Northeastern South China Sea Research Articles

Sedimentation in remnant ocean basin off SW Taiwan with implication for closing northeastern South China Sea

Taiwan is one of the most cited examples of modern mountain building in an oblique arc–continent collision. However, the recognition and significance of the Taiwan remnant ocean basin in the final stages of the collision of Luzon with the Chinese margin have long been ignored. This paper emphasizes the Taiwan remnant ocean basin as the main sink for sediments from the uplifted Taiwan suture zone and as a record of clues to the suturing history of the Taiwan collision zone. A part of the palaeo South China Sea has been closed by oblique collision of the Luzon Arc with the Chinese margin in the Taiwan region since about 4 Ma. As collision continued southward along the Taiwan–Luzon convergent zone the remaining South China Sea off SW Taiwan became the Taiwan remnant ocean basin. Beginning about 1.6 Ma much sediment from the longitudinal drainage of the Taiwan orogen was fed into this remaining basin and trapped in the intra-slope basins of the Kaoping slope, with some sediment bypassing via the Kaoping Canyon to the lower reach of the Penghu Canyon. This basin is characteristic of an axial canyon–deep-sea channel–oceanic trench system without significant sediment accumulation in submarine fans. Excess sediments from the Penghu Canyon mainly spill over out of the closing basin to the South China Sea and Manila Trench via the deep-sea Penghu Channel. The Taiwan–Luzon convergent zone continues along the NE South China Sea where the Taiwan remnant ocean basin exists. The implication of the convergence is that the Taiwan remnant ocean basin will keep shrinking in the next few million years southward to the northern end of the Manila Trench at around 20°N, where collision is replaced by subduction. The remnant ocean basin would be closed and deformed, and sediments would be accreted to uplifted collisional terranes north of the Manila Trench.

The northeastern South China Sea margin created by the combined action of down-slope and along-slope processes: Processes, products and implications for exploration and paleoceanography

Processes and products associated with the interplay of down-slope and along-slope processes are geographically widespread and yet poorly documented. Using a high-quality 2D database consisting of bathymetry, 2D seismic, piston cores, faunal, and radiocarbon data, six major depositional systems are recognized along the northeastern South China Sea margin, and from the upper slope to abyssal plain, they are erosional features, mass-flow systems, sediment gravity-flow (SGF) systems, mixed contourite-SGF systems, contourite systems and hemipelagic systems.Sedimentary processes on the studied margin show considerable spatial complexity, yielding a depositional model that must incorporate the interplay of down-slope and along-slope processes. Erosional features and mass-flow systems are common on upper and middle slopes, respectively, where high mass flows probably dominate over bottom currents. In the lower slope and continental rise where mass flows transform into SGFs, yielding SGF system. In the lower segment of the Taiwan canyon, there is a strong interplay of SGFs and bottom currents, forming the mixed contourite-SGF system. On the abyssal plain where SGFs are volumetrically overwhelmed by contour currents, contourite depositional systems are well developed. Our results highlight the complex interaction between down-slope and along-slope processes on continental margins, thus helping to better understand the deep-water sedimentation.Bottom-current reworked sands lacking ‘typical turbidite signatures’ are recognized, and are interpreted to be created by the interplay of SGF and bottom currents (including contour currents, bottom currents associated with internal solitary waves and deep-marine tidal bottom currents), which could potentially yield excellent hydrocarbon reservoirs after burial, affording an alternative for interpreting deep-marine non-turbidite reservoirs. Fine-grained bottom-current sediment waves are first recognized in the abyssal plain of the South China Sea and are interpreted to be created by contour currents resulting from the North Pacific Deep Water (NPDW–CCs), providing solid evidence for the intrusion of NPDW into the abyssal plain of the South China Sea. Preliminary bedform-velocity analysis suggests that NPDW–CCs have a maximum flow velocity up to 3～7 cm/s.

