Ventilation of the Sulu Sea Retrieved from Historical Data

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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 occurs 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 intermediate 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 conditions near the entrance of the MS in the SCS vary significantly from season to season, which make it possible 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. The process of intermediate con- LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11 3

vection is explored in Section 5, and the interbasin transforma- salinity and dissolved oxygen concentration (DO) at standard tion of water properties in Section 6. In Section 7, we argue that levels above 1 500 m (e.g., Fig. 3). Since the data are sparse at the SSDW is possibly derived from intermittent deep convec- depths below 500 m and do not allow a bin average for each tion of the MS-PS overflow; and in Section 8, some aspect about 1°×1° grid, all samples at a certain level were gridded directly the interbasin exchange in the near surface layer is discussed. using Kriging method with a search radius of two degrees to Finally a concluding remark is given in Section 9. generate the “mean” fields. Exceptional data points that are obvious outliers were discarded before the gridding. Considering 2 Data the strong seasonal variability near the surface and the con- The historical hydrographic data at standard depths and touring method used the property distribution for levels above the CTD (conductivity-temperature-depth) data from WEPOCS 200 m will not be discussed. III (Bingham and Lukas, 1994) were available from WOD 2005 The invasion and downward movement of the NPIW from (Boyer et al., 2006). The Sulu Sea CTD casts were taken in No- the SCS are obvious from these maps as illustrated in Fig. 2. vember 4–12, 1989 by R/V Shiyan 3 of the South China Sea In- The temperature difference (Fig. 3, upper panel) along the MS stitute of Oceanology, and the SCS casts are from the SCSMEX is minor at the 200 m level, but increases to greater than 3°C (South China Sea Monsoon Experiment) cruises of the State at the 400 m level, which manifests a cold tongue in the Panny Oceanic Administration of China (Wu et al., 2002). Trough indicating a bottom intensified intrusion through the The ten 1989 CTD casts in the SS plus one in the SWS were MS (Yaremchuk et al., 2009). In the interior SS, a southward conducted with a Neil Brown Mark III CTD while returning from temperature gradient was revealed for all available levels from a TOGA cruise. The data were calibrated by comparing T-S dia- the subsurface down to 1 300 m indicating a cold water source grams of Station 86 in the SWS (Fig. 1) with qualified CTD data originated in the northern basin sinking to the deep sea and of the same area from WEPOCS III (Lukas et al., 1991). From spreading southward (Quadfasel et al., 1990). From 1 000 to which a salinity bias of −0.05 was found by linear regression and 1 300 m, the warmest water was generally found along the Sulu was corrected accordingly. Results of the correction could be Archipelago in the southeast, but the pattern becomes indistin- verified by their T-S diagrams (Fig. 2), in which the corrected guishable farther below for lack of data. T-S relationship of Sta. 86 consists well with that of WEPOCS A large salinity gradient is also found in the MS at the 200 III, and those of the SS stations (Stas 87, 89–95) coincide with and 400 m levels (Fig. 3, middle panel) indicating intrusion of the mean curve. Moreover, the T-S relationship for the SS casts high salinity (>34.5) SLW from the SCS (Wyrtki, 1961). Below the meet precisely at σt=26.5 with those of the SCS casts showing 400 m level, however, the salinity distribution is not meaning- the SCS origin of SS waters in the deep, just as suggested by pre- ful presumably because the errors for scattering measurements vious studies (e.g., Wyrtki, 1961). spanning decades are greater than the actual horizontal ranges The surface drifter data were downloaded from the NOAA/ of salinity. AOLM Environmental Data Server (http://www.aoml.noaa.gov/ The distribution of DO is well consistent with that of the envids/gld/) except those deployed in the SCS during SCSMEX. temperature (Fig. 3, lower panel). Above the 200 m level, the oxygen content in the SS is generally high and is not distinguish- 3 Horizontal distribution of properties able from the SCS. At depth levels from 200 to 400 m, a high Based on available data from WOD2005, we managed to oxygen patch was observed in the northern SS, which intrudes construct horizontal maps for distributions of temperature, from the MS and extends southeastward toward the deep basin

