Variability of internal tides and near-inertial waveson the continental slope of the northwesternSouth China Sea

Zhenhua Xu,1,2 Baoshu Yin,1,2 Yijun Hou,1,2 and Yongsheng Xu1,2

Received 17 May 2012; revised 22 November 2012; accepted 29 November 2012; published 28 January 2013.

[1] Structure and variability of internal tides (IT) and near-inertial waves (NIW) onthe continental slope of the northwestern South China Sea were investigated, based on9-month moored current observations from autumn to early summer in 2008 and 2009.The diurnal IT kinetic energy, dominant over that of semidiurnal tides, is found to exhibitapparent seasonal variability—strongest in summer and weakest in winter—whereas thesemidiurnal variance remained nearly uniform throughout the observation period.Moreover, the diurnal IT were more coherent (i.e., phase-locked to the astronomicalforcing) than the semidiurnal constituents. Coherent diurnal variance accounts for about40% of diurnal motions, but semidiurnal tides contain a much smaller fraction (10%)of coherent motions. Further analysis demonstrates that the diurnal IT are dominated by thefirst mode, whereas the semidiurnal tides show a variable multimodal structure: the secondmode is dominant in summer and comparable to the first mode in spring and autumn,but the first mode predominates in winter. Multimodal semidiurnal IT are more influencedby varying stratification structures and background currents and thus exhibit highlyincoherent and intermittent behavior, which may wash out seasonal variability during theirlong propagation from the generation source. The observed NIW are seasonallyindependent and comparable to the semidiurnal motions. During the passage of TyphoonHagupit, however, the NIW became the most energetic component of the inertia-gravitywaveband motions. NIW energy and shear were significantly enhanced and exceededtidal counterparts by a factor of 2 to 3 in the upper layer.

Citation: Xu, Z., B. Yin, Y. Hou, and Y. Xu (2013), Variability of internal tides and near-inertial waves on the continentalslope of the northwestern South China Sea, J. Geophys. Res. Oceans, 118, 197–211, doi:10.1029/2012JC008212.

1. Introduction

[2] Internal inertial-gravity waves are waves in stratifiedwaters within the angular frequency band f (’)<o<N (x, z),where f is the local inertial or Coriolis frequency and N is thehorizontally varying and depth-dependent buoyancy fre-quency [Alford, 2003; Helfrich and Melville, 2006; Garrettand Kunze, 2007; Shaw et al., 2009; Zhao et al., 2010]. Inter-nal tides (IT) are usually generated in stratified waters bybarotropic tidal currents flowing over variable topographyfeatures, such as the continental shelf break, sills, or ridges[Nash et al., 2006; Garrett and Kunze, 2007; Zheng et al.,2007; Qian et al., 2010]. On the other hand, near-inertialwaves (NIW) mostly arise from wind forcing at the sea

surface, although some may be excited by geostrophicadjustment of currents in the ocean interior [Alford, 2003;van Aken et al., 2007]. Breaking IT and NIW are supposedto be possible energy sources for stimulating strong turbu-lence and vertical mixing and are thus important in main-taining the meridional circulation [St. Laurent and Garrett,2002]. NIW have often been found to generate greater shearthan IT over the continental slope zone and in the deepocean [Alford, 2010].[3] Higher-mode IT usually break and dissipate near their

source regions and thus generate local ocean mixing [Klymaket al., 2011; Vlasenko et al., 2010], whereas low-mode ITmay propagate for thousands of kilometers before dissipatingand usually dominate further from the source region[St. Laurent and Garrett, 2002; Rainville and Pinkel, 2006;Zhao et al., 2010]. In addition, after a long propagation fromthe source region, low-mode IT interact with backgroundconditions and integrate their effects through variable stratifi-cation and mesoscale activity [Park et al., 2006]. Then, theIT may exhibit intermittent feature and occur at frequenciesoutside the deterministic tidal frequencies, becoming inco-herent with the astronomical tides. The separation of incoher-ent IT from deterministic motions requires sufficiently longobservations. A variety of long-term current measurements

1Institute of Oceanology, Chinese Academy of Sciences, Qingdao,China.

2Key Laboratory of Ocean Circulation and Waves (KLOCAW), ChineseAcademy of Sciences, Qingdao, China.

Corresponding author: B.Yin, Institute ofOceanology, ChineseAcademy ofSciences, 7 Nanhai Road, Qingdao 266071, China.(bsyin@qdio.ac.cn; xuzhenhua@qdio.ac.cn)

©2012. American Geophysical Union. All Rights Reserved.2169-9275/13/2012JC008212

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JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 197–211, doi:10.1029/2012JC008212, 2013

have been made and used to examine the incoherent natureof IT across world seas in recent years. In the Bay of Biscay,incoherent IT of 5–7 day periods were found to account for30% of total tidal kinetic energy in the semidiurnal tidal bandand resulted from large-scale variations in the stratification[van Haren, 2004]. Eich et al. [2004] suggested that energeticincoherent semidiurnal IT could explain 40% of total semidi-urnal band variance in Mamala Bay and were attributable tothe remote generation and influence of variable stratificationand mesoscale features during the shoaling process of thedeep ocean internal wave field. Based on long-term mooredcurrent and temperature measurements above Kaena Ridge,Zilberman et al. [2011] found that eddy-related changes instratification and cross-ridge current speed contributed to in-coherent barotropic to baroclinic tidal energy conversion.[4] The northern South China Sea (SCS), as a semi-closed

