salt exchange coefficients and the buoyancy flux are estimated using the Osborne–Cox (1972) and Gregg (1989) parameterizations. The Turner angle for the verification of double diffusion processes is estimated. The contributions of diffusive convection and salt fingers to the buoyancy flux are studied. It is established that, due to differential diffusion, the layer from 80 to 170 m was involved in intense vertical mixing. Moreover, from midspring to early May, diffusive convection penetrates to a depth of 250 m, and then a process like salt fingers plays a leading role. Shear instability, caused, among other processes, by the weakly nonlinear interaction of internal waves, prevails in the underlying layers. [ABSTRACT FROM AUTHOR]

In this analysis the quantification of diapycnal diffusivity K ρ in stratified flows such as those found in the ocean and atmosphere is explored. There are two simplifications that are routinely made when estimating mixing rates in stably stratified flows. First, a constant value is commonly assumed for the (irreversible) mixing coefficient Γ . Second, dissipation rates of turbulent kinetic energy ϵ are inferred using either the Thorpe (or Ellison) length scales or from microstructure measurements using the isotropy assumption. Data from three independent direct numerical simulations of homogeneous stratified turbulence are used as a testbed to highlight impacts of these assumptions on estimates of K ρ . A systematic analysis compares the inferred diffusivities to exact DNS diffusivities as a function of the turbulent Froude number F r t . Use of a constant Γ results in an under-prediction of K ρ by up to a factor of 5 for strongly stratified conditions (low F r t ) and an over-prediction of K ρ by up to two orders of magnitude in weakly stratified conditions (high F r t ). The use of inferred dissipation rates ϵ based on the assumption of isotropy results in an over-prediction of K ρ by a factor of 2 for low F r t (which is within the instrumentation error) and converges on the exact K ρ for F r t ≥ 1 . However, the use of kinematic length scales, such as the Thorpe or Ellison scales, to infer ϵ result in significant errors. The implications of these findings are applied in a simple demonstration to show how these tools can be used for improved estimates of mixing rates in stably stratified flows. [ABSTRACT FROM AUTHOR]

Mesoscale eddies are known to enhance diapycnal mixing in the ocean, yet direct observation of this effect remains a significant challenge, especially in the robust Antarctic Circumpolar Current (ACC). To quantify the diapycnal mixing induced by mesoscale eddies in the standing meander of the ACC, satellite altimeter and Argo profile data were combined to composite eddies, where the 1.6 m dynamic height contour was used for the first time instead of the climatological Northern Sub-Antarctic Front (SAFN) to define the northern boundary of the ACC to eliminate the influence of frontal shift. The 3D structures of the composite anticyclonic/cyclonic eddy (CAE/CCE) were obtained. Both the CAE and CCE were similar in shape to Taylor columns, from sea surface to the neutral surface of 28.085 kgm−3 (1689 ± 66 dbar) for the CAE, and from sea surface to 28.01 kgm−3 (1491 ± 202 dbar) for the CCE. On the same neutral surface, the diffusivity (κ) inside the CCE was one to two orders of magnitude higher than that inside the CAE. Vertically, the maximum influence depth of the CCE on κ reached 1200 dbar, while for the CAE, it reached 800 dbar, where κ exceeded O(10−4) m2s−1, and κ gradually decreased from these depths downwards.

Several years of moored turbulence measurements from xpods 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, the DC layer is only indirectly linked to surface forcing. Yet, it varies diurnally in concert with daily changes in heating/cooling. Diurnal composites computed from 10-min averaged data at fixed xpod 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 m h-1, 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 m h-1. 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. [ABSTRACT FROM AUTHOR]

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 (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. [ABSTRACT FROM AUTHOR]

Diapycnal mixing impacts vertical transport rates of salt, heat, and other dissolved substances, essential for the overturning circulation and ecosystem functioning in marine systems. While most studies have focused on mixing induced by individual obstacles in tidal flows, we investigate the net effect of non‐tidal flow over multiple small‐scale (<1 km) bathymetric features penetrating a strongly‐stratified density interface in a coastal region. We combine high‐resolution broadband acoustic observations of turbulence microstructure with traditional shear microstructure profiling, to resolve the variability and intermittency of stratified turbulence related to the rough bathymetry. Scale analysis and acoustic imaging suggest that underlying mixing mechanisms are related to topographic wake eddies and breaking internal waves. Depth averaged dissipation rates (1.1 × 10−7 Wkg−1) and turbulent vertical diffusivities (7 × 10−4 m2s−1) in the halocline exceed reference values by two orders of magnitude. Our study emphasizes the importance of rough small‐scale bathymetric features for the vertical transport of salt in coastal areas. Plain Language Summary: Mixing of water across density interfaces is important for ecosystems and the circulation between basins. However, mixing related to rough small‐scale bathymetry is often not resolved in models and difficult to measure. In this study, we show high‐resolution acoustic observations of intense vertical mixing across a strong density interface, that separates the saltier bottom water from the fresher surface water in the northern Baltic Sea. In the study region, steep underwater hills and ridges extend into the density interface. As water flows over the region, the hills and ridges cause the water to mix. Measured values of mixing and vertical salt fluxes in this region are up to two orders of magnitude higher than at a nearby reference station with smooth bathymetry. Our analysis suggests that the observed high mixing is mainly caused by eddies in the wake of obstacles and secondarily by breaking internal waves, which are waves within the water that occur on interfaces between layers with different properties. Understanding mixing mechanisms and estimating their contribution to the total mixing is needed to implement mixing into ocean models. This study highlights the importance of rough small‐scale (<1 km) seafloor features for mixing and vertical transport of salt. Key Points: High‐resolution turbulence observations in a coastal, strongly stratified region with extremely rough seafloor topographyAcoustic turbulence imaging shows highly intermittent and localized mixing due to wake eddies and internal‐wave breaking near obstaclesTurbulent diffusivities and transport rates of salt increased by two orders of magnitude inside mixing hotspots near rough bathymetry [ABSTRACT FROM AUTHOR]

