Variability of the Deep-Water Overflow in the Luzon Strait*

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Variability of the Deep-Water Overﬂow in the Luzon Strait*

CHUN ZHOU,WEI ZHAO,JIWEI TIAN, AND QINGXUAN YANG Physical Oceanography Laboratory/Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, Qingdao, China

TANGDONG QU International Paciﬁc Research Center, School of Ocean and Earth Science and Technology, University of Hawai‘i, Honolulu, Hawai‘i

(Manuscript received 11 June 2014, in ﬁnal form 23 August 2014)

ABSTRACT

The Luzon Strait, with its deepest sills at the Bashi Channel and Luzon Trough, is the only deep connection between the Pacific Ocean and the South China Sea (SCS). To investigate the deep-water overflow through the Luzon Strait, 3.5 yr of continuous mooring observations have been conducted in the deep Bashi Channel and Luzon Trough. For the first time these observations enable us to assess the detailed variability of the deep- water overflow from the Pacific to the SCS. On average, the along-stream velocity of the overflow is at its 2 maximum at about 120 m above the ocean bottom, reaching 19.9 6 6.5 and 23.0 6 11.8 cm s 1 at the central Bashi Channel and Luzon Trough, respectively. The velocity measurements can be translated to a mean 2 volume transport for the deep-water overflow of 0.83 6 0.46 Sverdrups (Sv; 1 Sv [ 106 m3 s 1) at the Bashi Channel and 0.88 6 0.77 Sv at the Luzon Trough. Significant intraseasonal and seasonal variations are identified, with their dominant time scales ranging between 20 and 60 days and around 100 days. The intraseasonal variation is season dependent, with its maximum strength taking place in March–May. Deep-water eddies are believed to play a role in this intraseasonal variation. On the seasonal time scale, the deep-water overflow intensifies in late fall (October–December) and weakens in spring (March–May), corresponding well with the seasonal variation of the density difference between the Pacific and SCS, for which enhanced mixing in the deep SCS is possibly responsible.

1. Introduction (e.g., Hogg et al. 1999; Mercier and Speer 1998; Rudnick 1997; Meredith et al. 2011; Hansen and Østerhus 2000). Deep passages connecting deep basins and marginal Motivated by the need to understand the abyssal dy- seas, including the Vema and Hunter Channels, the namics in the northwestern Pacific Ocean and its role in Romanche Fracture Zone, the Samoan Passage, the the global thermohaline circulation, this study is focused Drake Passage, and the Faroe Bank Channel, have been on the Luzon Strait, a natural geographical constraint of repeatedly investigated, and the deep-water overflow the deep-water overflow from the Pacific to South China through these passages is believed to play a significant Sea (SCS). role in generating the global thermohaline circulation The SCS is the largest marginal sea in the northwestern Pacific, with a large, deep (.2000 m) basin of more than 1.0 3 106 km2 and a maximum water depth * School of Ocean and Earth Science and Technology Publica- over 5000 m. Diapycnal mixing in the deep SCS reaches tion Number 9196 and International Pacific Research Center 2 2 as large as O(10 3)m2 s 1, significantly larger than that Publication Number IPRC-1076. in the deep Pacific, because of the energetic internal waves and complicated bathymetry there (e.g., Tian Corresponding author address: Jiwei Tian, 238 Songling Road, et al. 2009; Alford et al. 2011). The corresponding up- Physical Oceanography Laboratory/Qingdao Collaborative Innovation 26 21 Center of Marine Science and Technology, Ocean University of China, welling in the deep SCS is on the order of 10 ms , Qingdao 266100, China. indicative of a resident time of less than 100 yr, making E-mail: [email protected] the SCS a potentially important pathway of the global

