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Zhongxiang Zhao

Senior Oceanographer

Email

zzhao@apl.washington.edu

Phone

206-897-1445

Department Affiliation

Ocean Physics

Education

B.S. Physics, Shandong University, 1994

Ph.D. Oceanography, University of Delaware, 2004

Publications

2000-present and while at APL-UW

Transition from partly standing to progressive internal tides in Monterey Submarine Canyon

Hall, R.A., M.H. Alford, G.S. Carter, M.C. Gregg, R.-C. Lien, D.J. Wain, and Z. Zhao, "Transition from partly standing to progressive internal tides in Monterey Submarine Canyon," Deep Sea Res. II, 104, 164-173, doi:10.1016/j.dsr2.2013.05.039, 2014.

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1 Jun 2014

Monterey Submarine Canyon is a large, sinuous canyon off the coast of California, the upper reaches of which were the subject of an internal tide observational program using moored profilers and upward-looking moored ADCPs. The mooring observations measured a near-surface stratification change in the upper canyon, likely caused by a seasonal shift in the prevailing wind that favoured coastal upwelling. This change in near-surface stratification caused a transition in the behaviour of the internal tide in the upper canyon from a partly standing wave during pre-upwelling conditions to a progressive wave during upwelling conditions. Using a numerical model, we present evidence that either a partly standing or a progressive internal tide can be simulated in the canyon, simply by changing the initial stratification conditions in accordance with the observations. The mechanism driving the transition is a dependence of down-canyon (supercritical) internal tide reflection from the canyon floor and walls on the depth of maximum stratification. During pre-upwelling conditions, the main pycnocline extends down to 200 m (below the canyon rim) resulting in increased supercritical reflection of the up-canyon propagating internal tide back down the canyon. The large up-canyon and smaller down-canyon progressive waves are the two components of the partly standing wave. During upwelling conditions, the pycnocline shallows to the upper 50 m of the watercolumn (above the canyon rim) resulting in decreased supercritical reflection and allowing the up-canyon progressive wave to dominate.

Internal solitary waves in the China seas observed using satellite remote-sensing techniques: A review and perspectives

Zhao, Z., B. Liu, and X. Li, "Internal solitary waves in the China seas observed using satellite remote-sensing techniques: A review and perspectives," Int. J. Remote Sens., 35, 3926-3946, doi:10.1080/01431161.2014.916442, 2014.

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19 May 2014

Internal solitary waves (ISWs) occur ubiquitously in China's waters: the South China Sea (SCS), the East China Sea (ECS), the Yellow Sea (YS), and the Bohai Sea (BS). ISWs have long attracted much research interest because of their important role in ocean acoustics, offshore engineering, ocean mixing, primary productivity, and submarine navigation. ISWs have sea surface signatures that can be detected by satellite synthetic aperture radar (SAR) and optical sensors. Satellite remote-sensing images provide excellent two-dimensional views of the ISW field. Our understanding of ISWs in the China Seas has been greatly improved using satellite remote-sensing techniques. The primary objectives of this paper are to review the development of remote-sensing techniques in the study of ISWs and to summarize ISW characteristics in the China seas, mainly demonstrated by remote-sensing techniques. In addition, several issues with remote-sensing techniques and interesting research topics are discussed.

Internal solitary wave propagation observed by tandem satellites

Liu, B., H. Yang, Z. Zhao, and X. Li, "Internal solitary wave propagation observed by tandem satellites," Geophys. Res. Lett., 41, 2077-2085, doi:10.1002/2014GL059281, 2014.

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28 Mar 2014

Internal solitary waves (ISWs) are observed 2 times within 30 min in synthetic aperture radar (SAR) image pairs from the Envisat and ERS-2 tandem satellites. Three pairs of SAR images were acquired in the South China Sea (SCS) in April 2007, August 2008, and March 2009, and 13 ISWs were tracked between the image pair in an ArcGIS environment. The phase speeds of these ISWs are calculated from their spatial displacement and time interval. The resultant ISW speeds agree well with the theoretical values estimated from the Sturm-Louisville equation using local bathymetric and monthly climatology ocean stratification data. This technique reveals the spatial variation in the ISWs speed in the water depth between 100 and 4000 m in the SCS. The study shows that ISWs speed is mainly affected by bottom topography and generally decreases from deep to shallow water from east to west and from south to north.