The Indo-Pacific Ocean appears exponentially stratified between 1- and 3-km depth with a decay scale on the order of 1 km. In his celebrated paper "Abyssal recipes," W. Munk proposed a theoretical explanation of these observations by suggesting a pointwise buoyancy balance between the upwelling of cold water and the downward diffusion of heat. Assuming a constant upwelling velocity w and turbulent diffusivity κ, the model yields an exponential stratification whose decay scale is consistent with observations if κ ∼ 10−4 m2 s−1. Over time, much effort has been made to reconcile Munk's ideas with evidence of vertical variability in κ, but comparably little emphasis has been placed on the even stronger evidence that w decays toward the surface. In particular, the basin-averaged w nearly vanishes at 1-km depth in the Indo-Pacific. In light of this evidence, we consider a variable-coefficient, basin-averaged analog of Munk's budget, which we verify against a hierarchy of numerical models ranging from an idealized basin-and-channel configuration to a coarse global ocean simulation. Study of the budget reveals that the decay of basin-averaged w requires a concurrent decay in basin-averaged κ to produce an exponential-like stratification. As such, the frequently cited value of 10−4 m2 s−1 is representative only of the bottom of the middepths, whereas κ must be much smaller above. The decay of mixing in the vertical is as important to the stratification as its magnitude. Significance Statement: Using a combination of theory and numerical simulations, it is argued that the observed magnitude and shape of the global ocean stratification and overturning circulation appear to demand that turbulent mixing increases quasi-exponentially toward the ocean bottom. Climate models must therefore prescribe such a vertical profile of turbulent mixing in order to properly represent the heat and carbon uptake accomplished by the global overturning circulation on centennial and longer time scales. [ABSTRACT FROM AUTHOR]

Coherent ocean vortices, or eddies, are usually tracked on the surface of the ocean. However, tracking subsurface eddies is important for a complete understanding of deep ocean circulation. In this study, we develop an algorithm designed for the detection of subsurface eddies in the Arabian Sea using Nucleus for European Modelling of the Ocean (NEMO) model simulations. We optimize each parameter of our algorithm to achieve favorable results when compared with an algorithm using sea surface height (SSH). When compared to similar methods, we find that using the rescaled isopycnal potential vorticity (PV) is best for subsurface eddy detection. We proceed to demonstrate that our new algorithm can detect eddies successfully between specific isopycnals, such as those that define the Red Sea Water (RSW). In doing so, we showcase how our method can be used to describe the properties of eddies within the RSW and even identify specific long-lived subsurface eddies. We conduct one such case study by discerning the structure of a completely subsurface RSW eddy near the Chagos Archipelago using Lagrangian particle tracking and PV diagnostics. We conclude that our rescaled PV method is an efficient tool for investigating eddy dynamics within the ocean's interior, and publicly provide our optimization methodology as a way for other researchers to develop their own subsurface detection algorithms with optimized parameters for any spatiotemporal model domain. Significance Statement: Eddies are a key part of ocean circulation both at the surface and in the subsurface. The purpose of our study was to design the first detection method comprehensively optimized for subsurface eddy detection from numerical simulations. We demonstrate that potential vorticity (PV) is the best field to use when algorithmically tracking eddies in subsurface water masses, using our new method to identify and track eddies in the Red Sea Water (RSW). Additionally, our method allows us to efficiently evaluate the dynamics of eddies through potential vorticity diagnostics, exemplified with a previously undescribed eddy near the Chagos Archipelago. Our methodology can be used by future researchers to study the eddy dynamics hidden within subsurface water masses around the world. [ABSTRACT FROM AUTHOR]

Simplified descriptions of the ocean are useful both for formulating explanatory theories and for conveying meaningful global attributes. Here, using a 26-yr average of a global state estimate from ECCO, the basis for Munk's "abyssal recipes" is evaluated on a global scale between 1000- and 3000-m depth. The two specific hydrographic stations he used prove untypical, with potential temperature and salinity more generally displaying different vertical scale heights, and thus differing in one-dimensional (in the vertical) values of mixing coefficients and/or vertical velocities. The simplest explanation is that the circulation is fully three-dimensional with temperature and salinity fields not describable with a one-dimensional steady balance. In contrast, the potential density and buoyancy are quantitatively describable through a one-dimensional exponential balance, and which calls for an explanation in terms of turbulent mixing processes. [ABSTRACT FROM AUTHOR]