DOI: 10.1175/JPO-D-14-0113.1

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FIG. 1. Bottom topography in (a) the SCS and (b) Luzon Strait (Smith and Sandwell 1997). The black stars in Fig. 1b denote the mooring locations. thermohaline circulation (e.g., Broecker et al. 1986; Qu mooring observations (Liu and Liu 1988; Chang et al. et al. 2006b; Yang et al. 2011). The Luzon Strait, with 2010; Tian and Qu 2012), and model simulations (Zhao a sill depth of about 2400 m, is the only deep connection et al. 2014). By analyzing the ADCP measurements from between the SCS and the Pacific. Across the Luzon repeat occupation stations, Zhao et al. (2014) recently Strait, there is a persistent pressure gradient that drives provided the first picture of vertical structure of the deep- a deep-water overflow from the Pacific into the SCS water overflow in the Luzon Strait, with the along- (e.g., Qu et al. 2006b; Tian et al. 2006; Song 2006). After channel velocity below the 36.82 isopycnal, increasing crossing the Luzon Strait, water of Pacific origin sinks to with depth above the 120-m height above bottom (HAB) the deep SCS (Wyrtki 1961). It then upwells as a result of and decreasing below the 120-m HAB in the deep Bashi enhanced mixing in the deep SCS (Tian et al. 2009) and Channel and Luzon Trough (Fig. 1). eventually exits the SCS as part of the SCS Throughflow However, the temporal variability of the deep-water (e.g., Qu et al. 2005, 2006a), exerting notable impacts on overflow in the Luzon Strait has been barely examined the Indonesia Throughflow and its associated heat and due to the lack of observations. To our best knowledge, freshwater fluxes from the Pacific to the Indian Ocean the only continuous observations available so far were (e.g., Tozuka et al. 2007, 2009; Gordon et al. 2012). from Liu and Liu (1988) and Chang et al. (2010). Liu and Given the key role it plays in the SCS Throughflow, Liu (1988) conducted an 82-day mooring observation the deep-water overflow through the Luzon Strait has with one active current meter in the Bashi Channel, been investigated by several earlier studies. Two kinds of which was apparently not long enough to study the sub- interfaces have usually been used to delimit the deep water inertial temporal variability of the deep-water overflow. from intermediate water, of which one is the bifurcation Another mooring observation was reported by Chang depth (;1500 m), calculated from mean density profiles et al. (2010), with double current meter moorings lasting on the east and west sides of the Luzon Strait (Qu et al. for 9 months at the Bashi Channel and Taltung Canyon, 2 2006b). The other is the 36.82 kg m 3 potential density as indicated in their work. Energetic variation with a pe- isopycnal referenced to 2000 m (s2), corresponding to riod spanning from 20 to 60 days was revealed, and the ;2000 m, estimated from repeat-occupation conductivity– visual correlation between the deep-water overflow and temperature–depth (CTD)/lowered acoustic Doppler sea surface height anomaly was identified in their study, current profiler (LADCP) profiles (Zhao et al. 2014). though the processes responsible for this variation remain These studies have arrived at mean transport estimates unknown. Based on a high-resolution regional model, 2 ranging from 1.0 to 2.5 Sverdrups (Sv; 1 Sv [ 106 m3 s 1), Zhao et al. (2014) suggested that seasonal variation might based on diagnostic calculation (e.g., Wang 1986; Qu exist in the deep-water overflow through the Bashi et al. 2006b; Song 2006), hydrographic data (e.g., Tian Channel and Luzon Trough, but this has not yet been et al. 2006; Yang et al. 2010, 2011; Zhao et al. 2014), confirmed by observations.