marginal sea extending from the western Pacific to the Chinashelf, has been found to have strong internal tides amongworld seas [Liu et al., 2004; Yang et al., 2004]. The northernSCS is also strongly affected by the East Asian monsoonsystem and frequently experiences strong tropical cyclonesoriginating from the western Pacific [Zheng et al., 2006].Strong vertical mixing and horizontal pressure gradientscaused by typhoon winds can produce obvious variationsof stratification structure and lead to the formation of strongnear-inertial internal waves in the ocean interior [Garrett,2001; Alford, 2010; Xu et al., 2011a]. However, given theshortage of long-term field measurements, few studies havecompared the relative contributions of IT and NIW to theSCS baroclinic field. Moreover, which of these is more im-portant in generating the shear field and leading to mixingin the northern SCS has not been addressed.[5] Until recently, most IT studies have been restricted to

the northeastern SCS (east of Dongsha Plateau). The IT in thisarea mostly originate from the Luzon Strait [Jan et al., 2007;Shaw et al., 2009], propagate westward across the deep basin[Duda et al., 2004; Liu et al., 2004; Zhao et al., 2004; Farmeret al., 2009], and disintegrate into nonlinear waves on the SCSshelf [Liu et al., 1998;Cai et al., 2002; Ramp et al., 2004; Lienet al., 2005; Zhao and Alford, 2006; Xu et al., 2010]. Moststudies indicate that first-mode motions dominate the IT field,although higher-mode signals may be significant during spe-cific cases or over short periods [Wang et al., 2007; Klymaket al., 2011; Vlasenko et al., 2010; Lee et al., 2012].[6] The northern SCS is unique for its mixed-tide charac-

ter, especially in its northwest area with dominant diurnalsurface tides and IT. Recent studies have revealed the pres-ence of multimodal structure IT in the northwestern SCS.Xu et al. [2011b] found that the diurnal constituents weredominated by first-mode motions, whereas the semidiurnalIT exhibited a higher modal structure with dominant sec-ond-mode signals on the shelf zone of the northwesternSCS (west of Dongsha Plateau and east of Hainan Island).[7] The incoherent nature of IT in the northern SCS has

been recently observed and studied. Based on 8-monthmoored acoustic Doppler current profiler observations inthe northeastern SCS, Lee et al. [2012] found that incoherentinternal tidal motions could explain about three fourths ofobserved tidal energy. However, they did not decomposethe incoherent signals into diurnal and semidiurnal compo-nents and examine their relative contributions. In particular,in the portion of the northwestern SCS that is over 600 km

from the Luzon Strait and 300 km southwest of the Leeet al.’ mooring site, diurnal and semidiurnal IT have beenfound to take the form of different modal structures [Xuet al., 2011b]. Thus, it is natural to question whether the diurnaland semidiurnal IT have the same incoherent feature. Nonethe-less, previous studies of IT in the northwestern SCS are mainlybased on short-term continuous measurements and are there-fore unable to precisely separate incoherent signals from coher-ent tides. In addition, relative contributions to the shear field byIT and NIW, and especially those under the influence oftyphoons, were also not determined for the northern SCS.[8] In this study, we use a 9-month continuous current data

set to investigate seasonal variability of IT and NIW on thecontinental slope of the northwestern SCS. We aim to makea comparison of the modal structure between diurnal andsemidiurnal IT and characterize the degree of coherence withastronomical forcing of semidiurnal IT versus diurnal IT.Long-term current observations also enable isolation of thetidal band and near-inertial band motions and examinationof their related kinetic energy and shear fields. NIW packetsduring the typhoon are also described and compared withthose during calm periods (termed as the periods withouttyphoon influence). We found that kinetic energy in the diur-nal tidal band has apparent seasonal variations, while semidi-urnal IT energy remained nearly unchanged throughout theobservational period. The reason for the different temporalvariability between diurnal and semidiurnal IT is discussed inrelation to their contrasting incoherent natures and vertical struc-tures. NIW energy and shear during the passage of TyphoonHagupit are also described and discussed.[9] The paper is organized as follows: section 2 describes

the data and methods; the rotary spectra of raw currents areexamined in section 3; sections 4 and 5 describe observa-tions of barotropic tides and IT, respectively; section 6 pre-sents the stratification structure and background currents;section 7 outlines the NIW analysis; section 8 gives the shearanalysis related to IT and NIW; and finally, discussions andconclusions are presented in section 9.

2. Data and Methods

[10] We used a 9-month (14 September 2008 to 17 June2009) time series of data obtained from an acoustic Dopplercurrent profiler (ADCP) at a station on the continental slopeof the northwestern SCS. Water depth at this station is405m. The study area and mooring position are indicated inFigure 1. The topography slope at the mooring site is subcrit-ical with respect to the diurnal and semidiurnal IT, estimatedon the basis of ETOPO5 5-min gridded data andWorld OceanAtlas 05 (WOA05) climatology data. The 75 kHz up-lookingADCP was positioned at 225m depth. The ADCP functionedwell with up-down movement within 5m during the entirerecording period, despite being subjected to the passage ofstrong internal tides and high-frequency solitary waves. Thedepth of available current data measured by the ADCP rangedfrom 28 to 220m, with vertical interval 4m. Current measure-ments were recorded with a precision of 5� 10�3m/s andtime interval 1 h. The horizontal velocity is used here and isdecomposed into along-isobath (positive denotes northeastdirection) and cross-isobath (positive denotes northwest)components, with the along-isobath direction defined toward35� north of east.