Subinertial, topographically trapped diurnal internal tides are an important energy source for turbulent mixing in the subarctic oceans. However, their generation may not be estimated by the conventional barotropic-to-baroclinic conversion because their vertical structure is sometimes barotropic, unlike superinertial internal tides that are always baroclinic. Here, a new energy diagram is presented, in which the barotropic mode is decomposed into the surface and topographic modes, with the latter being classified as part of the internal modes together with the baroclinic mode. The energy equation for the newly defined topographic mode is then derived, providing an appropriate formulation of the energy conversion rate from the subinertial surface tides to the topographically trapped internal tides. A series of numerical experiments confirm that the formulation successfully predicts the energy conversion rate for various cases, with the relative contribution of the baroclinic and topographic modes varying significantly depending on the bottom topography and stratification. Furthermore, this surface-to-internal conversion is demonstrated to give a significantly larger estimate than the barotropic-to-baroclinic conversion for subinertial tides. Applying the formulation to the results of a realistic numerical simulation in the Kuril Straits, an area with the strongest mixing due to subinertial diurnal tides, shows that the surface mode is converted into the baroclinic and topographic modes with comparable magnitudes, responsible for most of the energy dissipation in this area. These results indicate the need to reestimate the global distribution of the generation rate of the subinertial internal tides using our new formulation and to clarify their dissipation mechanisms. Significance Statement: Diurnal internal tides in mid- to high-latitude oceans are very different from semidiurnal internal tides, in that they are trapped by topographic features and characterized by uniform rotation throughout the water column rather than by vertical oscillations within the water column. Focusing on this character, we formulate for the first time the generation rate of diurnal internal tides trapped over variable bottom topography. A series of idealized numerical experiments and application to the Kuril Straits show the validity and usefulness of this formulation, which provides a significantly larger generation estimate than previous studies. The results of this study are important for accurate global mapping of turbulent mixing induced by the breaking of internal tides. [ABSTRACT FROM AUTHOR]

The rates of isopycnal stirring and water mass transport by mesoscale eddies, and of diapycnal mixing by small‐scale turbulence, across the Brazil‐Malvinas Confluence (BMC) are assessed from a set of microstructure and hydrographic measurements in the Argentine Basin. This assessment is founded on a theoretical framework that applies a triple decomposition to the temperature variance equation and assumes eddies to transfer potential vorticity downgradient. The BMC is found to host widespread intense isopycnal stirring at rates of O(103–104 m2 s−1), and generally weak diapycnal mixing at rates of O(10−6–10−5 m2 s−1). Despite such disparity, both diapycnal mixing and isopycnal stirring play roles of comparable importance in determining regional water mass properties within surface and mode waters. In deeper layers, isopycnal stirring prevails. Eddies are further diagnosed to effect an important cross‐BMC transport, at rates of O(1 m2 s−1). When scaled by the along‐stream extent of the BMC, these rates integrate to volume transports that may be as large as O(10 Sverdrups). This suggests that cross‐BMC transfers of waters are substantially effected by eddy‐induced flows. Plain Language Summary: Turbulent mixing plays an important role in large‐scale oceanic processes such as water mass modification, distribution of salinity and temperature properties, and the global overturning circulation. These mixing processes are the direct consequence of different physical phenomena, including mixing across density surfaces (i.e., diapycnal) by small‐scale turbulence and stirring along density surfaces (i.e., isopycnal) by mesoscale eddies. Here, we assess the strength of diapycal mixing and isopycnal stirring across the Brazil‐Malvinas Confluence (BMC). Our results suggest that both processes are key to determine regional water mass properties in the BMC. We find that mixing across density surfaces and stirring along density surfaces are both important in upper layers, while stirring is prevalent in deeper waters. Finally, we estimate the importance of eddies in the exchange of subtropical and subantarctic waters across the BMC, and thus in the Southern Ocean overturning circulation. We find that eddy‐induced flows contribute substantially to the cross‐BMC transfer of water masses. Key Points: We assess the rates of diapycnal mixing and isopycnal stirring and water mass transports by mesoscale eddies in the Brazil‐Malvinas ConfluenceIsopycnal stirring by eddies dominates water mass transformations across the BMC, especially below 500 mEddy‐indiced flows substantially contribute to the transfer of waters across the Brazil‐Malvinas Confluence [ABSTRACT FROM AUTHOR]

Recent work has shown that, when nontraditional (NT) effects associated with the horizontal component of the Coriolis parameter are taken into account, equatorial waves (EWs) experience critical reflection when they reflect off the seafloor at the latitude where their frequency is equal to the inertial frequency. As a result, the vertical shear associated with the wave is strongly enhanced locally and results in bottom-intensified mixing. Using an off-the-shelf parameterization for mixing, these studies have shown that this process could play an important role in driving diapycnal upwelling in the abyssal ocean, but the specific mechanisms generating the mixing have not been studied yet. In this work, we address this limitation by running two-dimensional, high-resolution, nonhydrostatic simulations of the critical reflection of internal waves modified by NT effects. These simulations can resolve the instabilities triggered when the wave reflects off the bottom, allowing us to characterize the energy cascade to smaller scales and to estimate the mixing it generates. We find that shear instabilities drive elevated turbulent diffusivities between 10−1 and −10−3 m2 s−1 over a critical layer of 100–300 m thick. The shear instabilities result directly from the enhancement of kinetic energy in the reflected wave that is confined against the seafloor during the critical reflection process. Simultaneously, higher harmonics are generated and flux energy upward in the water column. These higher harmonics are unstable to parametric subharmonic instability, which absorbs their energy and drive enhanced dissipation above the critical layer, to a height of O(1000) m off the bottom. We show how these results depend on key elements of the EWs and of the medium and discuss the implementation of a parameterization of these effects in global ocean models. [ABSTRACT FROM AUTHOR]