Morphology, sedimentary characteristics, and origin of the Dongsha submarine canyon in the northeastern continental slope of the South China Sea

The Dongsha submarine canyon is a large canyon belonging to a group of canyons on the northeastern South China Sea margin. Investigation of the Dongsha canyon is important for understanding the origin of this canyon group as well as the transport mechanism of sediments on the margin, and the evolution of the Taixinan foreland basin and the associated Taiwan orogenic belt. In this study, the morphology, sedimentary characteristics, and origin of the Dongsha canyon were investigated by integrating high-resolution multi-channel seismic reflection profiles and high-precision multibeam bathymetric data. This is a slope-confined canyon that originates in the upper slope east of the Dongsha Islands, extends downslope in the SEE direction, and finally merges with the South Taiwan Shoal canyon at a water depth of 3000 m. The total length and average width of the canyon are around 190 and 10 km, respectively. Eleven seismic sequence boundaries within the canyon fills were identified and interpreted as incision surfaces of the canyon. In the canyon fills, four types of seismic facies were defined: parallel onlap fill, chaotic fill, mounded divergent facies, and migrated wavy facies. The parallel onlap fill facies is interpreted as alternating coarser turbidites or other gravity-flow deposits and fine hemipelagic sediments filling the canyon valley. The chaotic fill facies is presumed to be debrites and/or basal lag deposits filling the thalwegs. The mounded divergent and migrated wavy seismic facies can be explained as canyon levees consisting mainly of overspilled fine turbidites and sediment waves on the levees or on the canyon-mouth submarine fans. Age correlation between the sequence boundaries and the ODP Site 1144 data suggests that the Dongsha canyon was initiated at approximately 0.9 Ma in the middle Pleistocene. Mapping of the canyon indicates that the canyon originated at the upstream portion of the middle reach of the modern canyon, and has been continuously expanding both upstream and downstream by retrogressive erosion, incision, and deposition of turbidity currents and other gravity transport processes. The ages of the sequence boundaries representing major canyon incision events are in good agreement with those of global sea-level lowstands, indicating that sea-level changes may have played an important role in the canyon’s development. The Dongsha canyon developed in a region with an active tectonic background characterized by the Taiwan uplifting and the development of the Taixinan foreland basin. However, no evidence suggests that the canyon formation is directly associated with local or regional faulting and magmatic activities. Turbidity currents and other gravity transport processes (including submarine slides and slumps) may have had an important influence on the formation and evolution of the canyon.

Ages and magnetic structures of the South China Sea constrained by deep tow magnetic surveys and IODP Expedition 349

AbstractCombined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1–2 Myr along the northern continent‐ocean boundary (COB). A southward ridge jump of ∼20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from ∼23.6 to ∼21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80 km/Myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70 km/Myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late‐stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100 m of the igneous basement.

South China Sea upper-slope sand dunes acoustics experiment

Very large subaqueous sand dunes were discovered on the upper continental slope of the Northeastern South China Sea. The spatial distribution and scales of these large sand dunes were mapped by two multibeam echo sounding (MBES) surveys, one in 2012 and the other in 2013. These two surveys represented two pilot cruises as part of a multiyear, US-Taiwan collaborative field study designed to characterize these sand dunes, the associated physical processes and the associated acoustic scattering physics. The main experiment will be carried out in 2014. The combination of MBES, coring and acoustic transmission data obtained from the two pilot cruises have provided vital initial knowledge of (1) the spatial and temporal scales of the sand dunes from objective analysis, (2) the geoacoustic properties of the dunes based on forward modeling to matching the measured levels, and (3) the anisotropy and translational variability of the transmission loss based on a signal energy analysis of the repeated 1–2 kHz and 4–6 kHz FM signals transmitted by a calibrated sound source towed along two circular tracks, each surrounding a receiver. The results from the pilot cruises are presented and discussed. [The research is sponsored by the US ONR and the Taiwan NSC.]

