A review of internal solitary wave dynamics in the northern South China Sea

A review of internal solitary wave dynamics in the northern South China Sea

Progress in Oceanography 121 (2014) 7–23 Contents lists available at SciVerse ScienceDirect Progress in Oceanography journal homepage: www.elsevier...

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Progress in Oceanography 121 (2014) 7–23

Contents lists available at SciVerse ScienceDirect

Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean

A review of internal solitary wave dynamics in the northern South China Sea C. Guo a,b, X. Chen a,⇑ a b

Ocean University of China, 238 Songling Road, Qingdao 266100, China School of Marine Science and Engineering, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

a r t i c l e

i n f o

Article history: Available online 18 April 2013

a b s t r a c t The current research into internal solitary waves (ISWs) and the related dynamic processes in the northern South China Sea (SCS) are reviewed in this paper. This unique wave phenomenon did not draw much interest until about one decade ago, and is now one of the hot topics in the field of wave dynamics. Three methods of investigation are summarized in the paper, namely, remote sensing images, in situ measurements, and numerical simulations. Previous works have primarily been based on one or two of these methods and have provided great insights into such wave phenomena. The lifetime of an ISW in the northern SCS, from its origin in the Luzon Strait (LS), through to its formation and evolution in the deep basin, and its transformation near the shelf break, up until its dissipation on the continental shelf, is summarized, with the illustration of different investigation approaches. Various factors that can affect the wave generation processes are summed up as well. Such factors as are barotropic tides in the LS, Kuroshio intrusion, temporal and spatial variation of stratification, and the modulation of the western ridge in the LS can significantly alter the wave fields and are extensively discussed. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Nonlinear internal solitary waves (ISWs) have received much attention in the past several decades due to their common occurrence in the oceans. They have been detected at numerous locations around the world’s oceans with a wide variety of in situ measurements and remote sensing images (Vlasenko et al., 2005; Apel et al., 2006; Helfrich and Melville, 2006). These waves are important since they produce shear currents and turbulence that contribute to the global energy transport and dissipation in the world’s oceans, and also due to their impacts on offshore engineering, biological activity, and military applications like submarine navigation and underwater acoustic propagation. It is generally believed that one of the most common mechanisms of ISW generation is due to the interaction of barotropic tides with the bottom topography under favourable oceanic conditions, after which baroclinic tides radiate out of the generation site. Their wave fronts steepen and become more nonlinear, which finally leads to the formation of high frequency nonlinear ISWs. ISWs are very ubiquitous features in the northern South China Sea (SCS) (Fig. 1) and have been drawing great attention in the past decade. Ever increasing investigations, including in situ measure⇑ Corresponding author. Address: College of Physical and Environmental Oceanography, Ocean University of China, 238 Songling Road, Qingdao, Shandong, China. Tel.: +86 13969718201. E-mail address: [email protected] (X. Chen). 0079-6611/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pocean.2013.04.002

ments, remote sensing images, and numerical simulations have been carried out, which indeed has provided great insight into this fascinating phenomenon, heightening our understanding and prompting further investigation. Moorings in the deep water and on the continental shelf have recorded passages of ISWs with different forms (Ramp et al., 2004; Klymak et al., 2006; Alford et al., 2010) and have shown that these waves are among the largest in the world. Klymak et al. (2006) observed waves with amplitudes of up to 170 m and phase speeds of 2.9 m s1, which is faster than their linear phase speed of 2.6 m s1, evidencing the high nonlinear behaviour of such waves. Both in situ observations (Alford et al., 2011) and numerical simulations (Jan et al., 2008; Buijsman et al., 2012) have confirmed the Luzon Strait (LS) as a major site for strong barotropic-to-baroclinic energy conversion, baroclinic energy radiation, and also turbulent dissipation. The LS (Fig. 1), which lies in the western Pacific Ocean, plays a decisive role in the dynamics of the SCS. It is a gateway connecting the SCS and the western Pacific Ocean. The most prominent feature of the bathymetry in the LS are two steep meridional ridges (hereafter referred to as the eastern ridge or Lan Yu Ridge, and the western ridge or Heng Chun Ridge) with a narrow, deep basin in between. The eastern ridge, dotted with several islands unevenly distributed along its length (i.e., the Batan Islands, the Babuyan Islands, etc.), is relatively higher and is thought to be the primary source of generation of large amplitude ISWs due to intense tide-topography interactions, while the western ridge, though not that efficient in generating internal waves due to its

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Fig. 1. Bathymetry of the northern SCS (500, 1000, and 3000 m isobaths are shown). The three dashed rectangles in the LS are the major generation sites of baroclinic tides, whereas the black solid rectangle on the continental shelf is the ASIAEX area. Four asterisks marked S7, B1, B2, and L1 are the mooring locations of the WISE/VANS experiment (Ramp et al., 2010). The grey solid line across the LS and the northern SCS is the approximate track along which 10 moorings were deployed as part of NLIWI (Alford et al., 2010). The two black dashed lines across the LS (marked N and S) are where measurements were concentrated as part of the IWISE (Alford et al., 2011). Also overlapped are the spatial distribution of multi-wave ISW packets (solid lines) near the LS and single-wave ISW packets (dashed lines) in the deep water derived from remote sensing images (Zhao et al., 2004). The vertical grey dashed line near the LS indicates the earliest appearance of ISWs according to statistics derived from remote sensing images. The red letters indicate the locations mentioned in the text: E denotes the eastern ridge (Lan Yu Ridge); W denotes the western ridge (Heng Chun Ridge); I denotes the Itbayat Island; Bt denotes the Batan Island; S denotes the Sabtang Island; Bb denotes the Babuyan Island; L denotes Lufeng. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (a) Zonal velocity from 15th April to 15th May, 2007 at 20.6°N, 122.0°E in the LS. The black line is the overall tide (including M2, S2, K1, and O1 harmonics), whereas the red and the blue lines are its semidiurnal and diurnal components, respectively. Meridional currents in the LS are weak. (b)–(d) Are three time periods featuring mixed tides, diurnal-dominant tides, and semidiurnal-dominant tides, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

great depth, also contributes somehow to the formation of ISWs in the northern SCS (Buijsman et al., 2010b; Vlasenko et al., 2010). Barotropic tides in the LS feature a fortnightly modulation with a mixture of both diurnal and semidiurnal tidal components.

Numerical experiments on barotropic tides in the LS (Jan et al., 2008) have shown that four principal harmonics, O1, K1, M2 and S2, constitute the complex barotropic tide structure there, with the first three components being dominant. Fig. 2 shows one

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month’s zonal velocity data from mid April to mid May, 2007 at one point located at 20.6°N, 122.0°E between Itbayat Island and Batan Island. It is clearly observed in the figure that barotropic tides at this site are highly asymmetric with stronger ebb (eastward) tides than flood (westward) tides. When decomposing the total tides into semidiurnal (M2, S2) and diurnal (K1, O1) components, it can be seen that during the spring phase the diurnal part dominates (Fig. 2c) while in the neap phase the semidiurnal counterpart is much more prominent (Fig. 2d). The shape of the tidal curve shown in Fig. 2 features one strong eastward peak and two westward peaks (one stronger and one weaker one) on certain days, and is typical of the barotropic tides in the LS, although the everchanging phase between diurnal and semidiurnal components leads to slight changes in the overall tides. The resultant asymmetry of the barotropic forcing from multiple tidal harmonics in the LS brings about the so called type-a and type-b waves observed in the northern SCS (Ramp et al., 2004). Above all, taking into account the active barotropic tides and the steep ridges, high rates of energy conversion in the LS would be expected, leading to strong baroclinic wave signals radiating out of the strait, both SCS-ward and western Pacific Ocean-ward. According to Jan et al. (2008), energy conversion rates from barotropic to baroclinic tides can reach 30% for diurnal tides and approximately 20% for semidiurnal tides. Furthermore, about 50% of baroclinic tidal energy is locally dissipated at a very fast rate. With the further radiation of baroclinic energy from the two ridges, ISWs riding on the troughs of internal tides start to emerge west of the LS in the deep water under the nonlinear effect. They are frequently observed in satellite Synthetic Aperture Radar (SAR) images. According to compilations of SAR images of different years (Hsu and Liu, 2000; Zhao et al., 2004; Jackson, 2009), the presence of ISWs is distributed from the neighbourhood of the LS all the way to the shelf of the northern SCS. Most waves are confined to a zonal band between 20° and 22°N. When reaching the abrupt continental shelf, the incoming internal tides can be partly reflected and partly locally dissipated, the extents of which depend on the inclination of the slope and the frequencies of the incident waves. ISWs start to deform dramatically under the effect of changing depth, and when first mode ISWs surpass a certain critical water depth which is close to the turning point at which the quadratic nonlinear coefficient in the Korteweg-de Vries (KdV) equation tends to be zero, wave polarities may change from depression to elevation (Orr and Mignerey, 2003). Finally, waves collapse on the continental shelf where wave breaking takes place due to the constant decrease of water depth (Chang et al., 2006). Farmer et al. (2011) have provided a general picture of the evolution of internal waves in the northern SCS at different stages, from generation in the LS until dissipation upon the continental shelf. Cai et al. (2012) have presented an overall but less detailed overview of ISW dynamics in the northern SCS, and have posed some problems that merit further investigation. The current paper is motivated by the fact that albeit the northern SCS has been a hot spot for the investigation of ISWs in the world’s oceans, an up-todate overall summary of the research progress is lacking. Several dozen papers have been published in the past decade, and much of what is known about the characteristics of ISWs in this area comes from these extensive and comprehensive studies. The present study summarizes most, if not all, of the past work that has been undertaken on the subject of ISWs and the related dynamic activities in the northern SCS, and so should serve as a useful reference in future works. The structure of the paper is organized as follows: Section 2 introduces normal approaches that are usually used to investigate ISWs in this area, whilst Sections 3 and 4 elaborate on the processes from their birth to their final dissipation. Finally in Section 5 factors that influence the generation of ISWs are discussed.

