JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, C03026, doi:10.1029/2008JC005109, 2009 Click Here for Full Article

Seasonal upper circulation in the Sulu from satellite altimetry data and a numerical model Shuqun Cai,1 Yinghui He,1,2 Shengan Wang,1 and Xiaomin Long1 Received 4 September 2008; revised 8 January 2009; accepted 27 January 2009; published 28 March 2009.

[1] Eight years of Absolute Dynamic Topography from satellite altimetry data are used to study the seasonal variability of the circulation in the (SS) through Empirical Orthogonal Function (EOF) analysis. It is revealed that first seasonal EOF mode shows a basin-scale anticyclonic/cyclonic circulation in summer/winter and second seasonal EOF mode shows a weak basin-scale anticyclonic/cyclonic meander flow from the Strait to the , and the typical surface circulation structure in the SS is shown as a basin-scale anticyclonic/cyclonic circulation (or meander) centered at about 120.8°E, 8.6°N in August and December. According to the numerical experiments by a connected single-layer and two-layer model, it is shown that the upper circulation in the SS is closely related to the outflow via the Sibutu Passage and seasonal local wind stress. Either an outflow via the Sibutu Passage or the summer monsoon may cause an anticyclonic circulation in the SS, while the winter monsoon may cause a cyclonic circulation. Either an outflow via the Sibutu Passage or the winter monsoon would push the water out of the SS via the but bring the water into the SS via the , while the summer monsoon would bring the water into the SS via the Balabac Strait but push the water out of the SS via the Mindoro Strait. Thus, in summer, the induced anticyclonic circulation with the negative relative vorticity is stronger in the SS but the water transports via the Mindoro and the Balabac straits are less, while in winter, the induced cyclonic circulation with the mainly positive relative vorticity is weaker but the water transports via the Mindoro and the Balabac straits are larger. The inflow via the Mindoro Strait is also significant since the outflow via the Sibutu Passage is mainly compensated by the inflow via the Mindoro Strait. The western strengthening near the Island and the asymmetry of the circulation in the SS is caused by the b effect. The transport via the Mindoro Strait is generally much larger than that via the Balabac Strait. An inflow into the SS via the Dipolog Strait contributes little to the circulation in the SS except for the current field near this strait. A stronger lower layer current than the upper layer one near the Mindoro Strait is also discussed. Citation: Cai, S., Y. He, S. Wang, and X. Long (2009), Seasonal upper circulation in the Sulu Sea from satellite altimetry data and a numerical model, J. Geophys. Res., 114, C03026, doi:10.1029/2008JC005109.

1. Introduction 30 m but the deepest depth about 960 m appears in the middle of the strait. The Dipolog Strait is very narrow, but its deepest [2] The Sulu Sea (SS) is almost a Mediterranean sea with depth is larger than 1000 m, it is connected with the Pacific via the greatest depth exceeding 5000 m in its southeast (Figure 1). the . The Sibutu Passage is wide and deep with its It mainly connects with the (SCS) through the deepest depth larger than 2000 m in the middle of the strait. Mindoro and the Balabac straits and with the Sulawesi Sea [3] Maybe the SS is too small to attract the oceanogra- through the Sibutu Passage. The Balabac Strait is narrow and phers’ attention; few studies on the circulation of the SS are shallow, across Section B (Figure 1a, 117°200E, 7° 8°200N),  reported. According to the current charts by Wyrtki [1961], the sill depths are only about 10 m. The Mindoro Strait is also the surface circulation in the SS is mainly anticyclonic in narrow but much deeper, across Section A (120°50 121°50E,  summer. Some regional model studies showed that the 12°150N),thesilldepthsatthewesternmostareshallowerthan annual mean upper circulation in the SS is cyclonic since the winter NE monsoon is dominant [Metzger and Hurlburt, 1LED, South China Sea Institute of Oceanology, Chinese Academy of 1996], and the southward transport flowing out of the SS via Sciences, Guangzhou, China. the Sibutu Passage into the Sulawesi Sea was more than 2 Graduate School of the Chinese Academy of Sciences, Beijing, China. 2.5Sv(1Sv=1Â 106 m3 sÀ1), but Wajsowicz [1999] suggested that the transport via the Sibutu Passage was only Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JC005109$09.00 0.7 Sv. The global circulation model study with 1/6°