15 Temperature/°C

33.5 34.0 34.5 35.0 Salinity

Fig.2. T-S diagram for the CTD casts taken in the SS (blue) and the SWS (green) during the 1989 cruise. For comparision, the mean T-S curve for the SS derived from WOA05 (light blue) and the referent casts of the SCS (Magenta) and the SWS (gold) are also plotted. The inset shows a blowup around the lower end of the T-S relationship of the SS revealing a weak salinity minimum there. 4 LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11

116° 118° 120° 122° 124°E116° 118° 120° 122° 124°E116° 118° 120° 122° 124°E116° 118° 120° 122° 124°E 14° N T TTT 9 12° 14 10 15 11

15 10° 16 10.4

10.5

8° 15 10.5 10.1 10.08 14 6°

4° 14° N S SSS 34.45 12° 34.5

10° 34.45 34.45 34.45

34.45 34.45 34.45

8° 34.45 34.45 34.45

34.4

6°

4° 14° N DO DO DO DO

12° 2.2 2 2 2 1.8 1.8 10° 1.6

1.8 1.6 1.8 1.6 1.6 8° 1.4

1.8 1.6 1.4

6° 2 1.6 1.8 200 m 400 m 600 m1 000 m 4°

Fig.3. Distributions of temperature (°C), salinity, and DO (mL/L) in the SS. The meridional gradients caused by intruding and sinking of the NPIW are in evidence. along the Panay Trough. The DO contents in the patch are very recently (Tessler et al., 2010; Sprintall et al., 2012), an overall close to those of the same levels in the SCS (Li and Qu, 2006) image of the overflows through the entire channel is yet to be suggesting their SCS origin. Similar to the temperature distribu- demonstrated. Based on historical data available from WOD05, tion, a meridional gradient of DO is also apparent in the SS, but we managed to construct a section along the main axis of the with higher oxygen contents in the north and the lowest values channel from the MS to the Panay Trough, in which data within found near the west coasts of Mindanao Island and the Sulu Ar- a 10′ grid around the sampling point were used (Fig. 4a). chipelago. There seems to be a pattern of cyclonic circulation As indicated by the figure, there are two sills along the chan- that transports the oxygen rich water along the coasts of Pala- nel: one at about 12°N to the south of Mindoro and another at wan. about 11.3°N to the west of Panay. The former appears shallower with a sill depth of about 420 m and thus acts as the 4 Mindoro and Panay Straits overflow controlling sill. The latter found near 11°19′N (Fig. 5) is about Although it is generally recognized that water in the deep SS 510 m in depth rather than 570 m as given by previous authors originates from the SCS through the MS and the Panay Trough (e.g., Tessler et al., 2010). To the north of the Panay Sill, there (Wyrtki, 1961), and the Panay Sill overflow has been explored is a small basin with depths greater than 1 000 m in between LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11 5

120° 121° 122°E 0 13° 4.0 N South 100 T S DO 20 34.30 3.0 China 18 34.40 MS 200 16 34.50 14 Sea 500 400

300 34.45 2 500 12 400 0.

12° 34.45 0 200 400 10 PS 2.0 500 400 500 50

100 Depth/m 600 PS 8

11° 700 2.0 Sulu Sea 500 800 10

0 50 200 6 500 1 000 900 34.45 b c d 1 000 a 1 000 10° 10°N 11°N 12°N 13°N 10°N 11°N 12°N 13°N 10°N11°N 12°N 13°N

Fig.4. The sea-bed topography of the Mindoro Strait (MS) and the Panny Strait (PS) showing sampling points (triangle) along the channel (a); and distributions of temperature (°C) (b), salinity (c), and dissolved oxygen (mL/L) (d) along the section. The red dot in Fig. 4a indicates the Semirara Sea.

100

200

300 11 °23′N 11 °22′N

Depth/m 400 11 °21′N 11 °20′N 11°19′N 500 11 °18′N 11 °17′N 600 11 °16′N 11 °15′N

121.6°E 121.7°E 121.8°E 121.9°E 122.0°E

Fig.5. Zonal depth profiles across the Panay Trough showing a sill depth of about 510 m, based on Smith and Sandwell (1997) and http://topex.ucsd.edu/marine_topo.