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[11] The barotropic current is defined as the depth-averagedflow, and the baroclinic current as the residual once the baro-tropic current is removed. Barotropic tidal currents were esti-mated by applying a least-squares fit method of harmonicanalysis to the depth-averaged currents [Pawlowicz et al.,2002]. Tidal harmonic analysis was also used to examine thevertical structure and temporal variation of internal tidal cur-rents. Harmonic analysis of barotropic and baroclinic tides wasapplied to the entire period of record. Baroclinic current timeseries were band-pass filtered to separate diurnal (0.8–1.2 cpd),semidiurnal (1.8–2.2 cpd), and near-inertial (0.5–0.9 cpd)motions, via a fourth-order Butterworth filter. We also usedempirical orthogonal function (EOF) method to characterizedetailed modal content of the baroclinic signals. This methoddoes not depend on any dynamical assumption but only onstatistics of the data. However, EOF analysis does demonstratea qualitative modal structure of baroclinic tides in the SCS tosome extent [Lee et al., 2012]. Furthermore, rotation spectraproperties of the observed current were calculated to describethe rotation components of motions at various frequencies,according to the method of Emery and Thomson [2001].[12] We divided the observational data into autumn

(14 September to 30 November 2008), winter (1 December2008 to 28 February 2009), spring (1 March to 31 May2009) and early summer (1–17 June 2009) months. Note thatour measurements only include 17 days of data during thesummer, while data were acquired from 75 to 90days duringthe other seasons. Nevertheless, as seen from the trend of tidalenergy variations (Figure 3), we see that the half-month datacan represent IT properties in the summer season to some de-gree, which is consistent with the enhancement of IT activityin summer described by many previous studies [Eich et al.,2004; Chavanne et al., 2010]. Further, we extended the sum-mer data to 25–30 days by including current data from 17 to22 May in late spring and analyzed elongated baroclinic cur-rents using the harmonic analysis method. The harmonicresults from late spring to summer are similar to those fromthe summer month only (June) and show a clearly intensifiedpattern relative to those in other months, especially the wintermonths. This once again indicates that the 17-day currentrecords can capture some properties of IT in summer.

[13] Our mooring also recorded the passage of TyphoonHagupit. Hagupit originated as a tropical depression over thewestern North Pacific on 19 September and then intensified intoa tropical storm on 20 September and moved west-northwest-ward. Hagupit strengthened into a typhoon on 21 Septemberand entered the SCS on 22 September; it directly crossed ourmooring position with wind speed 49m/s on 23 September. Itmade landfall in western Guangdong Province on the morningof 24 September, with wind speed 43m/s. Therefore, two dis-tinctly different periods of NIW events were distinguished,based on prevailing atmospheric conditions. The first was anenergetic period from 23 to 30 September, during which thetyphoon passed and induced strong NIW packets. The secondwas a calm period during the rest of the observational period,which we consider characteristic of nominal conditions withweak and irregular NIW.

3. Current Spectra

[14] Figure 2 shows rotary spectra of entire record rawcurrents at 48m depth, with 42 degrees of freedom. Spectrawere determined with a 30-day window and had frequencybandwidths about 0.076 cpd. Filtered spectra with frequencyresolution 0.22 cpd were added to the figure. The currentspectra show significant peaks at the near-inertial and diur-nal and semidiurnal frequencies, with dominant clockwiserotation. Since internal tidal variance is mainly containedin the clockwise component in the northern hemisphere,the peaks at near-inertial and tidal frequencies were thereforeconsistent with pronounced internal wave activity in thenorthern SCS. The clockwise component of the currents isdominated by motions in the diurnal frequency band (K1

and O1), suggesting that the diurnal IT are much strongerthan the NIW and semidiurnal IT in the study area. Addi-tional significant spectral peaks were also observed at fre-quencies of the combination of diurnal and semidiurnal tidesand higher harmonics (i.e., O1 +M2, O1 +M4), because ofnonlinear interaction and advection of the main diurnal andsemidiurnal tides. The spectra also show a discernible butnot significant peak at f +M2 frequency, owing to the M2

heaving of near-inertial shear layers [Alford, 2001]. Further,

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Figure 1. Topography of the northern South China Sea. Triangular symbol indicates mooring position.Contours show isobaths in meters. Track of Typhoon Hagupit from 22 to 24 September 2008 and windspeeds in the vicinity of the mooring are denoted.

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the diurnal tidal currents tend to be polarized (i.e., the clock-wise component is much larger than the counterclockwisecomponent), whereas the semidiurnal tidal currents are morerectilinear than their diurnal counterparts.[15] The NIW are purely clockwise and have energy com-

parable to the clockwise component of the semidiurnalmotions, which is commonly observed at lower frequenciesin the northern hemisphere. In addition, unlike the narrowtidal peaks shown in Figure 2, the NIW energy is containedin a broad frequency band [Fu, 1981; Alford and Whitmont,2006]. The central frequency band of the NIW is 0.74–0.80 cpd, with a blue shift about 6%–14% above the inertialfrequency (0.70 cpd) [Garrett, 2001].

4. Barotropic Tides

[16] Barotropic tidal currents in the study area are domi-nated by the two main diurnal constituents K1 and O1, andthe two semidiurnal constituents M2 and S2. The diurnalconstituents are dominant and much stronger than the semi-diurnal constituents. K1 is the largest barotropic tidal constit-uent, with semi-major axis length approximately 8.7 cm/s,followed by O1 with magnitude 6.65 cm/s (Table 1). TheM2 current ellipse has amplitude roughly one fourth that ofK1; the S2 has the weakest magnitude, roughly one third thatof M2. The K1 (O1) constituent is generally aligned with thecross-isobath (along-isobath) direction within a few degrees,the M2 with the comparable cross-isobath and along-isobathcomponents, and the S2 aligned mainly in the zonal direc-tion. The M2 tidal currents are almost rectilinear, whereasthe K1, O1, and S2 tidal ellipses are slightly circular. Further,diurnal velocity vectors rotate clockwise, whereas semidiur-nal velocity vectors turn anticlockwise.