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Parametric subharmonic instability of the internal tide at 29°N

MacKinnon, J.A., M.H. Alford, O. Sun, R. Pinkel, Z. Zhao, and J. Klymak, "Parametric subharmonic instability of the internal tide at 29°N," J. Phys. Oceanogr., 43, 17-28, doi:10.1175/JPO-D-11-0108.1, 2013.

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1 Jan 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 a critical latitude of 29°N, it does not do so catastrophically.

The latitudinal dependence of shear and mixing in the Pacific transiting the critical latitude for PSI

MacKinnon, J.A., M.H. Alford, R. Pinkel, J. Klymak, and Z. Zhao, "The latitudinal dependence of shear and mixing in the Pacific transiting the critical latitude for PSI," J. Phys. Oceanogr., 43, 3-16, doi:10.1175/JPO-D-11-0107.1, 2013.

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1 Jan 2013

Turbulent mixing rates are inferred from measurements spanning 25°–37°N in the Pacific Ocean. The observations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from Hawaii. Previous and companion results argue that, near a critical latitude of 29°N, the internal tide loses energy to high-mode near-inertial motions through parametric subharmonic instability. Here, the authors estimate mixing from several variations of the finescale shear–strain parameterization, as well as Thorpe-scale analysis of overturns. Though all estimated diffusivities are modest in magnitude, average diffusivity in the top kilometer shows a factor of 2%u20134 elevation near and equatorward of 29°N. However, given intrinsic uncertainty and the strong temporal variability of diffusivity observed in long mooring records, the meridional mixing pattern is found to be near the edge of statistical significance.

Internal tides and mixing in a submarine canyon with time-varying stratification

Zhao, Z., M.H. Alford, R.-C. Lien, M.C. Gregg, and G.S. Carter, "Internal tides and mixing in a submarine canyon with time-varying stratification," J. Phys. Oceanogr., 42, 2121-2142, doi:10.1175/JPO-D-12-045.1, 2012.

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1 Dec 2012

The time variability of the energetics and turbulent dissipation of internal tides in the upper Monterey Submarine Canyon (MSC) is examined with three moored profilers and five ADCP moorings spanning February–April 2009. Highly resolved time series of velocity, energy, and energy flux are all dominated by the semidiurnal internal tide and show pronounced spring-neap cycles. However, the onset of springtime upwelling winds significantly alters the stratification during the record, causing the thermocline depth to shoal from about 100 to 40 m. The time-variable stratification must be accounted for because it significantly affects the energy, energy flux, the vertical modal structures, and the energy distribution among the modes. The internal tide changes from a partly horizontally standing wave to a more freely propagating wave when the thermocline shoals, suggesting more reflection from up canyon early in the observational record. Turbulence, computed from Thorpe scales, is greatest in the bottom 50–150 m and shows a spring-neap cycle. Depth-integrated dissipation is 3 times greater toward the end of the record, reaching 60 mW m-2 during the last spring tide. Dissipation near a submarine ridge is strongly tidally modulated, reaching 10-5 W kg-1 (10–15-m overturns) during spring tide and appears to be due to breaking lee waves. However, the phasing of the breaking is also affected by the changing stratification, occurring when isopycnals are deflected downward early in the record and upward toward the end.

Internal waves on the Washington continental shelf

Alford, M.H., J.B. Mickett, S. Zhang, P. MacCready, Z. Zhao, and J. Newton, "Internal waves on the Washington continental shelf," Oceanography, 25, 66-79, doi:10.5670/oceanog.2012.43, 2012.