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To better understand the temporal variability of the 300 m from the transducers. Because of resource limita- deep-water overflow and its associated governing pro- tions, each mooring deployed from October 2009 to cesses, we deployed two moorings at two carefully se- March 2010 was equipped with only one current meter. lected sites in the Luzon Strait and acquired 3.5 yr of data. As additional resources became available, more in- The results from an analysis of these data are reported in struments were mounted on the moorings during the this study. The rest of this paper is organized as follows: latter period of the observations. Details pertinent to Section 2 is devoted to description of the experiment mooring design and configuration are shown in Table 1. configuration. The basic characteristics are presented in Since the conductivity sensor failed in some segments, the section 3, and results on temporal variability of the deep- present study only examines the temperature and hori- water overflow are presented in section 4. The results are zontal velocity data in the Luzon Strait. summarized in section 5. Strong deep current can cause the moorings to tilt. During the period of observation, over 99.9% of the tilt records of the instruments were below the designed limits, 2. Data which are 458 for current meters and 158 for ADCPs, suggesting that the velocity measurements were reliable. a. Mooring data The tilt of moorings could also cause vertical excursions As part of the SCS deep circulation experiment, two of the instruments monitored by the pressure sensors of bottom-anchored moorings were deployed in October the CTD. Unlike the case in open ocean where moorings 2009 and recovered in April 2013 at two sites of the Luzon tend to exhibit basically circular motion driven by quasi- Strait (Fig. 1). One (marked as BC in Fig. 1)wasposi- circular tidal and inertial currents (e.g., Alford and tioned in the Bashi Channel at a depth of 2720 m, on Whitmont 2007), the moorings at BC and LT swing like approximately the same site as where the mooring mea- pendulums because of the strong constraint of bathyme- surements were conducted by Liu and Liu (1988) and try. The typical pressure fluctuation between sequential Chang et al. (2010). The other (marked as LT in Fig. 1) records is about 6 dbar, which is the equivalent of a lateral was positioned in the central Luzon Trough, at a depth deflection of 37 m at 120-m HAB, suggesting a horizontal 2 of 3620 m, where the funnel-shaped topography leads velocity error of 0.5 cm s 1, significantly smaller than the to an intensification of the deep flow. The moorings velocity of the deep-water overflow in the Luzon Strait. were recovered and redeployed five times to refurbish b. CTD profiles and maintain the instruments. During each of the refurbishments, data were interrupted for less than a day. A Historical hydrographic data, including large numbers cubic spline fit was applied to fill the gaps between the of CTD profiles from the World Ocean Database 2009 refurbishments. Sea-Bird Electronics SBE 37-SM CTDs, (WOD09; Boyer et al. 2009) and from our own field Aanderaa Instruments Recording Current Meter (RCM) experiments, are also used in this study. After elimi- Seaguard current meters, and Teledyne RD Instruments nating those profiles not passing standard deviation Workhorse Long Ranger 75-kHz acoustic Doppler cur- checks or flagged as outliers and those extending shal- rent meters (ADCPs) were mounted on the moorings to lower than 1500-m depth, the hydrographic data used monitor the salinity, temperature, pressure, and hori- for this study consist of 150 temperature/salinity profiles zontal velocity of the deep flow. The accuracies of the in the northeastern SCS and northwestern Pacific (see 2 instruments are 0.0028C for temperature, 0.003 mS cm 1 section 4b for further details). for conductivity, and 0.1% of full-scale range for pressure (which is 7 m for the CTD used in this experiment). Ac- 2 3. General description curacies of velocity measurements were 0.15 cm s 1 for 2 current meters and 0.1% S 6 5mms 1 for ADCP (S Time series of horizontal velocity and temperature at stands for the water velocity relative to ADCP). The 120-m HAB of the BC and LT are shown in Fig. 2. sampling intervals were set to 2 h for all instruments from Notable tidal signals are visible both in horizontal ve- October 2009 to March 2010 and 1 h thereafter. Current locity and temperature during the period of observation. meters were configured with 150 pings per record in burst A Butterworth bandpass filter is performed to resolve mode, while the ADCPs sampled 37 ensembles with 24 the semidiurnal and diurnal tidal signals, of which the pings each in burst mode and a bin size of 16 m. Although standard deviations in the zonal (meridional) direction 2 the designed range of the 75-kHz ADCP amounts to are 12.1 and 8.7 (9.4 and 8.7) cm s 1, respectively, at the 2 600 m in long-range mode with a bin size of 16 m, poor BC and 4.0 and 6.2 (7.5 and 8.1) cm s 1, respectively, at scatterer concentration in the deep water weakened the the LT. This result suggests that both the semidiurnal echo intensity and reduced the maximum range to about and diurnal amplitudes at the BC are stronger than those

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TABLE 1. Design of moorings shown in Fig. 1.

Moorings Depth (m) Period Instruments Instrument depth (m) BC 2720 October 2009–March 2010 RCM 2600 CTD 2605 March 2010–August 2010 RCM 2200 CTD 2205 RCM 2600 CTD 2605 August 2010–April 2011 RCM 2000 CTD 2005 RCM 2200 RCM 2400 CTD 2405 RCM 2600 CTD 2605 April 2011–March 2012 ADCP 2665 CTD 2680 March 2012–March 2013 ADCP 2665 CTD 2680 LT 3620 October 2009–March 2010 RCM 3500 March 2010–August 2010 RCM 3500 CTD 3505 August 2010–April 2011 RCM 3100 CTD 3105 RCM 3500 CTD 3505 April 2011–March 2012 RCM 2700 CTD 2705 RCM 3100 CTD 3105 RCM 3300 RCM 3500 CTD 3505 March 2012–March 2013 RCM 1500 CTD 1505 RCM 2500 CTD 2505 RCM 3500 CTD 3505