5. Internal Tides

5.1. Temporal Variations

[17] Seasonal variability of internal tidal energy has beenexamined by a variety of field measurements and numerical

methods over many regions of the global oceans. Eich et al.[2004] observed seasonal changes in amplitude and phase ofthe semidiurnal IT in Mamala Bay based on moored currentmeter and temperature observations. At some long-termmoor-ing sites on the shelf between Timor Island and Australia,Katsumata et al. [2010] found that semidiurnal IT energyreached a maximum in June–July because of increased strat-ification.Chavanne et al. [2010] also suggested that observedM2 energy density had apparent seasonal variability in theKauai Channel. Using a numerical hydrostatic internal-tidegeneration model, Gerkema et al. [2004] showed that the sea-sonal thermocline was essential to the evolution and propaga-tion of IT, but it had a minor effect on the generation of ITfrom deep sources. Most of the aforementioned studies con-centrated on seasonal variations of semidiurnal tides; temporalvariations of the diurnal component have rarely been reported.[18] The stratification structure and background currents

in the northern SCS are seasonally dependent, under the influ-ence of solar radiation, the monsoon system, and Kuroshiointrusion. It is then natural to assume that internal tidal energyin this area might have highly temporal variations.Qian et al.[2010] used a three-dimensional nonhydrostatic numericalmodel to study generation of internal waves in the SCS. Theyfound that internal wave generation was most efficient underthe presence of strong upper ocean stratification duringspring and summer. Based on long-term current observationin the northeastern SCS, Lee et al. [2012] found that both di-urnal and semidiurnal IT showed large irregular fortnightlyvariations; however, they appeared to be seasonally invariantbecause of overwhelming incoherent motions. On the conti-nental slope of the northwestern SCS in particular, seasonalvariations of IT and NIW energy remain poorly understoodowing to the shortage of long-term field observations.[19] Thus, to examine temporal variations of IT and NIW

on the northwestern SCS slope, we isolated the filtered tidaland near-inertial components from the 9-month barocliniccurrent observations. The baroclinic current variances werethen calculated as Var = (u2, v2) with the 9-month window,to represent fluctuations for the diurnal (0.8–1.2 cpd), semi-diurnal (1.8–2.2 cpd), and near-inertial (0.5–0.9 cpd) bands.Figure 3 shows depth-averaged baroclinic current varianceat these frequency bands, from September 2008 to June2009. Low-frequency signals of the variances with a fort-night-period cutoff are also shown, to clearly indicatemonthly and seasonal variations of the baroclinic tides.[20] From the upper panel of Figure 3, we find the interest-

ing result that the diurnal tidal variance exhibits apparentseasonal variability. The diurnal variance gradually weakensfrom autumn to winter and reaches a minimum from Januaryto February during winter. The variance tends to strengthenfrom spring to early summer. Although the current data onlyspan 17 days in early summer, the apparent enhancement of

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Figure 2. Spectra for clockwise (red) and counterclock-wise (blue) components of raw velocity at 48m. Filteredspectra also noted with black lines.

Table 1. Ellipse Properties of the Major Diurnal and SemidiurnalBarotropic Tidal Constituents

ConstituentSemi-majorAxis, cm/s

Semi-minorAxis, cm/s Inclination, deg Phase, deg

K1 8.70 �5.36 121 175O1 6.65 �4.16 63 63M2 2.15 0.28 161 192S2 0.83 0.53 8 69

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kinetic energy from late spring to summer (10May to 17 June)clearly indicates the intensification of diurnal IT activity rela-tive to that in winter.[21] In surprising contrast, kinetic energy in the semidiur-

nal tidal band remains nearly uniform throughout the observa-tional period, with comparable energy in winter and summer(middle panel of Figure 3). Apart from their seasonal variabil-ity, diurnal IT exhibit fortnightly and monthly variations. Thesemidiurnal IT tend to be more intermittent, with a 3–5 day os-cillation. The contrasting seasonal behavior between diurnaland semidiurnal IT was an initial motivation for our study,which is detailed in the following sections.[22] Furthermore, during the passage and departure of

Typhoon Hagupit (23 to 30 September), NIW events becamethe most energetic component of inertia-gravity wavebandmotions. The mean NIW kinetic energy induced by strongtyphoon winds was significantly enhanced, by a factor of 10.It also exceeded the total diurnal and semidiurnal internaltidal energy, by a factor of 2 to 3. However, during the calmstage, the NIW energy was generally smaller than diurnal tideenergy and a little larger than that of the semidiurnal tides.The observed NIW variance appears seasonally invariant, incontrast to the winter intensification frequently observed inthe northern hemisphere deep sea [Alford, 2003].

5.2. Vertical Structures in Different Seasons

[23] To further address the seasonal variability and verticalstructure of diurnal and semidiurnal IT, half-month diurnaland semidiurnal cross-isobath currents in summer and winterare shown in Figure 4. Diurnal currents are much stronger insummer than in winter, which is consistent with the descrip-tion in the previous section. Diurnal currents with amplitude15–30 cm/s appear throughout the observed water columnduring the spring tides of the summer month. In winter, thelargest diurnal currents are of magnitude 20 cm/s and occuronly occasionally within the 180–220m depth range. In con-trast, the amplitude of the semidiurnal currents remains

nearly unchanged in summer and winter, in agreement withthe earlier results. The semidiurnal currents have the highestamplitude (roughly 15 cm/s), and the strong currents inducedby the semidiurnal tides occur highly intermittently duringboth seasons.[24] Furthermore, the vertical structure differs greatly

between diurnal and semidiurnal motions (Figure 4). DiurnalIT show the dominance of first-mode vertical structure, butthere are some seasonal differences between structures. Insummer, diurnal currents had a stable vertical structure, espe-cially during the spring tides from 11 to 16 June when a strongseasonal thermocline was present [Zheng et al., 2007]. Thecurrents have a more unstable vertical structure in winter.The diurnal current profile displays a reversal structure inthe vertical, with a zero crossing point at ~120m depth duringthe spring tides of summer and ~130m depth in winter. Thisis probably related to depth variations of the seasonal pycno-cline during different seasons.[25] Figure 4 clearly reveals the multimodal and intermittent

structure of semidiurnal IT. Moreover, during the period ofstrong semidiurnal currents, semidiurnal IT are observed withsecond-mode structure in summer but have a first-mode struc-ture in winter. For example, the semidiurnal current profileintensified from 8 to 10 June and showed two current-reversalstructures in the vertical, with zero crossing points at 80 and180m depths and vertical wavelength less than 100m. In con-trast, the strong semidiurnal packets showed a first-modestructure with zero crossing points at 120m depth from 11 to13 January and at 140m depth from 13 to 15 January.[26] Unlike the notable spring-neap cycle of diurnal

motions shown in the upper panel of Figure 4, the intermit-tent semidiurnal variance oscillates with the 3–5 day period,which agrees with the earlier analysis of velocity variance.