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(a) (b)

Figure 1. (a) Computational domain (including the SCS, SS, and part of the Pacific) surrounded by thick solid lines and coastal lines (double-dashed lines stand for open boundaries) and (b) topography of the Sulu Sea (unit is m). resolution by Fang et al. [2003] suggested that the annual the SS in depth. In the following, data and EOF analysis are mean transports from the SCS into the SS via the Mindoro given in section 2, the model description and choice of and the Balabac straits are 0.25 Sv and 1.35 Sv, respectively. parameters are given in section 3, in section 4, the numerical On the basis of the 900-year integration of a global ocean experimental results and discussions are presented, and circulation model with 1° coarse resolution driven by section 5 is the conclusion. ECMWF reanalysis wind data, Cai et al. [2005] studied the water exchange between the South China Sea and its 2. Data and Seasonal Variability of EOF Analysis adjacent , and the simulated current charts suggested that the surface circulation of the SS is basically cyclonic in [5] The present study is based on the high-resolution winter. According to the preliminary numerical experiments (0.25°) Delayed Time Maps of ADT data with up to four by a connected single-layer and two-layer model with a satellites (Topex/Poseidon, Jason-1+ERS, Envisat and resolution of 100, Cai et al. [2008] found that because of the GFO) at a given time from the Web site http://www.aviso. local monsoon stress curl, the upper circulation in the SS is oceanobs.com. The data with a time step of 7 days from dominated by a weak anticyclonic eddy in summer and a June 2000 to May 2007 for the SS are extracted, then the strong cyclonic eddy in winter. However, no further inves- climatologic monthly means from January to December are tigation of the vorticity dynamics on the SS circulation has calculated for the seasonal variability of EOF analysis. The been carried out, e.g., what about the contributions of the data at the water depth shallower than 50 m and near the water exchange via the Sibutu Passage, Dipolog Strait and coast in the SS are excluded in the analysis. Finally, there Balabac Strait to the vorticities of the SS circulation? What are 344 effective grid points data used for the EOF analysis. about the contribution of the monsoon wind stress to the [6] Figure 2 shows the first two EOF modes for the seasonal upper circulation in the SS? Can we estimate the basic variability of the surface circulation in the SS. First EOF mode distributions of the transports via the Mindoro and the accounts for 71.46% of the total variance, it shows the basin- Balabac straits? It is necessary to employ a numerical model scale anticyclonic/cyclonic circulation centered at 120.6°E, to study these problems. Meanwhile, no satellite altimetry 8.4°N, and the time series has a minimum in August data are used to testify their numerical results. corresponding to the summer anticyclonic circulation and an [4] Satellite altimetry, which provides high time-space opposite maximum in December–January corresponding to the coverage unavailable from in situ data, has been widely winter cyclonic circulation in the basin. Second EOF mode employed to study the oceanic circulation variability. The accounts for 21.3% of the total variance, it shows a weak basin- major objective is to present the basic characteristics of the scale anticyclonic/cyclonic meander from the Mindoro Strait to seasonal mesoscale circulation in the SS by Empirical the Sibutu Passage, and the time series has a minimum in April Orthogonal Function (EOF) analysis using 8 years of corresponding to the anticyclonic meander flow from the Sibutu Absolute Dynamic Topography (ADT) from satellite altim- Passage to the Mindoro Strait and an opposite maximum in etry data, then, a connected single-layer and two-layer November corresponding to the cyclonic meander flow in an model used by Cai et al. [2008] is again employed to study opposite direction. To facilitate easy comparison with the the vorticity dynamics of the seasonal upper circulation in following numerical results, Figure 3 shows the combined first

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Figure 2. Seasonal EOF modes of the Sulu Sea ADT variability: (a) spatial structure of first EOF, (b) time series of first EOF (unit is cm), (c) spatial structure of second EOF (unit is cm), and (d) time series of second EOF.

