Latitudinal Variation and Nonlinear Behavior of Internal Tides in the East China Sea (original) (raw)
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Parametric Subharmonic Instability of the Internal Tide at 29°N
Journal of Physical Oceanography, 2013
Observational evidence is presented for transfer of energy from the internal tide to near-inertial motions near 29°N in the Pacific Ocean. The transfer is accomplished via parametric subharmonic instability (PSI), which involves interaction between a primary wave (the internal tide in this case) and two smaller-scale waves of nearly half the frequency. The internal tide at this location is a complex superposition of a low-mode waves propagating north from Hawaii and higher-mode waves generated at local seamounts, making application of PSI theory challenging. Nevertheless, a statistically significant phase locking is documented between the internal tide and upward- and downward-propagating near-inertial waves. The phase between those three waves is consistent with that expected from PSI theory. Calculated energy transfer rates from the tide to near-inertial motions are modest, consistent with local dissipation rate estimates. The conclusion is that while PSI does befall the tide near...
The variability of internal tides in the Northern South China Sea
Journal of Oceanography, 2013
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.
Temporal variability of internal tides in the northeast South China Sea
Journal of Geophysical Research, 2012
1] The northeast South China Sea is perhaps the largest internal tide energy source in the world. The temporal variability of internal tides was investigated on the basis of 8-month moored acoustic Doppler current profiler observations on the continental slope at the Dongsha Plateau. The daily amplitude and phase of diurnal and semidiurnal internal tides were determined from complex demodulation, and the dominant spatial and temporal patterns were extracted with empirical orthogonal function (EOF) analysis. The EOF modal structures showed good agreement with the normal mode solution, although vertical phase propagations were apparent. The first semidiurnal EOF mode corresponded to the first normal mode, and the first two diurnal EOF modes corresponded to the second and third normal modes, respectively. The modal structure and energy flux also were consistent with previous observations near the shelf break from the Asian Seas International Acoustic Experiment. On the other hand, the amplitudes of diurnal and semidiurnal EOF modes both indicated large irregular fortnightly variations that were not phase locked (incoherent) with astronomical forcing. The study highlighted the importance of incoherent internal tidal motion, which accounted for about three fourths of the observed tidal energy.
Mooring ADCP current observations from August to November are used to study the barotropic tides, baroclinic tides and near-inertial motions in the upper 450 m layer of the northern South China Sea. The barotropic and baroclinic tides at the mooring station are all dominated by M 2 , K 1 , O 1 , P 1 , with the barotropic amplitudes being 7.8 cm·s −1 , 7.0 cm·s −1 , 5.4 cm·s −1 and 3.5 cm·s −1 respectively. The amplitudes of M 2 , K 1 , O 1 , P 1 internal tides vary greatly, which are 12 15 cm·s −1 in the thermocline, and then decrease with increasing depth. The amplitude of the barotropic near-inertial motions is less than 1 cm·s −1 , contributing little to the barotropic currents. However, that of the baroclinic nearinertial motions can be as large as 5 cm·s −1 . The inclination of tidal ellipse tends to increase with increasing depth, implying upward propagation of energy, while that of the near-inertial ellipse tends to decrease with increasing depth, implying downward propagation of energy.
Distinct Variability between Semidiurnal and Diurnal Internal Tides at the East China Sea Shelf
Remote Sensing
Breaking internal tides and induced mixing are critical to shelf dynamics, including heat and mass exchanges. Spatiotemporal variability of internal tides and modulation factors for the southern East China Sea shelf were examined based on a combination of a three-month mooring velocity and satellite altimeter data. Semidiurnal and diurnal internal tides exhibited distinct temporal trends, with the semidiurnal internal tides enhanced by an order of magnitude during the latter half of the record, while the diurnal internal tides followed quasi spring-neap cycles with a generally stable intensity except for two specific periods of strengthening. These internal tides probably originated remotely over the shelf-slope area northeast of Taiwan. Time-varying stratification was the most important factor for the internal tidal magnitude. In addition, varying background currents influenced the diurnal critical latitude band, which explains the slightly enhanced diurnal internal tides during th...
Episodes of nonlinear internal waves in the northern East China Sea
Geophysical Research Letters, 2006
Episodes of high-frequency internal waves, which lasted approximately 3 hours, were detected in the northern East China Sea during a specific phase of the barotropic tide (i.e., low tide at 32°N, 125°E). The observed internal waves influenced the whole water column. The wave packets were presumably generated near the ocean shelf break, approximately 200 km to the southeast of the test site. During several internal wave episodes, which coincided with the neap tide, large-amplitude solitary wave-like features emerged preceding higher frequency internal waves. Shear instability of tidal currents is explored as a possible mechanism for sustaining or regenerating internal waves in the packets during the course of their propagation. It is suggested that rotating velocity field of tidal current supports sufficient vertical shear within wave packets to cause outbreaks of K-H instability. These instabilities may gradually transition to more symmetric Hölmböe waves, following the increase of the bulk Richardson number.