(Fig. 4a) known as the Semirara Sea (Gordon et al., 2011). In Fig. 4b, a large horizontal temperature gradient below 500 m From the property distribution (Figs 4b, c and d), a com- can be seen in the Semirara Sea, which indicates a well-shaped plete image of the overflow system along the channel is pre- overflow of the first step that pours into the Semirara basin after sented that carries waters of temperature less than 11°C, sa- passing the MS sill and then proceeds to the Panay Sill. It is also linity below 34.45, and oxygen concentration greater than suggested that vigorous vertical mixing may occur in the deep 2.0 mL/L passing through the MS-PS and pouring into the SS Semirara basin (Fig. 4b) where little difference in both salinity downstream of the Panay Sill. The down-slope movement of and oxygen are also found (Figs 4c and d). the overflow is evidence by distributions of all three proper- After passing the Panay Sill, the overflow moves down the ties, in which the colder, fresher, and oxygen-rich waters over continental slope. The temperature distribution suggests that the slope can all be trace back to the SCS and hence confirms the intruded SCS water could reach a depth of about 1 000 m that SCS waters intruded from the MS is a major source of where an isolated cold core with temperature below 10°C is ob- deep waters of the SS. served on bottom of the lower Panay Trough. A low salinity core It is revealed further by Fig. 4 that the MS-PS overflow is in (< 34.41) is also found over the slope centered at 700 m with oxy- fact a two-step system consisted of the Panay Sill overflow (Tes- gen contents greater than 2.0. All these evidences suggest active sler et al., 2010) and an upstream overflow behind the MS sill. convection in the northern SS (Fig. 4). 6 LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11

5 Intermediate convection brackish surface, there is a core layer of salinity maximum with Convection appears to be the most important ventilation S>34.47 lying at about 200 m depth extending southward from process for the deep SS. It is believed that the invaded NPIW the Panay Trough where salinities greater than 34.5 were found from the SCS practically supplies its bottom water (Wyrtki, in its northern reach (Fig. 3). It thus confirms that this core layer 1961), which drives the deep convection of the SS. However, is the SLW originated from the MS and is not hampered by the Frische and Quadfasel (1990) found that, due to the observed sills (Quadfasel et al., 1990). higher temperature and lower salinity, water in the invaded Below the core layer of salinity maximum, isotherms and iso- plume is less dense than the deep ambient water in the south- pycnals generally tilt down southward from the Panay Trough ern basin and therefore deep convection may not occur. (Figs 6a and c) indicating sinking of the overflows toward the

To have a better understanding about the behavior of the deep. A low salinity core (Smin<34.45) was found between 500 invaded SCS waters after entering the SS, two hydrographic and 1 200 m in the northern basin, where the overflows occur sections extending southward from the Panay Trough were fur- (Fig. 6b). It is thus suggested that this low salinity water is origi- ther examined: the 121.8°E “mean” section re-sampled from the nated from the intruded NPIW of the overflow even though a horizontal analyses for standard levels of WOD2005, and the direct connection is not seen from the salinity contours, which “snap-shot” section (see Fig. 1) taken in early November, 1989 may finally reach intermediate depths by sinking or mixing and on the field. Restricted by data availability, only upper ocean result in a layer of salinity minimum at about 700 m. distributions are presented (Fig. 6). It should be noted that this is a very weak minimum: the sa- The property distributions along the “mean” section (Figs linity differences of the core with its surrounding is rather small 6a to d) generally consist with what given in Quadfasel et al. (<0.03) and could hardly be identified in the southern basin (1990)—the sole hydrographic section of the SS ever appeared (Fig. 6b). The oxygen distribution, in contrast, provides a much in literature. It is clearly demonstrated that below the warm, clearer image (Fig. 6d). It shows that the high oxygen water is

0 25 34.3 23 3.0 20 34.4 25 24 15 34.45 26

1.8 500 11 11 2.0

1.6 10.2 <34.44 26.5 Depth/m

34.45 1 000

4.1

a T b S c σt d DO 1 500 7°N8°N 9°N 10°N11°N 7°N8°N 9°N 10°N11°N 7°N8°N 9°N 10°N11°N7°N 8°N9°N 10°N11°N 0 25 22 34.3 24 23 20 34.4 25 15 >34.5 26