EOF and second EOF modes in August and December. It is Figure 4 shows that the SS is controlled by the southwest shown that there is a basin-scale anticyclonic circulation cen- monsoon with the mainly negative wind stress curl in August tered at about 120.8°E, 8.6°N in August and a strong cyclonic while by the northeast monsoon with the mainly positive meander also centered at about the same site in December. wind stress curl in December. [7] The synchronous QuikScat wind data at a resolution of 0.25° but with a time step of 3 days from June 2000 to May 3. Model Description and Choice of Parameters 2007 are extracted from the Web site http://www.remss.com, and then the climatologic monthly mean wind stress fields [8] The connected single-layer and two-layer model used and their curls in August and December are calculated and here is based on the connection of a single-layer model over interpolated into the whole domain with a resolution of 50. the shallow sea (less than 200 m) and a two-layer model

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Figure 2. (continued) over the deep basin. It was used to model the upper Here, subscript i is set 1 for upper layer and 2 for lower layer; ! ! circulation in the SCS and SS [Cai et al., 2007, 2008]. r is water density; V i and ui are vertical integral transport The controlling equations of the model are the same with and depth mean velocity, respectively; g0 = g(r À r )/r = !2 ! 1 ! those of the two-layer: gDr/r is reduced gravity acceleration; and t1, t2 and t3 denote wind stress, interfacial and bottom friction stress, ! which are computed using a quadratic law, i.e., @Vi ! ! ! ! ! ! ƒƒ! þðrÁVi þ Vi ÁrÞui þ k  fVi ¼ÀhirPi þðti À tiþ1Þ @t ! ! ! t1 ¼ raCDW W ; 2! =r þ AH r Vi ð1Þ ƒƒƒƒƒƒ! ƒƒƒƒƒƒ! ! t2 ¼ r1CI ðu1 À u2Þ ðu1 À u2Þ ;

@hi ! þrÁVi ¼ 0 ð2Þ ! ! ! @t t3 ¼ r2CBu2 u2 ;

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Figure 3. The combined first EOF and second EOF modes (unit is cm; here and subsequently, solid line stands for positive, while dashed line stands for negative) (a) in August and (b) in December.

! 0 where ra is the air density, W is the wind velocity, CD is the height anomaly (PHA). P1 = gh1, and P2 = P1 À g h1; f is the drag coefficient, and CI and CB are the coefficients of Coriolis parameter, and AH is the lateral friction coefficient. interface and bottom friction, respectively. Moreover, hi is [9] In the computation, the spatial and temporal steps are 0 depth, i.e., h1 = H1 + h1 À h2, h2 = H2 + h2, H1 and H2 are the 5 and 0.5h, respectively. The computational domain includes undisturbed initial depths within the upper and lower layer, most of the SCS, SS and part of the Pacific (Figure 1). For low À3 and h1 and h2 are free surface anomaly (FSA) and pycnocline wind speeds CD is 1.1 Â 10 , but for wind speeds over 6 m

Figure 4. Distributions of the wind stress field (vectors in N/m2; here and subsequently, one vector at every three grid points is plotted) and its curl (contours in 10À8 N/m3) (a) in August and (b) in December.

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Table 1. Idealization of Depth in the Computational Domain H (m) 0 < H 30 30 < H 150 150 < H 200 200 < H 250 250 < H