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1 Jun 2012

The low-frequency oceanography of the Washington continental shelf has been studied in great detail over the last several decades owing in part to its high productivity but relatively weak upwelling winds compared to other systems. Interestingly, though many internal wave-resolving measurements have been made, there have been no reports on the region's internal wave climate and the possible feedbacks between internal waves and lower-frequency processes. This paper reports observations over two summers obtained from a new observing system of two moorings and a glider on the Washington continental shelf, with a focus on internal waves and their relationships to lower-frequency currents, stratification, dissolved oxygen, and nutrient distributions. We observe a rich, variable internal wave field that appears to be modulated in part by a coastal jet and its response to the region's frequent wind reversals. The internal wave spectral level at intermediate frequencies is consistent with the model spectrum of Levine (2002) developed for continental shelves. Superimposed on this continuum are (1) a strong but highly temporally variable semidiurnal internal tide field and (2) an energetic field of high-frequency nonlinear internal waves (NLIWs). Mean semidiurnal energy flux is about 80 W m-1 to the north-northeast. The onshore direction of the flux and its lack of a strong spring/neap cycle suggest it is at least partly generated remotely. Nonlinear wave amplitudes reach 38 m in 100 m of water, making them among the strongest observed on continental shelves of similar depth. They often occur each 12.4 hours, clearly linking them to the tide. Like the internal tide energy flux, the NLIWs are also directed toward the north-northeast. However, their phasing is not constant with respect to either the baroclinic or barotropic currents, and their amplitude is uncorrelated with either internal-tide energy flux or barotropic tidal forcing, suggesting substantial modulation by the low-frequency currents and stratification.

Mapping low-mode internal tides from multisatellite altimetry

Zhao, Z., M.H. Alford, and J.B. Girton, "Mapping low-mode internal tides from multisatellite altimetry," Oceanography, 25, 42-51, doi:10.5670/oceanog.2012.40, 2012.

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1 Jun 2012

Low-mode internal tides propagate over thousands of kilometers from their generation sites, distributing tidal energy across the ocean basins. Though internal tides can have large vertical displacements (often tens of meters or more) in the ocean interior, they deflect the sea surface only by several centimeters. Because of the regularity of the tidal forcing, this small signal can be detected by state-of-the-art, repeat-track, high-precision satellite altimetry over nearly the entire world ocean. Making use of combined sea surface height measurements from multiple satellites (which together have denser ground tracks than any single mission), it is now possible to resolve the complex interference patterns created by multiple internal tides using an improved plane-wave fit technique. As examples, we present regional M2 internal tide fields around the Mariana Arc and the Hawaiian Ridge and in the North Pacific Ocean. The limitations and some perspective on the multisatellite altimetric methods are discussed.

Internal tides around the Hawaiian Ridge estimated from multisatellite altimetry

Zhao, Z., M.H. Alford, J. Girton, T.M.S. Johnston, and G. Carter, "Internal tides around the Hawaiian Ridge estimated from multisatellite altimetry," J. Geophys. Res., 116, doi:10.1029/2011JC007045, 2011.

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24 Dec 2011

Satellite altimetric sea surface height anomaly (SSHA) data from Geosat Follow-on (GFO) and European Remote Sensing (ERS), as well as TOPEX/Poseidon (T/P), are merged to estimate M2 internal tides around the Hawaiian Ridge, with higher spatial resolution than possible with single-satellite altimetry. The new estimates are compared with numerical model runs. Along-track analyses show that M2 internal tides can be resolved from both 8 years of GFO and 15.5 years of ERS SSHA data. Comparisons at crossover points reveal that the M2 estimates from T/P, GFO, and ERS agree well. Multisatellite altimetry improves spatial resolution due to its denser ground tracks. Thus M2 internal tides can be plane wave fitted in 120 km x 120 km regions, compared to previous single-satellite estimates in 4° lon x 3° lat or 250 km x 250 km regions. In such small fitting regions the weaker and smaller-scale mode 2 M2 internal tides can also be estimated.

