Submesoscale daily data from a non-hydrostatic OGCM at 1/90° resolution over Northern South China Sea in 2019.

Refraction of the M2 internal tides by mesoscale eddies in the South China Sea

The coherent and incoherent features of internal tides (ITs) in the north South China Sea (SCS) are investigated based on observations and numerical simulations. The 11-month (from May 2011 to March 2012) moored current observations indicate that coherent semidiurnal ITs are obviously amplified, which can be attributed to the interference of ITs. Interference enhances coherent motions of semidiurnal ITs, but weakens those of diurnal ITs. Moreover, observations also show that semidiurnal ITs are more incoherent than diurnal ITs. Variations of vertical stratification and surface tide forcing can hardly affect the incoherence of ITs. The increase of incoherent signal is largely due to the influence of mesoscale eddies. Mesoscale eddies affect both amplitude and phase of ITs, making them more incoherent. Mesoscale eddies not only increase the intensity of background currents, but also induce horizontal variations of density. Variations of horizontal density and the influence of background currents lead to the increase of incoherent signals. And semidiural ITs are more sensitive to the influence of mesoscale eddies, making them more incoherent than diurnal counterparts. Incoherent ITs, which induce strong current shear, play essential roles in cascading tidal energy to small-scale motions, and contribute to turbulent mixing eventually. The findings help to better understand ITs and may offer reference for the improvement of parameterization of ocean turbulent mixing in the northern SCS.

Any type of non-buoyant material in the ocean is transported horizontally by currents during its sinking journey. This lateral transport can be far from negligible for small sinking velocities. To estimate its magnitude and direction, the material is often modelled as a set of Lagrangian particles advected by current velocities that are obtained from Ocean General Circulation Models (OGCMs). State-of-the-art OGCMs are strongly eddying, similar to the real ocean, providing results with a spatial resolution on the order of 10 km on a daily frequency. While the importance of eddies in OGCMs is well-appreciated in the physical oceanographic community, other marine research communities may not. Further, many long term climate modelling simulations (e.g. in paleoclimate) rely on lower spatial resolution models that do not capture mesoscale features. To demonstrate how much the absence of mesoscale features in low-resolution models influences the Lagrangian particle transport, we simulate the transport of sinking Lagrangian particles using low- and high-resolution global OGCMs, and assess the lateral transport differences resulting from the difference in spatial and temporal model resolution. We find major differences between the transport in the non-eddying OGCM and in the eddying OGCM. Addition of stochastic noise to the particle trajectories in the non-eddying OGCM parameterises the effect of eddies well in some cases (e.g. in the North Pacific gyre). The effect of a coarser temporal resolution (once every 5 days versus monthly) is smaller compared to a coarser spatial resolution (0.1° versus 1° horizontally). We recommend to use sinking Lagrangian particles, representing e.g. marine snow, microplankton or sinking plastic, only with velocity fields from eddying Eulerian OGCMs, requiring high-resolution models in e.g. paleoceanographic studies. To increase the accessibility of our particle trace simulations, we launch planktondrift.science.uu.nl, an online tool to reconstruct the surface origin of sedimentary particles in a specific location.

Any type of non-buoyant material in the ocean is transported horizontally by currents during its sinking journey. This lateral transport can be far from negligible for small sinking velocities. To estimate its magnitude and direction, the material is often modelled as a set of Lagrangian particles advected by current velocities that are obtained from Ocean General Circulation Models (OGCMs). State-of-the-art OGCMs are strongly eddying, similar to the real ocean, providing results with a spatial resolution on the order of 10 km on a daily frequency. While the importance of eddies in OGCMs is well-appreciated in the physical oceanographic community, other marine research communities may not. Further, many long term climate modelling simulations (e.g. in paleoclimate) rely on lower spatial resolution models that do not capture mesoscale features. To demonstrate how much the absence of mesoscale features in low-resolution models influences the Lagrangian particle transport, we simulate the transport of sinking Lagrangian particles using low- and high-resolution global OGCMs, and assess the lateral transport differences resulting from the difference in spatial and temporal model resolution. We find major differences between the transport in the non-eddying OGCM and in the eddying OGCM. Addition of stochastic noise to the particle trajectories in the non-eddying OGCM parameterises the effect of eddies well in some cases (e.g. in the North Pacific gyre). The effect of a coarser temporal resolution (once every 5 days versus monthly) is smaller compared to a coarser spatial resolution (0.1° versus 1° horizontally). We recommend to use sinking Lagrangian particles, representing e.g. marine snow, microplankton or sinking plastic, only with velocity fields from eddying Eulerian OGCMs, requiring high-resolution models in e.g. paleoceanographic studies. To increase the accessibility of our particle trace simulations, we launch planktondrift.science.uu.nl, an online tool to reconstruct the surface origin of sedimentary particles in a specific location.