H1(m) 30 H 150 200 200 H2(m) 0 0 0 50 H À 200

À1 s a linear relation between CD and wind speed W is used both August and December, there is 3.1 Sv mean transport out [Gill, 1982], i.e., CD =(0.61+0.063W)/1000. The coefficients of the SS via the Sibutu Passage and 1.0 Sv mean transport of the interface and bottom friction are taken as 5 Â 10À5 and into the SS via the Dipolog Strait, respectively, since accord- 2 Â 10À3, respectively, the lateral friction coefficient is 100 m2 ing to our sensitivity experiments, it is shown that the sÀ1 (according to the sensitivity experiments, the modeled variations of the prescribed transports via the Sibutu Passage current field would be damped when these coefficients are and the Dipolog Strait only have a slight effect on the modeled larger, whereas they would be eddy-active when they are less). circulation gyre center in the SS (this can also be testified by [10] The northern boundary north of Taiwan Island and the the comparison of the following experiments with only eastern boundary in the Pacific are taken as the outflow and different prescribed transports at the same open boundary), inflow open boundaries for the Kuroshio, respectively; the and the SS circulation is slightly affected by the different Sunda Shelf in the SCS, the Sibutu Passage and the Dipolog prescribed transports at the other open boundaries in the SCS. Strait in the SS are also taken as open boundaries. At these To simplify the question, it is supposed that the upper and five open boundaries, the global outflows via the open lower layer transports in these two straits are equal, i.e., boundaries are balanced by the global inflows via the other 1.55 Sv outflows for the upper and lower layers of the Sibutu open boundaries, i.e., the net sum of the prescribed Kuroshio Passage, respectively, and 0.5 Sv inflows for the upper and inflow, the Dipolog Strait inflow, the Sunda Shelf outflow, the lower layers of the Dipolog Strait, respectively. The Sibutu Passage outflow and the northern open boundary corresponding inflow velocities at the open boundaries are outflow is zero. Thus, in the following numerical experiments, simply uniform, i.e., they are given as the depth and width the northern open boundary outflow could be calculated by mean of the inflow transport, whereas the outflow velocities the prescribed transports via the first four open boundaries. are geostrophically adjusted by the model itself [Hurlburt and The other boundaries are taken as solid boundaries with a no- Thompson, 1980]. slip boundary condition. Note that when no-slip boundary [11] The relative density difference value, between the conditions are applied, a viscous sublayer with a vorticity of upper and lower layers, is assumed as Dr/r = 0.004 on the opposite sign from that of the interior flow is generated and basis of Yang et al. [1988]. As stated above, the monthly advected along the boundary [e.g., O¨ zgo¨kmen et al., 1997]. It mean QuikScat winds data with a resolution of 0.25°in is very difficult to prescribe the inflows/outflows at the five August and December are interpolated into the computa- open boundaries owing to the lack of observational data, thus tional domain. The depth is based on the ETOPO5 Global we have to adopt the modeled results of the monthly mean Earth Topography with a resolution of 50. It is liable to be transports (in case the monthly mean transports are not divergent during the model’s running at shallow water; to available, the annual mean transports have to be prescribed) prevent it, the depth in the whole domain is somewhat at these open boundaries from the previous model studies. In idealized as shown in Table 1. In this case, the shallowest the western Pacific, the North Equatorial Current bifurcates depth in the model is 30 m. into the northward flowing Kuroshio and the southward [12] In the following, eleven experiments are designed to Mindanao Current as it encounters the coast. test whether the possible key dynamic factors, including the Yaremchuk and Qu [2004] suggested that the mean bifurca- water exchange via the straits (the Sibutu Passage, the tion position is about 14.3 ± 0.7°N. The annual mean transport Dipolog Strait and the Balabac Strait) and the local mon- of Kuroshio is about 30 Sv [Nitani, 1972]. In our model, it is soon stress affect the SS circulation or not. See Table 2 for supposed that in both August and December, the open the numerical experimental cases. The first five experiments boundary for the Kuroshio inflow is set from 14°200Nto are for the summer experiments, the next five for the winter 15°200N along the eastern boundary, with a transport of 18 Sv experiments and the last one for the no winds experiment. for the upper layer and 9 Sv for the lower layer, respectively, Two standard experiments are designed in summer and since Cai et al. [2007] suggested that the variation of the winter, respectively: experiment E1 is driven by August prescribed Kuroshio transport had little effect on the modeled winds, the Kuroshio inflow, inflow via the Dipolog Strait circulation in the SCS. According to the global ocean circu- and the outflows via the Sunda Shelf and the Sibutu lation model result [Cai et al., 2005], mean outflows of 0.8 Sv Passage; experiment E6 is driven by December winds, the in August and 3.5 Sv in December are prescribed at the open Kuroshio inflow, inflow via the Dipolog Strait and the boundary of the Sunda Shelf (here the depth is shallow and it outflows via the Sunda Shelf and the Sibutu Passage. Then, has only one layer in the model) in the numerical experiments. the other experiments are designed to shoot at the solution A recent numerical simulation based on the global Hybrid of the above questions: first, the Sibutu Passage (experi- Coordinate Ocean Model (HYCOM) with a resolution of ments E2 and E7), the Dipolog Strait (experiments E3 and 1/12° by Metzger et al. [2008] suggested that there are about E8) and the Balabac Strait (experiments E4 and E9) are 2.63.7 Sv annual mean transport out of the SS via the Sibutu blocked separately; second, the water exchange via all of Passage and about 0.91.1 Sv mean transport into the SS via the straits (experiments E5 and E10) and the local monsoon the Surigao Strait (which connects the Pacific with Bohol stress (experiment E11) are excluded, respectively. Finally, Sea), respectively. Thus, in our model, it is supposed that in by comparison of the standard experiments results with the

