Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11 DOI: 10.1007/s13131-014-0517-3 http://www.hyxb.org.cn E-mail: [email protected] Ventilation of the Sulu Sea retrieved from historical data LI Li1*, GAN Zijun2 1 Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China 2 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanoglogy, Chinese Academy of Sciences, Guangzhou 510301, China Received 8 April 2014; accepted 28 May 2014 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014 Abstract Based on historical observations, ventilation of the Sulu Sea (SS) is investigated and, its interbasin exchange is also partly discussed. The results suggest that near the surface the water renewal process not only oc- curs through the Mindoro Strait (MS) and the Sibutu Passage, but also depends on the inflows through the Surigao Strait and the Bohol Sea from the Pacific and through the Balabac Strait from the South China Sea (SCS). Both inflows are likely persistent year round and their transports might not be negligible. Below the surface, the core layer of the Subtropical Lower Water (SLW) lies at about 200 m, which enters the SS through the Mindoro Strait not hampered by topography. Moreover, there is no indication of SLW inflow through the Sibutu Passage even though the channel is deep enough to allow its passage. The most significant ventilation process of the SS takes place in depths from 200 m to about 1 200 m where intermediate convection driven by quasi-steady inflows through the Mindoro and Panay straits (MS-PS) dominates. Since the invaded water is drawn from the upper part of the North Pacific Intermediate Water (NPIW) of the SCS, it is normally not dense enough to sink to the bottom. Hence, the convective process generally can only reach some interme- diate depths resulting in a layer of weak salinity minimum (about 34.45). Below that layer, there is the Sulu Sea Deep Water (SSDW) homogeneously distributed from 1 200 m down to the sea floor, of which the salinity is only a bit higher (about 34.46) above the minimum. Observational evidence shows that hydrographic con- ditions near the entrance of the MS in the SCS vary significantly from season to season, which make it possi- ble to provide the MS-PS overflow with denser water of higher salinity sporadically. It is hence proposed that the SSDW is derived from intermittent deep convection resulted from property changes of the MS-PS inflow. Key words: Sulu Sea, ventilation, overflow, interbasin exchange Citation: Li Li, Gan Zijun. 2014. Ventilation of the Sulu Sea retrieved from historical data. Acta Oceanologica Sinica, 33(9): 1–11, doi: 10.1007/s13131-014-0517-3 1 Introduction in the SCS (Li and Qu, 2006). The Sulu Sea is a semi-enclosed basin in the southeastern By comparing water properties of the SS with the SCS Wyrtki Asian waters (Fig. 1), which is totally isolated, below a few hun- (1961) indicates that topography of the MS plays an important dred meters, from the Pacific and the other neighboring ba- role in ventilation of the SS. The sill depth allows the SLW from sins, e.g., the South China Sea (SCS), the Sulawesi Sea (SWS, the Western Pacific to spread, by way of the SCS, into the SS also known as Celebes Sea). The Mindoro and Panay Straits without obstruct causing a distinct salinity maximum in the (MS-PS), which connects the SS and the SCS with a sill of 420 SS at a depth of around 200 m. It also allows an inflow of the m deep lying at about 11.5°N to the west of Panay Island (Fig. NPIW from the SCS that supplies the SS intermediate and bot- 1), is the deepest channel for water exchange with neighboring tom waters through the Mindoro Strait (Wyrtki, 1961; Frische basins (Wyrtki, 1961). The other channels are relatively shallow and Quadfasel, 1990; Quadfasel et al., 1990; Chen et al., 2006; (Table 1). Gamo et al., 2007), though only the upper portion of the inter- The Sulu Sea, which is over 6 000 m deep in the central ba- mediate water of the SCS enters the SS quasi-steadily (Gamo sin, is hydrographically unique where water below the interme- et al., 2007). Comparison of hydrographic profiles below the diate layer is almost homogeneous vertically with temperature subsurface on both sides of the MS generally shows colder and very close to 10°C and salinity around 34.47 all the way down saltier waters with higher oxygen and nutrient contents on the to the bottom (Wyrtki, 1961; Quadfasel et al., 1990; Chen et al., SCS side (Wyrtki, 1961; Broecker et al., 1986 ; Chen et al., 2006; 2006; Gamo et al., 2007). Gamo et al., 2007; Qu and Song, 2009). Hence, intrusions of in- Earlier studies indicated that there are two type of water termediate water from the SCS will bring in