IEEE Journal of Oceanic Engineering, 2004
A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.
Significant internal waves and internal tides measured northeast of Taiwan
Journal of Marine Research, 2013
Internal gravity waves in an area northeast of Taiwan are characterized using data from multiple sensor types. The data set includes intermittent information collected from a ship and short time series from moorings. Modeled nonlinear waves are fitted to observed nonlinear waves to provide selfconsistent estimates of multiple wave parameters. A nonlinear internal wave of over 50 m amplitude, observed in deep water, is examined in detail. This wave was moving northward from the southern Okinawa Trough toward the continental shelf, and presumably formed from internal tides propagating northward from the Ilan Ridge area. A possible scenario for the formation of this wave from the internal tide is compared to related behavior south of Taiwan. On the outer continental shelf, a few large internal waves with maximum displacement greater than one-quarter of the water depth were measured with moorings. Sensors aboard ship and satellite recorded waves in this area traveling in many directions. Two possible causes (not mutually exclusive) for the multiple wave directions are scattering of nonlinear internal waves arriving from the south, and variable local generation of nonlinear gravity waves by the strong tidal and internal tidal currents. Internal tides on the shelf are relatively strong, among the strongest measured, having about 10 times greater kinetic energy density than numerous low-energy sites, which is consistent with the strong barotropic tides of the area. The ratio of diurnal baroclinic to barotropic kinetic energy found in this area is unusually high.
Barotropic Tide in the Northeast South China Sea
IEEE Journal of Oceanic Engineering, 2004
A moored array deployed across the shelf break in the northeast South China Sea during April-May 2001 collected sufficient current and pressure data to allow estimation of the barotropic tidal currents and energy fluxes at five sites ranging in depth from 350 to 71 m. The tidal currents in this area were mixed, with the diurnal O1 and K1 currents dominant over the upper slope and the semidiurnal M2 current dominant over the shelf. The semidiurnal S2 current also increased onshelf (northward), but was always weaker than O1 and K1. The tidal currents were elliptical at all sites, with clockwise turning with time. The O1 and K1 transports decreased monotonically northward by a factor of 2 onto the shelf, with energy fluxes directed roughly westward over the slope and eastward over the shelf. The M2 and S2 current ellipses turned clockwise and increased in amplitude northward onto the shelf. The M2 and S2 transport ellipses also exhibited clockwise veering but little change in amplitude, suggesting roughly nondivergent flow in the direction of major axis orientation. The M2 energy flux was generally aligned with the transport major axis with little phase lag between high water and maximum transport. These barotropic energy fluxes are compared with the locally generated diurnal internal tide and high-frequency internal solitary-type waves generated by the M2 flow through the Luzon Strait.
Nonlinear internal waves forced by tides near the critical latitude
Deep Sea Research Part I: Oceanographic Research Papers, 2003
Generation of semi-diurnal internal tides at high latitudes is investigated theoretically and on the basis of in situ data, collected in the Barents Sea. The experimental data had revealed evidence of semi-diurnal internal tides in records obtained at the critical latitude and above it, whilst the existing theories of baroclinic tides (linear or nonlinear) predict a suppression of tidal activity in the vicinity of the critical latitudes. The analytical solution for linear internal tides excited over the oceanic ridges predicts a substantial reduction of the efficiency of their generation. A similar result was obtained also in the frame of a fully nonlinear, nonhydrostatic model, developed and applied for typical conditions of the Barents Sea: weakly nonlinear baroclinic tides at high latitudes are suppressed by the Earth's rotation and have characteristics inherent to linear tides (small amplitude, large wavelength and small group speed). However, when applied to conditions of the southern slope of the Spitsbergen Bank, where the nonlinearity of the generated waves was estimated to be sufficiently strong, the mathematical model demonstrated generation of topography-scale short internal waves with characteristics close to those of unsteady lee waves. Propagating away from their source these waves produce wave patterns which may be considered as nonlinear internal tides of relatively short wavelength, although their genesis and features significantly differ from common internal tides. In particular, under conditions of strong nonlinearity and rotation the energy transfer from the barotropic tidal flux to the baroclinic modes leads to an effective generation of multiple tidal harmonics. r