Variability of coherent and incoherent features of internal tides in the north South China Sea

Abstract

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. Internal tides (ITs) are commonly generated in stratified oceans by barotropic tidal currents flowing over seamounts, ridges and continental shelf breaks 1-4. As an essential intermediate step of tide-to-turbulence cascade, ITs play important roles in dissipating surface tidal energy and enhancing mixing, which contribute to deep-water circulation 5-10. Munk found that the breaking of ITs provides nearly half of the energy necessary to maintain the global meridional overturning circulation 11. The Luzon Strait (LS), which is featured by two north-south oriented ridges, connects the western Pacific and South China Sea (SCS). At the LS, strong diurnal and semidiurnal ITs are generated when barotropic tides flow over the double ridges 12. ITs propagate westward into the SCS, making it a region with strong ITs 13-15. Wang et al. found that the ITs radiating from the LS dominate the tidal dissipation in the SCS. Without the ITs generated at the LS, the dissipation in the SCS will be at least one order of magnitude smaller 16. Coherent ITs are phase-locked with barotropic tides at the generation site. Variability of coherent ITs is primarily explained by spring-neap cycles in barotropic tides. During their propagation, incoherence grows and ITs lose coherence to surface tides 3,4,17,18. In the Bay of Biscay, incoherent semidiurnal signals explain 30% of the total motions 19. Eich et al. attributed the growing of incoherent semidiurnal ITs in the Mamala Bay to the influences of both variable stratification and mesoscale motions 17. Many scale...

Modulation of Internal Tides by Turbulent Mixing in the South China Sea

“…Distributions of density anomaly at the end of simulation duration at 100 m depth are shown in Figure 8. The horizontal variations of density can lead to the incoherence of ITs (Li et al, 2020). Differences of density anomaly emerge when turbulent mixing changes, which further influence the incoherence of ITs.…”

“…Moreover, background currents and mesoscale eddies are thought as essential factors, which modulate the incoherent feature of ITs. The mesoscale eddies can increase the intensity of background currents and induce horizontal variations of density, which would eventually add incoherence to ITs (Li et al, 2020). But, the relationship between incoherency of ITs and turbulent mixing has rarely been discussed.…”

Summertime M2 Internal Tides in the Northern Yellow Sea

“…According to Nash et al (2012), a broadband internal tide can be decomposed into two components, (1) the coherent component, which is largely predictable and can be described by a series of tidal frequency sinusoids, and (2) the incoherent component, which is largely unpredictable and associated with intermittent pulses of tidal-band energy that are arriving with different phases and amplitudes. The coherent component of the internal tides is often applied as a parameter to estimate the contribution of local generation from the barotropic tides (Nash et al, 2012;Kumar et al, 2019;Li et al, 2020).…”

Internal Tide Cycle and Topographic Scattering Over the North Mid‐Atlantic Ridge

“…The major mechanisms include (Eden & Olbers, 2014; Müller et al., 1986; Polzin & Lvov, 2011): (1) interaction with (sub)‐mesoscale ocean turbulence (Duda et al., 2018; Dunphy et al., 2017; Kafiabad et al., 2019; Savva & Vanneste, 2018), (2) scattering by topographic corrugations (Baines, 1971a; Kelly et al., 2013; Müller & Xu, 1992), and (3) nonlinear wave‐wave interactions such as Parametric Subharmonic Instability (e.g., McComas & Bretherton, 1977; MacKinnon et al., 2012; D. J. Olbers, 1976; D. Olbers et al., 2020; Onuki & Hibiya, 2018). As low mode internal tides (typically the first and, to a weaker extent, the second baroclinic modes) can propagate long distances, they lose coherence (which is associated with departure from an exactly repeating signal) through the above‐mentioned processes (Buijsman et al., 2017; B. Li et al., 2020; Nelson et al., 2019) and, in addition, they have a remote impact in the ocean. This greatly complicates the spatial distribution of the internal tide field and its impact in the global ocean, and in particular the distribution of the associated diapycnal mixing (Melet et al., 2016).…”

“…to a weaker extent, the second baroclinic modes) can propagate long distances, they lose coherence (which is associated with departure from an exactly repeating signal) through the above-mentioned processes (Buijsman et al, 2017;B. Li et al, 2020;Nelson et al, 2019) and, in addition, they have a remote impact in the ocean.…”