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2. Investigation approaches The nature of ISWs in the northern SCS has been the subject of intensive investigations, through both satellite and in situ observations, supplemented in more recent years by numerical simulations. All these three methods are indispensable and are illustrated separately in the following sub-sections. 2.1. Satellite imagery Currently there are many sensors aboard satellites that have the ability to acquire high spatial resolution images of the ocean’s surface, which can be used to detect ISWs above a certain amplitude. Such sensors include the European Remote-Sensing Satellite-2 (ERS-2) mounted SAR, the Environmental Satellite (ENVISAT) Advanced SAR (ASAR), the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Terra, etc., among which SAR is the most used and has revealed a wealth of ISW activities in the world’s oceans (Jackson, 2004). As is known, the undersea motions of ISWs can lead to the convergence/divergence of the sea surface water, which can modulate the surface roughness spectrum through the interaction of the current field with wind-generated waves, and manifest itself as alternating bright/dark bands in SAR images. The convergent zone is rougher, resulting in high backscatter of signals back to the sensor, and hence is manifested as bright stripes. On the contrary, the divergent zone is related to dark stripes. For first mode depression ISWs, the bright band is aligned in front of the dark one, whilst it is the opposite for elevation waves. As for the second mode ISWs, convex waves (as defined by Yang et al. (2010)) possess the same characteristics as first mode depression waves in SAR images due to their similar velocity structures in the upper layer, while concave waves resemble first mode elevation waves. Visualization of ISWs in SAR images is very straightforward. One can clearly identify the location, direction of propagating, along-crest width, number of waves per packet, and even wave amplitude through some theoretical methods (Zheng et al., 2001). The results are very robust since they accurately reproduce these wave properties. Fig. 3 shows three Envisat ASAR images featuring ISWs located near the LS, in the deep basin, above the shelf break, and on the continental shelf. A packet containing five ISWs (fragment c) shows up just west of the western ridge, and is also west of the locations of multi-wave packets near the LS compiled by Zhao et al. (2004) (represented by the solid black lines in Fig. 1). Fragment b features two extraordinarily wide ISW packets located in the deep basin and near the shelf break, respectively. The

Fig. 3. Three Envisat ASAR images compiled in one map showing: (a) ISW packets on the shallow continental shelf (21-JUN-2005, 14:09 UTC); (b) ISW packets near the shelf break and in the deep basin (12-AUG-2006, 14:04 UTC); (c) an ISW packet in the deep basin but near the LS (11-AUG-2006, 01:50 UTC). The arrow and the number in (b) indicate the distance between the two consecutive ISW packets. The four black lines in (a) show the decreasing distance between two neighbouring packets when shoaling onto the shelf.

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packet in the deep basin has an along-crest width of nearly 300 km and is among the widest ever observed. The two consecutive packets are 127.6 km apart, which corresponds to the wavelength of a semidiurnal internal tide at that depth. Lastly, five ISW packets, with decreasing distance between neighbouring pairs, and which are significantly refracted northward, are present in fragment a. Wave refraction by the Dongsha Atoll can be seen, and wave surface signals are weakened further north with the decrease of water depth on the continental shelf. The first study of ISWs in the northern SCS using satellite images was conducted by Fett and Rabe (1977), who exhibited a Very High Resolution (VHR) Defense Meteorological Satellites Program (DMSP) visible image of the SCS to illustrate the wave refraction by the Dongsha Atoll. In recent years, more and more satellite images, especially SAR images, have been acquired and compiled to study the spatial distribution of ISWs in this area (Ebbesmeyer et al., 1991; Hsu and Liu, 2000; Hsu et al., 2000; Zhao et al., 2004; Liu and Hsu, 2004; Du et al., 2008; Huang et al., 2008; Jackson, 2009). Statistical analyses of ISW occurrence with SAR images from 1995 to 2001 (Zheng et al., 2007) show that 22% of ISWs were distributed east of the 118°E meridian, whilst the remainder were located to its west. The occurrence of ISWs reveals both inter-annual and monthly variability, with April to July being the most favourable months for the appearance of ISWs. Temporal and spatial statistics of ISW occurence in this area was also made by Huang et al. (2008) based on 344 SAR images acquired between 1995 and 2007, and similar conclusions were drawn. Zhao et al. (2004) compiled a spatial distribution map of ISW packets from 1995 to 2001, in which ISWs propagating in a northerly or northwesterly direction from the LS can be observed both in the deep basin and in the shallow water. Two types of waves were classified in the map: multiple-wave ISW packets with rank-ordered waves and single-wave ISW packets containing only one wave without tails (Fig. 1). The former type mostly appears around the shelf break where disintegration occurs, while the latter type takes place in the deep basin. The inspection of SAR images has revealed some interesting features. An ERS-2 SAR image exhibited by Zhao et al. (2004) fortunately recorded a structure of ISW in which the bright strip width decreases dramatically from north to south, showing a gradual transition from internal tide to ISW. In addition, a transition process of first mode depression ISW to elevation ISW in the shallow water was illustrated by Zhao et al. (2003), who showed a SPOT-3 high resolution visible multi-spectral (HRV-XS) image in which both ISW packets of elevation and depression can be identified. Whereas Vlasenko et al. (2010) have numerically and theoretically identified a type of short internal waves that ride on a second mode ISW and trail a first mode ISW in the northern SCS, such a structure yet lacks observational evidence. However, by examining a number of Wide Swath Envisat ASAR images, Guo et al. (2012a) have provided robust evidence of the existence of such a structure, in which reasonably good similarity between the modelling results and ASAR images is reached. Satellite images have been very instrumental in investigating ISWs in the northern SCS. However, although the spatial resolution and coverage of many satellite images are very satisfying, they are restricted by the too long time sampling interval of a single sensor, which makes the study of ISWs with sequential images impossible, since most oceanic surface features (including ISWs) have relatively much shorter coherent time periods. Nonetheless, Zhao et al. (2008) have co-registered three images from different sensors, namely, an ERS-2 SAR, an ENVISAT ASAR, and a Terra MODIS image, which were acquired sequentially within a very short time interval (less than half an hour), and have studied the refraction process of ISWs caused by the Dongsha Atoll. Apart from the long sampling interval problem, satellite images are also of limited

value for characterizing the interior wave structure and the associated motions. 2.2. In situ measurements In situ measurements of ISWs in the northern SCS were first reported about two decades ago (Ebbesmeyer et al., 1991; Bole et al., 1994), but it is only in recent years that has much advance been made, following the SCS component of the large field experiment: the Asian Seas International Acoustic Experiment (ASIAEX) during 2000 and 2001. The ASIAEX experiment led to a number of publications (Orr and Mignerey, 2003; Beardsley et al., 2004; Liu et al., 2004; Ramp et al., 2004; Duda et al., 2004; Yang et al., 2004; Zhao and Alford, 2006; Duda and Rainville, 2008), after which increasingly more observational efforts have been put to this subject, including both long-term consecutive observations (Farmer et al., 2009; Yang et al., 2009; Alford et al., 2010; Ramp et al., 2010; Li and Farmer, 2011; Lee et al., 2012; Guo et al., 2012b; Xu et al., 2013), and short-term shipboard or mooring measurements (Orr and Mignerey, 2003; Lien et al., 2005, 2012; Chang et al., 2006; Liu et al., 2006; St. Laurent, 2008; Alford et al., 2011; Klymak et al., 2011; Fu et al., 2012; Pinkel et al., 2012). The observational sites are located not only in the LS and the deep water of the northern SCS, but also on the continental slope and shelf. Probably the earliest measurements of internal wave activities in the northern SCS were conducted in September 1990 when an Acoustic Doppler Current Profiler (ADCP) was deployed at Lufeng which lies on the continental shelf of the northern SCS (Ebbesmeyer et al., 1991). The recorded current speed data clearly showed the passages of ISW packets with the leading ISW inducing the fastest currents. Ebbesmeyer et al. (1991) correlated the ISWs measured at Lufeng with tidal forcing in the channel between the Batan and Sabtang Islands, and the several days’ lag time between tidal generation and the arrival of ISWs is consistent with the propagation time of the first mode ISWs. ASIAEX was systematically organized by multiple institutions in order to ‘‘understand acoustic interaction with the ocean volume in the presence of strong variability’’ (Lynch et al., 2004). Some moorings were anchored on the shelf of the northern SCS (area indicated with a black solid rectangle in Fig. 1) and recorded ISWs periodically passing by. Although constrained by the small observational area and short duration, ASIAEX is the first large-scale in situ research on ISWs in the SCS, providing considerable insight into the dynamics of ISWs in the northern SCS. Ramp et al. (2004) divided the observed packets into two types: the well-known typea and type-b ISWs (Fig. 4). ISWs belonging to type-a have larger amplitudes and arrive at the moorings regularly with a period of 24 h, while the type-b ISWs are relatively weaker and arrive consecutively one hour later every day. It was found that the composite barotropic tides featuring both diurnal and semidiurnal harmonics (Fig. 2) are responsible for the alternative appearances of such waves, and their characteristics will be further examined in Section 5.1. Zhao and Alford (2006) analyzed the arrival times of ISWs at two moorings, and related them to the tidal forcing over the eastern ridge in the middle LS. The comparisons showed that every ISW packet can be associated with a westward current peak in the LS rather than an eastward one. During the ASIAEX, moored records (Duda et al., 2004; Yang et al., 2004) and shipboard measurements (Orr and Mignerey, 2003) also observed wave polarity transitions from depression to elevation waves that resulted from the shoaling topography of the continental slope. In contrast with the extensive study of first mode ISWs, second or even higher mode ISWs have attracted little concern, presumably due to their rarity in the world’s oceans. However, they have indeed been observed on the shelf of the northern SCS (Ramp et al., 2004; Yang et al., 2004, 2009, 2010) and in the northern portion of

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Fig. 4. Stack plot of temperature recorded by a mooring during ASIAEX. Eleven days’ observation (May 4–14, 2001) showing the passages of stronger type-a ISWs and weaker type-b ISWs can be clearly seen (figure adopted from Ramp et al. (2004)). Type-a waves arrive almost at the same time each day, while type-b waves arrive one hour later each consecutive day.