Submesoscale processes usually have characteristic horizontal scales of O(0.1 to 10) km and timescales of O(0.1 to 10) days, and play significant roles in energy cascade and vertical tracer transport which affect ocean circulation, air-sea interactions, and biogeochemical cycles. As an effective way, high-resolution simulations have been conducted to study submesoscales. The hydrostatic approximation becomes unsuitable for high-resolution ocean modeling because the horizontal scales of the motions are comparable to the local vertical scales. Combining hydrostatic and non-hydrostatic pressure in the ocean general circulation models (OGCMs) contributes to accurate modeling. Based on the pressure correction method, the non-hydrostatic dynamics are implemented into the hydrostatic OGCM. Based on the numerical simulation, the dynamic characteristics and spatiotemporal Variations of submesoscales in the South China Sea (SCS) are analyzed, and two leading generation mechanisms, including strain-induced frontogenesis and mixed layer baroclinic instabilities, are discussed through the vertical buoyancy transport and potential vorticity budget analysis. The comparison also shows that the simulated internal tide signature by non-hydrostatic OGCM is more obvious, and the simulated temperature are significantly closer to the Argo data. The construction of non-hydrostatic OGCM greatly promotes high-resolution ocean modeling and is of great significance for the research on the multi-scale interaction.

Yang, Q.; Zhou, L.; Tian, J., and Zhao, W., 2014. The roles of Kuroshio intrusion and mesoscale eddy in upper mixing in the northern South China Sea. Upper mixing in the northern South China Sea (SCS) is derived from Argo profiles from 2006 to 2012, using the Gregg-Henyey-Polzin parameterization. Temporal and spatial mean estimates reveal clear spatial patterns of mixing over the northern SCS. Enhanced diffusivity on an order of 10−3 m2 s−1 exists in the Luzon Strait and adjacent areas. South of 20° N, diffusivity is one order of magnitude smaller on average than it is near the Luzon Strait. Such observations suggest that other than internal tides, both Kuroshio intrusion and mesoscale eddy might be key factors to the enhanced mixing. Shear instability associated with Kuroshio intrusion plays a major role in strengthening the turbulent mixing near the Luzon Strait. In addition, warm eddies could reinforce the downward-propagating near-inertial waves and thus promote the occurrence of strong mixing, whereas cold eddies could reduce this downward-propagating, inhibiting strong mixing. This work facilitates the contributions of mesoscale eddy and Kuroshio intrusion in the northern SCS.

Near-inertial waves (NIWs), which can be generated by wind or the parametric subharmonic instability (PSI) of internal tides, are common in the South China Sea (SCS). Moored current observations from the northern SCS have revealed that the PSI of semidiurnal (D2) internal tides is another source of NIWs. The objective of this study was to examine the energy variance in the PSI of D2 tides. The PSI of D2 internal tides generated NIWs and waves with frequencies around the difference frequency of D2 and f. The observed NIWs induced by PSI could be distinguished clearly from those elicited by typhoon Krosa. Shortly after Krosa entered the SCS, NIWs began to intensify on the surface and they propagated downward over subsequent days. The near-inertial currents were damped quickly and they became relatively weak before the waves were reinforced beneath the mixed layer when wind stress was relatively weak. Rotation spectra indicated an energy peak at exactly the difference frequency D2-f of the NIWs and D2, indicating nonlinear wave-wave interaction among D2, f, and D2-f. Depth-time maps of band-pass filtered velocities of D2-f showed the waves amplified when the NIWs were reinforced, and they intensified at depths with strong D2 tides. The energies of the NIWs and D2-f had high correlation with the D2 tides. The PSI transferred energy of low-mode D2 internal tides to high-mode NIWs and D2-f waves. For the entire observational period, PSI reinforcement was observed only when mesoscale eddies emerged and when D2 was in spring tide, revealing a close connection between mesoscale eddies and NIWs. Mesoscale eddies could increase the energy in the f-band by enhancing the PSI of D2 internal tides. Thus, this represents another mechanism linking the energy of mesoscale eddies to that of NIWs.