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Table 2. Model’s Running Cases and Their Experimental Parametersa Experiment Explanation E1 It is driven by August winds and the Kuroshio with an inflow of 18 Sv for the upper layer and 9 Sv for the lower layer; meanwhile, the upper layer outflow via the Sunda Shelf is 0.8 Sv, the upper and lower layers outflows via the Sibutu Passage are both 1.55 Sv, and the upper and lower layers inflows via the Dipolog Strait are both 0.5 Sv. E2 Same as E1, but there is no outflow via the Sibutu Passage. E3 Same as E1, but there is no inflow via the Dipolog Strait. E4 Same as E1, but the Balabac Strait is blocked. E5 It is driven only by August winds; all of the inflows/outflows at the open boundaries are 0. E6 It is driven by December winds and the Kuroshio with an inflow of 18 Sv for the upper layer and 9 Sv for the lower layer; meanwhile, the upper layer outflow via the Sunda Shelf is 3.5 Sv, the upper and lower layers outflows via the Sibutu Passage are both 1.55 Sv, and the upper and lower layers inflows via the Dipolog Strait are both 0.5 Sv. E7 Same as E6, but there is no outflow via the Sibutu Passage. E8 Same as E6, but there is no inflow via the Dipolog Strait. E9 Same as E6, but the Balabac Strait is blocked. E10 It is driven only by December winds, all of the inflows/outflows at the open boundaries are 0. E11 Same as E1, but there are no winds for the whole computational domain. aNote: the other parameters are the same except those given in Table 2. other ones, the contributions of the water exchange via each [15] In experiment E2, the case is the same as in experi- strait and the local monsoon to the upper circulation in the ment E1 except that now there is no outflow via the Sibutu SS could be known. Passage. The major difference of the current field from that in experiment E1 is that owing to the exclusion of the southward 4. Numerical Experimental Results and outflow via the Sibutu Passage, the anticyclonic circulation Discussions center is a little retreated northeastward (Figure 6a); a small- scale cyclonic meander is induced in the northern SS near the [13] Each experiment commences from the state of rest Mindoro Strait; the water exchange via the Mindoro Strait and reaches quasi-equilibrium after running about 600d. changes from an inflow to an outflow with a value of about The model is run continuously for 830d and the data, after 0.82 Sv (Table 3), and the outflow transport via the Balabac 720d, are saved for analysis and comparison. In the follow- Strait increases to a value of about 0.18 Sv. The upper layer ing, we only concern the simulated results of the circulation relative vorticity in the northwestern SS is basically similar to in the SS. that in experiment E1 (Figure 6b). [16] In experiment E3, the case is the same as in exper- 4.1. Summer Experiments E1–E5 iment E1 except that now there is no inflow via the Dipolog [14] The result of experiment E1 (Figure 5a) shows that Strait. The current field (Figure 7) and the distribution of the the SS is dominated by a basin-scale anticyclonic circula- upper layer relative vorticity (not shown) change little when tion centered at about (119.8°E, 8.8°N), somewhat west- compared with those in experiment E1, except that: the flow ward when compared with the anticyclonic circulation speed near the Mindoro Strait gets larger, and the center based on the combined EOF modes (Figure 3a). corresponding inflow transport increases to a value of about The west component of the anticyclone (i.e., the northeast- 3.09 Sv (Table 3); the flow speed near the Dipolog Strait ward current near the Palawan Island) is narrower but gets weaker. stronger than its eastern part (i.e., the southwestward [17] In experiment E4, when the Balabac Strait is blocked, current); this western boundary strengthening of the current the simulated current and the upper layer relative vorticity field is due to the b effect, which is demonstrated by an fields (not shown) are almost the same as those in experiment additional experiment when the b is removed from the E1, as expected. This is well understood since in experiment model (not shown). Table 3 shows the simulated mean E1, the water exchange via the Balabac Strait is very trivial. water transports across Section A of the Mindoro Strait and [18] In experiment E5, when the model is driven only by Section B of the Balabac Strait in the experiments, there is August winds, the current field (Figure 8a) is very similar to about 2.15 Sv water flowing into the SS via the Mindoro that in experiment E2: most of the SS is dominated by an Strait while only 0.05 Sv water flowing out of the SS via anticyclonic circulation; a small-scale cyclonic meander is the Balabac Strait. Although the simulated maximum FSA also induced in the northern SS near the Mindoro Strait; (Figure 5b) is slightly different from that based on the about 0.17 Sv water flows out of the SS via the Mindoro satellite altimetry data (Figure 3a), it is found that both their Strait (Table 3), and this outflow is compensated by the variation gradients and distributional trends are very similar. same amount of water flowing into the SS via the Balabac Figure 5c shows that the upper layer relative vorticity in the Strait. However, because of the exclusion of the southward SS is mainly negative with a negative maximum less than À9 À1 outflow via the Sibutu Passage, the current and the upper À3500 Â 10 s , corresponding to the anticyclonic layer relative vorticity fields are slightly weaker than those circulation center. Meanwhile, a weak tongue-like band in experiments E1, e.g., the area covered by the À2000 Â with positive vorticities extending from north to southwest 10À9 sÀ1 contour line in Figure 8b is smaller than that in is induced and inserted into the middle of the SS. Note that Figure 5c. along the western boundary of the SS, the positive vorticity is generated within the sublayer. As stated above, it is 4.2. Winter Experiments E6–E10 caused by the application of no-slip boundary conditions [19] The result of experiment E6 (Figure 9a) shows that [O¨ zgo¨kmen et al., 1997]. the SS is mainly dominated by a cyclonic circulation