Water mass transport through the northern Bashi Channel in the northeastern South China Sea affects copepod assemblages of the Luzon Strait

Abstract Background It is controversial to what extent the intrusion of the Kuroshio Branch Current to the Luzon Strait and the South China Sea circulation can shape copepod assemblages around southern Taiwan. We tested the hypothesis that currents such as the Kuroshio Current bring marine zooplankton and copepods from subtropical and tropical waters to the south of Taiwan. We studied copepod assemblages from the Bashi Channel in the northeastern South China Sea at the coast of southwest Taiwan during early October 1996. Results A total of 77 copepod species were identified from 34 genera that included calanoids, cyclopoids, harpacticoids, and poecilostomatoids. Several indicator species suggest that the study area is highly influenced by water masses from the northern South China Sea as well as from the Kuroshio Current. Acrocalanus gracilis (Paracalanidae, Calanoida) was most abundant (with a relative abundance (RA) of 22.07 and an occurrence rate (OR) of 100%), followed by Paracalanus aculeatus (Paracalanidae, Calanoida) and Oncaea venusta (Cyclopoida). The stations close to the Kuroshio Current showed a higher species diversity (H') and a higher species richness with 3.9 to 4.6 at moderate abundance, whereas station 11 showed lowest species diversity (H') with (2.1), accompanied by the far lowest species number (14) and abundance (880 ind./100 m3). Conclusions Several indicator species suggest that the study area is highly influenced by water masses from the northern South China Sea as well as from the Kuroshio Current.

Simulation of export production and biological pump structure in the South China Sea

The export flux of particulate organic carbon (POC) consumes upwelled dissolved inorganic carbon (DIC), which hinders surplus CO2 being released to the atmosphere. The export flux of POC is therefore crucial to the carbon and biogeochemical cycles. This study aims to model the long-term (1958–2009) variation of export flux and structure of the biological pump in the South China Sea (SCS) using a three-dimensional physical-biogeochemical coupled (ROMS-CoSiNE) model. The modeled POC export flux in the northeastern and north central SCS is high in winter and low in summer, whereas the flux in the central, southwestern and southern SCS varies following a “W” shape: two maxima in winter and summer, and two minima in spring and autumn. The pattern follows the variation of the East Asian monsoon and is consistent with observations. On the interannual scale, export flux is anti-phased with the El Nino-Southern Oscillation such that El Nino (La Nina) conditions correspond to low (high) export flux. Modeled annual mean POC export flux reaches up to 1.95 mmol m–2 day–1, which is underestimated comparing with field observations. The f-ratio is estimated to be ~0.4. The b value of the Martin equation for POC is 1.18±0.03. Remineralization rate of POC is greater than the classical Martin equation but is consistent with its subtropical counterparts. The modeled results indicate that the SCS is a weak source of atmospheric CO2 with a flux estimated at 1.0 mmol m–2 day–1. The modeled results provide an insight of the temporal and spatial variability of the carbon cycle in this monsoon-driven, semi-enclosed basin.

Reply to Chang et al., 2014, Evolution of the South China Sea: Revised ages for breakup and seafloor spreading

Chang et al. introduce some new lines of evidence for an end of seafloor spreading in the South China Sea not before 15 Ma. However, we find strong indications that their interpretation is not conclusive. The origin of the East Taiwan Ophiolite from the SCS as assumed by Chang et al. is not in agreement with recent studies which find continental crust in the deep-water domain of the northeastern South China Sea. The age dating of the East Taiwan Ophiolite is based on one single K–Ar age of questionable accuracy. The calculation of Te-derived ages leaves a thermal rejuvenation of the oceanic crust in the South China Sea by post-spreading volcanism out of consideration. Biostratigraphically determined ages of sediments recovered directly above igneous crust of the South China Sea during IODP 349 drilling expedition are inconclusive so far. The final key to unlocking the spreading history of the SCS will be the Ar–Ar age dating of the basaltic rocks recovered during IODP Expedition 349.

