Diapycnal Diffusivity Research Articles

Enhanced internal tidal mixing in the Philippine Sea mesoscale environment

Abstract. Turbulent mixing in the ocean interior is mainly attributed to internal wave breaking; however, the mixing properties and the modulation effects of mesoscale environmental factors are not well known. Here, the spatially inhomogeneous and seasonally variable diapycnal diffusivities in the upper Philippine Sea were estimated from Argo float data using a strain-based, fine-scale parameterization. Based on a coordinated analysis of multi-source data, we found that the driving processes for diapycnal diffusivities mainly included the near-inertial waves and internal tides. Mesoscale features were important in intensifying the mixing and modulating of its spatial pattern. An interesting finding was that, besides near-inertial waves, internal tides also contributed significant diapycnal mixing in the upper Philippine Sea. The seasonal cycles of diapycnal diffusivities and their contributors differed zonally. In the midlatitudes, wind mixing dominated and was strongest in winter and weakest in summer. In contrast, tidal mixing was more predominant in the lower latitudes and had no apparent seasonal variability. Furthermore, we provide evidence that the mesoscale environment in the Philippine Sea played a significant role in regulating the intensity and shaping the spatial inhomogeneity of the internal tidal mixing. The magnitudes of internal tidal mixing were greatly elevated in regions of energetic mesoscale processes. Anticyclonic mesoscale features were found to enhance diapycnal mixing more significantly than cyclonic ones.

Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?

AbstractA generic shortcoming of constant‐depth (or “z‐coordinate”) ocean models such as MOM5 and Nucleus for European Models of the Ocean (NEMO) is a tendency for the advection scheme to produce unphysical numerical diapycnal mixing, which may exceed the explicitly parameterized mixing based on observed physical processes. Megann (2018, https://doi.org/10.1016/j.ocemod.2017.11.001) estimated the effective diapycnal diffusivity in the Global Ocean Version 5.0 (GO5.0) 0.25° global implementation of the NEMO model and showed that this was up to 10 times the explicit diffusivity used in the model's mixing scheme and argued that this was at least partly caused by large transient vertical velocities on length scales comparable to the horizontal grid scale. The current UK global NEMO configuration GO6, as used in the Global Coupled Model version 3.1 (GC3.1) and UK Earth System Model (UKESM1), is integrated in forced mode at 0.25° resolution with a range of viscosity parameterizations. In the present study, the effective diffusivity is evaluated for each integration and compared with the explicit value from the model mixing scheme, as well as with that in the control (using the default viscosity). It is shown that there is a strong correspondence between lower viscosity and enhanced numerical mixing and that larger viscosities lead to a marked reduction in the unrealistic internal temperature drift seen in the control configuration, without incurring excessive damping of the large‐scale circulation, mixed layer depths, or sea ice cover. The results presented here will inform the choices made in global ocean configurations used in climate and Earth System models following the sixth Coupled Model Intercomparison Project (CMIP6).

Observational estimates of turbulent mixing in the southeast Indian Ocean

AbstractThis study investigates the spatio-temporal variability of turbulent mixing in the eastern South Indian Ocean using a collection of data from EM-APEX profiling floats, shipboard CTD and microstructure profilers. The floats collected 1566 profiles of temperature, salinity and horizontal velocity data down to 1200 m over a period of about four months. A fine-scale parameterization is applied to the float and CTD data to estimate turbulent mixing. Elevated mixing is observed in the upper ocean, over bottom topography and in mesoscale eddies. Mixing is enhanced in the anticyclonic eddies due to trapped near-inertial waves within the eddy. We found that cyclonic eddies contribute to turbulent mixing in the depth range of 500 – 1000 m, which is associated with downward propagating internal waves. The mean diapycnal diffusivity over 250 – 500 m depth is O(10−6) m2 s−1 and it increases to O(10−5) m2 s−1 in 500 – 1000 m in cyclonic eddies. The turbulent mixing in this region has implications for watermass transformation and large-scale circulation. Higher diffusivity (O(10−5) m2 s−1) is observed in the Antarctic Intermediate Water (AAIW) layer in cyclonic eddies whereas weak diffusivity is observed in the Subantarctic Mode Water (SAMW) layer (O(10−6) m2 s−1). Counter-intuitively, then, the SAMW watermass properties are strongly affected in cyclonic eddies whereas the AAIW layer is less affected. Comparatively high diffusivity at the location of the South Indian Countercurrent (SICC) jets suggests there are wave-mean flow interactions in addition to the wave-eddy interactions that warrant further investigation.