The formation of a sharp oceanic front located south-southeast of Sri Lanka during the southwest monsoon is examined through in situ and remote observations and high-resolution model output. Remote sensing and model output reveal that the front extends approximately 200 km eastward from the southeast coast of Sri Lanka toward the southern Bay of Bengal (BoB). This annually occurring front is associated with the boundary between the southwest monsoon current with high-salinity water to the south, and a weak flow field comprised of relatively fresh BoB water to the north. The front contains a line of high chlorophyll extending from the coastal upwelling zone, often for several hundred kilometers. Elevated turbulent diffusivities ∼10−2 m2 s−1 along with large diapycnal fluxes of heat and salt were found within the front. The formation of the front and vertical transports are linked to local wind stress curl. Large vertical velocities (∼50 m day−1) indicate the importance of ageostrophic, submesoscale processes. To examine these processes, the Ertel potential vorticity (PV) was computed using the observations and numerical model output. The model output shows a ribbon of negative PV along the front between the coastal upwelling zone and two eddies (Sri Lanka Dome and an anticyclonic eddy) typically found in the southern BoB. PV estimates support the view that the flow is susceptible to submesoscale instabilities, which in turn generate high vertical velocities within the front. Frontal upwelling and heightened mixing show that the seasonal front is regionally important to linking the fresh surface water of the BoB with the Arabian Sea. Significance Statement: Within the ocean, motions span extraordinarily wide ranges of sizes and time scales. In this study we focus on a narrow, intensified feature called a front. This front occurs in the southern Bay of Bengal during the summer monsoon and forms a boundary between fresher water to the north and saltier water to the south. Features such as this are difficult to study, however, by combining observations made from ships and satellites with output from numerical models of the ocean, we are able to better understand the front. This is important because fronts like the one studied here play a role in determining the pathways of heat within the ocean, which, in turn, may feedback into the atmosphere and weather patterns. [ABSTRACT FROM AUTHOR]

Abstract Diapycnal mixing impacts vertical transport rates of salt, heat, and other dissolved substances, essential for the overturning circulation and ecosystem functioning in marine systems. While most studies have focused on mixing induced by individual obstacles in tidal flows, we investigate the net effect of non‐tidal flow over multiple small‐scale (

The filaments of the African Eastern Boundary Upwelling System (EBUS) are responsible for feeding nutrients to the oligotrophic waters of the northeastern Atlantic. However, turbulent mixing associated with nutrient uplift in filaments is poorly documented and has been mainly evaluated numerically. Using microstructure profiler measurements, we detected enhanced turbulent kinetic energy dissipation rates (ε) within the Cape Ghir upwelling filament. In contrast to previous studies, this enhancement was not related to symmetrical instabilities induced by down-front winds but to an increase in vertical current shear at the base of the mixed layer (hρ). In order to quantify the impact of vertical shear and the influence of the active mixing layer depth (hε) in the filament, a simple one-dimensional (1D) turbulent entrainment approach was used. We found that the effect of turbulent enhancement, together with the isopycnal morphology of the filament front, drove the formation of local positive entrainment zones (Δh=hε−hρ), as hε was deeper than hρ. This provided suitable conditions for the entrainment of cold, nutrient-rich waters from below the filament pycnocline and the upward transport of biophysical properties to the upper boundary layer of the front. We also found that diapycnal nutrient fluxes in stations influenced by the filament (1.35 mmol m-2 d-1) were two orders of magnitude higher than those of stations not affected by the filament front (0.02 mmol m-2 d-1). Despite their importance, the effects of vertical shear and hε have often been neglected in entrainment parameterizations. Thus, a modified entrainment parameterization was adapted to include vertical shear and observed ε, which are overestimated by existing parameterizations. To account for the possible role of internal waves in the generation of vertical shear, we considered internal wave scaling to parameterize the observed dissipation. Using this adapted parameterization, the average entrainment velocities were six times (6 m d-1) higher than those obtained with the classic parameterization (1 m d-1).

AbstractThe time-mean zonal and meridional overturning circulations of the entire Mediterranean Sea are studied in both the Eulerian and residual frameworks. The overturning is characterized by cells in the vertical and either zonal or meridional planes with clockwise circulations in the upper water column and counterclockwise circulations in the deep and abyssal regions. The zonal overturning is composed of an upper clockwise cell in the top 600 m of the water column related to the classical Wüst cell and two additional deep clockwise cells, one corresponding to the outflow of the dense Aegean water during the Eastern Mediterranean Transient (EMT) and the other associated with dense water formation in the Rhodes Gyre. The variability of the zonal overturning before, during, and after the EMT is discussed. The meridional basinwide overturning is composed of clockwise, multicentered cells connected with the four northern deep ocean formation areas, located in the Eastern and Western Mediterranean basins. The connection between the Wüst cell and the meridional overturning is visualized through the horizontal velocities vertically integrated across two layers above 600 m. The component of the horizontal velocity associated with the overturning is isolated by computing the divergent components of the vertically integrated velocities forced by the inflow/outflow at the Strait of Gibraltar.

Abstract Diapycnal mixing shapes the distribution of climatically important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation‐based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation (AMOC). The rate of water mass transformation in the Atlantic Ocean's interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW, neutral density γn ≃ 27.6–28.15), with a diapycnal circulation of 0.5–8 Sv between 48°N and 32°S in the Atlantic Ocean. Moreover, tracers within the southward‐flowing NADW may undergo a substantial diapycnal transfer, equivalent to a vertical displacement of hundreds of meters in the vertical. This result, confirmed with a zonally averaged numerical model of the AMOC, indicates that mixing can alter where tracers upwell in the Southern Ocean, ultimately affecting their global pathways and ventilation timescales. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.