Size-based analysis of a zooplankton community under the influence of the Pearl River plume and coastal upwelling in the northeastern South China Sea

To examine zooplankton responses to the Pearl River plume (PRP) and coastal upwelling on the northern shelf of the South China Sea (SCS), zooplankton (>160 µm) were investigated in the northeastern SCS during 30 June to 11 July 2008. Zooplankton biomass size spectra (BSS) were constructed to describe zooplankton communities. The BSS parameters such as intercept and slope values were 8.659 ± 0.727 and −0.816 ± 0.074, respectively. On the basis of the BSS parameters, the sampling sites were classified into two main groups, which corresponded to the plume- and upwelling-affected areas, respectively. In addition, a positive correlation between zooplankton and phytoplankton abundance (P < 0.05) was observed in the study waters. These results indicate that (1) zooplankton BSS could be used as an indicator for distinguishing the physical processes, and (2) the PRP and coastal upwelling could regulate zooplankton communities differently through altering nutrient supply for the phytoplankton. The BSS slopes were flatter than expected for a steady-state community and the dominance of jellyfish in the larger size fractions indicated that an energy accumulation in gelatinous organisms occurred in the study waters. Since gelatinous organisms are not food for fish, this energy accumulation may be detrimental to the fishery production in the study area.

Crustal structure and inferred rifting processes in the northeast South China Sea

TAIGER project deep-penetration seismic reflection profiles acquired in the northeastern South China Sea (SCS) provide a detailed view of the crustal structure of a very wide rifted continental margin. These profiles document a failed rift zone proximal to the shelf, a zone of thicker crust 150 km from the shelf, and gradually thinning crust toward the COB, spanning a total distance of 250–300 km. Such an expanse of extended continental crust is not unique but it is uncommon for continental margins. We use the high-quality images from this data set to identify the styles of upper and lower crustal structure and how they have thinned in response to extension and, in turn, what rheological variations are predicted that allow for protracted crustal extension. Upper crustal thinning is greatest at the failed rift (βuc ≈ 7.5) but is limited farther seaward (βuc ≈ 1–2). We interpret that the lower crust has discordantly thinned from an original 15–17 km to possibly less than 2–3 km thick beneath the central thick crust zone and more distal areas. This extreme lower crustal thinning indicates that it acted as a weak layer allowing decoupling between the upper crust and the mantle lithosphere. The observed upper crustal thickness variations and implied rheology (lower crustal flow) are consistent with large-scale boudinage of continental crust during protracted extension.

Rifting and magmatism in the northeastern South China Sea from wide‐angle tomography and seismic reflection imaging

AbstractWe present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern South China Sea. These data reveal moderately stretched crust ~25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin. Along the continental slope, crust rapidly thins to ~4 km in a basin characterized by tilted fault blocks that sole into a low‐angle detachment. Strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading. Crust in the distal margin is ~12–15 km thick. Few extensional structures are apparent in the distal margin, but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust. The magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata, sills intruded into the postrift sedimentary section, and a high‐velocity (~6.9–7.5 km/s) lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions. These features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading. The postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the South China Sea.

A modeling study of the formation, maintenance, and relaxation of upwelling circulation on the Northeastern South China Sea shelf

We investigated persistent summer upwelling circulation, in response to upwelling and downwelling favorable winds, in the Northeastern South China Sea (NSCS). We used a validated three-dimensional ocean circulation model that was forced by realistic atmospheric fluxes and downscaled coupling of real-time lateral fluxes. We found that upwelling in the NSCS was formed and maintained by the presence of an intensified westward along-isobath pressure gradient force (PGFm) and bottom frictional effect that led to cross-isobath currents over the unique NSCS widened shelf. The upwelling favorable PGFm and the frictional effect arose from the interaction between the eastward shelf current and the shelf topography. These intrinsic upwelling dynamics in the NSCS were largely sustained during episodic downwelling winds in the upwelling season because the retreat of the eastward shelf current from pre-existing upwelling was significantly slowed by the unique widened shelf topography. Furthermore, the upwelling dynamics could also be maintained and, even developed, during downwelling favorable winds, when the eastward shelf current was sustained by the pumping of downstream outflow.