Abstract. A practical scheme is proposed to explicitly introduce tides into ocean general circulation models (OGCM). In this scheme, barotropic linear response to the tidal forcing is calculated by the time differential equations modified for ocean tides, instead of the original barotropic equations of an OGCM. This allows for the usage of various parameterizations specified for tides, such as the self-attraction/loading (SAL) effect and energy dissipation due to internal tides, without unintentional violation of the original dynamical balances in an OGCM. Meanwhile, secondary nonlinear effects of tides, e.g., excitation of internal tides and advection by tidal currents, are fully represented within the framework of the original OGCM equations. That is, this scheme drives the OGCM by the barotropic linear tidal currents which are predicted progressively by a tuned tide model, instead of the equilibrium tide potential, without large additional numerical costs. We incorporated this scheme into Meteorological Research Institute Community Ocean Model and executed test experiments with a low-resolution global model. The results showed that the model can simulate both the non-tidal circulations and the tidal motion simultaneously. Owing to the usage of tidal parameterizations such as a SAL term, a root-mean-squared error in the tidal heights is found to be as small as 10.0 cm, which is comparable to that of elaborately tuned tide models. In addition, analysis of the speed and energy of the barotropic tidal currents is found to be consistent with that of past tide studies. The model also showed active excitement of internal tides and tidal mixing. In the future, the impacts of internal tides and tidal mixing should be examined using a model with a finer resolution, since explicit and precise introduction of tides into an OGCM is a significant step toward the improvement of ocean models.

A mooring system and two sites of bottom currents were deployed over the slope and near the shelf break on the propagating paths of internal solitary waves (ISWs), west off Dongsha Atoll in the northern South China Sea. Data indicated that energetic ISWs obliquely shoaled onto the shelf west off Dongsha Atoll in an approximately 290 degrees direction, causing strong reversing currents (some exceeding 80 cm/s) near the bottom. Two types of sandwaves and short scour channels are discernible on the seafloor near the shelf break, which have reasonable correlations with the obliquely incident ISWs and internal tides. Type 1 sandwaves, featured by ISWs at the depths of 130-150 m, have flat crests interacting with the isobaths at an angle of nearly 45 degrees which slightly incline and migrate upslope. Type 2 sandwaves are associated with internal tides, which have crests parallel to the isobaths and distinctly incline and migrate downslope. Short channels are parallel to the depth contours and truncate the strata, which could be formed and maintained by along-slope currents that are probably produced by the obliquely ISWs on a large gradient (gamma>0.8 degrees). The ISWs can move coarse grains or suspend fine grains but do not change the long-term trend of sediment transport on the seabed with larger gradients (gamma/c>1), which is dominated by internal tides. These features are likely widespread near the shelf break in the northern South China Sea and other seas but are limited on mild slopes where ISWs do not break.

Coherent and incoherent features, seasonal behaviors and spatial variations of internal tides in the northern South China Sea

The mode-2 internal solitary waves (ISWs) generated by mode-2 internal tide (IT) are identified by mooring observations in the northern South China Sea (SCS) from 2016 to 2017. Two mode-2 ISWs with a re-appearance period of 24.9 h observed on 29 and 30 July 2016 are characterized by type-b ISWs. They occurred when the isotherms compressed obviously in the vertical direction. Modal decomposition of IT horizontal currents shows that the vertical compression of the isotherms is mainly caused by diurnal mode-2 IT. The analysis of the role of the density stratification reveals that a deeper and thinner pycnocline is favorable for generation of mode-2 ISWs rather than pycnocline intensity. By comparing the mode-2 nonlinear, dispersion coefficients and the Ursell numbers calculated based on the stratification associated with different kinds of ITs with the observation results, it is shown that the diurnal mode-2 IT plays a crucial role in the generation of the mode-2 ISWs. When the diurnal mode-2 IT interacts with the semidiurnal IT and causes a deeper and thinner pycnocline, the mode-2 ISWs are easily excited.