The higher spatial resolution leads to a clearer view of the M2 internal tide field around the Hawaiian Ridge. Discrete generation sites and internal tidal beams are clearly distinguishable, and consistent with the numerical model runs. More importantly, multisatellite altimetry produces larger M2 internal tidal energy fluxes, which agree better with model results, than previous single-satellite estimates. This study confirms that previous altimetric underestimates are partly due to the more widely spaced ground tracks and consequently larger fitting region. Multisatellite altimetry largely overcomes this limitation.

A perfect focus of the internal tide from the Mariana Arc

Zhao, Z., and E.A. D'Asaro, "A perfect focus of the internal tide from the Mariana Arc," Geophys. Res. Lett., 38, doi:10.1029/2011GL047909, 2011.

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30 Jul 2011

The Mariana Arc of ridges and islands forms an ~1300-km-long arc of a circle, ~630 km in radius centered at 17N, 139.6E. The hypothesis that the westward-propagating internal tides originating from the arc converge in a focal region is tested by examining the dominant M2 internal tides observed with air-launched expendable bathythermographs (AXBTs) and altimetric data from multiple satellites. The altimetric and AXBT observations agree well, though they measure different aspects of the internal tidal motion. M2 internal tides radiate both westward and eastward from the Mariana Arc, with isophase lines parallel to the arc and sharing the same center. The westward-propagating M2 internal tides converge in a focal region, and diverge beyond the focus. The focusing leads to energetic M2 internal tides in the focal region. The spatially smoothed energy flux is about 6.5 kW/m, about four times the mean value at the arc; the spatially un-smoothed energy flux may reach up to 17 kW/m. The size of the focus is close to the Rayleigh estimate; it is thus a perfect focus.

Long-range propagation of the semidiurnal internal tide from the Hawaiian Ridge

Zhao, Z., M.H. Alford, J.A. MacKinnon, and R. Pinkel, "Long-range propagation of the semidiurnal internal tide from the Hawaiian Ridge," J. Phys. Oceanogr., 40, 713-736, 2010.

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1 Apr 2010

The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), Hawaii, is studied with an array of six simultaneous profiling moorings spanning 25.5–37.1 deg N (~1400 km) and 13-yr-long Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter data processed by a new technique. The moorings have excellent temporal and vertical resolutions, while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide's coherent portion. Together these two approaches provide a unique view of the internal tide's long-range propagation in a complex ocean environment. The moored observations reveal a rich, time-variable, and multimodal internal tide field, with higher-mode motions contributing significantly to velocity, displacement, and energy. In spite of these contributions, the coherent mode-1 internal tide dominates the northeastward energy flux, and is detectable in both moored and altimetric data over the entire array. Phase and group propagation measured independently from moorings and altimetry agree well with theoretical values. Sea surface height anomalies (SSHAs) measured from moorings and altimetry agree well in amplitude and phase until the northern end of the array, where phase differences arise presumably from refraction by mesoscale flows. Observed variations in SSHA, energy flux, and kinetic-to-potential energy ratio indicate an interference pattern resulting from superposed northeastward radiation from Hawaii and southeastward from the Aleutian Ridge. A simple model of two plane waves explains most of these features.

New altimetric estimates of mode-1 M2 internal tides in the central North Pacific Ocean

Zhao, Z., and M.H. Alford, "New altimetric estimates of mode-1 M2 internal tides in the central North Pacific Ocean," J. Phys. Oceanogr., 39, 1669-1684, doi:10.1175/2009JPO3922.1, 2009.

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1 Jul 2009

New estimates of mode-1 M2 internal tide energy flux are computed from an extended Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter dataset that includes both the original and tandem tracks, improving spatial resolution over previous estimates from O(500 km) to O(250 km). Additionally, a new technique is developed that allows separate resolution of northward and southward components. Half-wavelength features previously seen in unseparated estimates are shown to be due to the superposition of northward and southward wave trains.