the western ridge in the LS (Ramp et al., 2012). The observations of second mode ISWs have been reported in several other regions of the world’s oceans (for example, by Konyaev et al. (1995), Moum et al. (2008), and Shroyer et al. (2010)). A brief summary of pioneering investigations on this type of ISW, including theoretical, laboratory, numerical, and field investigations has been conducted by Yang et al. (2010), and the reader is referred to the literature therein. The first event of second mode ISWs in the northern SCS was reported by Yang et al. (2004), based on data obtained during ASIAEX. Time series of velocity and temperature on April 10, 1999 clearly recorded the passage of a second mode wave with a ‘‘bulge’’ close to a depth of 120 m in the thermocline. The amplitude of this wave (maximum isopycnal displacement) can reach 50 m, and its vertical temperature and zonal velocity profiles reasonably fit the KdV solutions. More recently in 2005 and 2006, some long-term moorings were deployed across the continental shelf of the northern SCS under the joint Taiwan/US program Variations Around the Northern South China Sea (VANS) and the Windy Islands Soliton

Experiment (WISE) supported by Taiwan and the US, respectively (Yang et al., 2010). The anchored velocity and temperature moorings recorded very active second mode ISW activities on the continental slope in the neighbourhood of the ASIAEX area. According to the measurements, there was notable seasonal variation in the occurrence frequencies of the second mode ISWs. They were detected only occasionally in summer, and 90% of them followed first mode ISWs, which, as was speculated by Yang et al. (2009), indicates that second modes might be related to the shoaling process of the first modes. On the contrary, during winter second modes were frequently recorded but not correlated with the first modes, which were actually rare at the mooring site. Yang et al. (2009) attributed such a seasonal variation to different thermal structures in the two seasons: a deeper thermocline in the winter, closer to the middle depth of the water, is favourable for the generation of second modes but at the same time suppresses the first modes. The above-mentioned second modes are all convex waves with upward (downward) displacement of isotherms in the upper (lower) water column. Nonetheless, four concave waves with the

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opposite waveforms were recorded as well (Yang et al., 2010), marking the first documentation of such waves. A three-layer analytical model was utilized to qualitatively study the characteristics of second mode ISWs, and it was found that a broad middle layer thickness (larger than half the water depth) is necessary for the generation of concave waves, while a relatively thinner middle layer is favourable for convex waves. It was also reasoned that concave waves are rarely observed because stratified water with a large middle layer is rare, however, such a structure has indeed been observed on the continental shelf of the northern SCS (Yang et al., 2004), providing the necessary conditions for the appearance of concave waves. The WISE/VANS program had four moorings deployed across the northern SCS (two in the deep basin, one on the upper continental shelf, and one in the LS; as shown in Fig. 1) and obtained more than one year’s high-resolution observations of temperature, salinity, and velocity. Ramp et al. (2010) described the observations from this program, with special attention paid to wave arrival patterns and their seasonal variations. ISW properties such as amplitude, propagation directions and speeds for both type-a and type-b waves were statistically documented, and the generation mechanism of the ISWs was discussed based on the data collected from the aligned moorings. The Nonlinear Internal Waves Initiative (NLIWI) was another substantial measurement program in the northern SCS (Alford et al., 2010). Ten moorings, including Pressure Inverted Echo Sounders (PIES), ADCP moorings and profiling moorings were deployed during 2006–07, and recorded 14 nonlinear ISWs as they passed by the synchronized array of these 10 moorings from the LS onto the shelf (indicated by the solid line in Fig. 1). Larger, narrower type-a waves and smaller, wider type-b waves alternated during the measurements. The waves’ properties, such as their speed, wavelength, number of crests, and energy were tracked over 560 km of their transit. Based on a couple of PIES in the LS and the deep basin along a line in a west-northwest direction, Farmer et al. (2009) and Li and Farmer (2011) acquired data for three months in 2005 and six months in 2007. Several ISW shapes were obtained under different barotropic conditions at LS. Wave properties like speeds and arrival times were analyzed, and the effects of rotation and nonlinearity were extensively scrutinized by applying numerical models with weakly nonlinear and fully nonlinear capacities. Based on two lines of short-term observational stations across the middle and the southern reaches of the LS in 2010 (shown as two dashed lines in the LS in Fig. 1), which are part of the Internal Waves in Straits Experiment (IWISE), Alford et al. (2011) presented some of the first observational studies of the internal tide generation and dissipation dynamics at the LS. Very strong barotropic-tobaroclinic conversion and baroclinic energy flux were recorded and proved to be among the strongest in the world’s oceans. Measured wave energy and energy flux exhibit a standing wave pattern across the middle LS, which is a consequence of the resonant effect of the two ridges. Strong overturning and a high turbulent dissipation rate were observed at the stations. A calculation of the fraction of the barotropic conversion to local dissipation yielded a value of 0.39, implying that the LS is a highly dissipative site. As for the observations above the continental slope and shelf, Chang et al. (2006) showed a strong divergence of the energy flux of the incoming internal waves by analyzing three sets of ADCP measurements. They concluded that most of the energy is already dissipated before the waves reach the continental shelf, and a dissipation rate of O(107–106) W kg1 was estimated, indicating that the northern SCS margin is a very dissipative area. A similar conclusion regarding the high dissipation level in this area was reached by St. Laurent (2008), who made turbulent dissipation observations along the continental slope and shelf northeast of

the Dongsha Atoll during a 10-day cruise. St. Laurent (2008) found that the most intense turbulence takes place around the shelf break, where near-bottom dissipation accounts for nearly 30% of the total depth integrated dissipation. Klymak et al. (2011) carried out a two-week in situ experiment east of the Dongsha Atoll and observed strong near-bottom isopycnal displacement (>200 m), with a pronounced cycle of spring-neap tides. Such large overturning events near the bottom induce large turbulent dissipations, as is consistent with the findings of Chang et al. (2006) and St. Laurent (2008). Klymak et al. (2011) also measured a beam-like structure impacting the continental shelf, rather than a more simple and common first baroclinic mode intrusion. Decomposition of the measured signals showed a multi-modal structure, with both diurnal and semidiurnal characteristics, and pronounced wave reflection occurred for diurnal waves due to the supercritical slopes. In situ measurements of ISWs in the northern SCS, though limited, have provided quite valuable information and have laid the foundation for our present understanding of such wave activities. However, to build an instantaneous observation system with good accuracy, long-time moorings covering the whole domain with fine spatial resolution are expected to be deployed across the LS, deep basin, and the continental slope and shelf area, with the aim to monitor wave processes in a comprehensive and systematic way. This seemingly ambitious aim requires the persistent efforts of the oceanographers that have been devoted to this research and a massive funding support from publicly funded research bodies. 2.3. Numerical simulations Complementary to these satellite and in situ observations, there have been advances in numerical simulations of ISWs in this area. The increase of satellite and in situ observations both in quantity and quality has been of great motivation and beneficial to the development and validation of numerical models. Especially in the recent years, modelling efforts have been increasingly synthesized with field observations (Alford et al., 2011; Buijsman et al., 2012; Zhang et al., 2011) and the capability of numerical simulations has consequently been substantially improved. Simmons et al. (2011) have reviewed the latest modelling efforts for the simulation of ISWs in the northern SCS and has shed light on the predictability of ISW characteristics under increasingly realistic conditions. It is known that the demand on the model performance largely depends on the phenomena of interest and the desired accuracy. For instance, when simulating the dynamics of internal tides, which feature scales of O(100) km, hydrostatic models with relatively coarse horizontal resolutions (O(1)  O(10) km) are adequate for the purpose. However, when it comes to the simulation of large nonlinear ISWs, a non-hydrostatic model with a fine resolution (O(100) m) is required to accurately delineate wave characteristics like profile, amplitude, phase speed, etc. It was shown by Vitousek and Fringer (2011) that coarse resolution will introduce considerable spurious numerical dispersion that can overwhelm the actual physical dispersion, seriously undermining the fidelity of the simulated results. Indeed, the 3D simulation by Zhang et al. (2011) with relatively coarse resolutions did yield a much smaller ISW amplitude (up to 50%) compared to in situ observations. However, this model does a good job in simulating the arrival times of ISWs due to the relatively weak nonlinear dispersion. In the northern SCS, a large number of numerical investigations have been carried out during the past decade. Most of the early models were 2D and often used some approximations, such as ideal bathymetry, forcing, and stratification (Cai et al., 2002; Du et al., 2008; Shaw et al., 2009; Cai and Xie, 2010; Qian et al., 2010; Wang et al., 2010). Emphasis was placed on the wave generation mechanisms and processes, and various factors that could