at the LT. The mean magnitudes of the U and V com- The cross-stream direction is defined as 908 counter- 2 ponents are 215.2 and 213.0 cm s 1 at the BC and 28.1 clockwise from the along-stream direction. By this defini- 2 2 and 221.6 cm s 1 at the LT, respectively, representing tion we have a mean velocity of 19.9 6 6.5 cm s 1 at 120-m 2 a strong, deep-water overflow from the Pacific to the HAB of the BC and 23.0 6 11.8 cm s 1 at 120-m HAB of SCS through the Luzon Strait. The mean temperature the LT, where the values after the signs represent the around 120-m HAB is 2.038C at the BC and 2.228C at the standard deviations of the daily mean velocities. Figure 3 LT. Once crossing the Bashi Channel, the deep water shows the histograms of velocities in both directions. While experiences a substantial descending, with the depth of the distributions of Va at the BC and Vc at both the BC and maximum velocity following the topography and in- LT basically follow a Gaussian distribution, Va at the LT creasing from 2600 to 3500 m (Zhao et al. 2014). tends to display obvious negative skewness, indicating that Given the strong topography constraint, we remap the the velocity anomalies are much stronger in the negative horizontal velocity into the along-stream Va and cross- direction than those in positive direction. We will return to stream Vc components, with the along-stream direction this point later (section 4a). defined as the mean current direction, which is 2308 Further analysis of current measurements at shallower and 2018 clockwise from north at the BC and LT, re- depths allows us to investigate the vertical structure of the spectively, and is basically parallel to the local isobaths. deep-water overflow in the Luzon Strait. Similar to what

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FIG. 2. Bihourly time series of U, V, and temperature observed at 120-m HAB of the (left) BC and the (right) LT. The black lines indicate the daily mean time series. has been revealed by Zhao et al. (2014),resultsfrom of deep inflow, we average all the profiles during the August 2010 to April 2011, the segment with the largest period October 2009–March 2013 at the BC and LT and vertical coverage of current measurements, show that linearly interpolate the velocity vertically between the the mean velocity at the BC is about zero at 2000 m and current meters with an assumption of zero velocity at the 2 increases gradually with depth to about 22.6 cm s 1 at sea floor. The cross-stream topography is from Smith 120-m HAB (Fig. 4a). Owing to the low-scattering envi- and Sandwell (1997), which has been widely used for the ronment in the deep water, the vertical coverage of region studied (e.g., Qu et al. 2006b). After digitizing the ADCP measurements at the BC from April 2011 to vertical profiles to a vertical grid of 20 m and in- March 2013 is limited to about 300-m HAB. Despite terpolating the velocity horizontally using a cubic spline a relatively minor vertical shear between 2400 and with the assumptions of zero velocity at two sidewalls, 2600 m, the mean profile during this period shows a sim- the volume transport of the deep-water overflow is es- ilar vertical structure to that from August 2010 to April timated to be 0.83 6 0.46 Sv at the BC and 0.88 6 0.77 Sv 2011. Vertical structure of the deep-water overflow at the at the LT. These estimates are generally comparable LT also displays a bottom-intensified structure the same with the previous studies (e.g., Liu and Liu 1988; Tian as that at the BC (Fig. 4b), that is, increasing from 1600 to et al. 2006; Qu et al. 2006b; Chang et al. 2010; Yang et al. 3500 m or about 120-m HAB. Results from the two seg- 2011; Zhao et al. 2014). ments are basically consistent. In contrast to the vertical structure of the mean current, it is noticeable that the 4. Variability standard deviations of velocity decrease with depth at both the BC and LT, with their maxima found around To focus on the subinertial signals, a 15-day Butter- 2000 m at the BC and 2700 m at the LT, reflecting strong worth low-pass filter is applied to the time series of influence of bottom topography. Because of this topo- remapped velocity and temperature (Fig. 5). Low- graphic constraint, the velocity profiles of Vc at both the passed time series of Va and temperature at the BC 2 BC and LT are fairly weak. (LT) are shown to fluctuate between 1 and 37 cm s 1 2 According to Zhao et al. (2014), the Rossby radii at (217 and 36 cm s 1) and between 1.788 and 2.368C (2.138 the BC and LT, 19 and 20 km, are comparable with the and 2.328C), respectively, visually characterized by en- local channel widths, 20 and 22 km, respectively, and so ergetic variations from intraseasonal to interannual it seems reasonable to discuss the volume transport at time scales. The result also shows that there is not only the BC and LT with individual profiles. Based on the a substantial increase in temperature but also a sub- velocity profiles mentioned above, we employ 2000 and stantial increase in the homogeneousness of the water 1600 m to be the upper interface of the deep water at the property from the BC to LT. Because of the strong to-

BC and LT, respectively. To construct the mean proﬁle pography constraint, the low-passed Vc at the BC and

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FIG. 3. Histogram of subinertial Va and Vc at 120-m HAB of the (a),(c) BC and (b),(d) LT with the skewness of the subinertial Va or Vc series indicated in each panel.