5.3. EOF Analysis

[27] We used EOF analysis of the cross-isobath currents toreveal the detailed modal structure of IT. Figures 5–7 show

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the modal structure and spectral density of temporal variancesfor the first three modes of IT in summer, winter, and springseasons, respectively. The spectral density can representkinetic energy of the various modes of IT. The autumn resultis similar to that of spring and is not shown. The modal struc-ture of IT also differed greatly between semidiurnal and diur-nal constituents. In the diurnal frequency band, the first modewas dominant throughout the observation period, consistentwith the earlier description. The reversal layer depth of the firstmode and depths of the uppermost reversal layer of the highermode are deeper in winter than in summer and spring. Thisdoes not coincidewith the specific depths indicated in Figure 4,but it does have a qualitatively similar pattern.[28] In contrast, semidiurnal tides show a variable, multi-

modal structure. Second-mode motions are dominant insummer (Figure 5). First-mode and second-mode motionshave comparable magnitudes in spring (Figure 6) andautumn (not shown), but the first-mode component predomi-nates in winter (Figure 7); this matches the analysis of timeseries cross-isobath currents shown in Figure 4. The modalstructure of the second-mode semidiurnal component inFigure 5 also displays two reversal layers in the vertical, withzero crossing points at 75 and 180m depth. This is in quali-tative agreement with contours of the semidiurnal cross-iso-bath currents in summer (Figure 4; oscillating layers around80 and 180m).

5.4. Coherent Motions

[29] Tidal-current ellipses of the major semidiurnal anddiurnal constituents are derived from harmonic analysis of

baroclinic current measurements over the entire recordperiod [Emery and Thomson, 2001]. The harmonic analysisresults are mainly coherent baroclinic tidal waves (i.e.,phase-locked to the astronomical tides). We also used vary-ing windows (one to nine months) to calculate coherent andincoherent components of the baroclinic tides. The scales ofcoherent diurnal and semidiurnal motions over the incoher-ent parts, which are the focus of the present study, changeslightly with varying windows. Thus, the harmonic analysisover the entire record period was used. In this section, weuse the two major constituents K1 and M2 to represent thediurnal and semidiurnal band motions, respectively, accord-ing to most previous studies of the northern SCS (Figure 8).[30] Consistent with the domination of diurnal currents for

the barotropic tides, here the diurnal internal tidal currents arealso much larger than those of the semidiurnal constituents.Despite the large amplitude of the K1 component in bothsummer and winter, the summer K1 and M2 internal currentswere all higher than those in winter, by a factor approaching2. The M2 internal currents were weak in both seasons, withamplitudes ~2 cm/s in summer and ~1 cm/s in winter.[31] In addition, there are remarkable differences of modal

structure between the diurnal and semidiurnal IT. The K1

resembles a steady first-mode structure, i.e., with two sepa-rate layers oscillating 180� out of phase, around 120m depthin summer and 140m in winter. In contrast, M2 appears toshow a variable and intermittent multimodal structure inboth summer and winter.[32] The K1 IT in both seasons mainly aligned with the

meridional direction but deflected slightly east in the upper

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layer and west in the lower layer. The M2 internal tides gener-ally propagated in the northwest direction, but with an ambigu-ity of 180�.

5.5. Incoherent Feature

[33] We subtracted the coherent components calculated bythe harmonic analysis method from the raw baroclinic currents

to obtain the incoherent tidal currents and then calculated thecoherent and incoherent velocity variances. Figure 9 shows thedepth-averaged coherent and incoherent portions of the diurnaland semidiurnal variances over the entire observational period.[34] The time series coherent and incoherent variance

once again reveals the seasonally dependent diurnal motionsand seasonally independent semidiurnal kinetic energy. The

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(m

2 /s2 )

(b)Mode 1

Mode 2

Mode 3

Figure 6. As in Figure 5, but for results in spring.

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coherent diurnal variance gradually decreased from autumnto winter, reaching a minimum from January to March dur-ing winter. This variance tended to increase from spring toearly summer, attaining a maximum between 20 May and15 June. Similarly, the incoherent diurnal variance reacheda maximum from May to June but remained uniform fromautumn to early spring. In contrast, the coherent and incoher-ent semidiurnal variances showed strongly intermittent be-havior and did not have clear seasonal variation.[35] The coherent diurnal signals showed a pronounced

spring-neap cycle with the fortnightly period, whereas the inco-herent motions displayed an irregular spring-neap cycle withsuperposition of oscillation signals with the period 3–5days.Coherent semidiurnal signals also showed an irregular spring-neap cycle, modulated by a 3–10day period, whereas incoher-ent semidiurnal motions were highly intermittent with a 1–2day period. The dominant incoherent nature of semidiurnalIT probably accounts for the lack of apparent spring-neap andseasonal modulations of semidiurnal motions, as shown inFigures 3 and 4.[36] We also calculated time averages of variances of

coherent and incoherent components in the diurnal andsemidiurnal bands (Figure 10). The incoherent diurnal vari-ance accounts for about 40% of diurnal motions, but thesemidiurnal tidal variance contains a much smaller fraction(10%) of the coherent motions.[37] Figure 11 also shows time-averaged coherent and inco-

herent variances in the diurnal and semidiurnal frequencybands, in summer andwinter. One sees that the diurnal coherentand incoherent motions display a first-mode structure in thevertical, during both summer and winter. This is consistent withthe time series currents and EOF analysis results of previoussections. In the semidiurnal frequency band, both coherent andincoherent signals exhibit a second-mode structure in summer

and first-mode structure in winter, also consistent with the ear-lier findings.