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affect such processes were heavily discussed. In the past few years, 2D numerical models with more and more realistic configurations have gradually arisen. A high-resolution 2.5-D non-hydrostatic model (Lamb, 1994) was employed to study the generation of internal waves induced by the tidal currents (K1, M2, and O1 tidal components were included) at LS (Warn-Varnas et al., 2009). Large amplitude ISWs were simulated and Kelvin–Helmholtz type instabilities were visible above the sills during very strong tidal flows. Model-predicted wave properties like amplitude and half width were compared to in situ measurements (primarily from ASIAEX) and analytical solutions, and reasonable agreement was obtained. Moreover, factors that can influence the generation of ISWs, including tidal current strength, geostrophic background current, initial density field, etc., were also checked. The non-hydrostatic Regional Ocean Modelling System (ROMS) was employed by Buijsman et al. (2010a) to study nonlinear internal waves generated at LS. Various idealized/realistic ridge configurations, barotropic tidal amplitudes and shapes, and Coriolis effects were taken into account. Meanwhile, the same model was applied again by Buijsman et al. (2010b) to quantitatively study ISW asymmetry east and west of the LS caused by spatial and temporal variations of the thermocline, bathymetry, and asymmetric modulated barotropic tides, adequately forecasting the inferior likelihood of ISWs being generated east of the LS, corroborating their seldom appearance in the SAR imagery. As was introduced in the previous subsection, Farmer et al. (2009) and Li and Farmer (2011) have studied the generation and evolution of internal waves based on in situ measurements, which they compared to several theoretical models. Farmer et al. (2009) used weakly nonlinear theory to explore the effects of rotation and nonlinearity and found that the disintegration of propagating internal tides to ISWs largely depends on the initial slope of the internal tides near the generation site. Li and Farmer (2011) further extended the investigation of internal wave propagation by using a fully nonlinear two-layered model (Helfrich and Grimshaw, 2008) coupled with a linear internal tide generation model by Hibiya (1986). The simulation discrepancies were assessed between weakly and fully nonlinear modelling contexts. Characteristics of the asymmetric barotropic forcing and the resultant ISW structure, effects of the Kuroshio, and the modulation of the western ridge were all considered and discussed. All the models discussed thus far are 2D, whereas 3D simulation on ISW generation and structures in the northern SCS was first presented by Vlasenko et al. (2010), who employed the fully nonlinear, non-hydrostatic Massachusetts Institute of Technology general circulation model (MITgcm) (Marshall et al., 1997). Clear evidence of 3D wave fields is visible, especially around the two ridges in the LS. The main focus of their paper is on the multimodal structure of the waves. To be more specific, large amplitude first mode ISWs (120 m), evident second mode concave ISWs (80 m), and some short internal waves (10 m) riding on the second mode are highlighted. The large scale of second mode concave ISWs obtained demonstrates that the second mode ISWs observed by Yang et al. (2010) were very likely to originate from the LS, rather than from the local bottom bathymetry or the shoaling first mode ISWs, and that the convex waves are produced due to the polarity change caused by the shoaling topography. Evidence of second mode signals was also provided by the Hovmöller diagram (Buijsman et al., 2010a; Zhang et al., 2011), in which propagation of second modes from the LS can be clearly seen. A numerical study of the propagation of a strongly nonlinear second mode ISW over a straight slope, with the model parameters taken close to those of the northern SCS, was presented by Guo and Chen (2012), who simulated the whole transformation process from the incident concave wave in the deep water to the convex wave train on the shelf. However, in situ measurements in the deep basin have not

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reported any evidence of second mode ISWs, which calls for more studies to be conducted on their characteristics with more robust evidence, both numerically and via in situ observations. Another more comprehensive 3D, nonlinear, and nonhydrostatic simulation has been performed recently by Zhang et al. (2011), who employed the parallel, unstructured grid model SUNTANS (Fringer et al., 2006) which solves the Navier–Stokes equation with Boussinesq approximation on a finite volume grid. The whole LS and the northern SCS continental slope was covered in the model domain, which is unprecedented in studying the 3D spatial variety of the generation and evolution of ISWs in this area. Eight harmonics were included to drive the model during a fortnight run, and qualitative comparisons with in situ measurements and satellite imagery show good similarity, demonstrating the capability of the model to simulate internal wave dynamics in this whole domain. Considerable attention has been given to the spatial and temporal characteristics of the generation and propagation of type-a and type-b waves. It was shown that the generation of type-a waves is stronger in the southern reach of the LS and that they propagate predominantly in a northerly direction in the northern SCS, whereas type-b wave generation is stronger in the northern reach and they propagate predominantly in a southerly direction. However, the relatively coarse spatial grids make the simulation somewhat inaccurate in the sense that the amplitudes of the predicted ISWs are largely underestimated (up to 53% at one mooring), being unable to correctly capture the nonlinearity and dispersion during the evolution process. Besides the simulations of ISWs in the northern SCS, some modelling efforts have also been performed to investigate properties of internal tides near the LS with the 3D hydrostatic Princeton Ocean Model (POM) (Niwa and Hibiya, 2004; Jan et al., 2007, 2008; Zu et al., 2008). Diagnostic quantities like baroclinic wave flux, conversion rate and energy balance for different tidal harmonics were calculated and the results showed that both diurnal and semidiurnal internal tides are very active at LS, and pronounced isopycnal disturbances are registered in the far field, with amplitudes up to 20 m for K1 and M2, 10 m for O1, and 5 m for S2 (Jan et al., 2008). In parallel to satellite imagery and in situ observations, efforts to provide complementary information from numerical simulations must persist. Fortunately, the development of numerical models on ISWs in the northern SCS in recent years has been progressing rapidly, which is largely motivated by the increasing availability of in situ observations which are essential in refining and validating numerical models. On the other hand, the improvement of numerical models could in turn provide macroscopic information and aid the implementation of observations. For example, Pinkel et al. (2012) introduced a cruise between the Itbayat and Batan Islands devoted to searching for lee waves which were predicted by numerical simulations. Although the models mentioned above are basically able to reveal the underlying physics and address the issues that primarily concern the wave generation and propagation, they are not competent to model the ISW field with sufficient accuracy for operational uses such as prediction. While the resolution and the size of the modelling domain are essential for the prediction of ISW characteristics, knowledge of correct stratification, bathymetry, and barotropic tidal forcing is desirable, and how substantial these can be to enhance the fidelity of modelling results remains to be seen. This calls for the support from massive in situ observations which can be expected to be assimilated into the model. Moreover, complexity is added by the impacts of Kuroshio, which itself has been unclear in terms of its flowing patterns and structures in the LS. Taking into account the potential impacts of the pycnocline inclination and background currents related to the Kuroshio and the attendant eddy-shedding, a remedy could be that the internal

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wave generation model be coupled or nested with a general circulation model. However, its feasibility and outcome remain to be evaluated. Challenges remain, but with the rapid development of computer techniques, it is foreseeable in the near future that the difficulties will be overcome and a comprehensive prediction system will be built. 3. Generation sites, processes, and mechanisms It has been acknowledged that the middle LS (i.e., around the Batan Islands) is the primary region for the generation of large amplitude ISWs in the northern SCS, while the rest of the strait generates comparatively weak waves. In situ observations (Alford et al., 2011) and numerical simulations (Jan et al., 2008) have shown that very strong energy conversion and energy flux occur here (Fig. 5). However, the other two regions, i.e., the southern reach of the LS (the Babuyan Islands) and the northern part of the western ridge, although not that efficiently, are also believed to be able to produce significant baroclinic signals (Jan et al., 2008; Alford et al., 2011). Internal waves radiating from different sources at the same time can meet and connect very rapidly with each other and propagate westward together. Nonlinear wave– wave interactions take place during the merging process, and this has been numerically illustrated by Cai and Xie (2010) with a weakly 2D KdV type model, and by Chen et al. (2011) based on a MODIS image and the Kadomtsev–Petviashvili equation. It has been illustrated by Chen et al. (2011) that under the resonant effect, the nonlinear interaction of two obliquely propagating ISWs from two sources leads to the emergence of a third ISW that possesses larger amplitude than both the original waves. Different generation mechanisms of ISWs have been proposed under various oceanic conditions. These mechanisms include: (1) the nonlinear steepening of internal tides (Lee and Beardsley, 1974; Gerkema and Zimmerman, 1995; Farmer et al., 2009); (2) the formation of lee waves (Maxworthy, 1979; Nakamura et al., 2000); (3) the interaction of tidal beams with the thermocline that leads to the ‘‘local’’ formation of ISWs (New and Pingree, 1990, 1992; Gerkema, 2001); (4) the collapse of mixed water that produces internal waves which further develop into ISWs due to nonlinear steepening (Maxworthy, 1979). As for the case in the northern SCS, past publications have provided inconsistent conclusions on the generation processes and

mechanisms of internal waves, presumably due to the intricate bottom topography and various oceanic dynamics therein. As has been described earlier, the strong tide-topography interaction in the double-ridged LS is responsible for the spawning of ISWs in the northern SCS. The periodic or quasi-periodic appearance of the packets in numerical models and in situ measurements show that they are closely related to the asymmetric tidal forcing in the LS (Ebbesmeyer et al., 1991; Zhao and Alford, 2006; Warn-Varnas et al., 2009), which features a fortnightly period with stronger and weaker peaks appearing alternately. Cai et al. (2002) suggested with their two-layered numerical model that waves generated in this area much like lee waves. Pinkel et al. (2012) have made direct observations in an outflow channel in between the Itbayat and Batan Islands where flows are very strong. Very large overturnings (250 m) associated with high dissipation was observed, which, together with support from numerical simulations, manifests features of breaking lee waves. However, whether these lee waves could escape the topographic feature and quickly develop into ISW packets remains to be answered. Lien et al. (2005) argued that, since a mooring just west of the LS recorded no passages of ISWs, which contradicts the theory of lee wave generation, ISWs observed in the northern SCS are subject to nonlinear steepening of internal tides. Zhao and Alford (2006) made a comparison between the observed ISWs on the shelf and the corresponding westward tidal current peaks in the LS and found that they correlate very well. Since the eastward and westward tidal currents in the LS are asymmetrical and no obvious correlation between ISWs and eastward tidal currents is identified, it was also suggested that ISW packets are generated by nonlinear steepening of internal tide rather than lee wave mechanism. This conclusion was further verified by a numerical model (Warn-Varnas et al., 2009) in which a sensitivity run was performed and the results indicated that the formation of a depression on the lee side of the sill plays little role in the generation of ISW packets. Using a linear analytical model, a western boundary current instability generation mechanism was suggested by Yuan et al. (2006b). It was found that the west wing of the Kuroshio is unstable and can serve as a disturbance source from which internal waves absorb energy, propagate westward, and finally lead to the generation of ISWs. Zheng et al. (2007) proposed that the generation of ISWs is subject to necessary and sufficient conditions, namely, the formation

Fig. 5. Spatial distribution of barotropic-to-baroclinic energy conversion rate and baroclinic energy flux averaged in three days for (a) K1 and (b) M2 tidal harmonics in summer (figure adopted from Jan et al. (2008)). Several sites with high energy conversion can be seen, and the discrepancy between the spatial distributions of K1 and M2 baroclinic energy fluxes is obvious.