LT are weak. Spectrum analysis of the low-passed Va in the deep Vema Channel, Zenk (2008) revealed that, manifests peaks in the frequency field (Fig. 6). Despite although eddy kinetic energy is smaller than the mean some quantitative differences between the BC and LT, kinetic energy, high velocity variance falls in the sub- both time series are dominated by intraseasonal varia- mesoscale range between 14 and 20 days. tions, with their periods ranging between 20 and 60 days. Considering that variations with a period of 20–100 While not obvious at the BC, a peak around 100 days days dominate the fluctuation of the deep-water overflow also stands out at the LT. On seasonal time scale, the in the Luzon Strait, a Butterworth bandpass filter with variation satisfies the 95% confidence level at the LT, a window of 15–120 days was applied to the Va time series but is relatively minor at the BC. In the following, the to study the intraseasonal variations (Fig. 7). Because of intraseasonal and seasonal variations are discussed the constraint of bathymetry, significant correlation is separately. found between the time series at the BC and LT, with a correlation coefficient of 0.49 for the period of obser- a. Intraseasonal variations vation. The amplitudes of variations are different at the 2 Intraseasonal variations associated with topographic two channels, with a standard deviation of 5.0 cm s 1 at 2 waves and eddies have been observed in many parts of the BC and a standard deviation of 9.1 cm s 1 at the LT, the deep ocean (e.g., Hamilton 2009; Arhan et al. 2002). respectively, suggesting that the intraseasonal variations High-frequency fluctuations in abyssal passages that at the LT are more energetic than those at the BC. connect deep basins have also been reported by previous It is interesting to note that the intraseasonal varia- studies. At the Samoan Passage in the Pacific, for exam- tions exhibit obvious nonstationary features (Fig. 7). ple, Rudnick (1997) claimed that the 30-day fluctuation Wavelet analysis following the method of Torrence and dominates the variability of transport and temperature Compo (1998) is conducted to investigate how the deep beneath 4000 m. Based on a 687-day current meter record flow varies in amplitude and frequency during the period

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FIG. 4. Vertical profile of Va at the (a) BC and (b) LT. Thick black (red) lines indicate the mean profile from the mooring during the periods October 2010–April 2011 at the BC and April 2011–March 2012 at the LT (April 2011–March 2013 at the BC and March 2012–March 2013 at the LT). Blue bars indicate the standard deviations. Green pentagrams and triangles display the maxima and minima of Va at different depth. Yellow pentagrams indicate the mean Va at 120-m HAB. of observation. In Fig. 8 it seems obvious that the fre- the fact that the deep water experiences continuous quency peaks shown in Fig. 6 are strongly dependent on warming as it penetrates toward the SCS along the deep season. At the LT, the 30-day oscillation is enhanced Luzon Trough, the positive temperature anomaly im- substantially in boreal spring (March–May), except for plies that relatively warm downstream water be pushed 2012, when the most energetic period seemed to shift back to the northern Luzon Trough and sometimes all from 30 to 50 days. A similar oscillation can be seen at the way back to the Bashi Channel, like the case noted 21 the BC, though not as regular as that at the LT. above when the mean Va reached 210.4 cm s and the The positive skewness shown in the Va distribution at duration of the reversal exceeded 13.5 days. Similar re- the LT (Fig. 3) could be interpreted by its bandpassed versals have also been reported in the Vema and Hunter filtered time series (Fig. 7), with stronger anomalies in Channels (e.g., Zenk et al. 1993; Hogg et al. 1999; Zenk the negative direction. These negative anomalies are et al. 1999). The similarities and differences between basically found during March–May each year, when these reversals in unidirectional deep channel flows short-term reversals of the inflow, with a typical period need to be investigated further by research. of 5–10 days, occur at the LT. These reversals are usually Interestingly, a lagged anticorrelation can be found accompanied by weakened inflow at the BC. The causes between the temperature and Va at the BC. To further of these reversals are not understood. When a 50-day investigate this anticorrelation, we perform a lag corre- oscillation was enhanced in March 2012, the flow at the lation analysis within the time window of 90 days. BC was also reversed (Fig. 5). As an example, Fig. 9 Figure 10a shows that the variation of temperature cor- shows the details of how this reversal took place. For this responds well with the variation of velocity during the period, the temperature fluctuation was anticorrelated period of observation at the BC, with the former lagging with Va both at the BC and LT. The time series of sub- the latter by 2–6 days. Around March 2012, this time lag inertial velocity at the LT indicates that the reversal increased to about 7 days. No obvious time lag is found took place from 5 March to 15 March and was associated at the LT (Fig. 10b). In fact as one can see in Fig. 9,the with a positive temperature anomaly of 0.048C. Given reversals of velocity at the BC took place at a slightly