34.47 500 11 34.46

Depth/m 10.2 26.5

1 000

e T f S g σt 1 500 7°N8°N 9°N 10°N11°N7°N 8°N9°N 10°N11°N7°N 8°N9°N 10°N11°N

Fig.6. Distributions of temperature (a), salinity (b), σt (c), and dissolved oxygen (mL/L) (d) along 121.8°E derived from WOD2005; and that of temperature (e), salinity (f), and σt (g) for the November 1989 section (see Fig. 1). LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11 7

from Panay Trough, which sinks along the northern slope and kas et al., 1991). The maximum reduces to about 34.65 in the spreads southward at intermediate depths around 700 m form- SCS, and to 34.55 in the SS reflecting the great influence of fresh ing a core layer of about 700 m thick. water input and the vigorous mixing in these land-surrounded, Instead of the “mean” pattern, the 1989 CTD section pro- semi-enclosed basins. Accompanying with the reduction of vides a snap-shot (Figs 6e–g). Like what appears in the “mean” salinity, the core shifts from the 24.0 sigma-t surface to around state, the core of SLW is clearly visible at about 200 m depth with 25.2 in the SCS and about 25.5 in the SS. These changes also salinity greater than 34.50 in the north. It extends southward signify strong vertical mixing in the upper layer that deepens beneath the seasonal thermocline and weakens gradually on the salinity maximum. its way south. Below that, there is a layer of salinity minimum It should also be noted that even though the Sibutu Passage as well with salinity less than 34.46, lying at depths from 600 is deep enough to allow the SLW to enter the SS from the SWS, to 1 400 m. The core of the layer (S<34.456 with T<10.1°C) also there is no hint of that in either Fig. 3 or Fig. 6. It seems that the locates on the slope revealing its northern origin from the Panay Sibutu Passage is mainly an outflow channel of the SS. Trough. For intermediate waters around the salinity minimum the As to the density structure, a downward tilt of isopycnals situation appears more complex. Several studies have reported close to the northern slope is found in the “mean” case suggest- the low salinity core at intermediate depths in the SS and gener- ing sinking of the overflow forced by negative buoyancy. Un- ally attributed to the salinity minimum of NPIW in the SCS (e.g., fortunately, this is not observed in the snap-shot owing to the Wyrtki (1961). Unlike the SCS, however, the low salinity core in sparse sampling interval of the 1989 cruise. the SS is extremely weak, and its salinity minimum can hardly The available observations indicate that, as pointed out by be identified from a T-S diagram since the salinity difference previous studies (Frische and Quadfasel, 1990; Gordon et al., between the invaded NPIW and the SS deep water (SSDW) is 2011), the NPIW plume invaded from the SCS through the lower less than 0.02, which is about one order smaller than that in the layer of the MS does not plunge down directly to the bottom SCS (Quadfasel et al., 1990; Nozaki et al., 1999). Moreover, the and the convection driven by negative buoyancy effect is likely temperature difference between the two water masses is also just intermediate. very small. The invaded plume is most apparent as a high oxygen All these features are well presented in Fig. 2. The salinity tongue extending down the northern slope after leaving the MS minimum is clearly defined in the SWS and the SCS, which is (Fig. 6d). It stagnates, however, at intermediate depths where it generally below 34.44 with σt of 26.55 in the SWS (May become takes a turn and spreads southward forming a high oxygen core less than 34.40 in some cases caused by intrusions of the NPIW layer lying at depths around 700 m. If we take the 1.6 mL/L con- (Bingham and Lukas, 1994) but increases to above 34.44 with σt tour as the boundary of the plume, the convection could gen- of 26.75 in the SCS showing a pathway from pacific to the SCS erally reach a maximal depth of about 1 200 m only, although and the deepening of the minimum as a result of mixing with deep waters. In contrast to both the SWS and the SCS, the salin- deeper convection may takes place in certain occasion. Since ity minimum is rather difficult to be identified for the SS from its the oxygen contents either above or to the south of the core are full T-S diagram (Fig. 2, main frame) even though a low salinity lower, the tongue manifests an intermediate convective pattern layer does exist at intermediate depths (Figs 6b and f). in the upper ocean. A blow-up of the SS T-S diagram (Fig. 2, inset) reveals an This pattern is further supported by the salinity distribution interesting fine structure near its low-temperature endpoint, of both cases (Figs 6b and f) that shows a similar core of low which shows that, below σ =26.5, the SS T-S relationship is very salinity water at intermediate depths stretching southward and t different from both the SCS and the SWS. It shows that the SS being surrounded by more saline waters, although its upstream T-S relationship takes a sharp turn near σ =26.5 becoming par- connection is not resolved by the contours. t allel to the density contour that manifests a weak salinity mini- In summary, the available historical evidence supports the mum at 34.45. This turning point is found at depths around viewpoint that convective process of the MS-PS overflow is 1 000 m where the low salinity core locates, and is obviously mostly intermediate. The invaded NPIW generally appears as a associated with the low salinity layer. Below the turning point pool of cold, oxygen-rich and relatively fresh waters spreading salinity increases slightly to greater than 34.46. In addition to from the northern basin at intermediate depths between 600 salinity, temperature also rises a little bit below the turning and 1 400 m (Fig. 6). point as a consequence of adiabatic heating in the deep. This fine structure could hardly be seen in the main diagram since 6 Interbasin transformation of water properties waters in the SS are almost homogeneous below 800 m (Fig. 6). In addition to the analysis above, the observed behavior and It is apparent that the sharp turn signifies a weak salinity transformation of the SLW and the NPIW from the Pacific to- minimum of the SS caused by the low salinity layer of interme- ward the SS can be further examined by means of their temper- diate convection and the SSDW water of higher salinity in below ature-salinity (T-S) characteristics. (e.g., Figs 6b and f). It is also noted that the density of waters at