This study focuses on the statistical features of dissipation flux coefficient Γ in the upper South China Sea (SCS). Based on the microscale measurements collected at 158 stations in the upper SCS and derived dissipation rates of turbulent kinetic energy and temperature variance ε and χT, via a modified method, we estimate Γ and analyze its spatiotemporal variation in an energetic and a quiescent region. We show that Γ is highly variable, which scatters over three orders of magnitude from 10−2 to 101 in both regions. Ιn the energetic region, Γ is slightly greater than in the quiescent region; their median values are 0.23 and 0.17, respectively. Vertically, Γ presents a clear increasing tendency with depth in both regions, though the increasing rate is greater in the energetic region than in the quiescent region. In the upper SCS, Γ positively depends on the buoyancy Reynolds number Reb and negatively depends on the ratio of the Ozmidov scale to the Thorpe scale ROT and is scaled as Γ ∝ Re ⁡ b 1 / 2 R OT − 4 / 3 , which holds for both regions. The vertical decreasing of ROT is observed, which yields parameterization of ROT = 10−0.002z; this parameterization improves the performance of the Thorpe-scale method by reducing at least 50% of the bias between the observed and parameterized ε. These results shed new light on the spatiotemporal variability and modulating mechanism of Γ in the upper ocean. Significance Statement: The great global ocean conveyor is maintained by vertical mixing. Turbulent kinetic energy released by local internal wave breaking goes into two parts: one part is used to furnish this vertical mixing, and the rest is dissipated into irreversible heat. The ratio of these two parts is termed as the dissipation flux coefficient and is usually treated as a constant. Our measurements suggest that this coefficient is highly spatiotemporally variable. Specific relationships are obtained when scaling this coefficient with other parameters, and mechanisms modulating this coefficient are also explored. This study sheds light on how much turbulent kinetic energy contributes to elevating the potential energy and its associated influences not only in marginal seas but also in open oceans. [ABSTRACT FROM AUTHOR]

The strength of the Atlantic meridional overturning circulation (AMOC) at subpolar latitudes is dominated by water mass transformation in the eastern subpolar North Atlantic Ocean (SPNA). However, the distribution of this overturning across the individual circulation pathways of both the Subpolar Gyre (SPG) and the Nordic seas overflows remains poorly understood. Here, we introduce a novel Lagrangian measure of the density-space overturning to quantify the principal pathways of the time-mean overturning circulation within an eddy-permitting ocean model hindcast. By tracing the trajectories of water parcels initialized from the northward inflows across the Overturning in the Subpolar North Atlantic Program (OSNAP) East section, we show that water mass transformation along the pathways of the eastern SPG accounts for 55% of the mean strength of the eastern subpolar AMOC. Water parcels following the dominant SPG pathway, sourced from the Subarctic Front, form upper North Atlantic Deep Water by circulating horizontally across sloping isopycnals in less than 2 years. A slower SPG route, entrained by overflow waters south of the Iceland–Faroes Ridge, is a crucial conduit for subtropical-origin water masses to penetrate the deep ocean on subdecadal time scales. On reproducing our findings using time-averaged velocity and hydrographic fields, we further show that the Nordic seas overflow pathways integrate multiple decades of water mass transformation before returning across the Greenland–Scotland Ridge. We propose that the strong disparity between the overturning time scales of the SPG (interannual) and the Nordic seas overflows (multidecadal) has important implications for the propagation of density anomalies within the eastern SPNA and hence the sources of AMOC variability. [ABSTRACT FROM AUTHOR]

Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles. [ABSTRACT FROM AUTHOR]

Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in situ turbulence suggest that mixing is bottom enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer release experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer's diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer's diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in situ diffusion as well as an additional nonlinear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (<20%), especially compared to the O (1) structural errors due to the omission of distortion effects in inverse models. These results suggest that TREs, although indispensable, should not be treated as "unambiguous" constraints on diapycnal mixing. [ABSTRACT FROM AUTHOR]

The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed water-mass transformations are dominated by rough topography "hotspots," where the bottom enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger upwelling in a thin bottom boundary layer (BBL). These water-mass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downward diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough's stratification. We propose simple modifications to the 1D boundary layer model that approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the nonlocal coupling to the basin-scale circulation. [ABSTRACT FROM AUTHOR]

Diapycnal mixing in the upper western equatorial Pacific (WEP) plays an important role in the tropical air–sea interactions and in the formation of the global climate system. Yet, the WEP is uniquely rich in water masses originating from the two hemispheres and in multiscale processes of different dynamical nature, thus creating a complex regime of mixing remains to be fully characterized by elaborate observations. Here, on the basis of microstructure measurements in the WEP, we report the observations on a strong deep cycle turbulence extending well into the upper thermocline by westerly wind event, with the turbulent kinetic energy dissipation rate ε~O(10−8–10−7) W kg−1 and diapycnal diffusivity Kρ∼O(10−4) m2 s−1. Below the deep cycle turbulence layer, turbulence and mixing are generally weak with ε~O(10−10–10−9) W kg−1 and Kρ∼O(10−7–10−6) m2 s−1, a prototype of the weak mixing nature of the low-latitude western Pacific. The observed turbulence below the deep cycle turbulence layer can be satisfactorily scaled by either the MacKinnon–Gregg model or the Richardson number–based model with tuned model parameters.

The equatorial Pacific zonal circulation is composed of westward surface currents, the eastward equatorial undercurrent (EUC) along the thermocline, and upwelling in the eastern cold tongue. Part of this upwelling arises from water flowing along isotherms sloping up to the east, but it also includes water mass transformation and consequent diabatic (cross-isothermal) flow (wci) that is a key element of surface-to-thermocline communication. In this study we investigate the mean seasonal cycle and subseasonal variability of cross-isothermal flow in the cold tongue using heat budget output from a high-resolution forced ocean model. Diabatic upwelling is present throughout the year with surface-layer solar-penetration-driven diabatic upwelling strongest in boreal spring and vertical mixing in the thermocline dominating during the rest of the year. The former constitutes warming of the surface layer by solar radiation rather than exchange of thermal energy between water parcels. The mixing-driven regime allows heat to be transferred to the core of the EUC by warming parcels at depth. On subseasonal time scales the passage of tropical instability waves (TIWs) enhances diabatic upwelling on and north of the equator. On the equator the TIWs enhance vertical shear and induce vertical-mixing-driven diabatic upwelling, while off the equator TIWs enhance the sub-5-daily eddy heat flux which enhances diabatic upwelling. Comparing the magnitudes of TIW, seasonal, and interannual wci variability, we conclude that each time scale is associated with sizeable variance. Variability across all of these time scales needs to be taken into account when modeling or diagnosing the effects of mixing on equatorial upwelling. [ABSTRACT FROM AUTHOR]

