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

2972 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 44 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 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 09/28/21 03:38 PM UTC NOVEMBER 2014 Z H O U E T A L . 2973 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. Unauthenticated | Downloaded 09/28/21 03:38 PM UTC 2974 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 44 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.

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