Figure 2 shows the T-S diagram for CTD profiles of the 1989 and below the salinity minimum (σt≈26.5) is less than that of the cruise along with qualified historical profiles of the SCS and SCS salinity minimum (σt=26.75), which indicates that waters of the SWS for comparison. In the figure, the variation of the SLW the MS-PS overflow can only derived from the upper part of the salinity maximum is well documented along the pathway from SCS intermediate layer. Pacific to the SS via the SCS. The salinity maximum appears to These results also suggest that the salinity minimum of the be the highest in the SWS, where it could reach 34.9 or higher SS is not simply “copied” or “advected” from the SCS. In stead, it because the SWS communicates freely with the western Pacific is more likely a result of interaction between the invaded NPIW and its water properties are similar to the nearby Pacific (Lu- and the pre-existing SSDW. 8 LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11

7 Intermittent convection: a possible mechanism that de- tent. Since the Panay sill overflow is persistent year round (Tes- rives the SSDW sler et al., 2010), the intermittency should primarily depend on Since convection of the MS-PS overflow normally may not the density of the MS-PS overflow, which may change from time reach the deep SS, a question still left open is how the SSDW is to time and so may the depth of convection. derived. From the property distributions, it is obvious that the In the 1989 case, for example, waters of the invaded NPIW source water of the SSDW should not only be dense enough to plume at the time of measurement is probably warmer (though descend to the bottom but also be saltier than the intermediate saltier) and hence less dense than that of the pre-existing inter- water. mediate core, and could only sink to a shallower depth of about In an effort to answer this question, Quadfasel et al. (1990) 700 m lying above the core (Fig. 6f). suggested that water of the MS overflow may get loaded with Previously studies have indicated that the density of the suspended sediment becoming denser on its way, and plunge NPIW inflow depends mainly on waters available at the en- down the slope as turbidity current events. From records on a trance of the MS on the SCS side, which is closely related with sediment core they found evidences of events with an average the thermocline depth in the adjoining SCS (Quadfasel et al., interval of about 70 years and speculated that passage of tropi- 1990; Gamo et al., 2007). They suggested that passage of tropical cal cyclonic over the MS may responsible for triggering these cyclones may uplift the thermocline and allows denser SCS wa- events. Gamo et al. (2007), on the other hand, argued that geo- ter entering the SS. For situation like the 1989 case, thermocline thermal heating may causes overturning of the SS bottom water in the adjoining SCS might be deeper than usual, and hence the below a depth of 3 000 m. However, both assumptions do not MS inflow draws warmer water of higher salinity from a shal- explain why the observed salinity in the deep SS is higher than lower portion of the NPIW in the SCS. that of the intermediate convective layer. The question is what kind of process might pump the ther- To resolve the paradox, Gordon et al. (2011) proposed that, mocline up and provide the MS-PS overflow with water that is instead of the SCS, the deep SS ventilation is derived from the dense enough to sink below the low salinity intermediate layer south (the SWS) through the Sibutu Passage. However, the inset and is saltier than the core layer at the same time,. of Fig. 2 shows that, along the 26.5 sigma-t surface, salinity of It is well known that, driven by the alternating East Asia the SWS water is less than that of the SSDW with a difference of monsoons, the general circulation of the SCS varies dramati- about 0.03 (measured from their medians). Hence, the Sibutu cally with seasons (Wyrtki, 1961; Li et al., 2003) and so does the Passage is unlikely a source of the SSDW. thermocline (Liu et al., 2000). A feature worth noting in both Figs 4 and 6 is that the low- Figure 7 compares two CTD casts near the entrance of the salinity core on the slope appears isolated from the MS with no MS (at 12.0°N, 118.5°E) taken on June 23, 1998 and December 5, visible contours connected to the strait. This hydrographic pat- 1998 respectively, which shows the seasonal fluctuation of the tern suggests that the convective process is possibly intermit- thermocline and the variation of water properties associated