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FIG. 5. 15- and 120-day low-passed (a) Va and (b) temperature at 120-m HAB of the BC and LT. different time as those at the LT. Following Figs. 10a and observation it does not correspond to the variability of

10b, we also calculate the lag correlation of Va between along-stream velocity. So, we suggest that the intra- the two channels, as shown in Fig. 10c. Presumably be- seasonal variations observed in the deep Luzon Strait cause of the strong constraint of topography, the two time cannot originate from the surface. series correspond well most of the time. In boreal spring and sometimes in October, when intraseasonal variations are intensified, the time series at the LT leads that at the BC by about 30 h, implying that the signals propagate against the main stream of the overflow. The high correlation of intraseasonal variations between the BC and LT suggests that they are possibly regulated by the same mechanism. Surface detectable eddies have been assumed to have notable influence on fluctuations of deep-water overflows through relatively shallow passages like the Faroe Bank Channel and Denmark Strait (e.g., Høyer and Quadfasel 2001). In the deep Bashi Channel and Taltung Canyon, Chang et al. (2010) indicated that the intraseasonal variations of deep-water overflow could also be related to the mesoscale processes in the upper ocean. Here, based on the merged satellite altimeter data of Jason-2, Jason-1, and EnviSat from Archiving, Validation, and In- terpretation of Satellite Oceanographic (AVISO) data, we also compare the local sea level anomaly (SLA) with the along-stream velocity observed at the BC and

LT. As one can see in Fig. 11, the local SLA is domi- FIG. 6. Spectrum analysis of the 20-day low-passed Va at 120-m nated by seasonal cycle, and during most periods of the HAB of the BC and LT.

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FIG. 7. 15–120-day bandpassed Va at 120-m HAB of the BC and LT, with standard deviations of the corresponding series indicated.

One possible forcing mechanism is related to deep kinetic energy of the deep flow reaches its maximum at eddies. Based on hydrographic sections, Arhan et al. about 3100 m at the LT, and from there it slightly de- (2002) have reported the existence of deep eddies cen- creases both upward and downward. Figure 12 shows the tered at the interface between the North Atlantic Deep vertical structure of bandpassed flow at the LT. The flow Water and Antarctic Bottom Water. By examining the beneath 2500 m shows a vertically coherent phase, while current and temperature time series from the mooring the flow above it (1500 m) also shows a good corre- observation, they indicated that the occasional reversals spondence with that at 2500 m. These are consistent with spotted at the deep Vema Channel could result from the the characteristics of deep eddies (e.g., Arhan et al. passage of deep eddies. According to Fig. 4b, eddy 2002), which, if exist in the Luzon Strait, could lie in the

FIG. 8. Wavelet analysis of Va at the (a) BC and (b) LT. The color-ﬁlled contours are the log10-scaled variance 2 22 (cm s ) of the wavelet transform of normalized Va. The black solid contours are the 95% conﬁdence level, and the black dashed line in each panel indicates that the areas below it are subject to the edge effect.