Using observationally based hydrographic and eddy diffusivity datasets, a volume budget analysis is performed to identify the main mechanisms governing the spatial and seasonal variability of Antarctic Intermediate Water (AAIW) within the density range γn = (27.25–27.7) kg m−3 in the Southern Ocean. The subduction rates and water mass transformation rates by mesoscale and small-scale turbulent mixing are estimated. First, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter-variety AAIW and Subantarctic Mode Water (SAMW). The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. The circumpolar-integrated total subduction yields from −5 to 19 Sv (1 Sv ≡ 106 m3 s−1) of AAIW volume loss. Second, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing (2–13 Sv) near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing (1–10 Sv). Last, 3–14 Sv of AAIW are exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the ACC and northward into the subtropical gyres. The volume budget analysis reveals strong seasonal variability in the rate of subduction, vertical mixing, and volume transport driving volume change within the AAIW layer. The nonzero volume budget residual suggests that more observations are needed to better constrain the estimate of geostrophic flow and mesoscale and small-scale mixing diffusivities. [ABSTRACT FROM AUTHOR]

The middepth ocean temperature profile was found by Munk in 1966 to agree with an exponential profile and shown to be consistent with a vertical advective–diffusive balance. However, tracer release experiments show that vertical diffusivity in the middepth ocean is an order of magnitude too small to explain the observed 1-km exponential scale. Alternative mechanisms suggested that nearly all middepth water upwells adiabatically in the Southern Ocean (SO). In this picture, SO eddies and wind set SO isopycnal slopes and therefore determine a nonvanishing middepth interior stratification even in the adiabatic limit. The effect of SO eddies on SO isopycnal slopes can be understood via either a marginal criticality condition or a near-vanishing SO residual deep overturning condition in the adiabatic limit. We examine the interplay between SO dynamics and interior mixing in setting the exponential profiles of σ2 and ∂zσ2. We use eddy-permitting numerical simulations, in which we artificially change the diapycnal mixing only away from the SO. We find that SO isopycnal slopes change in response to changes in the interior diapycnal mixing even when the wind forcing is constant, consistent with previous studies (that did not address these near-exponential profiles). However, in the limit of small interior mixing, the interior ∂zσ2 profile is not exponential, suggesting that SO processes alone, in an adiabatic limit, do not lead to the observed near-exponential structures of such profiles. The results suggest that while SO wind and eddies contribute to the nonvanishing middepth interior stratification, the exponential shape of the ∂zσ2 profiles must also involve interior diapycnal mixing. [ABSTRACT FROM AUTHOR]

Understanding the contribution of ocean circulation to glacial-interglacial climate change is a major focus of paleoceanography. Specifically, many have tried to determine whether the volumes and depths of Antarctic- and North Atlantic-sourced waters in the deep ocean changed at the Last Glacial Maximum (LGM; ∼22-18 kyr BP) when atmospheric pCO2 concentrations were 100 ppm lower than the preindustrial. Measurements of sedimentary geochemical proxies are the primary way that these deep ocean structural changes have been reconstructed. However, the main proxies used to reconstruct LGM Atlantic water mass geometry provide conflicting results as to whether North Atlantic-sourced waters shoaled during the LGM. Despite this, a number of idealized modeling studies have been advanced to describe the physical processes resulting in shoaled North Atlantic waters. This paper aims to critically assess the approaches used to determine LGMAtlantic circulation geometry and lay out best practices for future work. We first compile existing proxy data and paleoclimate model output to deduce the processes responsible for setting the ocean distributions of geochemical proxies in the LGM Atlantic Ocean. We highlight how small-scale mixing processes in the ocean interior can decouple tracer distributions from the large-scale circulation, complicating the straightforward interpretation of geochemical tracers as proxies for water mass structure. Finally, we outline promising paths toward ascertaining the LGMcirculation structure more clearly and deeply. [ABSTRACT FROM AUTHOR]

The propagation of internal waves (IWs) of tidal frequency is inhibited poleward of the critical latitude, where the tidal frequency is equal to the Coriolis frequency (f). These subinertial IWs may propagate in the presence of background vorticity, which can reduce rotational effects. Additionally, for strong tidal currents, the isopycnal displacements may evolve into internal solitary waves (ISWs). In this study, wave generation by the subinertial K1 and M2 tides over the Yermak Plateau (YP) is modeled to understand the linear response and the conditions necessary for the generation of ISWs. The YP stretches out into Fram Strait, a gateway into the Arctic Ocean for warm Atlantic-origin waters. We consider the K1 tide for a wide range of tidal amplitudes to understand the IW generation for different forcing. For weak tidal currents, the baroclinic response is predominantly at the second harmonic due to critical slopes. For sufficiently strong diurnal currents, ISWs are generated and their generation is not sensitive to the range of f and stratifications considered. The M2 tide is subinertial yet the response shows propagating IW beams with frequency just over f. We discuss the propagation of these waves and the influence of variations of f, as a proxy for variations in the background vorticity, on the energy conversion to IWs. An improved understanding of tidal dynamics and IW generation at high latitudes is needed to quantify the magnitude and distribution of turbulent mixing, and its consequences for the changes in ocean circulation, heat content, and sea ice cover in the Arctic Ocean. [ABSTRACT FROM AUTHOR]