Water mass transport through the northern Bashi Channel in the northeastern South China Sea affects copepod assemblages of the Luzon Strait

Water mass transport through the northern Bashi Channel in the northeastern South China Sea affects copepod assemblages of the Luzon Strait

Inter-shelf nutrient transport from the East China Sea as a major nutrient source supporting winter primary production on the northeast South China Sea shelf

Abstract. The East China Sea (ECS) and the South China Sea (SCS) are two major marginal seas of the North Pacific with distinct seasonal variations of primary productivity. Based upon field observations covering both the ECS and the northern SCS (NSCS) during December 2008–January 2009, we examined southward long-range transport of nutrients from the ECS to the northeastern SCS (NESCS) carried by the China Coastal Current (CCC) driven by the prevailing northeast monsoon in wintertime. These escaped nutrients from the ECS shelf, where primary production (PP) was limited in winter, might however refuel the PP on the NESCS shelf at lower latitude, where the water temperature remained favorable, but river-sourced nutrients were limited. By combining the field observation of nitrate+nitrite (NO3+NO2, DIN) with our best estimate of volume transport of the CCC, we derived a first-order estimate for DIN flux of 1430 ± 1024 mol s−1. Under the assumption that DIN was the limiting nutrient, such southward DIN transport would have stimulated 8.84 ± 6.33 × 1011 gC of new production (NP), accounting for 33–74% of the NP or 14–22% of PP in winter on the NESCS shelf shallower than 100 m.

Effects of Kuroshio intrusions on nonlinear internal waves in the South China Sea during winter

During winter the Kuroshio tends to cross Luzon Strait, penetrating the northeastern South China Sea where it forms energetic mesoscale structures. Luzon Strait is also a site where westward‐propagating large‐amplitude internal waves are generated. We describe observations of these waves acquired in the deep basin of the South China Sea during the winter of 2010−2011, with the goal of assessing the influence of mesoscale variability on their properties. Combining tidal current simulations with an internal wave generation and evolution model, we obtain time series of deviations between our observations and the model simulations. These deviations are analyzed in terms of mesoscale variability based on the data‐assimilated HYbrid Coordinate Ocean Model (HYCOM) simulations. We find that simplified models of nonlinear internal wave response to changes in horizontal stratification gradients and vertical shear provide at best weak simulations of amplitude modulation. In contrast to these results, deviations of internal wave arrival time, occurring up to 2 h early during Kuroshio inflows, are quite well simulated when derived from integration of the first internal mode phase speed along two‐dimensional ray‐traced paths using HYCOM simulations of velocity and density fields together with bathymetry. Refraction of the internal waves by Kuroshio intrusions can lead to substantial distortion of the paths, sufficient to provide a potential explanation for the apparent suppression of waves during previous winter measurements. Our results suggest that the internal wave field can be a sensitive indicator of mesoscale variability in Luzon Strait and the northeastern South China Sea.

A mesoscale eddy pair southwest of Taiwan and its influence on deep circulation

A pair of mesoscale eddies, namely, an anticyclonic eddy (AE) and a cyclonic eddy (CE), generated southwest of Taiwan was investigated using long‐term moored observations augmented with satellite and reanalysis data. AE (CE) increased the upper‐ocean current to 100 cm/s (50 cm/s) and generated temperature anomaly (T′) up to 7.5°C (−3.0°C). The vertical structure of AE presented a phase difference between T′ and velocity, with the core of T′ located south of the zero‐velocity line. This phase difference induced a −28.5 ± 12.0 TW (1 TW = 1012 W) westward eddy heat transport southwest of Taiwan. AE also presented strong nonlinearity in the upper ocean and yielded an annual mean volume transport of 0.86 ± 0.15 Sv. Energy analysis showed that both baroclinic and barotropic instabilities were important for the generation and growth of the eddy pair, and contribution from wind work was relative small. In the deep sea, AE (CE) intensified the deep currents and generated large positive (negative) T′. The deep and surface currents flowed in opposite directions during the eddy events, which is consistent with the interpretation that the eddy pair is dominated by the first baroclinic mode. The observed deep‐penetrating eddy pair could greatly influence the deep circulation of the northeastern South China Sea.