Diapycnal Mixing in the Subthermocline of the Mariana Ridge from High-Resolution Seismic Images

AbstractThe Mariana Ridge is one of the prominent mixing hotspots of the open ocean. The high-resolution underway marine seismic reflection technique provides an improved understanding of the spatiotemporal continuous map of ocean turbulent mixing. Using this novel technique, this study quantifies the diapycnal diffusivity of the subthermocline (300–1200-m depth) turbulence around the Mariana Ridge. The autotracked wave fields on seismic images allow us to derive the dissipation rate ε and diapycnal diffusivity Kρ based on the Batchelor model, which relates the horizontal slope spectra with +1/3 slope to the inertial convective turbulence regime. Diffusivity is locally intensified around the seamounts exceeding 10−3 m2 s−1 and gradually decreases to 10−5–10−4 m2 s−1 in ~60-km range, a distance that may be associated with the internal tide beam emanating paths. The overall pattern suggests a large portion of the energy dissipates locally and a significant portion dissipates in the far field. Empirical diffusivity models Kρ(x) and Kρ(z), varying with the distance from seamounts and the height above seafloor, respectively, are constructed for potential use in ocean model parameterization. Geographic distributions of both the vertically averaged dissipation rate and diffusivity show tight relationships with the topography. Additionally, a strong agreement of the dissipation results between seismic observation and numerical simulation is found for the first time. Such an agreement confirms the suitability of the seismic method in turbulence quantification and suggests the energy cascade from large-scale tides to small-scale turbulence via possible mechanisms of local direct tidal dissipation, near-local wave–wave interactions, and far-field radiating and breaking.

Energetics‐Based Estimation of the Diapycnal Mixing Induced by Internal Tides in the Andaman Sea

AbstractThe Andaman Sea (AS) is characterized by surprisingly weak stratification in its deep basin in drastic contrast to the nearby Bay of Bengal (BoB), presumably due to strong diapycnal mixing fertilized by the dissipation of internal tides. Here, we report on the first estimates of tidal mixing in the AS using idealized numerical simulations resolving the generation and evolution of low‐mode internal tides. The estimation is based on a diagnostic analysis of the energetics of the simulated internal tides of both local and non‐local origins. The results highlight the dominant role of the internal tides generated in the channels along the western boundary island chain in energizing diapycnal mixing in the entire AS. Tidal dissipation and mixing in the deep basin of the AS are found to be highly elevated, with the depth‐integrated dissipation rate and the diapycnal diffusivity being O(10−2–10 ) W m−2 and O(10−3–10−2) m2 s−1, respectively, both of which are 1–2 orders of magnitude larger than those in the deep BoB. These model‐based estimates of tidal mixing are in general consistent with those from in situ conductivity‐temperature‐depth (CTD) measurements using the fine‐scale parameterization. Our results suggest that the oft‐used tidal mixing parameterization considering only local internal tide generation would underestimate the depth‐integrated tidal dissipation in the AS by several orders of magnitude, highlighting therefore the necessity of considering contributions of non‐local internal tides in parameterizing tidal dissipation and mixing in marginal seas like the AS.

Quantifying mixing and available potential energy in vertically periodic simulations of stratified flows

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Inverse estimation of diffusivity coefficients from salinity distributions on isopycnal surfaces using Argo float array data

Distributions of diapycnal, isopycnal, and thickness diffusivity coefficients were estimated using long-term (2001–2015) mean salinity distributions on neutral density (γ) surfaces in seven oceanic basins (except the equatorial region). Diffusivity amplitudes and spatial differences were consistent with distributions previously estimated by various parameterizations of fine-scale structures. This result suggests that these parameterizations are consistent with water mass modifications shown by salinity distributions. Although the estimated diapycnal diffusivity coefficients were the lowest among those used by general circulation models to reproduce tracer distributions, they were closer to directly observed values. Because diapycnal diffusivity was generally small, meridional transports in the upper layers (above about 1600 dbar) associated with diapycnal mixing were relatively small. This result may indicate that diapycnal mixing in coastal regions has relatively large effects on meridional transport, along with water mass formation and mixing and wind forcing at the sea surface. The major water mass transports in subtropical gyres estimated in this study were consistent with previous estimates and corresponded to subduction rates determined using mixed layer thickness budgets and transient tracer inventories. These results, based on recent observation network data, show that understanding of water mass formation and modifications can be improved by taking into account diffusive effects, but more detailed assessments of estimation errors are needed to clarify the role of mixing in water mass modification.