Temperature/°C ΔTw-s/°C Δσt 0010 20 30 −4 −2 24−1 −0.5 0 0.5 1 0

100

200

300

400 Depth/m

500

600

700 abc

800 34.2 34.3 34.4 34.5 34.6 34.7 −0.05 0 0.05 −0.2 0 0.2

Salinity ΔSw-s Δσt

Fig.7. Vertical profiles for Temperature (solid line) and salinity (dash line) taken near the entrance of the MS in the SCS at (12.0°N,

118.5°E) in summer (black) and winter (grey) (a); their seasonal differences (b); and the difference of σt (c). Shaded area indicates higher values in winter. LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11 9

with the fluctuation. It shows that, because of surface cooling 2011; Ohlmann, 2011) and vigorous mixing during winter, the surface mixed layer is Figure 1 presents trajectory of some surface drifting buoys colder and the thermocline appears deeper in winter than in that either drifted into or deployed inside the SS in the past two summer (Fig. 7a). Interestingly though, the water temperature decades. Though only fifteen drifters are available, they do give below the mixed layer (from about 70 m to the sill depth of the a hint about interbasin exchange in some aspects. The trajec- MS, approximately 420 m) is considerably higher in winter, and tories of these drifters generally fall into two groups (with one the salinity below 150 m is higher as well (Fig. 7b). Furthermore, exception that entered from the SWS through the Jolo Strait it is revealed that there are three main zero-crossing points in in winter). The major group includes six buoys (five in winter the profile of density difference at about 50, 410, and 720 m plus one in summer) from the western Pacific and five deployed depths (Fig. 7c). The result indicates that both density and salin- locally (three in the SS plus two in the Dipolog Strait), which ity at intermediate depths (about 400– 700 m) near the entrance manifest a surface throughflow from the Pacific that enters the of the MS is higher in winter, which might provides the MS-PS Philippine Archipelago via the Surigao Strait, passing through overflow with source water that can cause a deeper convection the Bohol Sea and the Dipolog Strait reaching the interior of the in the SS and thus derive the SSDW seasonally. SS. The second group includes three buoys from the SCS that The tricky part of the problem is that the depth of the second entered through the Balabac Strait suggesting a flow toward the zero-crossing point is very close to the sill depth. It is likely that SS through the strait as well. It is also noted that, though most some other process, such as tidal and wind-induced fluctua- drifters passed through the straits in winter, some cases did oc- tion, must have taken effect as well. They may help in pumping cur in summer. the water up to the channel and hence enhancing the convec- An important phenomenon revealed here is that, despite tion. the seasonal reverse of monsoon, all available drifters move in In brief, observational evidence indicates that, driven by the the same direction directing toward the SS (Fig. 1). It is thus East Asian monsoon, water properties near the entrance of the suggested that surface inflows may persist throughout the year MS are subject to significant seasonal variation, where water at along both paths. As roughly estimated from the existing mea- intermediate depths (400– 700 m) is denser and saltier in winter surements, the mean current speed through the Bohol Sea is than in summer. With the help of tidal and meteorological forc- about 0.5 m/s in winter, but could boost to about 1.0 m/s as in- ing, the MS-PS overflow is possible to bring in denser waters of dicate by the sole drifter in summer, and that gives a simple an- higher salinity in certain periods leading to deep convection. It nual mean around 0.6 m/s. Assuming barotropic in nature and is hence proposed that the SSDW may derive from intermittent a cross section of 40 km×50 m, it was estimated that the trans- deep convection of the MS-PS overflow. port through the Surigao Strait is about 1.0×106 m3/s in winter and could reach 2.0×106 m3/s in summer. For the Balabac Strait, 8 Interbasin exchange in the near surface layer on the contrary, the through flow appears stronger (about 0.9 A question left unaddressed here is how the near surface m/s) in winter and weakening in summer (about 0.3 m/s), with