6. Stratification Structure and BackgroundCurrents

[38] Buoyancy frequency in the study area was calculatedfrom WOA05 temperature and salinity data (Figure 12). Thestratification structure had evident seasonal variability, ow-ing to effects of the seasonal monsoon system and Kuroshiointrusion. Upper ocean stratification was strongest in sum-mer and weakest in winter, consistent with apparent en-hancement of the coherent diurnal IT in summer relative tothat in winter (Figure 9a). This obvious seasonal variationof coherent diurnal energy in close relation to stratificationstrength of the upper water column probably resulted inthe seasonal variability of diurnal IT (Figure 3).[39] Sub-inertial along-isobath and cross-isobath currents

were also low-pass filtered from the raw current time seriesusing a 0.6 cpd cutoff frequency, via a fourth-order Butter-worth filter. The raw and depth-averaged currents are shownin Figure 13. One sees that strong background currents withan average amplitude of about 20 cm/s occurred from Febru-ary toMarch 2008. Combined with one-layer temperature datafromADCP and AVISO elevation data (not shown), we foundthat the strong background currents were caused by a warmeddy. During eddy passage, the along-isobath currents flowednortheast in the second half of February and turned southwestfrom 1 to 20 March. Strong onshore cross-isobath currentsdominated from 10 to 28 February and then reversed to an off-shore direction from 1 to 15March. However, during the otherobservational period with no influence from the strong eddy,sub-inertial background currents were weaker, with an averagevalue of about 10 cm/s and no notable seasonal variation.

−0.4 −0.2 0 0.2 0.4

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Spe

ctra

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Figure 7. As in Figure 5, but for results in winter.

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Along-isobath currents averaging 10 cm/s generally flowedtoward the southwest, whereas onshore and offshore cross-isobath currents alternately occurred.[40] The winter weakening of IT has been suggested to be

closely related to a weak pycnocline (Figure 12). Moreover,diurnal and semidiurnal IT were both observed to be weakest inthe second half of February, throughout the observational per-iod (Figure 3). This might be due in part to the effect of strongnortheast eddy currents (Figure 13). In addition, Figure 9 showsthat incoherence of both diurnal and semidiurnal baroclinictides increased during the first half of March, which might co-incide with refraction of southward background currents

induced by the eddy. To gain a clear idea how backgroundcurrents and eddies influence the baroclinic tides, more fieldobservations and numerical work are needed.

7. Near-inertial Wave Packets

[41] Typhoon Hagupit propagated westward across ourmooring site, and east-west velocity was much stronger thanthe north-south component during the typhoon-wake period.Hence in this section, we use the east-west component torepresent the baroclinic velocity structure. Figure 14 showsseveral examples of NIW packets during the typhoon-wakeand calm periods. We see that there were weak NIW currentsof amplitude ~5 cm/s prior to typhoon arrival. These currentsincreased to 30 cm/s on 23 September, reaching maximumvalues of 60 cm/s on 25 September in the upper layer, duringthe typhoon-wake period. The NIW events generated by thetyphoon appeared from 23 to 29 September and radiateddownward, consistent with the mechanism of wind forcingfrom the surface. NIW currents caused by the typhoon exhib-ited a steady first-mode structure, with shear interface around80m depth, upper layer currents of 60 cm/s, and lower layercurrents about 40 cm/s.[42] In contrast, during the calm period, the NIW signals were

weak and very intermittent. Both upward (23 to 27 November)and downward (30 September to 3 October) radiating NIWmotions were observed, suggesting that the NIW in the SCScould arise from wind forcing at the sea surface, or be excitedby the geostrophic adjustment of currents in the ocean inte-rior, or survive multiple reflections from the thermocline andocean bottom.

8. Shear Analysis

[43] Figure 15 shows temporal mean baroclinic currentvariances in the diurnal, semidiurnal, and near-inertial fre-quency bands during summer, winter, and the typhoon-wakeperiod. To compare IT and NIW contributions to the shearfield, we also calculated the time-average shear S = ((@u/@z)

2,(@v/@z)

2) produced by currents in the three bands at 4m verti-cal scale over each period and compared them (Figure 13,lower panel).[44] Relative magnitudes between internal and near-iner-

tial band motions in spring and autumn are similar to thosein summer and winter and thus are not shown. During thecalm stage in summer and winter, NIW kinetic energy wasgenerally smaller than diurnal tide energy and a little largerthan that of semidiurnal tides (Figure 13). In addition, NIWshear was smaller than diurnal and semidiurnal IT shear.On the other hand, during the typhoon passage, NIW packetsbecame the most energetic component of the inertia-gravitywaveband motions. The NIW energy and shear induced bystrong typhoon winds were significantly enhanced andexceeded internal tidal energy and shear by a factor of 2 to3 in the upper layer, having implications for local mixing.The NIW shear reached a maximum at 70–90m depths,where the first-mode node in the vertical of NIW events dur-ing the typhoon occurred (Figure 12).[45] Diurnal internal tidal currents were much larger than

those of the semidiurnal constituents in the study area, asdescribed previously. However, shear caused by semidiurnalIT is suggested to be comparable to diurnal tide shear,

−20 −10 0 10 20220

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20K1−Summer

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th (

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M2−Winter

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b

a

Figure 8. Current ellipses at different depths for K1 and M2

in summer and winter; ellipse angles are relative to north.