Structure and drivers of ocean mixing north of Svalbard in summer and fall 2018

Abstract. The Arctic Ocean is a major sink for heat and salt for the global ocean. Ocean mixing contributes to this sink by mixing the Atlantic- and Pacific-origin waters with surrounding waters. We investigate the drivers of ocean mixing north of Svalbard, in the Atlantic sector of the Arctic, based on observations collected during two research cruises in summer and fall 2018. Estimates of vertical turbulent heat flux from the Atlantic Water layer up to the mixed layer reach 30 W m−2 in the core of the boundary current, and average to 8 W m−2, accounting for ∼1 % of the total heat loss of the Atlantic layer in the region. In the mixed layer, there is a nonlinear relation between the layer-integrated dissipation and wind energy input; convection was active at a few stations and was responsible for enhanced turbulence compared to what was expected from the wind stress alone. Summer melting of sea ice reduces the temperature, salinity and depth of the mixed layer and increases salt and buoyancy fluxes at the base of the mixed layer. Deeper in the water column and near the seabed, tidal forcing is a major source of turbulence: diapycnal diffusivity in the bottom 250 m of the water column is enhanced during strong tidal currents, reaching on average 10−3 m2 s−1. The average profile of diffusivity decays with distance from the seabed with an e-folding scale of 22 m compared to 18 m in conditions with weaker tidal currents. A nonlinear relation is inferred between the depth-integrated dissipation in the bottom 250 m of the water column and the tidally driven bottom drag and is used to estimate the bottom dissipation along the continental slope of the Eurasian Basin. Computation of an inverse Froude number suggests that nonlinear internal waves forced by the diurnal tidal currents (K1 constituent) can develop north of Svalbard and in the Laptev and Kara seas, with the potential to mix the entire water column vertically. Understanding the drivers of turbulence and the nonlinear pathways for the energy to turbulence in the Arctic Ocean will help improve the description and representation of the rapidly changing Arctic climate system.

Roles of Shear and Convection in Driving Mixing in the Ocean

AbstractUsing field, numerical, and laboratory studies, we consider the roles of both shear and convection in driving mixing in the interior of the density‐stratified ocean. Shear mixing dominates when the Richardson number Ri < 0.25, convective mixing dominates when Ri > 1.0, and in the intermediate regime when 0.25 < Ri < 1.0 both shear and convection can contribute to mixing. For pure shear mixing the mixing efficiency Rif approaches 0.5, while for pure convective mixing the mixing efficiency Rif approaches 0.75. The diapycnal diffusivities for the two mechanisms are given by very different expressions. Despite these complexities, a simple mixing length model using the mean flow shear S provides robust estimates of diffusivity across the range 0 < Ri < 2. To account for the roles of both shear and convection over this range of Ri, we also formulate a modified version of the empirical KPP model for parameterizing ocean mixing in numerical models.