The new technique and higher spatial resolution afford a new view of mode-1 M2 internal tides in the central North Pacific Ocean. As with all altimetric estimates, only the coherent or phase-locked signals are detectable owing to the long repeat period of the tracks. Emanating from specific generation sites consistent with predictions from numerical models, internal tidal beams 1) are as narrow as 200 km and 2) propagate a longer distance than previously observed. Two northward internal tidal beams radiating from the Hawaiian Ridge, previously obscured by coarse resolution and the southward Aleutian beam, are now seen to propagate more than 3500 km across the North Pacific Ocean to reach the Alaskan shelf. The internal tidal beams are much better resolved than in previous studies, resulting in better agreement with moored flux estimates.

Internal solitary waves in the northwestern South China Sea inferred from satellite images

Li, X., Z. Zhao, and W.G. Pichel, "Internal solitary waves in the northwestern South China Sea inferred from satellite images," Geophys. Res. Lett., 35, doi:10.1029/2008GL034272, 2008.

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12 Jul 2008

Internal solitary waves (ISWs) in the northwestern South China Sea are studied from three spaceborne synthetic aperture radar images. ISWs are observed in the same area 18.5–20.5°N, 112–114°E. The common characteristics of the ISWs are: 1) their propagation directions are 270 ~ 300 degrees with respect to north; 2) the wavelength is about 1.2–1.6 km; 3) the distance between two neighboring ISW packets is about 10 km, but it is not a constant; 4) in two images, the easternmost ISWs evolve into multiple rank-ordered soliton on the shelf (ISW fission); and 5) near Shenhu Shoal, a local uplift at 19.5°N, 112.9°E, one ISW packet splits into two ISW packets. Based on their propagation direction and barotropic tidal forcing analysis, we suggest that these ISWs originate from tide-topography interactions in the Luzon Strait. It takes the internal tide about 100 hours to propagate 880 km from the Luzon Strait to the observation site.

Internal waves across the Pacific

Alford, M.H., J.A. MacKinnon, Z. Zhao, R. Pinkel, J. Klymak, and T. Peacock, "Internal waves across the Pacific," Geophys. Res. Lett., 34, doi:10.1029/2007GL031566, 2007.

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18 Dec 2007

The long-range propagation of the semidiurnal internal tide northward from the Hawaiian ridge and its susceptibility to parametric subharmonic instability (PSI) at the "critical latitude," λc = 28.8°N, were examined in spring 2006 with intensive shipboard and moored observations spanning 25–37°N along a tidal beam. Velocity and shear at λc were dominated by intense vertically-standing, inertially-rotating bands of several hundred meters vertical wavelength. These occurred in bursts following spring tide, contrasting sharply with the downward-propagating, wind-generated features seen at other latitudes. These marginally-stable layers (which have inverse 16-meter Richardson number Ri16-1 = 0.7) are interpreted as the inertial waves resulting from PSI of the internal tide. Elevated near-inertial energy and parameterized diapycnal diffusivity, and reduced asymmetry in upgoing/downgoing energy, were also observed at and equatorward of λc . Yet, simultaneous moored measurements of semidiurnal energy flux and 1-km-deep velocity sections measured from the ship indicate that the internal tide propagates at least to 37°N, with no detectable energy loss or phase discontinuity at λc . Our observations indicate that PSI occurs in the ocean with sufficient intensity to substantially alter the inertial shear field at and equatorward of λc, but that it does not appreciably disrupt the propagation of the tide at our location.

Global patterns of low-mode internal-wave propagation. Part I: Energy and energy flux

Alford, M.H., and Z.X. Zhao, "Global patterns of low-mode internal-wave propagation. Part I: Energy and energy flux," J. Phys. Oceanogr., 37, 1829-1848, doi:10.1175/JPO3085.1, 2007.

