Spatial distribution of turbulent diapycnal mixing along the Mindanao current inferred from rapid-sampling Argo floats

The Mindanao Current (MC) bridges the North Pacific low-latitude western boundary current system region and the Indonesian Seas by supplying the North Pacific waters to the Indonesian Throughflow. Although the previous study speculated that the diapycnal mixing along the MC might be strong on the basis of the water mass analysis of the gridded climatologic dataset, the real spatial distribution of diapycnal mixing along the MC has remained to be clarified. We tackle this question here by applying a finescale parameterization to temperature and salinity profiles obtained using two rapid-sampling profiling Argo floats that drifted along the MC. The western boundary (WB) region close to the Mindanao Islands and the Sangihe Strait are the two mixing hotspots along the MC, with energy dissipation rate ε and diapycnal diffusivity Kρ enhanced up to ~ 10–6 W kg−1 and ~ 10–3 m2 s−1, respectively. Except for the above two mixing hotspots, the turbulent mixing along the MC is mostly weak, with ε and Kρ to be 10–11–10–9 W kg−1 and 10–6–10–5 m2 s−1, respectively. Strong mixing in the Sangihe Strait can be basically attributed to the existence of internal tides, whereas strong mixing in the WB region suggests the existence of internal lee waves. We also find that water mass transformation along the MC mainly occurs in the Sangihe Strait where the water masses are subjected to strong turbulent mixing during a long residence time.

Multi-timescale control of Southern Ocean diapycnal mixing over Atlantic tracer budgets

Oceanic cross-density (diapycnal) mixing helps sustain the ocean den- sity stratification and its Meridional Overturning Circulation (MOC) and is key to global tracer distributions. The Southern Ocean (SO) is a key region where different overturning cells connect, allowing nutri- ent and carbon rich Indian and Pacific deep waters, and oxygen rich Atlantic deep waters to resurface. The SO is also rife with localized intense diapycnal mixing due to breaking of internal waves induced by the interaction of energetic eddies and currents with rough topogra- phy. SO diapycnal mixing is believed to be of secondary importance for the MOC. Here we show that changes to SO mixing can cause sig- nificant alterations to Atlantic biogeochemical tracer distributions over short and long timescales in an idealized model of the MOC. While such alterations are dominated by the direct impact of changes in diapycnal mixing on tracer fluxes on annual to decadal timescales, on centennial timescales they are dominated by the mixing-induced variations in the advective transport of the tracers by the Atlantic MOC. This work sug- gests that an accurate representation of spatio-temporally variable local and non-local mixing processes in the SO is essential for climate mod- els’ ability to i) simulate the biogeochemical cycles and air sea carbon fluxes on decadal timescales, ii) represent the indirect impact of mixing- induced changes to MOC on biogeochemical cycles on longer timescales.

Enhanced diapycnal mixing with polarity-reversing internal solitary waves revealed by seismic reflection data

Shoaling internal solitary waves near the Dongsha Atoll in the South China Sea dissipate their energy and enhance diapycnal mixing, which have an important impact on the oceanic environment and primary productivity. The enhanced diapycnal mixing is patchy and instantaneous. Evaluating its spatiotemporal distribution requires comprehensive observation data. Fortunately, seismic oceanography meets the requirements, thanks to its high spatial resolution and large spatial coverage. In this paper, we studied three internal solitary waves in reversing polarity near the Dongsha Atoll and calculated their spatial distribution of diapycnal diffusivity. Our results show that the average diffusivities along three survey lines are 2 orders of magnitude larger than the open-ocean value. The average diffusivity in internal solitary waves with reversing polarity is 3 times that of the non-polarity reversal region. The diapycnal diffusivity is higher at the front of one internal solitary wave and gradually decreases from shallow to deep water in the vertical direction. Our results also indicate that (1) the enhanced diapycnal diffusivity is related to reflection seismic events, (2) convective instability and shear instability may both contribute to the enhanced diapycnal mixing in the polarity-reversing process, and (3) the difference between our results and Richardson-number-dependent turbulence parameterizations is about 2–3 orders of magnitude, but its vertical distribution is almost the same.

Spatial variability of diapycnal mixing in the South China Sea inferred from density overturn analysis

The nominal spatial distribution of diapycnal mixing in the South China Sea (SCS) is obtained with Thorpe-scale analysis from 2004 to 2020. The inferred dissipation rate ε and diapycnal diffusivity Kz between 100 and 1500 m indicated that the strongest mixing occurred in the Luzon Strait and Dongsha Plateau regions, with ε ~ 3.0 × 10-8 W/kg (εmax = 5.3 × 10-6 W/kg) and Kz ~ 3.5 × 10-4 m2/s (Kz max = 4.2 = 10-2 m2/s). The weakest mixing occurred in the thermocline of the central basin, with ε ~ 6.2 × 10-10 W/kg and Kz ~ 3.7 × 10-6 m2/s. The ε and Kz in the continental slope indicated that the mixing in the northern part [O(10-8) W/kg, O(10-4) m2/s] was comparatively stronger than that in the Xisha and Nansha regions [O(10-9) W/kg, O(10-5) m2/s]. The Kz in the continental slope region (200–2000 m) decayed at a closed rate from the ocean bottom to the main thermocline when the measured depth D was normalized by the ocean depth H as D/H, whether in the shallow or deep oceans. The diapycnal diffusivity was parameterized as Kz = 3.3 × 10−4 (1 + )−2 − 6.0 × 10−6 m2/s. The vertically integrated energy dissipation was nominally as 15.8 mW/m2 for all data and 25.6 mW/m2 for data at stations H < 2000 m. This was about one order higher than that in the open oceans (3.0–3.3 mW/m2), which confirmed the active mixing state in the SCS.

Energetics and mixing of stratified, rotating flow over abyssal hills

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 theory and a constant value for mixing efficiency. However, previous oceanographic measurements found that non-linear dynamics may be important close to topography. In order to investigate the role of non-linear interactions, we conduct idealized 3D numerical simulations of steady flow over 1D topography and vary the topographic height, which correlates to the degree of flow non-linearity. We analyze spatial distribution of energy transfer rates between internal waves and the non-geostrophic portion of time-mean flow, and of dissipation and diapycnal mixing rates. In our simulations with taller, more non-linear 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, vertical profile of mixing efficiency is different for linear and non-linear topographic regimes, which may need to be incorporated into parameterizations of small-scale processes in models and estimates of ocean energy loss.