Persistence of bioconvection‐induced mixed layers in a stratified lake

AbstractIn situ observations of biophysical interactions in natural waters typically focus on physical mechanisms influencing biological activity. Yet, biological activity can also drive physical processes in aquatic environments. A community of photoautotrophic, motile and heavy bacteria—Chromatium okenii, which requires light, sulfide, and anoxic conditions to perform anoxygenic photosynthesis, accumulates below the chemocline of the meromictic Lake Cadagno (Switzerland). Upward vertical migration drives bioconvection, which modifies the physical environment of the bacteria‐populated water to create a deep and homogeneous mixed layer of up to 1 m thickness. Continuous convection within the mixed layer and diapycnal diffusivity from its adjacent stratified surroundings determine ecologically relevant gradients. The daytime vertical migration that induce convective instabilities is well‐established. It consists in bacteria swimming upward towards light and accumulating at the upper part of the anoxic layer, leading to a locally‐unstable density excess. However, nocturnal activity has not yet been analyzed. An intensive 48‐h survey was conducted in August 2018 using standard and microstructure profilers, as well as a moored high‐resolution current meter coupled with temperature and turbidity sensors deployed across the mixed layer depth. This survey revealed a persistent mixed layer also during nighttime hours. Using a mixed layer shape model, vertical velocity observations and turbulent dissipation estimates, we conclude that photoautotrophic bacteria continue their vertical migration at night. This nocturnal activity thereby drives “dark bioconvection” and maintains the subsurface mixed bacterial layer in Lake Cadagno throughout the diel cycle.

Parametrization of irreversible diapycnal diffusivity in salt-fingering turbulence using DNS

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Turbulence and Mixing in the Arctic Ocean’s Amundsen Gulf

AbstractThis study uses CTD and microstructure measurements of shear and temperature from 348 glider profiles to characterize turbulence and turbulent mixing in the southeastern Beaufort Sea, where turbulence observations are presently scarce. We find that turbulence is typically weak: the turbulent kinetic energy dissipation rate ε has a median value (with 95% confidence intervals in parentheses) of 2.3 [2.2, 2.4] × 10−11 W kg−1 and is less than 1.0 × 10−10 W kg−1 in 68% of observations. Variability in ε spans five orders of magnitude, with indications that turbulence is bottom enhanced and modulated in time by the semidiurnal tide. Stratification is strong and frequently damps turbulence, inhibiting diapycnal mixing. Buoyancy Reynolds number estimates suggest that turbulent diapycnal mixing is unlikely in 93% of observations; however, a small number of strongly turbulent mixing events are disproportionately important in determining net buoyancy fluxes. The arithmetic mean diapycnal diffusivity of density is 4.5 [2.3, 14] × 10−6 m2 s−1, three orders of magnitude larger than that expected from molecular diffusion. Vertical heat fluxes are modest at O(0.1) W m−2, of the same order of magnitude as those in the Canada Basin double-diffusive staircase, however, staircases are generally not observed. Despite significant heat present in the Pacific Water layer in the form of a warm-core mesoscale eddy and smaller, O(1) km, temperature anomalies, turbulent mixing was found to be too low to release this heat to shallower depths.

Generation, propagation and dissipation of internal tides on the continental shelf and slope off the west coast of India

The present study focuses on the dynamics of M2 internal tides on the shelf and slope off the west coast of India (WCI) using a high-resolution (1/48∘) numerical ocean circulation model configured for the eastern Arabian Sea. The WCI is characterized by the presence of large internal tides where most of the internal tides generated on shelf-slope regions propagate offshore as beams. Baroclinic energy budget estimates show that about 2.4 GW of barotropic tides gets converted to baroclinic tides over the region. The regions of shelf breaks and upper slope around 16–18.5∘ N and the northern part of the Chagos-Laccadive Ridge (8–14∘ N) are the two most important regions of internal tide generation along the WCI, which together contribute to more than 50% of total area-integrated M2 conversion over the domain. The diapycnal diffusivity for internal tide dissipation exceeds 10−3m2/s in these regions. The central and northern slope regions off the WCI are dominated by locally generated internal tides, whereas part of internal tides in the southern region is characterized by the presence of remotely-generated internal tides coming from the Chagos-Laccadive Islands. Internal tides experience significant seasonal variation on the shelf owing to the variation in stratification, while their variations are negligible on the slope, where the seasonal variation in stratification is not significant.