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Figure 5. (a) Upper layer current field (unit is cm sÀ1), (b) distribution of FSA (unit is cm), and (c) upper layer relative vorticity (unit is 10À9 sÀ1) in the Sulu Sea of experiment E1 after running 720d (note that because of the sharp variation of vorticity along the shore, only the vorticity contour within 118°122°E, 6°10°300N is shown, here and subsequently). centered at about (120.4°E, 6.8°N). The circulation center is are different from those based on the satellite altimetry data more southward when compared with the cyclonic meander (Figure 3b), their spatial patterns seem similar. The difference center based on the combined EOF modes (Figure 3b). The between the simulated FSA and the observed one based on current fields in the western and northern SS are also the satellite altimetry data may be caused by the following stronger than that in the eastern SS. There is about 2.46 Sv factors: first, in the model, the prescribed transports and the water flowing into the SS via the Mindoro Strait while corresponding inflows velocities at the open boundaries are 0.36 Sv water flowing out of the SS via the Balabac Strait simply uniform via the Sibutu Passage and the Dipolog Strait, (Table 3). As in the summer experiments, although the and some other small channels near the Sibutu Passage at the simulated maximum FSA and variation gradients (Figure 9b) southeastern SS could not be described in the model; second,

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Table 3. Comparison of the Simulated Mean Water Transport in Some Straitsa Section A Along Section B Along the Experiment the Mindoro Strait Balabac Strait E1 À2.15 0.05 E2 0.82 0.18 E3 À3.09 À0.00 E4 À2.09 0.00 E5 0.17 À0.17 E6 À2.46 0.36 E7 0.72 0.28 E8 À3.33 0.23 E9 À2.10 0.00 E10 À0.29 0.29 E11 À2.21 0.11 aNote: Positive means flowing out of the SS, while negative means flowing into the SS. Mean water transport values are in Sv.