Copepod assemblages in the northern South China Sea during inter-monsoon transition periods

We investigated the influence of permanent oceanographic features in structuring copepod assemblages in the northern South China Sea during the inter-monsoon transition periods, spring and autumn. A total of 25 families, 48 genera and 88 species, were recorded, as well as a decrease in species richness along with the seasonal temperature decrease. We show that copepod assemblages are influenced by quasi-permanent oceanographic conditions governing the Northeastern South China Sea, i.e. China Coastal Current and the Kuroshio Current intrusion. This study provides a synoptic picture of the seasonal changes in the community structure of copepods during spring and autumn in the northern South China Sea.

Segmentation of the Manila subduction system from migrated multichannel seismics and wedge taper analysis

Based on bathymetric data and multichannel seismic data, the Manila subduction system is divided into three segments, the North Luzon segment, the seamount chain segment and the West Luzon segment starts in Southwest Taiwan and runs as far as Mindoro. The volume variations of the accretionary prism, the forearc slope angle, taper angle variations support the segmentation of the Manila subduction system. The accretionary prism is composed of the outer wedge and the inner wedge separated by the slope break. The backstop structure and a 0.5–1 km thick subduction channel are interpreted in the seismic Line 973 located in the northeastern South China Sea. The clear decollement horizon reveals the oceanic sediment has been subducted beneath the accretionary prism. A number of splay faults occur in the active outer wedge. Taper angles vary from 8.0° ± 1° in the North Luzon segment, 9.9° ± 1° in the seamount segment to 11° ± 1° in the West Luzon segment. Based on variations between the taper angle and orthogonal convergence rates in the world continental margins and comparison between our results and the global compilation, different segments of the Manila subduction system fit well the global pattern. It suggests that subduction accretion dominates the north Luzon and seamount chain segment, but the steep slope indicates in the West Luzon segment and implies that tectonic erosion could dominate the West Luzon segment.

Study of the propagation direction of the internal waves in the South China Sea using satellite images

Internal wave propagation carries considerable vertical shear which can lead to turbulence and mixing. Based on the analysis of more than 2 500 synthetic aperture radar (SAR) and optical satellite images, the internal wave propagation in the whole South China Sea was investigated systematically. The results show that (1) in the northeastern South China Sea, most internal waves propagate westward from the Luzon Strait and are diffracted by coral reefs near the Dongsha Islands. Some impinge onto the shelf and a few are reflected; (2) in the northwestern South China Sea, most internal waves are generated at the shelf and propagate northwestward or westward to the coast; (3) in the western South China Sea, most internal waves propagate westward to the Vietnamese coast, except a few propagate southward to the deep sea; and (4) in the southern South China Sea, most internal waves propagate southwestward to the coast. Some propagate southeastward to the coast of Kalimantan Island, and a few propagate southeastward because of the influence of the Mekong River.

Observations of enhanced nonlinear instability in the surface reflection of internal tides

Enhanced vertically standing waves formed by the superposition of two upward and downward going near‐diurnal (D1) waves are observed during one semidiurnal (D2) spring tide in an approximately 75 day long velocity record from the northeastern South China Sea. Bicoherence estimates suggest that the enhanced D1 waves are likely due to nonlinear parametric subharmonic instability of D2 internal tides. The timescale for energy growth by an order of magnitude is about 2.5 days for these waves. In addition to subharmonics, higher harmonics D4 (=D2 + D2) and a mean flow are generated by a different nonlinear interaction during the same D2 spring tide. The separation of coherent from incoherent internal tidal signals and a rotary spectral decomposition in the vertical direction reveal that D2 waves with opposite vertical propagation directions in the region of internal tide reflection from the surface may be responsible for the pronounced nonlinear instability.