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of initial disturbance and the growth of wave amplitude. In the northern SCS the former condition could be fulfilled by the tidetopography interaction or the boundary current instability, whilst the latter could be due to the thermocline shoaling near the LS. It was also illustrated on this premise that the growth of eastward propagating initial disturbance is hampered by dissipation effects when propagating into the deepening thermocline east of the LS. Two possible generation mechanisms were proposed by Du et al. (2008). One of them links to the evolution of depression waves and much resembles an internal tide mechanism, whereas the other mechanism is due to the internal mixing disturbance, as was elucidated by Maxworthy (1979). The latter mechanism takes place when the tidal forcing is strong enough to make depression waves near the eastern ridge break down quickly and lead to mixing, after which the disturbance of the mixed water will directly create groups of ISWs in the LS. The generation of ISWs by the interaction of internal tidal beams with the thermocline was illustrated by Shaw et al. (2009) in the conditions of the northern SCS. A two-step process was proposed, i.e., the generation of tidal beams and their subsequent interaction with a sharp thermocline, after which ISWs emerge due to the nonlinear interactions in the thermocline. Using the non-hydrostatic ROMS, Buijsman et al. (2010a) proposed a mixed lee wave regime which features both characteristics of internal tides and lee waves. However, contrary to the conclusion reached by Zhao and Alford (2006), westward propagating ISWs modelled in their paper follow strong eastward barotropic tidal currents in the LS. This is further corroborated by mooring measurements near the two ridges by Alford et al. (2010), who showed that westward type-a/type-b ISWs align with eastward strong/weak tidal peaks. However, at the same time they also addressed the potential difficulty in assessing the phase of the generated waves with the complicated current field above the ridges. By correlating observational data in the northern SCS and in the LS, Ramp et al. (2010) drew somewhat different conclusions that much stronger eastward tides at LS correspond to the generation of more nonlinear type-a ISWs, whereas type-b ISWs are associated with the weaker westward tides at LS. This idea was later corroborated by the 3D numerical simulation by Zhang et al. (2011), who also proposed that both types of ISWs arise from the nonlinear steepening of semidiurnal internal tides from the LS instead of lee wave mechanism. Zhang et al. (2011) attributed the stronger typea wave generation at the southern LS to the augmentation of semidiurnal type-a waves by the diurnal internal tidal beams, whereas stronger type-b waves in the north LS result from the resonant effect of the two ridges. According to the above discussions, various conclusions have been reached in terms of the generation processes and mechanisms of ISWs in the northern SCS. Whether these results are conflicting or could co-exist remains to be evaluated. As a matter of fact, it would be helpful to first diagnose the generation regime with the aid of the magnitude of two parameters: the tidal excursion ku0/x and the slope parameter kh0/a (Vlasenko et al., 2005; Garrett and Kunze, 2007), where k1 is the topographic scale, u0 is the magnitude of the barotropic tide, x is the tidal frequency, h0 is the topographic height, and a is the slope of the tidal beam. In the case of the LS, Pinkel et al. (2012) illustrated via numerical simulations that above the eastern ridge, the tidal flow goes around, rather than over, the ridges. In the region between the Itbayat and Batan Islands, the ebb tides (from SCS towards the Pacific) flow out along two channels with large velocity. With a rough estimate in regions like this the tidal excursion has the value close to or greater than unity, which implies a lee-wave regime. Indeed, Pinkel et al. (2012) directly observed the formation of strong breaking lee waves along one channel with strong flow. On the other hand, in relatively deeper water along the two ridges, the ti-

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dal excursion is much smaller than unity, indicating the tendency for a internal tide regime. In the LS the proportion of shallower water with tidal excursion near or larger than unity is much smaller than deep water with very small tidal excursion. Given that the wide ISWs originate from the tide-topography interaction in the whole LS, internal tide regime associated with small tidal excursion is expected to dominate. As for the slope parameter, critical and supercritical slopes for M2 waves exist along the eastern ridge (Guo et al., 2011), which is favourable for the formation of tidal beams, and their later interaction with the thermocline may give rise to the emergence of ISWs. However, to what extent this mechanism contributes to the formation of ISWs in the northern SCS remains unclear. Given the spatially and temporally variable distribution of the tidal excursion and the slope parameter, it is not surprising that different conclusions have been made regarding the wave generation processes and mechanisms. This does not necessarily mean that these results are conflicting, rather, they might refer to different geographical locations and oceanic conditions.

4. Wave evolution in the deep basin and wave shoaling After internal waves are produced in the LS, they propagate westward or northwestward in the deep basin. Both in situ measurements (Alford et al., 2010; Farmer et al., 2009; also see Section 2.2) and SAR images (Zhao et al., 2004; Jackson, 2009; also see Section 2.1) have revealed that ISWs are already well formed in the deep basin before they reach the shoaling continental slope (Fig. 3). Normally there exists only one single wave in a packet, while multi-wave packets emerge near the shelf break (the multi-wave packet in the deep basin in Fig. 3 is an exception). The observed shapes have been examined and compared to analytical KdV theory (Bole et al., 1994; Yang et al., 2004; Klymak et al., 2006; Warn-Varnas et al., 2009). Although the KdV solution is not very accurate in delineating such large waves, the observed shape and phase speed of ISWs are basically consistent with it in the above literature. A point that should be raised is where and when ISWs start to emerge west of the LS. Geographic distribution of ISWs (Zhao et al., 2004) reveals that single ISWs in the deep water appear closely to the west of the 120°E meridian, whereas multi-wave ISW packets exist between this area and the western ridge. Using a simple model based on the phase speed difference between ISWs and internal tides, Zhao and Alford (2006) derived a distance from the LS, 260 ± 40 km (at about 120°E), over which ISWs develop in the frontal face of the internal tides due to nonlinear steepening. This distance is consistent with the easternmost single ISWs in the deep water captured on SAR images. Using the shock wave equation, Farmer et al. (2009) predicted the earliest breaking location (when the waves become steep enough that non-hydrostatic effects come into play) as 120.3°E when treating the internal tides as purely semidiurnal. Internal tides are progressively shaped by nonlinear and rotational effects after propagating out of the source area. ISWs are formed under the continuous steepening of internal tides when non-hydrostatic effects come into play. These effects on the ISWs in the northern SCS were examined by Farmer et al. (2009) and Li and Farmer (2011). Among them, the Earth’s rotation plays a key role in the propagation of the generated internal waves and its effect has been examined in numerous publications (c.f., Ostrovsky, 1978; Gerkema, 1996; Helfrich and Grimshaw, 2008). Farmer et al. (2009) and Li and Farmer (2011) examined rotational effects in the northern SCS by comparing measured time series and modelled results with and without rotation. It was found that ISWs are much more developed without the presence of rotation, and in

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such a case they have larger amplitude, more waves, and arrive later than the observed waves. These conclusions are expected considering the damping effects of rotation, and this also addresses the importance of including rotations when studying baroclinic waves in the northern SCS. During propagation in the deep basin, internal tides are likely to lose coherence when the ambient stratification is changed, or when passing through mesoscale eddies, which are ubiquitous in the northern SCS. Lee et al. (2012) discussed this case based on eight months’ moored ADCP observations and concluded that incoherent internal tidal motion accounts for three fourths of the observed tidal energy. Similar conclusions were also reached by Xu et al. (2013) based on nine months’ moored current observations, and it was found that for the semidiurnal tides, the coherent component only accounted for about 10% of the total semidiurnal motions. When reaching the shallower margin of the northern SCS, internal wave signals, which include both high-frequency ISWs and long internal tides, interact strongly with the shoaling topography. The wave energy scatters upon the shoaling topography, either advancing further upslope, being partially reflected, or being locally dissipated. Of the strong transmitted wave energy onto the continental slope and shelf, a great proportion is locally dissipated near the shelf break (Orr and Mignerey, 2003; Lien et al., 2005; Chang et al., 2006; St. Laurent, 2008; Klymak et al., 2011; St. Laurent et al., 2011). As was discussed in Section 2.2, pronounced near-bottom isopyncal fluctuations with spring-neap cycles have been measured, accompanied by a high rate of turbulent dissipation. As a consequence, much of the incoming internal tidal energy originating from the LS is lost before the waves reach the shallow continental shelf. Before introducing properties of internal tides or ISWs in shallower water, a point worth mentioning is the reflection and diffraction effect of the Dongsha Atoll, which is located at about 116°450 E, 20°400 N, right in the propagation path of ISWs from the LS. The Dongsha Atoll features steep slopes around it and is connected to the deep sea on the east and the continental shelf on the west. With an intensive array of thermister moorings and an ADCP, Fu et al. (2012) measured the shoaling process of large amplitude (100 m) ISWs just east of the Dongsha Atoll in water depths between 100 m and 285 m. The instruments recorded rapid transformations of wave profiles over the steep slope (3°), perhaps marking the largest wave shoaling events that have been documented. When a large amplitude ISW of first mode or second mode impinges on the Dongsha Atoll, it is split into two fragments which separately bypass the atoll and rejoin behind it. The process is accompanied by the generation of secondary waves and backward reflection, which makes the wave field become very complicated in the vicinity of the atoll. Fig. 6 shows an ASAR image in which multiple dynamic processes related to ISWs can be seen: the ISW packets being refracted by the atoll, a weak reflected ISW, intricate wave–wave interactions between the previously refracted ISWs and incoming ISWs further north formed in the next cycle, and weak ISW packets shoaling onto the continental shelf. The scenarios of wave refraction and reflection are frequently visible in satellite images, as was shown by Fett and Rabe (1977), Bole et al. (1994), Liu and Hsu (2004), Ramp et al. (2004) and Zhao et al. (2008), who all exhibited SAR images in which very prominent diffraction effects can be seen. Moreover, observational data provided by Ramp et al. (2004) during ASIAEX is also very revealing: histograms of current direction for all ISWs show two peaks, one of which heads towards the northwest and points right at the Batan Islands in the LS, which is the primary origin for transbasin waves. The other direction, however, is abundant with relatively small waves that can be traced back to the Dongsha Atoll,

