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We present a method to infer ocean diapycnal diffusivity based on high‐resolution ocean model predictions of the depth‐dependent viscous dissipation associated with internal wave shear. This method relies on recent advances in modeling and the parameterization of stratified turbulent mixing processes. Especially important in the latter regard is the distinction between irreversible and reversible mixing processes associated with internal wave breaking. Utilizing the Bouffard–Boegman (BB) compilation of data, we derive depth‐dependent profiles of diapycnal diffusivity from viscous dissipation rates obtained from downscaled internal wave fields of the global ocean simulation LLC4320. Our methodology displays some skill in matching observationally‐informed inferences of diapycnal diffusivity and demonstrates that the KPP‐based production of diapycnal diffusivity fails to account for the distinction between reversible and irreversible mixing components. This work provides a framework for further improving the parameterization of mixing processes in large scale climate models through simulations of the background internal wave field.

The southwest Atlantic gyre connects several distinct water masses, which means that this oceanic region is characterized by a complex frontal system and enhanced water mass modification. Despite its significance, the distribution and variability of vertical mixing rates have yet to be determined for this system. Specifically, potential conditioning of mixing rates by frontal structures, in this location and elsewhere, is poorly understood. Here, we analyze vertical seismic (i.e., acoustic) sections from a three-dimensional survey that straddles a major front along the northern portion of the Brazil-Falkland Confluence. Hydrographic analyses constrain the structure and properties of water masses. By spectrally analyzing seismic reflectivity, we calculate spatial and temporal distributions of the dissipation rate of turbulent kinetic energy, ε, of diapycnal mixing rate, K , and of vertical diffusive heat flux, F H . We show that estimates of ε, K , and F H are elevated compared to regional and global mean values. Notably, cross-sectional mean estimates vary little over a 6 week period whilst smaller scale thermohaline structures appear to have a spatially localized effect upon ε, K , and F H . In contrast, a mesoscale front modifies ε and K to a depth of 1 km, across a region of O (100) km. This front clearly enhances mixing rates, both adjacent to its surface outcrop and beneath the mixed layer, whilst also locally suppressing ε and K to a depth of 1 km. As a result, estimates of F H increase by a factor of two in the vicinity of the surface outcrop of the front. Our results yield estimates of ε, K and F H that can be attributed to identifiable thermohaline structures and they show that fronts can play a significant role in water mass modification to depths of 1 km.

A deliberate tracer release experiment in 2008–2010 was used to study diapycnal mixing in the tropical northeastern Atlantic. The tracer (CF3SF5) was injected on the isopycnal surface σΘ = 26.88 kg m−3, which corresponds to about 330 m depth. Three surveys, performed 7, 20, and 30 months after the release, sampled the vertically and laterally expanding tracer patch. The mean diapycnal mixing estimate over the entire region occupied by the tracer and the period of 30 months was found to be (1.19 ± 0.18) × 10−5 m2 s−1, or, alternatively, (3.07 ± 0.58) × 10−11 (kg m−3)2 s−1as computed from the advection‐diffusion equation in isopycnal coordinates with the thickness‐weighted averaging. The latter method is preferable in the regions of different stratification for it yields local diapycnal mixing estimates varying less with stratification than their Cartesian coordinate counterparts. Results of this study are comparable to the results of the North Atlantic tracer release experiment (NATRE). However, the internal wave‐wave interaction models predict reduced mixing from the breaking of internal waves at low latitudes. Thus, the diapycnal diffusivity found in this study is higher than parameterized by the low latitude of the site (4°N–12°N). Key Points Tracer release experiment yields the diapycnal diffusivity of Kz = 0.12 cm2/s It is advantageous to use isopycnal coordinates in a tracer release experiment Higher than expected mean diapycnal diffusivity estimate was found

The Indonesian seas, with their complex passages and vigorous mixing, constitute the only route and are critical in regulating Pacific–Indian Ocean interchange, air–sea interaction, and global climate events. Previous research employing remote sensing and numerical simulations strongly suggested that this mixing is tidally driven and localized in narrow channels and straits, with only a few direct observations to validate it. The current study offers the first comprehensive temporal microstructure observations in the south of Lombok Strait with a radius of 0.05° and centered on 115.54oE and 9.02oS. Fifteen days of tidal mixing observations measured potential temperature and density, salinity, and turbulent energy dissipation rate. The results revealed significant mixing and verified the remotely sensed technique. The south Lombok temporal and depth averaged of the turbulent kinetic energy dissipation rate, and the diapycnal diffusivity from 20 to 250 m are ε = 4.15 ± 15.9) × 10–6 W kg–1 and Kρ= (1.44 ± 10.7) × 10–2 m2s–1, respectively. This Kρ is up to 104 times larger than the Banda Sea [Kρ = (9.2 ± 0.55) × 10–6 m2s–1] (Alford et al. Geophys Res Lett 26:2741–2744, 1999) or the “open ocean” Kρ= 0.03 × 10–4 m2s−1 within 2° of the equator to (0.4–0.5) × 10–4 m2s−1 at 50°–70° (Kunze et al. J Phys Oceanogr 36:1553–1576, 2006). Therefore, nonlinear interactions between internal tides, tidally induced mixing, and ITF plays a critical role regulating water mass transformation and have strong implications to longer-term variations and change of Pacific–Indian Ocean water circulation and climate.