Interannual summer mixing processes in the Ilulissat Icefjord, Greenland

In the stratified water column inside the Ilulissat ice fjord, oceanic mixing processes can promote an upward transport of heat and salt from the subsurface to the surface mixed layer. Our objective was to map the temporal variation of vertical mixing inside the fjord by estimating the diapycnal mixing rates derived from oceanographic data collected during summer research cruises between 2009 and 2017. We applied the Thorpe scale method based on local density inversions. We found enhanced mixing in the deep water at 400 m sill depth inside the fjord, with diapycnal diffusivity reaching a maximum value of 1× 10 –3.5 m 2 s −1 . The strongest mixing rates in 2013, and the weakest during 2016, were mainly due to the Glacier's melting conditions and melange. The highest values of diapycnal diffusivity values occurred during years when low melange concentrations inside the ice fjord increased ventilation, which favored the intrusion of Polar and Irminger waters that perturbed the water structure. These years were also associated with a decrease in diffusive convection processes. Our results give insight into how the intrusion of waters from Disko Bay to the Ilulissat ice fjord may affect the stability of the Jakobshavn Isbræ Glacier. • · Water mixing interaction in the Ilulissat Ice Fjord, increase diapycnal diffusivities by more than one order of magnitude. • We used the Thorpe method to estimate mixing rates in the IIF system. • A warm water intrusion in 2013 lifted the isopycnals reducing the ice melange leading to the retreat of the glacier front.

Application of Three-Dimensional Interpolation in Estimating Diapycnal Diffusivity in the South China Sea

Diapycnal diffusivity is an important parameter to characterize oceanic turbulent mixing and vertical transport. However, due to the challenging accessibility of field observations, the observation of diapycnal diffusivity in the South China Sea (SCS) is rare. In this study, a three-dimensional field of diapycnal diffusivity in the SCS with high spatial resolution is performed by interpolating the rare field observations, which aims to provide a reference for the value of diapycnal diffusivity in ocean models. Given the anisotropy of diapycnal diffusivity and its rapid change in the magnitude in the vertical direction, several typical interpolation methods are compared in this study. Results of two cross-validation methods demonstrate that the three-dimensional (3D) thin-plate spline interpolation method yields the most reasonable and accurate results among a total of five typical methods used in this study.

Abyssal Pathways and the Double Silica Maximum in the Northeast Pacific Basin

AbstractThis study examines causes of the double silica maximum in the deep interior Northeast Pacific Basin using a stochastic Lagrangian tracer model based on steady‐state advective fields and diapycnal diffusion established by a hydrographic inverse method that conserves potential vorticity and salinity. Lateral diffusion, unresolved by the inverse model, is adjusted for overall agreement with radiocarbon distribution. The double silica maximum in vertical profiles arises from an eastern‐intensified single maximum in the North Pacific Deep Water along the northern domain boundary (originating in the western Pacific), and a strong subarctic bottom source supplying silica to Upper Circumpolar Deep Water density surfaces that successively intersect the seafloor over a broad area east of 150°W, associated geostrophically with southward flow. The existence of the double silica maximum requires weak diapycnal transport in the deep interior, with broader implications for the conceptual picture of meridional overturning circulation in the North Pacific.

Hydraulics and Mixing of the Deep Overflow in the Lifamatola Passage of the Indonesian Seas

AbstractHydrographic measurements recently acquired along the thalweg of the Lifamatola Passage combined with historical moored velocity measurements immediately downstream of the sill are used to study the hydraulics, transport, mixing, and entrainment in the dense overflow. The observations suggest that the mean overflow is nearly critical at the mooring site, suggesting that a weir formula may be appropriate for estimating the overflow transport. Our assessment suggests that the weir formulas corresponding to a rectangular, triangular, or parabolic cross section all result in transports very close to the observation, suggesting their potential usage in long-term monitoring of the overflow transport or parameterizing the transport in numerical models. Analyses also suggest that deep signals within the overflow layer are blocked by the shear flow from propagating upstream, whereas the shallow wave modes of the full-depth continuously stratified flow are able to propagate upstream from the Banda Sea into the Maluku Sea. Strong mixing is found immediately downstream of the sill crest, with Thorpe-scale-based estimates of the mean dissipation rate within the overflow up to 1.1 × 10−7 W kg−1 and the region-averaged diapycnal diffusivity within the downstream overflow in the range of 2.3 × 10−3 to 10.1 × 10−3 m2 s−1. Mixing in the Lifamatola Passage results in 0.6–1.2-Sv (1 Sv ≡ 106 m3 s−1) entrainment transport added to the overflow, enhancing the deep-water renewal in the Banda Sea. A bulk diffusivity coefficient estimated in the deep Banda Sea yields 1.6 × 10−3 ± 5 × 10−4 m2 s−1, with an associated downward turbulent heat flux of 9 W m−2.