the monthly wind stress data set with a resolution of 0.25° are interpolated into the space with a resolution of 50 for the computational domain, this may also cause some errors on the modeled circulation gyre center in the SS; and third, the shortcomings of the model itself (e.g., no water mass exchange between the two layers ocean) would make it Figure 7. Same as Figure 5a but for experiment E3. difficult to simulate the SS circulation well. Figure 9c shows that the upper layer relative vorticity in the SS is mainly À9 À1 positive with a maximum of about 2000 Â 10 s ,whichis clonic eddy near the Palawan Island and a cyclonic eddy distinctly less than the summer experiments. The mainly near the Mindoro Strait (Figure 10a), respectively; the positive vorticity in the southern SS near the Sibutu Passage cyclonic eddy and the corresponding positive vorticity is corresponding to the cyclonic eddy. There are also some (Figure 10b) in the southern SS is weaker; the water weak negative vorticity bands induced in the middle of the exchange via the Mindoro Strait changes from an inflow SS and near the Dipolog Strait. to an outflow with a value of about 0.72 Sv (Table 3), and [20] In experiment E7, the case is the same as in exper- the outflow transport via the Balabac Strait is about 0.28 Sv. iment E6 except that now there is no outflow via the Sibutu [21] In experiment E8, the case is the same as in exper- Passage. The major difference of the current field from that iment E6 except that now there is no inflow via the Dipolog in experiment E6 is distinct: there are a small-scale anticy- Strait. The current field and the upper layer relative vorticity

Figure 6. (a) Upper layer current field (unit is cm sÀ1) and (b) upper layer relative vorticity (unit is 10À9 sÀ1) for experiment E2.

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Figure 8. Same as Figure 6 but for experiment E5.

(not shown) change little when compared with that in anticyclonic circulation centered at about (119.6°E, 9.2°N) experiment E6, except that: the flow speed near the Mind- (Figure 13a); there is about 2.21 Sv water flowing into the oro Strait gets larger, and the corresponding inflow transport SS via the Mindoro Strait while 0.11 Sv water flowing out increases to a value of about 3.33 Sv (Table 3); the outflow of the SS via the Balabac Strait (Table 3). Figure 13b shows transport via the Balabac Strait is about 0.23 Sv; the flow that the upper layer relative vorticity is negative in the speed near the Dipolog Strait gets weaker. northwestern SS but positive in the southern SS. [22] According to the above winter experiments, the outflow transports via the Balabac Strait are greater than 4.4. Discussions 0.23 Sv, thus in experiment E9, when the Balabac Strait is [25] According to the above numerical experiments, the blocked, some difference from that in experiment E6 can be observational spatial circulation structure shown by the noted in the simulated current and the upper layer relative combined EOF modes is basically simulated by the model, vorticity fields (Figure 11), especially near the Balabac it is inferred that the circulation in the SS is closely related Strait and the Palawan Island: there is a weak anticyclonic to the outflow via the Sibutu Passage and the seasonal local eddy near the Palawan Island, which extends to the northern wind stress. An inflow into the SS via the Dipolog Strait has SS at about 10°N. By comparison with the result in an unimportant effect on the circulation in the SS except for experiment E6, it seems to suggest that in winter, the the current field near this strait (in experiments E3 and E8). northeast monsoon (Figure 4b) is liable to induce a south- When there is an outflow via the Sibutu Passage (in no westward boundary current along the Palawan Island which winds experiment E11) or the SS is driven by the southwest flows out of the SS via the Balabac Strait. monsoon with the negative wind stress curl (in experiment [23] In experiment E10, when the model is driven only by E5), an anticyclonic circulation with the mainly negative December winds, the SS is dominated by a cyclonic circulation relative vorticity would be induced in the SS; while when centered at about (119.4°E, 7.2°N) (Figure 12a); about 0.29 Sv the SS is driven by the northeast monsoon with the positive water flows into the SS via the Mindoro Strait (Table 3), and wind stress curl (in experiment E10), a cyclonic circulation this inflow is compensated by the same amount of water with the mainly positive relative vorticity would be induced. flowing out of the SS via the Balabac Strait. Figure 12b shows In summer experiments, the effect of an outflow via the that the upper layer relative vorticity in the SS is mainly Sibutu Passage on the circulation is similar to that of the positive with a maximum greater than 3500 Â 10À9 sÀ1, summer monsoon (experiment E5 versus E11), while in corresponding to the cyclonic circulation center. Meanwhile, a winter experiments, the effect of an outflow via the Sibutu weak tongue-like band with negative vorticities extending Passage on the circulation is counter to that of the winter from south to north is induced and inserted into the middle monsoon. Thus, in August, the couple linear effect of an of the SS. outflow via the Sibutu Passage and the southwest monsoon would induce a stronger anticyclonic circulation with the 4.3. No Winds Experiment E11 negative relative vorticity (in experiment E1), while in [24] In experiment E11, when the model is driven by the December, the couple linear effect of an outflow via the Kuroshio inflow, inflow via the Dipolog Strait, outflows via Sibutu Passage and the northeast monsoon would induce a the Sunda Shelf and the Sibutu Passage, while the seasonal weaker cyclonic circulation with the positive relative vor- winds are not considered. The SS is dominated by an ticity (in experiment E6). This conclusion is also supported