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suggest that downward-propagating Kelvin and Rossby waves can contribute to the variability in the deep equatorial Atlantic and Pacific (e.g., Thierry et al. 2006). In the upper layer of the Luzon Strait, a strong seasonal cycle has been reported relating to variability of the pressure gradient across the Luzon Strait, which is attributed to the pileup of water from the prevailing monsoon (e.g., Metzger and Hurlburt 1996; Qu 2000). The vertically synchronous seasonal cycle, however, does not seem to favor the downward propagation assumption noted above. Persistent density difference exists between the deep Pacific and SCS, which sustains a baroclinic pressure gradient across the Luzon Strait that in turn drives the deep-water overflow from the Pacific into the SCS (Qu et al. 2006b). Two boxes, one lying east (EA; 218–238N, FIG. 9. An example of (top) horizontal velocity distribution and 1228–1248E) of the Luzon Strait and the other lying west the (bottom) corresponding temperature time series at the BC (WA; 188–208N, 1198–1218E) of it, are chosen to exam- (blue) and LT (red) when reversals of the overflow take place. ine this density difference. As shown in Fig. 14a, the deep water is more weakly stratified in the SCS than that in the northwestern Pacific below the bifurcation depth depth range centered at about 3100 m, where the maxi- of about 1500 m. By averaging the density profiles in mum eddy kinetic energy is attained. boreal summer and winter separately, it is noticed that the density below the bifurcation depth in boreal winter b. Seasonal variation is notably lower than that in boreal summer (with a s2 Seasonal variation of the deep-water overflow is also difference ;0.01 in 2400 m) in the WA, while the density shown in Fig. 5 (the bold lines). By averaging the 3.5 yr in the EA is basically the same for the two seasons. This of 100-day low-passed time series of Va and temperature result suggests that the pressure gradient across the for each month, we obtain the mean seasonal cycle of the Luzon Strait is larger in boreal winter, and this possibly flow. The flow appears to attain its seasonal maximum in drives a stronger deep-water overflow than in boreal late boreal fall (October–December) and its seasonal summer. 2 2 minimum in boreal spring (March–May) (Fig. 13a). Enhanced mixing of up to 10 2 m2 s 1 is revealed in Since deep water in the northwestern Pacific is signifi- the deep (.1000 m) SCS (Qu et al. 2006b; Tian et al. cantly cooler than that in the Luzon Strait (e.g., Qu et al. 2009), which could explain the weak stratification

2006b), the maximum Va is always accompanied by there. Recently, Zhao et al. (2014) conducted a series minimum temperature and vice versa (Fig. 13b). Stan- of numerical experiments, and their results confirmed dard deviations of the mean seasonal cycle of Va reach the hypothesis that the enhanced mixing in the deep 2 2 1.6 cm s 1 at the BC and 3.3 cm s 1 at the LT, indicating SCS is a major process driving the deep-water overflow that the seasonal variation is more energetic at the LT in the Luzon Strait (Qu et al. 2006b). The question that than at the BC, similar to what has been discussed for the may arise immediately is if its seasonal variation intraseasonal variation. But, the magnitude of the sea- (Figs. 13, 14a) is also related to the mixing in the deep sonal variation of Va is significantly weaker than that of SCS. the intraseasonal variation (Fig. 5a), implying that the To address this question, we use one of the most com- velocity variability of deep-water overflow in the Luzon monly used models for turbulent diapycnal diffusivity k, Strait is dominated by fluctuations within the intra- suggested by Osborn (1980), seasonal period band, which differs from the case of « temperature variability (Fig. 5b). Similar to the intra- k 5G , N2 seasonal variation, the phase of the seasonal variation is also vertically consistent in the depth range from the where G is the mixing efficiency and typically taken to be bottom to about 2000 m at the BC and to about 2500 m 0.2 (Osborn 1980), N is the buoyancy frequency, and « is at the LT. the turbulence dissipation rate that can be estimated The seasonal cycle is ubiquitous in the deep ocean, from density overturns (Thorpe 1977; Alford and Pinkel especially in the equatorial area. Evidence exists to 2000):

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FIG. 10. Lag correlation between temperature and Va at the (a) BC and (b) LT and (c) between Va of the two channels during the observation with a time window of 90 days. Positive values indicate the former lags the latter.

« 5 2 2 3 consequently a stronger deep-water overﬂow across the a LT N , Luzon Strait. The details need to be carefully examined where a 5 0.8 is a constant of proportionality (Dillon when more observations become available.

1982), and LT is the Thorpe scale (Thorpe 1977). It must be noted that both the spatial resolution of the mea- 5. Summary surements and the noise of the instruments may impose constraints on the overturn detection. In this paper, the Based on 3.5 yr of measurements from two current overturn size criteria proposed by Galbraith and Kelley meter moorings deployed in the Bashi Channel and (1996) and the modified profile preprocessing method central Luzon Trough, we have investigated the mean and overturn ratio Ro criterion proposed by Gargett and structure and temporal variability of the deep-water Garner (2008) are employed to reject those spurious overflow in the Luzon Strait. Averaged over the period overturns. of observation, the along-stream velocity is at its maxi- 2 Using this Thorpe scale method, diffusivity of the mum near 120-m HAB, reaching 19.9 6 6.5 cm s 1 at the 2 deep SCS is examined based on the available density BC and 23.0 6 11.8 cm s 1 at the LT. As it penetrates profiles with fine vertical resolution. Figure 14b shows from the Bashi Channel to the central Luzon Trough, that diapycnal diffusivity generally increases with the deep water experiences a significant water property 2 2 depth below 1500 m, from the order of 10 4 m2 s 1 at transformation, with its mean temperature increasing 2 2 1500 m to 10 3 m2 s 1 at 3000 m. From this figure one from 2.028 to 2.228C at 120-m HAB, indicative of strong can see that diffusivity in boreal winter is stronger than mixing processes in the deep Luzon Strait. Whereas the in boreal summer. The enhanced mixing in boreal vertical stratification is weakened by the enhanced winter may generate a weaker stratification in the deep mixing as the deep water penetrates into the Luzon SCS, resulting in a stronger pressure gradient and Strait (Zhao et al. 2014), the bottom-intensified vertical