Turbulent mixing at the sub‐meter scale is an essential component of the ocean's meridional overturning circulation and its associated global redistribution of heat, carbon, nutrients, pollutants, and other tracers. Whereas direct turbulence observations in the ocean interior are limited to a modest collection of field programs, basic information such as temperature, salinity, and depth is available globally. Here, we show that supervised machine learning algorithms can be trained on the existing turbulence data to develop skillful predictions of the key properties of turbulence from T, S, Z, and topographic data. This constitutes a promising first step toward a hybrid physics‐artificial intelligence approach to parameterization of turbulent mixing in ocean and climate models. Plain Language Summary: Ocean turbulence plays an important role in sustaining the general ocean circulation and in the mixing of heat, carbon, nutrients, and other tracers within the ocean interior. Turbulent mixing is technically challenging to measure and is often inferred from measurable quantities using parameterizations that are based on numerous simplifying assumptions about the physics of turbulence. In this study, we show that artificial intelligence (more specifically, various machine learning algorithms) can be successfully employed to infer turbulent mixing from quantities measured routinely by global observational programs. Key Points: Machine learning can be used to infer ocean turbulent mixing from basic seawater and geometric propertiesThe machine learning models are trained based on limited available direct turbulence measurementsThe trained models can be applied to data from global observational programs, which do not sample turbulence directly [ABSTRACT FROM AUTHOR]

This study presents novel observational estimates of turbulent dissipation and mixing in a standing meander between the Southeast Indian Ridge and the Macquarie Ridge in the Southern Ocean. By applying a finescale parameterization on the temperature, salinity, and velocity profiles collected from Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats in the upper 1600 m, we estimated the intensity and spatial distribution of dissipation rate and diapycnal mixing along the float tracks and investigated the sources. The indirect estimates indicate strong spatial and temporal variability of turbulent mixing varying from O(10−6) to O(10−3) m2 s−1 in the upper 1600 m. Elevated turbulent mixing is mostly associated with the Subantarctic Front (SAF) and mesoscale eddies. In the upper 500 m, enhanced mixing is associated with downward-propagating wind-generated near-inertial waves as well as the interaction between cyclonic eddies and upward-propagating internal waves. In the study region, the local topography does not play a role in turbulent mixing in the upper part of the water column, which has similar values in profiles over rough and smooth topography. However, both remotely generated internal tides and lee waves could contribute to the upward-propagating energy. Our results point strongly to the generation of turbulent mixing through the interaction of internal waves and the intense mesoscale eddy field. [ABSTRACT FROM AUTHOR]

Shipboard observations of upper-ocean current, temperature–salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in a cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6f. The mode regression method was proposed to extract NIWs from the shipboard ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the "inertial chimney" effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs' downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean. Significance Statement: We provide observational evidence for the downward propagation of near-inertial waves enhanced by mesoscale eddies. This is significant because the down-taking of wind energy by the near-inertial waves is an important energy source for turbulent mixing in the interior ocean, which is essential to the shaping of ocean circulation and climate. The anticyclonic eddies are widely regarded as a conduit for the downward near-inertial energy propagation, while the cyclonic eddies activity influencing the near-inertial waves propagation lacks clear cognition. In this study, enhanced near-inertial waves and turbulent dissipation were observed inside both cyclonic and anticyclonic eddies in the Kuroshio Extension region, which has significant implications for improving the parameterization of turbulent mixing in ocean circulation and climate models. [ABSTRACT FROM AUTHOR]

Microstructure measurements from two cruises during winter and spring 2019 documented the importance of double-diffusion processes for small-scale mixing in the upper 400 m of the open-ocean region of the eastern Arabian Sea (EAS) below the mixed layer. The data indicated that shear-driven mixing rates are weak, contributing diapycnal diffusivity (Kρ) of not more than 5.4 × 10−6 m2 s−1 in the EAS. Instead, signatures of double diffusion were strong, with the water column favorable for salt fingers in 70% of the region and favorable for diffusive convection in 2%–3% of the region. Well-defined thermohaline staircases were present in all the profiles in these regions that occupied 20% of the water column. Strong diffusive convection favorable regime occurred in ∼45% of data in the barrier layer region of the southern EAS (SEAS). Comparison of different parameterizations of double diffusion with the measurements of vertical heat diffusivity (KT) found that the Radko and Smith salt fingering scheme and the Kelley diffusive convection scheme best match with the observations. The estimates based on flux law show that the combination of downward heat flux of approximately −3 W m−2 associated with salt fingering in the thermocline region of the EAS and the upward heat flux of ∼5 W m−2 due to diffusive convection in the barrier layer region of the SEAS cools the thermocline. [ABSTRACT FROM AUTHOR]

One of the proposed mechanisms for energy loss in the ocean is through dissipation of internal waves, in particular above rough topography where internal lee waves are generated. Rates of dissipation and diapycnal mixing are often estimated using linear internal wave generation theory and a constant value for mixing efficiency. However, previous oceanographic measurements found that nonlinear dynamics may be important close to topography. To investigate the role of nonlinear interactions, we conduct idealized 3D direct numerical simulations (DNS) of steady flow over 1D topography and vary the topographic height, which correlates to the degree of flow nonlinearity. We analyze the spatial distribution of energy transfer rates between internal waves and the nongeostrophic portion of the time-mean flow, and of dissipation and diapycnal mixing rates. In our simulations with taller, more nonlinear topographies, energy transfer rates are similar to previously unexplained oceanographic observations near topography: internal waves gain energy from time-mean flow through horizontal straining and lose energy through vertical shearing. In the tall topography simulations, buoyancy fluxes also play a significant role, consistent with observations but contrary to linear wave theory, suggesting that quasigeostrophy-based approximations and linear theory may not hold in some regions above rough topography. Both dissipation and mixing rates increase with topographic height, but their vertical distributions differ between topographic regimes. As such, the vertical profile of mixing efficiency is different for linear and nonlinear topographic regimes, which may need to be incorporated into parameterizations of small-scale processes in models and estimates of ocean energy loss. [ABSTRACT FROM AUTHOR]