Fig. 6. An Envisat ASAR image (03-NOV-2005, 14:15 UTC) showing the complicated wave field near the Dongsha Atoll. Incoming ISWs, refracted multi-wave packets, reflected single ISW, wave–wave interactions, and shoaling waves can be clearly spotted on the image. The two dashed lines indicate the position of the leading wave in the refracted ISW packet. The dashed ellipses show where pronounced wave–wave interactions take place.

which actually reflects and diffracts the incoming trans-basin waves to almost all the directions. In addition, a couple of numerical models were also implemented to look into this issue (Lynett and Liu, 2002; Chao et al., 2006; Cai and Xie, 2010; Chen et al., 2010). For example, using a 3D non-hydrostatic model, Chao et al. (2006) investigated the interaction of an incident ISW with a cylinder, whose radius is 15 km and is somewhat smaller than the Dongsha Atoll. Refraction and diffraction processes of both first and second mode ISWs are evident in the runs. As was also discussed in Section 2.2, according to the mooring measurements just east of the atoll (Klymak et al., 2011), the broad plateau of the Dongsha Atoll is able to obstruct the incoming energy contained in tidal beams. After the decomposition of first and higher mode internal tides of both diurnal and semidiurnal frequencies, it was found that one third of the incoming diurnal energy is reflected backwards, with most of the reflection in the first mode, whereas for the incoming semidiurnal energy, the observed reflection is weak due to the subcriticality of the bathymetry with respect to the semidiurnal tides. This process was corroborated by a linear scattering model which investigated the phenomenon more thoroughly in terms of the height of the supercritical topography and the phase between the incoming first and second mode internal tide flux. It was found that the phase between the incoming components significantly alters the wave transmission and reflection at the shelf break. Based on four mooring sites upon the shallow continental slope during ASIAEX, the internal tidal energy flux of diurnal and semidiurnal frequency was scrutinized by Duda and Rainville (2008). They found that the diurnal flux is substantially larger than the semidiurnal flux, and the ratio between the two far exceeds the ratio between their barotropic tidal kinetic energy. Strong temporal and spatial variations of the internal tidal flux were seen during the measurements, highlighting the irregular nature of baroclinic wave dynamics in this area. Similar conclusions were drawn by Guo et al. (2012b), who obtained ADCP current data at three moor-

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ings and studied temporal and seasonal characteristics of internal tides on the continental shelf. According to Duda and Rainville (2008), the diurnal energy flux onto the shelf was greater than that measured at the deeper site, whilst semidiurnal flux decreased monotonically upslope. The above facts imply the great likelihood of local generation of diurnal internal tides, which originate from barotropic tide-topography interaction at this area. As is known, barotropic tides in the northern SCS are mainly driven by the tidal energy flux from the western Pacific Ocean through the LS, with diurnal tides dominating (characteristics of the principle tidal constituents in the SCS were summarized by Fang et al. (1999) and Beardsley et al. (2004)). According to the analyses of the barotropic tides in the ASIAEX area (Beardsley et al., 2004), four tidal harmonics, i.e., O1, K1, M2, and S2, co-exist, with the diurnal currents dominant over the upper slope and semidiurnal currents dominant over the shelf. Recorded tidal currents at all mooring sites exhibited elliptical features with clockwise turning with time. According to the calculation of the internal wave ray characteristic angles for both diurnal O1 and semidiurnal M2 tides (Duda et al., 2004), critical slopes for the diurnal tide in this region are 0.16–0.3°, much shallower than those for the semidiurnal tide, which are of the order 0.5–1° (a similar map is also given by Klymak et al. (2011)). Considering that the shallower slopes that are critical for diurnal tides are very typical, this area is suitable for diurnal tidal generation rather than semidiurnal. A further interpretation was made by Duda and Rainville (2008), with more details and robust observational data. Meanwhile, Klymak et al. (2011) noted that such a critical value of diurnal internal tides upon the fringes of the northern SCS implies that these play a large role in the shaping of the continental shelf. In addition, complexity is added by the correlation between the remotely-generated shoaling internal tides and the local barotropic tides (Kelly and Nash, 2010), which has great impact on the local energy conversion of barotropic to baroclinic tides and thus affects the subsequent characteristics of baroclinic waves and the incoming ISWs. However, this idea has not been addressed yet in the SCS margin, and more quantitative estimations will be helpful in assessing the significance of such a process. A further complicating factor is that the dominant diurnal internal tides can somehow modulate the propagation of incoming ISWs, as was interpreted by Alford et al. (2010) who, by solving the Taylor–Goldstein equation which takes into account the background shear process (diurnal internal tides in this case), found that upon the upper continental shelf type-b waves move faster than type-a waves although with larger amplitudes. The profiles of ISW transform when they shoal onto the continental slope and shelf. In the northern SCS, measurements during ASIAEX have illuminated many fascinating results on the transformation processes of the incident waves. Such features were extensively studied by Duda et al. (2004) and Ramp et al. (2004), who showed that beginning as large narrow depression ISWs, they became much broadened when shoaling onto the continental shelf, accompanied by the trailing of oscillations and later appearance of elevation waves. Yang et al. (2004) reported four forms of ISWs during this process, including the near-breaking stage in which they were in the transition zone close to the time-varying turning point. Lien et al. (2012) observed the formation of a trapped core within a shoaling ISW above the continental slope. As the ISW shoals, the maximum along-wave current speed exceeds the wave phase speed, which triggers wave breaking that leads to the formation of two counter-rotating vortices feeding a jet inside the core. Very high overturnings (10–50 m) were observed, and the water within the core was found to be highly turbulent and dissipative. Two more intriguing polarity conversion events were presented by Orr and Mignerey (2003) with shipboard measurements in the ASIAEX area. Visualization of the acoustic flow from depression

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to elevation waves is quite illustrative, and wave properties like position and width were described during the shoaling process to the shallower water. A two-layered KdV model was also applied by Orr and Mignerey (2003) but proved to be inaccurate for the description of the observed waves. Large shear instabilities riding on the trough of an ISW were present during the conversion process, with their amplitudes up to 40 m. Similar wave structures were also observed by Klymak et al. (2006) during shipboard measurements in the deep basin, consisting of some small-scale oscillations riding on the frontal face of a first mode ISW but without a region of low Richardson number. In addition to in situ observations, Liu et al. (1998), by scrutinizing SAR images, identified elevation waves in the shallow water where the upper mixed layer was relatively thicker, and a two-layered KdV type model was also employed to qualitatively describe the conversion process. Zhao et al. (2003) exhibited a satellite image in which ISWs converting polarity was clearly seen. Two phases of the conversion process were verified by Zhao et al. (2003), i.e., the flattening of the original depression waves and the appearance of new elevation waves. Generated elevation waves either break up or are dispersed depending on the changes in water depth and local stratification, marking the termination of the life cycle of an ISW from its origin in the LS. Breaking internal waves at the shelf break and upon the shallow shelf may be the primary source for turbulent mixing, which can effectively weaken the stratification and affect the local dynamic processes. 3D numerical modelling with realistic conditions have been lacking, with the exception of one study by Shen et al. (2009), who simulated the shoaling process of ISWs in the ASIAEX area. Nonetheless, this final stage of evolution of ISWs is also crucial to the inshore dynamics and engineering (for example, the formation of numerous very large subaqueous sand dunes generated by the shoaling ISWs on the upper continental slope of the northern SCS was discovered by Reeder et al. (2011)), and should draw more attention. 5. Factors affecting the generation of ISWs From the above sections it can be seen that the generation of large amplitude ISWs in the northern SCS is primarily due to the strong tide-topography interactions above the eastern ridge in the LS. However, their characteristics are modulated by, but not limited to, the composite LS barotropic tides, the Kuroshio, the seasonal and inter-annual variations of stratification in the northern SCS, and the western ridge of the LS. Some of the related dynamic processes have been extensively studied, whereas others are still not well understood. In this section, the factors affecting the generation and evolution processes of ISWs in the northern SCS will be summarized and discussed. 5.1. Barotropic tidal strength and frequency in the LS As is shown in Fig. 2, barotropic tides in the LS feature both diurnal and semidiurnal harmonics with east–west asymmetry and fortnightly modulation. The complex effects of the impact of the barotropic tides in the LS are related to the amplitude of each tidal harmonic and their frequency and phase lag. Using 3D POM, Jan et al. (2008) showed that both diurnal and semidiurnal constituents are able to produce pronounced internal tide signals from the LS (Fig. 5). Using a fully nonlinear, weakly nonhydrostatic two-layered theoretical model with rotation, Helfrich and Grimshaw (2008) showed that with the conditions of the northern SCS, initial semidiurnal internal tides can disintegrate into ISWs west of the LS, whereas internal tides of diurnal frequency remain largely intact without disintegration. A similar conclusion was drawn by Li and Farmer (2011), who found that when semidiurnal