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because of its higher-mode structure and smaller verticalscale (Figure 13, lower panel). Particularly, in the upper layershallower than 110m depth where the first-mode current re-versal in the vertical occurs, the shear induced by semidiurnal

IT was larger than that induced by the diurnal variations inboth seasons. Deeper than 120m, shear caused by diurnaltides was comparable to that by semidiurnal tides in winterbut a little larger than that by those tides in summer. The

0

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ianc

e (c

m2 /

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nce

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Diurnal−coherent

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500

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30Semidiurnal−coherent

Sep Oct Nov Dec Jan Feb Mar Apr May Jun0

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Figure 9. Time series of coherent and incoherent current variances at diurnal and semidiurnal frequencybands, fromSeptember 2008 to June 2009. Blue linemarks raw current variance. Red line shows low-frequencycomponents obtained by low-pass filter with fortnight-period cutoff, to investigate seasonal variations.

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Figure 10. Temporal mean coherent and incoherent current variances at diurnal and semidiurnal fre-quency bands during entire record period.

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semidiurnal IT shear was larger within the upper layer insummer but reached a maximum at 120–150m depths in win-ter. Meanwhile, diurnal IT shear was a little larger in summerthan in winter, in close relation to the strong diurnal IT activ-ity in summer. Shear induced by the diurnal componentreached maximum values at 100–120m depths in summerand at 120–160m in winter.

9. Discussion and Conclusion

[46] Our observations lead to the interesting findings thatdiurnal IT kinetic energy exhibited apparent seasonal vari-ability, whereas semidiurnal variance remained nearly uni-form throughout the record period on the northwestern SCSslope. The present study also highlights the different incoher-ent behavior and modal structure between semidiurnal anddiurnal IT. Coherent diurnal variance was found to constituteabout 40% of diurnal motions, whereas the coherent semidi-urnal component explains a much smaller fraction (10%) ofsemidiurnal motions.[47] Further analysis demonstrates that diurnal IT were

dominated by first-mode signals. In contrast, semidiurnal tidesshowed a variable multimodal structure—the second modewas dominant in summer and comparable to the first modein spring and autumn, but the first mode predominates in win-ter. We note that diurnal IT propagate in a more horizontaldirection than the semidiurnal constituents, under the samestratification structure of the water column. Thus, the semidi-urnal IT are more subject to reflection from the surface andbottom, and more energy is scattered to higher-mode signals

thanwith diurnal tides, as observed in the present study [Klymaket al., 2011]. Moreover, the upper layer structure of the watercolumn is important in affecting propagation characteristicsof the IT [Shaw et al., 2009]. From spring to autumn, theupper water column in the northern SCS was intermittentand pycnocline layer thickness was often comparable to oreven greater than the thickness of adjacent layers [Zhenget al., 2007]. Mesoscale eddies also frequently appear in theSCS, which may result in temporal changes of backgroundstratification and background currents (Figure 13). Thus,incident first-mode semidiurnal IT from the deep basin mayintegrate the effects of stratification changes and backgroundcurrents, radiating outward and disintegrating into highermodes during their long-range propagation to the continentalslope, as observed in this study. Further, semidiurnal IT werealso found to disintegrate into strong internal solitary waves inthe northern SCS, thereby weakening and exhibiting unsteadywaveforms. In contrast, strong and horizontally propagatingfirst-mode diurnal IT are less affected by background condi-tions and retain most of their vertical structure, even within avariable stratification and mesoscale activity system [Farmeret al., 2009]. Furthermore, the seasonal pycnocline weakenedand even disappeared in winter, and only the steady permanentpycnocline may have remained, under which conditions bothdiurnal and semidiurnal IT are less influenced by the homoge-neous environment and retain a first-mode vertical structure(Figure 12).[48] Incoherence of IT generally increases, as a result of

refraction of the mesoscale eddies. The higher-mode semidi-urnal IT are also more affected by varying stratification

0 100 200 300 400

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th (

m)

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Summer

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Dep

th (

m)

Variance (cm2/s2) Variance (cm2/s2)

0 10 20 30 40

Semidiurnal−incoherent

Figure 11. Temporal mean coherent and incoherent current variances at diurnal and semidiurnal fre-quency bands, during summer (1 to 17 June 2009) and winter (1 March to 31 May 2009), respectively.

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structure and background currents and thus exhibit highlyincoherent and intermittent behavior [Ray and Zaron,2011]. Similar results have recently been observed by bothsatellite altimeters and field observations. Based on a 17-yearcombined record of Topex/Poseidon and Jason satellite alti-meters, Ray and Zaron [2011] demonstrated that globally inco-herent variance accounted for 25% or less of average varianceat wavenumber characteristics of mode-1 signal, while mode 2contained a larger fraction of non-stationary variance, typically50% or more. Based on simultaneous profiling moorings andaltimeter data from the Hawaiian Ridge, Zhao et al. [2010] alsofound that coherence of semidiurnal IT decreased and thatcontributions from less coherent higher modes increased withdistance from the source area. Further, Zilberman et al.[2011] suggested that eddy-related changes in stratificationcontributed to observed phase variations and variable

conversion but affected mode 2 more than mode 1 becauseof smaller mode 2 propagation and phase speed.[49] Our findings are complementary to those of Lee et al.