Subtidal motions, such as large-scale circulations and mesoscale eddies, frequently occupy on the propagation path of internal waves in the ocean. Through solving a modified Taylor–Goldstein equation with subtidal currents, density stratification, and the earth rotation, this study investigates the impacts of subtidal motions and the earth rotation on the modal characteristics of the semidiurnal internal tide (SIT) based on long-term mooring measurements in the northern South China Sea (SCS). It is shown that the modal characteristics of the SIT are significantly influenced by the time-varying subtidal current and density stratification associated with energetic subtidal motions in the northern SCS. The earth rotation plays a minor role in modulating the modal characteristics of the SIT in the northern SCS, but it becomes effective at middle and high latitudes when strong subtidal current shear associated with subtidal motions is present. Moreover, the strong subtidal current associated with subtidal motions may significantly affect the critical latitude of the SIT. The observational results reported here will help to improve our understanding of the modal characteristics of internal tides under the influence of subtidal motions in global oceans.

An array of three bottom-mounted ADCP moorings was deployed on the prevailing propagation path of strong internal tides for nearly 1 year across the continental slope in the northern South China Sea. These velocity measurements are used to study the intra-annual variability of diurnal and semidiurnal internal tidal energy in the region. A numerical model, the Luzon Strait Ocean Nowcast/Forecast System developed at the U.S. Naval Research Laboratory that covers the northern South China Sea and the Kuroshio, is used to interpret the observed variation of internal tidal energy on the Dongsha slope. Internal tides are generated primarily at the two submarine ridges in the Luzon Strait. At the western ridge generation site, the westward energy flux of the diurnal internal tide is sensitive to the stratification and isopycnal slope associated with the Kuroshio. The horizontal shear at the Kuroshio front does not modify the propagation path of either diurnal or semidiurnal tides because the relative vorticity of the Kuroshio in Luzon Strait is not strong enough to increase the effective inertial frequency to the intrinsic frequency of the internal tides. The variation of internal tidal energy on the continental slope and Dongsha plateau can be attributed to the variation in tidal beam propagation in the northern South China Sea.

The variations in the internal tide (IT) and near‐inertial waves (NIWs) that occurred in the summer of 2006 over the continental shelf of the northern South China Sea were observed using three bottom‐mounted moorings equipped with acoustic Doppler current profilers. Due to the strong nonlinear interaction, large portions (~40–60%) of the semidiurnal IT energy at the continental shelf were found to be incoherent; this proportion gradually increased from the shelf break to the offshore region. More importantly, our observations showed that parametric subharmonic instability (PSI) can enhance the f‐band energy in the shallow water (~100 m) on the continental shelf. Using plane‐wave fitting and the bicoherence spectrum, we confirmed that the intensified NIWs and the observed D2‐f waves are nonlinear coupled waves derived from the semidiurnal internal tidal PSI. These two subharmonic waves were observed in the near field of a strong semidiurnal IT generation site at the shelf break. By tracking the M2‐ray path, we verified that the PSI occurred in the pycnocline in the path of the reflected wave beam that was characterized by strong nonlinear instabilities and it could be modulated by mesoscale eddies in this shallow shelf water.

Reflection of the K1 internal tides (ITs) in the northern South China Sea (SCS) is investigated using a high‐resolution three‐dimensional numerical model. The simulated results indicate that the westward K1 ITs generated at the Luzon Strait (LS) first impact the continental slope in the northern SCS, and thereafter are reflected. There is a total of 0.31 GW of K1 energy that are reflected, suggesting a weak reflection considering 3.54 GW of westward K1 energy from the LS. Two reflected K1 beams are revealed. The southeast one is the more energetic and can propagate across the deep basin, agreeing with that extracted from satellite altimeter data. The eastward one propagates toward the LS, but it is absent in altimeter results because of its lower intensity. Both the two reflected beams are radiated from the supercritical region in shallow water. Different from the incident waves that are dominated by mode‐1, the reflected waves exhibit a multimodal structure. Contribution of high modes to reflected waves is comparable to that of mode‐1. Moreover, modal content is different for the two reflected beams, with higher proportion of high‐mode energy flux for eastward reflected beam. In contrast to the K1 ITs, no remarkable reflected O1 ITs are detected from either simulated or altimeter results. This is due to the weak generation of O1 ITs at the LS, which leads to less O1 energy reaching the continental slope. The results of this work can provide essential insight on IT dynamics in the SCS.