oxygen and nutrient masses in the SS can be traced back to the SCS: the North Pacific rich waters of lower temperature and higher salinity into the SS. Subtropical Lower Water (Wyrtki, 1961) (SLW, also known as the It has been noted that water mass of the MS inflow is less North Pacific Tropical Water (Fine et al., 1994), which appears as dense than the deep SS water which could not have reached the salinity maximum, and the North Pacific Intermediate Wa- the sea floor of its own accord because of its buoyancy. And, a ter (NPIW), which forms the salinity minimum (Wyrtki, 1961), mechanism of deep-water renewal by turbidity currents was both originated from the Pacific and with properties modified suggested (Quadfasel et al., 1990). It was also proposed that oc- Fundation item: The Chinese Ministry of Science and Technology through the National Basic Research Program under contract No. 2009CB421205. *Corresponding author, E-mail: [email protected] 2 LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11 115° 117° 119° 121° 123° 125° 127° E 14° N Sibuyan Mindoro St. Sea 12° Visayan Panay Sea South China 95 Sea 94 00 Surigao 2 10° 93 St. 92 Palawan 97 96 91 Bohol Sea 90 8° 89 Balabac St. Mindanao Sulu Sea 87Basilan St. 86 6° Jolo St. Sulawesi Kalimantan Sibutu Passage Sea 4° Fig.1. Map of the Sulu Sea showing locations of the hydrographic stations (■ represents the 1989 CTD casts and + the referent casts from a SCSMEX cruise in the SCS and the WEPOCS III cruise in the SWS for calibration and inter-comparison). Trajectories of surface drifters in winter (from October to April, blue and green lines) and summer (from May to September, orange and pink lines) are also presented. ① Panay Strait, ② Guimaras Strait, ③ San Bernardino Strait, ④ Dipolog Strait. Table 1. Major channels around the Sulu Sea connected with other basins1) Channel name Bearing Sill depth/m Basin connected Mindoro Strait (Panay Trough) North 420 (510) South China Sea Balabac Strait West 105 South China Sea Sibutu Passage South 270 Sulawesi Sea Jolo Strait Southeast 200 Sulawesi Sea Basilan Strait Southeast about 75 Sulawesi Sea Surigao Strait East 65 Pacific (through Dipolog Strait and Bohol Sea) Guimaras Strait Northeast <50 Pacific (through Visayan Sea) San Bernardino Strait North 110 Pacific (through Sibuyan Sea) Note: 1) Based on Wyrtki (1961), Broecker et al. (1986), Smith and Sandwell (1997) and http://topex.ucsd.edu/marine_topo. casional inflow of denser intermediate water may occur when the area and from which several importance aspects about the the thermocline in the South China Sea is uplifted by tropical current topic were revealed (Gordon et al., 2011). A persistent cyclones, which could further intensify the turbidity currents overflow from Panay Strait (PS) into the SS was evidenced for (Quadfasel et al., 1990; Gamo et al., 2007). the first time (Tessler et al., 2010; Sprintall et al., 2012); a gen- An important question related to ventilation of the SS being eral pattern of the water exchange between the SCS and the SS explored is the inter-basinal transport through the numerous through Mindoro and Panay Straits (MS-PS) is given (Gordon et straits around the basin, because the subsurface inflow through al., 2011); and the active interaction of the SS with the western the MS must be balanced by shallower outflows. Based on nu- Pacific by way of the shallow San Bernardino and Surigao Straits merical modeling Metzger and Hurlburt (1996) suggested a net were noted (Gordon et al., 2011). cyclonic flow around the Philippines, which enters the SS via Even though, our knowledge about the SS is yet so limited the MS and returns to the Pacific by way of Sibutu passage. The that a systematic description is rarely available and information transports through the two channels were recently estimated regarding the general pattern of property distribution could of about 2.4×106 m3/s and 2.8×106 m3/s respectively (Qu and hardly be found. To improve the understanding, we have re- Song, 2009), which appears to be part of the South China Sea cently examined a number of historical observations available throughflow system and plays a role in the large scale inter-ba- for the SS with focus on its ventilation. The remainder of the sinal exchange between the Pacific and the Indian Oceans (Fang paper is structured as follows. We begin with a brief description et al., 2003, 2005; Qu et al., 2005, 2006). of the data set. Next, the horizontal distribution of properties in In late last decade, the PhilEx (Philippine Straits Dynam- the SS (Section 3) and the vertical structure of the MS-PS over- ics Experiment) project conducted a comprehensive survey of flow (Section 4) are presented.