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FIG. 11. 15-day low-passed time series of local SLA (gray) compared with Va [(a) BC and (c) LT] and temperature [(b) BC and (d) LT].

structure seems not to be modified substantially, with Va known that mixing in the deep SCS is about two orders increasing monotonically from around 2000 m (1600 m) larger than that in the deep Pacific (e.g., Tian et al. 2009; to 120-m HAB at the BC (LT), contributing to a trans- Alford et al. 2011). So, the seasonal fluctuation of port estimate of 0.83 6 0.46 Sv at the BC and 0.88 6 stratification can be largely attributed to the mixing in 0.77 Sv at the LT. the deep SCS. Significant temporal variability on time scales from Withatimeseriesof3.5yr,weareunabletochar- intraseasonal to interannual is observed in the along- acterize the interannual variation of the deep-water stream velocity of the deep Luzon Strait. The dominant overflow in the Luzon Strait. Nevertheless, noticeable time scales of this variability are between 20 and 60 interannual variation can be identified in the 100-day days and around 100 days, while the seasonal variation low-passed time series of along-stream velocity (Fig. 5). is relatively weaker. The intraseasonal variation is During 2012/13, for example, the typical annual cycle strongly dependent on season, with its maximum am- was replaced by a double-peak feature: one around late plitude taking place in boreal spring. During this period, the deep-water overflow can sometimes reverse its direction, and associated with these reversals are positive temperature anomalies in both the LT and BC. The causes for these reversals are not clear. They are possibly related to deep eddies, but the details require further investigation. Seasonal variation is dominant in the upper-layer circulation of the Luzon Strait (e.g., Qu 2000; Yaremchuk and Qu 2004). Though also present in the deep-water overflow, with a seasonal maximum in fall and a seasonal minimum in spring, no downward propagation signals from the surface can be identified. Since the deep-water overflow is primarily forced by a persistent pressure gradient between the Pacific and the SCS (Qu et al. 2006b), it is speculated that the seasonal fluctuation of stratification in the deep SCS is a key process responsible for the seasonal variation of deep-water overflow through the Luzon Strait. With the presence FIG. 12. Horizontal velocities against depth at the LT [(top) April of energetic internal tides in the SCS, it has already been 2011 ; March 2012; (bottom) March 2012 ; April 2013].

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FIG. 13. Mean seasonal cycle of (a) Va and (b) temperature at the BC and LT based on monthly averaged time series. Standard deviations are indicated by gray bars.

July and the other around January. The amplitudes of Acknowledgments. The authors thank the two anon- intraseasonal variation in spring are noticeably differ- ymous reviewers for their constructive comments and ent from year to year. Interannual variability in the suggestions. The altimeter products used in this work Luzon Strait have been examined by several earlier are produced by SSALTO/DUCAS and distributed studies (e.g., Qu et al. 2004; Tozuka et al. 2009), in- by AVISO, with support from CNES (available online dicating that the Luzon Strait transport, especially in at www.aviso.oceanobs.com/duacs). Author Jiwei Tian the upper layer, have signiﬁcant correlation with shift wishes to acknowledge the support from National in the bifurcation of North Equatorial Current owing to Natural Science Foundation of China (91028008), au- the ENSO events. As for the deep layer, what domi- thor Wei Zhao wishes to acknowledge the support from nates the interannual variability and if there is any re- the National Natural Science Foundation of China lationship to the ENSO have not been investigated yet. (41176010) and the Program for New Century Excel- We will leave these for future studies. lent Talents in University (NCET-10-0764), and author

FIG. 14. (a) Mean density proﬁles in summer and winter in the deep Paciﬁc (EA: 218–238N, 1228–1248E) and SCS (WA: 188–208N, 1198–1218E); (b) Mean diapycnal diffusivity in the deep northeastern SCS (188–218N, 1178–1208E).

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