[2012], who described the incoherent nature of IT in thenortheastern SCS. We further examined incoherent featuresof the diurnal and semidiurnal tides, finding that diurnal ITwere more coherent than semidiurnal IT in the northwesternSCS. IT in the northwestern SCSwere shown to differ greatlyin character from those in the northeastern area investigatedby Lee et al. [2012], which may be attributed to their differentpropagation paths or modulation by variable backgroundconditions.[50] Both diurnal and semidiurnal IT showed large irregular

fortnightly variations in the northeastern SCS, as described byLee et al. [2012]. In the northwestern SCS area, however, co-herent diurnal signals showed a pronounced spring-neap cyclewith a fortnightly period, and incoherent diurnal motions exhib-ited an irregular spring-neap cycle with the superposition of os-cillation signals. Coherent semidiurnal signals also showed anirregular spring-neap cycle modulated by a 3–10 day period,whereas incoherent semidiurnal motions were highly intermit-tent with a 1–2 day period (Figure 9). We suggest that the dom-inant incoherent nature of semidiurnal IT probably accounts forthe lack of apparent spring-neap and seasonal modulations ofsemidiurnal motions, as shown in Figures 3 and 4.[51] The diurnal IT in our study area are comparable to

those in the northeastern area as observed by Lee et al.[2012], whereas the semidiurnal IT are much weaker thanthose in that area. This may be because the Lee et al.’ moor-ing site was located in the main IT beam originating from theLuzon Strait, whereas our study area is outside the mainbeam from Luzon Strait [Lien et al., 2005]. Further, thesemidiurnal beam from the Luzon Strait mainly travelsnorthwest and propagates into the northeastern SCS, butthe diurnal beam mostly travels in the west and southwestdirections and is more likely to propagate into the

Buoyancy frequency (cph)

Dep

th (

m)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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200 8

10

12

14

16

18

20

22

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30

Figure 12. Buoyancy frequency distributions during entireyear in upper 200m water column, estimated from WOA05data.

Dep

th (

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−20

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plitu

de (

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)

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th (

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Sep Oct Nov Dec Jan Feb Mar Apr May Jun−20

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20

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plitu

de (

cm/s

)

Depth average V

Figure 13. Along- and cross-isobath sub-inertial currents and corresponding depth-averaged compo-nents. Color bar units are cm/s.

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northwestern SCS [Simmons et al., 2011]. Therefore, a smallfraction of semidiurnal IT from the Luzon Strait could travelinto the northwestern SCS during a long propagation. Asdiscussed previously, weak semidiurnal IT in our study areaare thus more incoherent and exhibit a multimodal structure,

owing to refraction and scattering from the backgroundconditions.[52] The higher-mode IT usually break near their source

region, because of their small-scale wavelength. Numericalsimulations of the northeastern SCS also suggest that higher

Dep

th (

m)

2008/09/21 09/24 09/27 09/30 10/03

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Time (year/month/day)

Dep

th (

m)

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50

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−60

−20

20

60

−30

−10

10

30

a

b

Figure 14. Contours of near-inertial east-west meridional currents during typhoon-wake period from 21September to 3 October 2008 (a) and during calm period from 23 November to 8 December 2008 (b).Color bar units are cm/s.

0 200 400

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Current variance (cm2/s2) Current variance (cm2/s2) Current variance (cm2/s2)

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Figure 15. Vertical structure of mean variance and shear corresponding to semidiurnal and diurnal ITand NIW during different periods.

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baroclinic modes originating from the Luzon Strait arealmost undetectable in the far field [Vlasenko et al., 2010].Thus, higher-mode semidiurnal IT in our study area arelikely generated by scattering or reflection of low-mode ITfrom the Luzon Strait but are probably not directly generatedin the strait area. Note that the local generation of lowest-mode diurnal IT is also possible and probably weak in thestudy area, which are beyond the scope of the present studysince the single mooring observation prevents drawing astrong conclusion.[53] Incoherent IT were found to explain three fourths of

observed tidal energy in the northeastern area and were sug-gested as the reason for the disappearance of seasonal vari-ability [Lee et al., 2012]. However, in our study area, coher-ent diurnal IT explained a significant fraction of thediurnal energy. Coherent diurnal IT that retain lowest-modestructure were more stable, which is the likely reason for theobserved seasonal variability of diurnal IT in the study area.[54] Our study suggests that multimodal semidiurnal IT

are more influenced by varying stratification structure andbackground currents and therefore have more incoherentand intermittent behavior than diurnal tides. Furthermore,incoherent semidiurnal IT, especially the higher-modemotions, travel with small phase speed, which might obscureseasonal variability during their long propagation from thegeneration source.[55] Specific mechanisms for the multimodal structure and

incoherent features of semidiurnal IT activity observed in acomplex ocean environment remain unclear. These topics,as well as generation sources and propagation features ofIT in the northwestern SCS, require further study based onmore concurrent in situ observations and numericalsimulations.[56] During the passage of Typhoon Hagupit, NIW energy

and shear induced by strong typhoon winds were signifi-cantly enhanced, and they exceeded IT energy and shearby a factor of 2 to 3 in the upper layer. This could triggerinstabilities and wave breaking and contribute to the localocean mixing. The observed NIW variance appeared season-ally independent. NIW energy caused by the typhoon radi-ated downward and took the form of a first-mode structure.During the calm period, NIW signals were intermittent andradiated upward and downward, suggesting different gener-ation mechanisms for NIW in the SCS. NIW in the SCScould arise from direct wind forcing at the sea surface, orbe excited by the geostrophic adjustment of currents in theocean interior, or survive multiple reflections from the ther-mocline and ocean bottom. More concurrent velocity andturbulence dissipation data are needed to examine the rela-tive contributions of IT and NIW to the shear field and localmixing in the northern SCS.

[57] Acknowledgments. Funding for this study was provided by theNational Natural Science Foundation of China (No. 41106017, No.41030855), the Natural Science Foundation of Jiangsu Province of China(No. BK2011396), and the Knowledge Innovation Program of the ChineseAcademy of Sciences (No. KZCX1-YW-12). Constructive and helpful com-ments from two anonymous reviewers are gratefully acknowledged.

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