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tides at LS dominate, ISWs in the deep basin appear twice a day, whereas when diurnal tides dominate, ‘‘corner waves’’ are observed but ISWs do not appear. In addition, when the magnitude of semidiurnal tides is comparable to that of diurnal tides, the observed ISWs alternate between single and multiple waves, i.e., type-a and type-b ISWs. Nevertheless, in spite of the inefficiency of diurnal tide generation, its modulation on the resultant ISW generation, namely, the extensively discussed type-a and type-b waves, is notable (Zhang et al., 2011; Vlasenko et al., 2012; Wang, 2012), as will be discussed in this section. To begin with, it is apparent that the absolute value of the barotropic tidal currents directly and substantially affects the resulting wave field. A large number of studies have been devoted to the analysis of the impact of barotropic tidal strength at LS, mostly by numerical and observational means. Ramp et al. (2004) showed that the appearance of two clusters of ISWs only turned up during spring tides in the LS, whereas during neap time ISWs were hardly identifiable. This idea has been corroborated by a number of succeeding publications, c.f., Lien et al. (2005), Zhao and Alford (2006), and Ramp et al. (2010). The very large ISWs shown in Fig. 3 (fragment b), when being traced back to the LS, also correspond to the spring tide. The aforementioned literatures imply that there must exist a threshold of the tidal forcing strength at LS to induce ISWs. Such a threshold, which shall vary in the meridional direction, was established by Ramp et al. (2010) based on observations from a long-term mooring between the Itbayat and Batan Islands above the eastern ridge, which is considered to be a major generation site. Ramp et al. (2010) found this critical value for current speed to be equal to 71 cm s1 at this mooring site, below which no pronounced ISWs were observed. Zhang et al. (2011) introduced a simple linear model to correlate the amplitude of the generated ISWs to the Froude number at the generation site, and no ISWs emerge when the strength of the tidal currents is below a certain value. On the other hand, as was introduced in Section 3, the channels between the Itbayat and Batan Islands are conductive of pronounced lee waves due to the strong tidal flows that flow around the ridges (Pinkel et al., 2012). Instantaneous current speeds as large as 283 cm s1 were recorded in this region (Ebbesmeyer et al., 1991). Lien et al. (2005) mentioned that an abnormally strong barotropic tidal event occurred in 1995 which finally led to the unusual long-crest ‘‘big wave’’. Such atypical currents occurring in the LS are very likely to result in earlier appearances and larger scales of ISWs, such as the ISW packet just west of the LS (fragment c in Fig. 3), as was discussed at the beginning of Section 4. But whether it is indeed due to the large forcing in the LS cannot be asserted at present without robust evidence, and more studies need to be conducted. Apart from the amplitudes of different tidal harmonics, the phase difference between diurnal and semidiurnal tides at LS, which varies with time (Li and Farmer, 2011), can lead to such a composite asymmetrical structure as the tidal curve shown in Fig. 2. In recent years, due to this east–west asymmetry, there has been much debate about the alternate generation of type-a and type-b waves, which appear both in the deep basin and on the continental slope and shelf. As was reviewed in Section 3, quite a few papers (Zhao and Alford, 2006; Alford et al., 2010; Buijsman et al., 2010a,b; Ramp et al., 2010; Zhang et al., 2011) have been trying to correlate type-a and type-b waves with either the eastward or westward current peaks, accompanied by the diverse conclusions on the generation processes and mechanisms. Recently Li and Farmer (2011) and Vlasenko et al. (2012) found that, when decomposing the total barotropic tides at LS into the sum of semidiurnal and diurnal components (as shown in Fig. 2) and establishing a correlation with the observed ISWs, it is very clear that the ISWs always intermittently follow the maxima of

the semidiurnal tidal envelopes, regardless of the magnitude and the phase lag of the diurnal tides. This indicates that semidiurnal tides play a decisive role in the generation of ISWs in the northern SCS. Nonetheless, as was shown by Vlasenko et al. (2012), who numerically studied the characteristics of type-a and type-b waves during two different months, that the overlapping diurnal components are somehow able to substantially modulate the occurrence of ISWs, by leading to not only the generation of these two types of waves, but also to a near-monthly periodical transition of them. They argued that, although semidiurnal tides could only result in one hour’s time lag of the passing ISW at one fixed point, which is an important feature of type-b waves (Ramp et al., 2004), it fails to explain the same arrival times of type-a waves and the transition from one to another. By analyzing the complex occurring times of type-a and type-b waves, they proposed that it is the diurnal modulation on the semidiurnal tidal harmonics that leads to such features, and the temporal varying phase lag between the two species of harmonics can result in different scenarios of ISW occurrence. Furthermore, Vlasenko et al. (2012) also showed that the asymmetrical barotropic tides at LS can also lead to pronounced asymmetrical distribution of ISWs east and west of the LS. Arrival times of type-a and type-b waves exhibit different characteristics east of the strait, and the time interval between two successive waves in one day can even be as small as two or three hours. ISWs are ubiquitous west of the LS but are seldom reported east of it. With non-hydrostatic ROMS, Buijsman et al. (2010b) comprehensively studied the factors that can induce such an unequal east–west distribution by performing a series of sensitivity runs. They found that the asymmetrical barotropic tide in the LS contributes most to the lesser occurrence of ISWs east of the LS, and solely under this factor it is seen that eastward propagating solitary waves are 45% smaller than westward waves. 5.2. Impact of the Kuroshio intrusion The Kuroshio is a very intense western boundary current originating from the North Equatorial Current, with a velocity in the order of 1 m s1. Several manifestations have been identified when the Kuroshio passes by the LS (Liu et al., 2008): the main stream flowing northward between the two ridges and along the east flank of the eastern ridge without intrusion into the SCS; the intrusion of a branch; the formation of a ‘‘loop’’ style circulation, i.e., the main stream intruding into the SCS through the southern and middle reaches of the LS, meandering clockwise, and flowing out in the northern reach of the LS. Yuan et al. (2006a) argued that this anticyclonic intrusion of Kuroshio, accompanied by active eddy-shedding, is a transient process rather than a persistent one. Observational data has shown that the sea water in the LS features a sandwich-like inhomogeneous vertical structure, with net transport from the western Pacific into the SCS in the upper and bottom layers, and the opposite in the middle layer (Tian et al., 2006). So far, the water exchange across the LS and the Kuroshio intrusion are still not well understood, presumably due to the lack of longterm observational data to study such a complicated problem. As a consequence, currents in the middle LS, where the generation of baroclinic tides is the most efficient, can be uncertain, which undoubtedly affects the generation process of internal waves in the LS and clouds our understanding of this problem. Numerical runs performed have confirmed the influence of the Kuroshio on the generation of ISWs west of the LS (Cai et al., 2002; Du et al., 2008; Warn-Varnas et al., 2009; Buijsman et al., 2010b; Li and Farmer, 2011; Jan et al., 2012), and the results showed that the intrusion does have impacts on the generation process, although they are not crucial. Du et al. (2008) argued that the westward propagating branch of the Kuroshio can result in stronger

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westward background currents in the LS, which can increase the chance of producing internal mixing and thus groups of ISWs. With model results from ROMS, Buijsman et al. (2010b) showed that the Kuroshio, which can cause westward thermocline shoaling due to thermal wind balance, is one of the four factors that result in the highly unequal east–west distribution of ISWs on the two sides of the LS. On the contrary, numerical simulations conducted by WarnVarnas et al. (2009) showed that eastward propagating background steady currents can actually favour the generation of ISW trains. According to Warn-Varnas et al. (2009), although an eastward current reduces the strength of westward tidal currents and thus results in smaller wave depressions above the eastern ridge, ISWs appearing in the far field are significantly larger after they have evolved. They argued that with the inclusion of the background current, when depression waves generated by the eastern ridge reach the western ridge, the barotropic tides therein would have just reversed direction, making the amplitudes of the incoming waves larger due to the enhancement of the steady flow. Li and Farmer (2011) proposed that due to the Doppler effect brought by the Kuroshio, its westward component superimposed on the tidal currents reduces the amplitudes of the internal tides propagating with the steady current, which means that the Kuroshio plays a negative role in the production of ISWs in the northern SCS. That westward flowing Kuroshio currents weaken the generation of ISWs was also corroborated by Ramp et al. (2004) and by Li (2010), who examined the observational data at different periods but with similar tidal flows in the LS. They attributed the differentials to the different Kuroshio flows during the two periods. The effect of Kuroshio on the generation and propagation of baroclinic M2 and K1 tides in the LS was recently examined in depth by Jan et al. (2012), who used the 3D POM with idealized configurations to study the influences of the Kuroshio transport, location, and separation of the two ridges. It was found that the westward

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baroclinic energy flux is obviously enhanced when the Kuroshio is located in the middle or the west of the LS, whereas it is decreased when the Kuroshio is located to the east of the eastern ridge. The modulation of the Kuroshio on the wave generation exhibits different scenarios for M2 and K1 components, in the sense that the distance between the two ridges favours the resonance of M2 internal tides. The understanding of the influences of the Kuroshio on the generation and propagation of internal waves in the northern SCS is, to a large extent, controversial and incomplete, presumably because the understanding of the Kuroshio characteristics and water exchange at LS is not very clear. Current studies mostly remain superficial and are of a qualitative nature. A complete and accurate understanding calls for more support from observations and sophistication of numerical models (Section 2.3). 5.3. Variation of stratification in the northern SCS Oceanic stratification is crucial to internal wave properties. In the northern SCS, monthly-averaged climatological temperature/ salinity dataset from the World Ocean Atlas (2009) shows relatively spatial homogeneity but exhibits obvious seasonal variations (Fig. 7). The maximum buoyancy frequency in summer is located around 100 m and can reach 0.017 rad s1, while in winter stratification is somehow weaker owing to the existence of a larger upper mixed layer caused by the winter monsoon. In addition, stratification exhibits spatial variation on the two sides of the LS, with a deepening thermocline inclined eastwards due to the Kuroshio (Zheng et al., 2007; Buijsman et al., 2010b). Statistical analyses of SAR images have clearly shown the monthly and inter-annual variations of ISW occurrence. According to Zheng et al. (2007), April to July are the months most favourable for the generation of ISWs, while from December to February they are much less observed. Also, analysis of yearly distribution of

Fig. 7. Climatologically averaged monthly potential density (left) and buoyancy frequency (right) profiles in the northern SCS. Data is from the World Ocean Atlas (2009). The two thick dashed lines in the right panel indicate the weakest pycnocline with the thickest mixed layer in January, and the strongest pycnocline with the thinnest mixed layer in June, respectively.