Application of the IDEMIX Concept for Internal Gravity Waves in the Atmosphere

AbstractThe model Internal Wave Dissipation, Energy and Mixing (IDEMIX) presents a novel way of parameterizing internal gravity waves in the atmosphere. IDEMIX is based on the spectral energy balance of the wave field and has previously been successfully developed as a model for diapycnal diffusivity, induced by internal gravity wave breaking in oceans. Applied here for the first time to atmospheric gravity waves, integration of the energy balance equation for a continuous wave field of a given spectrum, results in prognostic equations for the energy density of eastward and westward gravity waves. It includes their interaction with the mean flow, allowing for an evolving and local description of momentum flux and gravity wave drag. A saturation mechanism maintains the wave field within convective stability limits, and a closure for critical-layer effects controls how much wave flux propagates from the troposphere into the middle atmosphere. Offline comparisons to a traditional parameterization reveal increases in the wave momentum flux in the middle atmosphere due to the mean-flow interaction, resulting in a greater gravity wave drag at lower altitudes. Preliminary validation against observational data show good agreement with momentum fluxes.

Time‐Lapse Acoustic Imaging of Mesoscale and Fine‐Scale Variability within the Faroe‐Shetland Channel

AbstractWe describe and analyze the results of a three‐dimensional seismic (i.e., acoustic) reflection survey from the Faroe‐Shetland Channel that is calibrated with near‐coincident hydrographic and satellite observations. 54 vertical seismic transects were acquired over a period of 25 days. On each transect, a 250‐ to 400‐m‐thick band of reflections is observed within the water column. Hydrographic measurements demonstrate that this reflective band is caused by temperature variations within the pycnocline that separates warm, near‐surface waters of Atlantic origin from cold, deep waters that flow southward from the Nordic Seas. Tilting of reflective surfaces records geostrophic shear between these near‐surface and deep waters. Measurements of temporal changes of pycnoclinic depth and of reflection tilt are used to infer the existence of an anticyclonic vortex that advects northeastward. Comparison with satellite measurements of sea‐surface temperature and height suggests that this vortex is caused by meandering of the Continental Slope Current. A model of a Gaussian vortex is used to match seismic and satellite observations. This putative vortex grows to have a core radius of 40–50 km. It has a maximum azimuthal velocity of 0.3–0.4 m s−1 and translates at 0.01–0.1 m s−1. Within the pycnocline, diapycnal diffusivity, K, is estimated by analyzing the turbulent spectral subrange of tracked reflections. K varies between 10−5.7 m2 s−1and 10−5.0 m2 s−1 in a pattern that is broadly consistent with translation of the vortex. Our integrated study demonstrates the ability of time‐lapse seismic reflection surveys to dynamically resolve the effects that mesoscale activity has upon deep thermohaline structure on scales from meters to hundreds of kilometers.

The KPP Trigger of Rapid AMOC Intensification in the Nonlinear Dansgaard‐Oeschger Relaxation Oscillation

AbstractMillennium time scale Dansgaard‐Oeschger oscillations of glacial climate, clear evidence for the occurrence of which was first provided on the basis of oxygen isotopic data from a Greenland ice core 25 years ago, have recently been shown to arise naturally (without explicit freshwater forcing) in a fully coupled modern climate model description of the interactions between the overturning circulation of the oceans, the atmosphere, and sea ice under maximum glacial conditions. The fast transitions from cold stadial to warm interstadial conditions in a typical D‐O oscillation are characterized by the appearance of an extensive polynya in the stadial sea ice cover of the Irminger Sea south of Denmark Strait that opens due to the onset of intense vertical mixing below the sea ice lid. Through detailed stability analysis of the water column in the region where the polynya first forms, together with analysis of the action of the KPP (Kappa Profile Parameterization) of stratified turbulence employed to represent water column diapycnal diffusivity, the authors show that the opening of the polynya is controlled by this turbulence parameterization. The relative contributions of the different components of the parameterization to polynya opening are investigated in order to better understand the rapid climate change that ensues. The authors furthermore show that the characteristic period of the model predicted Dansgaard‐Oeschger oscillation is also controlled by the detailed nature of this parameterization, a characteristic of the oscillation not previously explained.