The present study provides a theoretical linkage between the Lagrangian dispersion diffusivity and the diapycnal diffusivity in the context of vertical mixing, although previous studies have demonstrated their equivalence under the assumptions of stationary, homogeneous, and stratified turbulence. This is achieved in a new coordinate in which the fluid density is adiabatically sorted in the vertical direction. In the density-sorted coordinate, 1) the vertical motion of Lagrangian particles is solely subjected to irreversible diffusion process; 2) relations between Lagrangian dispersion diffusivity, diapycnal diffusivity, and the generalized Osborn diffusivity are exact; and 3) a generalization of the classical Munk balance between vertical advection and diffusion is also illustrated in an exact sense. Since the adiabatic sorting of the fluid does not require the turbulence to be statistically stationary, homogeneous, and stably stratified, the present solution eliminated these requirements and is thus more general than previous studies. Upon this, a new Lagrangian diagnostic is proposed to quantify the local, instantaneous, and irreversible mixing. Applications are demonstrated in a turbulent scenario of internal wave breaking induced by current-topography interaction, in which the turbulence is intermittent, nonstationary, and inhomogeneous. [ABSTRACT FROM AUTHOR]

This study presents the characteristics and spatiotemporal structure of near-inertial waves and their interaction with Leeuwin Current eddies in the eastern south Indian Ocean as observed by Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats. The floats sampled the upper ocean during July–October 2013 with a frequency of eight profiles per day down to 1200 m. Near-inertial waves (NIWs) are found to be the dominant signal in the frequency spectra. Complex demodulation is used to estimate the amplitude and phase of the NIWs from the velocity profiles. The NIW energy propagated from the base of the mixed layer downward into the ocean interior, following beam characteristics of linear wave theory. We visually identified a total of 15 near-inertial internal wave packets from the wave amplitudes and phases with a mean vertical wavelength of 89 ± 63 m, a mean horizontal wavelength of 69 ± 85 km, a mean horizontal group velocity of 3 ± 2 cm s−1, and a mean vertical group velocity of 9 ± 7 m day−1. A strong near-inertial packet with a kinetic energy of 20–30 J m−3 found propagating below 700 m suggests that the NIWs can contribute to deep ocean mixing. A blue shift of 10%–15% in the energy spectrum of the NIWs is observed in the upper 1200 m as the floats move toward the equator. The impacts of mesoscale eddies on the characteristics and propagation of the observed NIWs are also investigated. The elevated near-inertial shear variance in anticyclonic eddies suggests trapping of NIWs near the surface. Cyclonic eddies, in contrast, were associated with weak near-inertial shear variance in the upper 400 m. [ABSTRACT FROM AUTHOR]

The energetics of thermobaricity- and cabbeling-powered deep convection occurring in oceans with cold freshwater overlying warm salty water are investigated here. These quasi-two-layer profiles are widely observed in wintertime polar oceans. The key diagnostic is the ocean convective available potential energy (OCAPE), a concept introduced in a companion piece to this paper (Part I). For an isolated ocean column, OCAPE arises from thermobaricity and is the maximum potential energy (PE) that can be converted into kinetic energy (KE) under adiabatic vertical parcel rearrangements. This study explores the KE budget of convection using two-dimensional numerical simulations and analytical estimates. The authors find that OCAPE is a principal source for KE. However, the complete conversion of OCAPE to KE is inhibited by diabatic processes. Further, this study finds that diabatic processes produce three other distinct contributions to the KE budget: (i) a sink of KE due to the reduction of stratification by vertical mixing, which raises water column's center of mass and thus acts to convert KE to PE; (ii) a source of KE due to cabbeling-induced shrinking of the water column's volume when water masses with different temperatures are mixed, which lowers the water column's center of mass and thus acts to convert PE into KE; and (iii) a reduced production of KE due to diabatic energy conversion of the KE convertible part of the PE to the KE inconvertible part of the PE. Under some simplifying assumptions, the authors also propose a theory to estimate the maximum depth of convection from an energetic perspective. This study provides a potential basis for improving the convection parameterization in ocean models.

Thermobaric convection (type II convection) and thermobaric cabbeling (type III convection) might substantially contribute to vertical mixing, vertical heat transport, and deep-water formation in the World Ocean. However, the extent of this contribution remains poorly constrained. The concept of ocean convective available potential energy (OCAPE), the thermobaric energy source for type II and type III convection, is introduced to improve the diagnosis and prediction of these convection events. OCAPE is analogous to atmospheric CAPE, which is a key energy source for atmospheric moist convection and has long been used to forecast moist convection. OCAPE is the potential energy (PE) stored in an ocean column arising from thermobaricity, defined as the difference between the PE of the ocean column and its minimum possible PE under adiabatic vertical parcel rearrangements. An ocean column may be stably stratified and still have nonzero OCAPE. The authors present an efficient strategy for computing OCAPE accurately for any given column of seawater. They further derive analytical expressions for OCAPE for approximately two-layer ocean columns that are widely observed in polar oceans. This elucidates the dependence of OCAPE on key physical parameters. Hydrographic profiles from the winter Weddell Sea are shown to contain OCAPE (0.001-0.01 J kg-1), and scaling analysis suggests that OCAPE may be substantially enhanced by wintertime surface buoyancy loss. The release of this OCAPE may substantially contribute to the kinetic energy of deep convection in polar oceans.