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1 Jul 2007

Extending an earlier attempt to understand long-range propagation of the global internal-wave field, the energy E and horizontal energy flux F are computed for the two gravest baroclinic modes at 80 historical moorings around the globe. With bandpass filtering, the calculation is performed for the semidiurnal band (emphasizing M2 internal tides, generated by flow over sloping topography) and for the near-inertial band (emphasizing wind-generated waves near the Coriolis frequency). The time dependence of semidiurnal E and F is first examined at six locations north of the Hawaiian Ridge; E and F typically rise and fall together and can vary by over an order of magnitude at each site. This variability typically has a strong spring–neap component, in addition to longer time scales. The observed spring tides at sites northwest of the Hawaiian Ridge are coherent with barotropic forcing at the ridge, but lagged by times consistent with travel at the theoretical mode-1 group speed from the ridge. Phase computed from 14-day windows varies by approximately ±45° on monthly time scales, implying refraction by mesoscale currents and stratification. This refraction also causes the bulk of internal-tide energy flux to be undetectable by altimetry and other long-term harmonic-analysis techniques. As found previously, the mean flux in both frequency bands is O(1 kW m-1), sufficient to radiate a substantial fraction of energy far from each source. Tidal flux is generally away from regions of strong topography. Near-inertial flux is overwhelmingly equatorward, as required for waves generated at the inertial frequency on a β plane, and is winter-enhanced, consistent with storm generation. In a companion paper, the group velocity is examined for both frequency bands.

Global patterns of low-mode internal-wave propagation. Part II: Group velocity

Alford, M.H., and Z.X. Zhao, "Global patterns of low-mode internal-wave propagation. Part II: Group velocity," J. Phys. Oceanogr., 37, 1849-1858, doi:10.1175/JPO3086.1, 2007.

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1 Jul 2007

Using a set of 80 globally distributed time series of near-inertial and semidiurnal energy E and energy flux F computed from historical moorings, the group velocity is estimated. For a single free wave, observed group speed should equal that expected from linear wave theory. For comparison, the latitude dependence of perceived group speed for perfectly standing waves is also derived. The latitudinal dependence of observed semidiurnal group speed closely follows that expected for free waves at all latitudes, implying that 1) low-mode internal tides obey linear theory and 2) standing internal-tidal waves are rare in the deep ocean for latitudes equatorward of about 35°. At higher latitudes, standing waves cannot be easily distinguished from free waves using this method because their expected group speeds are similar. Near-inertial waves exhibit scattered group speed values consistent with the passage of events generated at various latitudes, with implied frequencies ω ≈ 1.05 – 1.25 x f, as typically observed in frequency spectra.

Source and propagation of internal solitary waves in the northeastern South China Sea

Zhao, Z.X., and M.H. Alford, "Source and propagation of internal solitary waves in the northeastern South China Sea," J. Geophys. Res., 111, 10.1029/2006JC003644, 2006.

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22 Nov 2006

Large-amplitude internal solitary waves (ISWs) observed near Dongsha Island in the South China Sea originate in tide-topography interactions at Luzon Strait. Their arrival times at two moorings (S7 at 117°17'E, 21°37'N, and Y at 117°13.2'E, 21°2.8'N) are investigated, with respect to model-predicted barotropic tidal currents over Lan-Yu ridge at Luzon Strait. Each ISW packet can be associated with a westward tidal current peak. The time lags between the ISWs and the barotropic tidal currents are 57.6 ± 0.9 hours at S7 and 55.1 ± 1.0 hours at Y, consistent with the mode-one internal waves propagating nondispersively through the region's bathymetry and climatological stratification. Larger ISWs usually arrive earlier than smaller ones, consistent with the theoretical relation between nonlinear wave speed and wave amplitude. The observation that the ISWs are associated with westward tidal currents, with/without the presence of earlier eastward tidal currents, suggests that they are generated by nonlinear steepening of internal tides, rather than by the lee-wave mechanism. An idealized nonlinearization distance, over which the ISWs are generated in internal tide troughs, is estimated to be 260 ± 40 km from Luzon Strait.

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