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.

We present observations from deployments of turbulent microstructure instrument and CTD package in the northern South China Sea from April to May 2010. From them we determined the turbulent mixing (dissipation rate ε and diapycnal diffusivity κ ), nutrients (phosphate, nitrate, and nitrite), nutrient fluxes, and chlorophyll a in two transects (A and B). Transect A was located in the region where turbulent mixing in the upper 100 m was weak ( κ ∼10 −6 −10 −4 m 2 /s). Transect B was located in the region where the turbulent mixing in the upper 100 m was strong ( κ ∼10 −5 −10 −3 m 2 /s) due to the influence of internal waves originating from the Luzon Strait and water intrusion from the western Pacific. In both transects, there was a thin subsurface chlorophyll maximum layer (SCML) (>0.25 mg/m 3 ) nested in the upper 100 m. The observations indicate that the effects of turbulent mixing on the distributions of nutrients and chlorophyll a were different in the two transects. In the transect A with weak turbulent mixing, nutrient fluxes induced by turbulent mixing transported nutrients to the SCML but not to the upper water. Nutrients were sufficient to support a local SCML phytoplankton population and the SCML remained compact. In the transect B with strong turbulent mixing, nutrient fluxes induced by turbulent mixing transported nutrients not only to the SCML but also to the upper water, which scatters the nutrients in the water column and diffuses the SCML.

Time‐varying diapycnal diffusivity signals are estimated using the fine‐scale parameterization method from high‐resolution CTD data along three repeat sections in the northwestern Pacific Ocean. Indicative of the parametric subharmonic instability (PSI), locally‐elevated diapycnal mixing is detected alongallthree sections within the 25°–29°N band where bottom topography is relatively featureless. Due to their proximity to the semidiurnal internal tide generation sites, the two sections along ∼137°E have a time‐mean diffusivity value 2.5 × 10−5 m2 s−1 in the 300–2,000 m upper ocean. In contrast, it is 1.2 × 10−5 m2 s−1along the 165°E section. The time‐varying diffusivity along both sections is dominated by signals whose vertical structure resembles the local first dynamic normal mode profile. At the 137°E site of 25°–29°N, the local spring‐neap modulated semidiurnal tidal current is found to lead the CTD‐derived diffusivity time series by 6 days and can alter the diffusivity level by 0.24 × 10−5 m2 s−1. Similarly, the concurrent surface wind work is found to modify the diffusivity level by 0.35 × 10−5 m2 s−1. The combined spring‐neap tide and wind work forcing explains 47% of the observed, time‐varying diffusivity signals. Key Points Indicative of PSI, elevated mixing is detected along three repeat CTD sections The vertical structure resembles the local first dynamic normal mode profile Relations to spring‐neap modulated M2+S2 current and local wind forcing

We investigate the spatial distribution of diapycnal mixing and its drivers in the central South Atlantic thermocline between the Rio-Grande Rise to the Mid-Atlantic Ridge. Diapycnal mixing in the ocean interior influences the slowly evolving meridional circulation, yet there are few observations of its variability with space and time or its drivers. To overcome this gap, seismic reflection data are spectrally analyzed to produce a 1,600 km long full-thermocline vertical section of diapycnal diffusivity, that has a vertical and horizontal resolution of O (10) m and spans a period of 4 weeks. We compare seismic-derived diffusivities with CTD-derived diffusivities and direct observations from 1996, 2003, and 2011. In the mean and on decadal scales, we find that thermocline diffusivities have changed little in this region, retaining a background value of 1 × 10 –5 m 2 s –1 . Imprinted upon the background rates, mixing is heterogeneous at mesoscales. Enhanced mixing, exceeding 10 × 10 –5 m 2 s –1 and spreading between 200 and 700 m depth, is found above the Mid-Atlantic Ridge suggesting the ridge enhances diffusivity by at least one order of magnitude across the entire water column. Rapid decay of diffusivities within 30 km of the ridge implies local dissipation of tidal energy. Above smooth topography, patches of enhanced mixing are possibly caused by a recent storm that injects near-inertial energy into the water column and elevates mixing from 3 × 10 –5 m 2 s –1 to 50 × 10 –5 m 2 s –1 down to depths of more than 600 m. The propagation speed of near-inertial energy varies substantially from 17 to 27 m/day. Faster speed, and therefore greater penetration depths of 800 m, are probably facilitated by an eddy. Together, these data extend the observational record of central South Atlantic thermocline mixing and provide insights into drivers of mesoscale variability.