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ISWs on SAR images (1995–2001) exhibits pronounced inter-annual variation, with frequencies in 1995, 1998, and 2000 between two to four times higher than in other years. The lower occurrence of ISWs in winter and their inter-annual variations have also been corroborated by in situ mooring observations. As was summarized in Section 2.2, a long-term mooring located on the continental slope of the northern SCS (Yang et al., 2009) showed that first mode ISWs were rarely seen in winter, in contrast to the activeness of this feature during the summer, whereas second mode ISWs were observed more frequently in winter. The long-term continuous data (from April, 2005 to June, 2006) recorded during WISE/VANS is very illustrative of the seasonal and inter-annual variations of the occurrence of ISWs (Ramp et al., 2010). It was found that the wave speed and amplitude increased steadily from April to November 2005, with the largest waves occurring in October and November, and not during the summer. Meanwhile, waves were mostly absent from December 2005 to February 2006, and reappeared in March. Moreover, a comparison between May 2005 and May 2006 also revealed strong inter-annual variability, with waves in May 2006 twice as large as those observed in May 2005. Ramp et al. (2010) attributed the lack of ISWs in winter to the formation of the deep winter mixed layer, and they also speculated that the sporadic occurrence of ISWs in winter can be correlated with some brief re-stratification events, like passing by warm spells and mesoscale eddies. However, Zheng et al. (2007) and Huang et al. (2008) argued that the low occurrence of ISWs on SAR images in winter has to do with frequent unfavourable high sea states that are likely to result in the underestimation of ISW numbers in SAR images. This gains support from numerical efforts. Jan et al. (2008) concluded that the baroclinic energy budget at LS shows little seasonal variation, but the generated baroclinic waves propagate 10% faster in summer than in winter. Li (2010) and Vlasenko et al. (2010) found that ISWs generated in winter are no smaller than those in summer, but they travel a bit more slowly although with almost the same amplitudes. Buijsman et al. (2010b) obtained monthly averaged density profiles based on Simple Ocean Data Assimilation (SODA) and did a series of experiments. Seasonal variation is pronounced in their model runs and they drew the conclusion that ISWs in summer are about 18% larger than in winter. Note that the multiple intrusion patterns of the Kuroshio (more likely to intrude in winter), as was discussed in the last subsection, might play a significant role in the seasonal and inter-annual variations of the occurrence of ISWs in this region. Inconsistent conclusions have been reached on the seasonal variation of ISW activities in the northern SCS. Most papers tend to believe that winter is the unfavourable season for the formation of ISWs. However, this claim requires verification with more longterm observations in order to provide more robust evidence. 5.4. Modulation of the western ridge As was mentioned before, the western ridge (Heng Chun Ridge) is much deeper than the eastern ridge (Lan Yu Ridge) except at the northern end where it connects to the shelf and its depth is comparable to the eastern ridge. Hence the western ridge is expected to play a less significant role in generating baroclinic waves. Numerical calculations of barotropic-to-baroclinic energy conversion rate (Niwa and Hibiya, 2004; Jan et al., 2008; Alford et al., 2011) show that the middle and southern reaches of the western ridge are relatively less efficient in generating baroclinic tides, whereas the northern reach serves as a pronounced generation site which contributes positively to the baroclinic tide field. The role of the western ridge should never be neglected, since it acts as a modulator for the generation of baroclinic tides at the two-ridge LS and thus alters the characteristics of ISWs formed beyond the strait in

the deep water. The average distance between the two ridges (80 km) is approximately equal to the wavelength of semidiurnal internal tides, which implies that resonant effect is expected to augment the strength of semidiurnal tides. However, different conclusions on the effects of the western ridge have been drawn, as will be shown below. The manner in which the western ridge can influence the distribution of the resulting wave field has been discussed in quite a few papers based on numerical methods (Chao et al., 2007; Du et al., 2008; Jan et al., 2008; Warn-Varnas et al., 2009; Vlasenko et al., 2010; Buijsman et al., 2010b, 2012; Echeverri and Peacock, 2010; Li and Farmer, 2011; Zhang et al., 2011). Chao et al. (2007) found that, apart from the northern reach, the middle part damps the incoming M2 waves emanating from the eastern ridge. A somewhat contrary conclusion was drawn by Warn-Varnas et al. (2009) who showed that the western ridge can enhance the generated wave trains. Echeverri and Peacock (2010) developed an analytical internal tide generation model with 2D arbitrary topography and applied it to the LS. Their modelling results indicate that the southern end of the western ridge plays a minor role, but further north it is more substantial. Meanwhile, Echeverri and Peacock (2010) also emphasize that the 2D radiated internal tides are very sensitive to the bathymetry and stratification, and need to be assessed on a case-by-case basis. The effects of the double-ridge system on the generation of ISWs were scrutinized by Vlasenko et al. (2010) in a 3D context, who performed two sensitivity runs by truncating the eastern and the western ridge, respectively. Both prominent first and second mode ISWs are generated with the two ridges, however, when the western ridge is truncated, the magnitude of the first mode ISWs is roughly the same, whilst second modes are hardly discernible. They concluded that it is the nonlinear interference of the wave signals generated by the two ridges separately that leads to the enhanced multimodal wave structures from the LS. Farmer et al. (2009) showed that the semidiurnal tidal beams emitted from the two ridges exhibit an in-phase superposition that implies resonance and enhanced wave generation. However, this does not take place for diurnal tides. Buijsman et al. (2010b) evaluated the role of the western ridge by conducting a series of sensitivity experiments regarding the height and its distance to the eastern ridge, and also drew the conclusion that the resonance due to tidal beams emanating from the two ridges enhances the development of ISWs west of the LS. This phenomenon was further scrutinized by Buijsman et al. (2012) who did a more thorough investigation and drew similar conclusions in terms of the double-ridge construction for semidiurnal internal tides and destruction for diurnal tides. Specifically, for the two ridge case, the barotropic-to-baroclinic tidal conversion, energy flux divergence, and dissipation are stronger than the sum of the two single-ridge cases when semidiurnal tides dominate, whereas it is the opposite for the diurnal case. In situ observational evidence on the effects of the western ridge was provided by Alford et al. (2011) who, based on a line of stations across the middle LS, observed nearly zero net semidiurnal energy flux but very strong energy between the two ridges, indicating an obvious standing wave pattern resulting from the interference of waves generated at the western ridge propagating eastward and waves generated at the eastern ridge propagating westward. However, such a pattern does not exist on the other line of stations at the southern reach of the LS due to the non-resonant characteristics along that line. Furthermore, the existence of the western ridge supports the formation of internal tide attractors in a two-ridge configuration by geometric focusing and trapping of internal wave rays which are generated locally by the two ridges (Tang and Peacock, 2010; Echeverri et al., 2011). It was shown that a great portion of the

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western ridge is less supercritical, undermining the formation of attractors. However, along the norther portion of the LS, the ridge bathymetry is favourable for their generation, as was studied theoretically by Tang and Peacock (2010) with the support of observational data. The fact that the average distance between the two ridges is approximately half the width of the semidiurnal internal tides suggests a potential resonance of the tides generated by the two ridges separately and thus an enhanced generation and dissipation should be expected. Indeed, current studies have basically proved this, especially the numerical work by Buijsman et al. (2012). They have shown the robustness of their model by comparing to the observations of Alford et al. (2011), and then performed detailed analyses regarding internal tide interference and its influence on wave generation and dissipation. Future works on this subject call for 3D modelling, given that the distance of the two ridges and the meridional location of the eastern ridge vary. Also, the diversion of tidal flows by islands and ridges leads to uneven distribution of wave generation conditions along the zonal transects, which may give rise to diverse scenarios of wave generation.

6. Conclusions In this paper an overall review of investigations on ISWs in the northern SCS is conducted, with the aim of summarizing recent fruitful explorations on this hot topic. ISWs and the related dynamics, including their origin in the LS, propagation in the deep northern SCS basin, and evolution above the continental slope/shelf, are reviewed based on different approaches. Various factors that can affect the characteristics of the generated ISWs are considered and described. It is concluded that basically three approaches have been employed to investigate the phenomenon, namely, satellite imagery, in situ measurements, and numerical simulations. Any one of these three approaches is indispensable and they are mutually supportive. Compilations of numerous historic satellite images have unveiled the impressive occurrence and spatially wide distribution of ISWs in this area on a statistical basis. The previous and ongoing ASIAEX, VANS/WISE, NLIWI, and IWISE programs have provided robust evidence of ISW characteristics in this area and have illuminated a number of novel results. Numerical models employed to study ISW generation and propagation in this area have been progressing at a rapid pace. Wave generation processes and mechanisms are primarily focused on, and a variety of factors that can affect ISW dynamics are taken into account via conducting multiple sensitivity runs. ISWs in the northern SCS originate from the LS due to strong tide-topography interactions. The generation mechanisms have been extensively studied but conclusions vary from one study to another. Generally it is believed that ISWs are formed primarily due to nonlinear steepening of long depression waves formed above the eastern ridge. Normally there is only one wave in the deep basin, but multi-wave packets emerge near the shelf break, which are mostly seen on satellite images. When getting close to the shelf break, the incoming internal tides, on which ISWs ride, are partly dissipated and reflected backwards, whilst the remaining energy (only a small portion) goes up onto the continental shelf. ISW polarity conversions were observed by both shipboard and long-term moorings under the topographic effects. The local generation of baroclinic tides, especially diurnal tides, is proved to exist and contributes to the local wave fields. ISW properties in the northern SCS are affected by various factors. The fortnightly periodic feature of barotropic tides in the LS, which comprise diurnal and semidiurnal harmonics of comparable amplitudes, impacts significantly on the resulting wave fields.

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ISWs are generated following the maxima of the semidiurnal envelope, whereas diurnal tides, which cause the asymmetry of the total tides, lead to the generation of type-a and type-b waves and a transition from one to another. Kuroshio intrusion in the LS intensifies the background currents and thus affects the wave generation processes. However, the intrusion is highly time-varying and bears complex vertical features, which makes the problem perplexing and its role remains to be assessed with more studies. Wave occurrence in the northern SCS also shows obvious seasonal and inter-annual variations, with waves in winter less active than in summer, indicating the impact of seasonal stratification on the generated waves. It is generally accepted that the western ridge plays a minor role in the southern reach of the LS, but is more substantial further north. Although still sort of controversial, foregoing studies tend to favour the belief that the western ridge (mainly the middle part) can augment the baroclinic wave signal due to the resonant effect of the double-ridge system.

Acknowledgements This work was supported by the Natural Science Foundation of China Project No. 41276008. We thank European Space Agency for providing the ASAR images.

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