Dong, Yang, Li and Li (2016) reported the first satellite evidence of eddy-induced mode-2 internal solitary waves (ISWs) in the South China Sea (SCS), associated with an anticyclonic mesoscale eddy, and proposed two generation processes: eddy-driven modification of stratification, and resonant interaction between mode-1 internal tides and the eddy that excites mode-2 internal tides which then disintegrate into mode-2 ISWs. They left open several linked issues: the dynamical conditions and parameter ranges under which eddy-induced resonant excitation actually produces mode-2 ISWs; whether mode-2 ISWs generated by different mechanisms produce distinguishable synthetic-aperture-radar (SAR) signatures; the occurrence statistics of mode-2 ISWs in the SCS; and their implications for upper-ocean mixing, material transport, and acoustic propagation. Later work has partially advanced individual pieces (one numerical study of anticyclonic-eddy/semidiurnal-internal-tide interaction, mooring observations of eddy-modulated internal tides, and a 342-event optical-satellite census of mode-2 ISWs), but no study has simulated or observationally closed the eddy-resonance-to-mode-2-ISW chain, determined its parameter ranges, or demonstrated mechanism-level discrimination from imagery, so the problem remains open. The research target is to resolve these four linked issues: a parameter-range characterization and an April 2001 attribution test for (1); a mechanism-distinguishability finding, positive or negative, for (2); occurrence statistics for (3); and a quantified impact assessment for (4).

Background

The ocean is density-stratified, and disturbances can propagate along lines of constant density as internal waves. When tide-driven currents flow over rough topography, they generate internal tides, whose steepening and nonlinear evolution can produce internal solitary waves (ISWs): localized, rapidly propagating packets of large-amplitude internal-wave displacement. Internal waves are organized in vertical modes: mode-1 displaces the water column with a single vertical structure peaked at the pycnocline (the subsurface density-gradient maximum), whereas mode-2 has one internal node and concentrates its displacement within the pycnocline itself. Mode-2 ISWs are therefore shorter, slower, and confined to the pycnocline, which makes them much harder to observe. Known mode-2 generation mechanisms include shoaling and modal conversion of mode-1 ISWs on slopes, local tide-topography generation where sills cut the pycnocline, disintegration of mode-2 internal tides, and scattering by topography. Internal waves in the SCS are dominated by mode-1 ISWs radiated westward from the Luzon Strait, where strong barotropic tides cross two parallel ridges; earlier field measurements had also documented winter mode-2 ISWs on the upper continental slope of the northern SCS, attributed to a shoaling mode-2 internal tide (as analyzed by Liang and Li, 2019). ISWs are widely mapped by synthetic-aperture radar (SAR): the waves' surface currents modulate centimeter-scale wind waves (the roughness scale to which SAR is sensitive), so the waves imprint as bright/dark bands even though the waves themselves are subsurface. Mesoscale eddies are roughly 100-km-scale swirling vortices that persist for weeks to months; an anticyclonic eddy (AE) rotates clockwise in the Northern Hemisphere and depresses (deepens) the thermocline, altering the stratification and background currents through which internal waves propagate; a cyclonic eddy (CE) is its counterclockwise-rotating counterpart, which raises (shoals) the thermocline. Before 2016, eddy-ISW interaction had been studied mainly through regional simulations of mode-1 ISWs (e.g., Xie, He, Chen, Xu and Cai, 2015, who simulated ISWs encountering mesoscale eddies in the northeastern SCS). Dong, Yang, Li and Li (2016) analyzed two SAR images from April 2001 in the SCS showing ISW patterns whose morphology indicated mode-2 structure, coinciding with an AE documented in sea-level-anomaly and sea-surface-temperature data; one month later the same area showed regular mode-1 ISWs. Using Argo profiles (vertical temperature-and-salinity profiles from Argo, a global array of autonomous profiling floats) inside SCS anticyclonic eddies over 1990-2014, they argued that the eddy's modification of vertical stratification created favorable conditions for mode-2 ISW generation, and proposed that resonance between the mode-1 internal tide and the AE excites mode-2 internal tides that disintegrate into mode-2 ISWs. In this context, eddy-tide resonance denotes the condition under which the eddy-modified stratification and background flow bring a mode-1 internal tide and a mode-2 internal tide into eigenmode coincidence - for example, a Doppler-shifted mode-1 tidal frequency matching an eddy-modified mode-2 eigenfrequency, as when the eddy's horizontal scale is comparable to that of the mode-1 internal tide - so that energy is transferred efficiently from mode 1 to mode 2. This was the first reported evidence of eddy-induced mode-2 ISWs in the SCS, but it rested on two image pairs and inference from climatological stratification, not on process modeling or in situ measurement of the waves themselves.

Problem Statement

Building on the SAR observations and proposed generation processes of Dong, Yang, Li and Li (2016) for the South China Sea, and still unresolved: determine how mode-2 internal solitary waves are generated in the SCS and what their observable and environmental consequences are, with the following linked components. (1) Quantify, under realistic SCS stratification and bathymetry, how mesoscale eddies (both anticyclonic and cyclonic) interact with mode-1 internal tides and internal solitary waves, and determine the parameter ranges (eddy scale and strength, stratification anomaly, tidal amplitude, background currents) within which the resonant excitation of mode-2 internal tides and their subsequent disintegration into mode-2 ISWs occur - including whether this pathway explains the April 2001 eddy-coincident mode-2 ISW observations. (2) Establish the relationship between mode-2 generation mechanisms (shoaling-induced conversion, eddy-resonance-driven excitation, bathymetric scattering, impinging tidal beams) and their two-dimensional SAR signature morphologies: specifically, determine whether or not mode-2 ISWs produced by different mechanisms produce distinguishable SAR patterns that allow mechanistic discrimination from imagery alone. (3) Establish the occurrence statistics of mode-2 ISWs in the SCS - incidence rates, amplitude distributions, and the role of tidal phase (spring versus neap, the strong and weak phases of the fortnightly tidal cycle) in whether mode-2 events are imaged. (4) Quantify the implications of large-amplitude mode-2 ISWs for upper-ocean mixing, material transport, and acoustic propagation in the SCS.

References

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