layer is “ventilated”. In other words, how does it exchange with an annual mean of 0.6 m/s roughly and a transport comparable the neighboring basins. Apart from the MS-PS system, little with the Surigao Strait. solid information is available regarding exchanges through nu- Although the number of available drifter is not enough to merous shallower channels around the SS, such as the Sibutu make a firm conclusion, the flows through the Surigao Strait/ Passage, the Balabac Strait, and the Surigao Strait through the Bohol Sea and the Balabac Strait estimated from the measure- Bohol Sea (Fig. 1, Table 1), even though some authors has tack- ments are in good agreement with the model results of Han et led problem based on model analysis or limited observations. al. (2009), who found strong westward currents in the Bohol Apart from the deep ventilation, the near surface exchange Sea carrying the surface water of the western Pacific from the is important for material and energy balance of a semi-enclosed Surigao Strait into the Sulu Sea and inflow from the SCS in the basin. Based on early observation Wyrtki (1961) suggested that, Balabac Strait. Nevertheless, more observation is necessary be- forced by the seasonally alternating monsoons, surfaces cur- fore firm conclusions can be drawn. rents through numerous straits around the SS are highly sea- There is another fact worth mentioning: though there are sonal except for the Surigao Strait, where water from the west- quite a number of drifters that entered the SWS (not shown), ern Pacific flows through the Bohol Sea entering the SS all year they all drifted into the Makassar Strait following the path of round. Yet, model results suggest that, as part of the low lati- the Indonesian Throughflow, and none of them got into the SS tude western current, water of Pacific origin enters the Sulu Sea through the Sibutu Passage. It is thus suggested that there is through the MS and returns to the Pacific through the Sibutu little surface water entering the SS through the Sibutu Passage. Passage (Metzger and Hurlburt, 1996). Their mean transport The passage is mainly an outflowing channel as well in the near estimated lately from satellite data is about 2.4×106 m3/s and surface layer. 2.8×106 m3/s respectively (Qu and Song, 2009). On other hand, by using a high-resolution model Han et al. (2009) found that, 9 Concluding remarks on the annul mean, there are surface inflow from the SCS at The SS is a mediterranean sea in the southeastern Asia sur- both the Mindoro and Balabac Straits and outflow at Sibutu rounded with islands (Fig. 1), which is unique because its water Passage, and in addition, there is a strong westward current in column is almost homogeneous below the main thermocline the Bohol Sea, which carries the surface water from the WP near (about 600 m). With a maximal sill depth, among numerous the Surigao Strait into the Sulu Sea. And, some recent observa- others, of only 420 m for the MS connecting with the SCS, venti- tion also suggested that direction of flows through the Surigao lation and interbasin exchange of the SS are subjects of special Strait is likely into the Philippine interior seas (Gordon et al., interest to marine scientists. In this paper, both the processes 10 LI Li et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 1–11

are examined based on historical data, but with an emphasis that is dense enough and of higher salinity at the same time, to on ventilation. sink below the intermediate layer of the SS in winter seasons. The results suggest that water renewal process in the SS is It is hence proposed that the SSDW is derived from intermit- basically composed of four major cells, which includes a near tent deep convection of the MS-PS overflow, which is seasonally surface cell where multi-channel, interbasin throughflows oc- modulated by the alternating monsoon. cur, and three ventilation cells: the subsurface cell in which the high salinity SLW spreads; the intermediate convective cell in References which the MS-PS overflow invaded from the SCS descends; and Bingham F M, Lukas R. 1994. The Southward Intrusion of North Pacific the abyssal cell where the SSDW lies. The ventilation pattern of Intermediate Water along the Mindanao Coast. 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