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Figure 9. Same as Figure 5 but for experiment E6. by the experiments without an outflow via the Sibutu et al. [2008] but larger than the simulated 0.551.13 Sv Passage, in experiment E2, the simulated anticyclonic mean transport based on the variable-grid global ocean circulation pattern is almost the same as that in experiment circulation model with the finest resolution of 1/6° by Fang E1 except that the corresponding anticyclonic center seems et al. [2003]. The water transport out of the SS via the to be pushed slightly northward, while in experiment E7, Balabac Strait is small, it is about 0.05 Sv in August and the simulated circulation pattern in the western SS is very 0.36 Sv in December (in experiments E1 and E6), which is different from that in experiment E6. also different from the simulated 1.442.91 Sv mean [26] The simulated transport into the SS via the Mindoro transport into the SS by Fang et al. [2003]. The pure winter Straits (Table 3) is about 2.15 Sv in August and 2.46 Sv in monsoon seems to push the water out of the SS via the December (in experiments E1 and E6), which is the same Balabac Strait (in experiment E10) while the pure summer order to the simulated 1.82.6 Sv mean transport based on monsoon brings the water into the SS via the Balabac Strait the Global HYCOM with a resolution of 1/12° by Metzger (in experiment E5), and in no winds experiment E11 the

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Figure 10. Same as Figure 6 but for experiment E7. water is also driven out of the SS via the Balabac Strait, thus Mindoro Strait, thus the transport into the SS via the the couple linear effect would make the transport via the Mindoro Strait (Table 3) is larger in winter than that in Balabac Strait in summer experiment E1 is weakened but summer (in experiments E1 and E6). No matter how, the strengthened in winter experiment E6, correspondingly. transport via the deeper Mindoro Strait is much larger than Therefore, if the Balabac Strait is blocked, the circulation that via the shallower Balabac Strait. pattern near the strait would change more largely in winter [27] Although the above discussion is confined to just the than in summer (in experiments E4 and E9). Similarly, the upper layer, it is interesting to talk something about the pure winter monsoon would bring the water into the SS via lower layer current field in the SS. Both in summer and the Mindoro Strait (in experiment E10) while the pure winter (in experiments E1 and E6), the lower layer current summer monsoon would push the water out of the SS via fields (not shown) are very similar, and in most of the basin the Mindoro Strait (in experiment E5), and in no winds the lower layer current is weak with a depth mean velocity experiment E11 the water is also brought into the SS via the less than 2 cm sÀ1, except that: within the isodepths of

Figure 11. Same as Figure 6 but for experiment E9.

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Figure 12. Same as Figure 6 but for experiment E10.

200 m (Figure 1b) in the narrow channel from section A to gets weaker with a depth mean velocity of about 3 cm sÀ1, about 10.5°N near the Mindoro Strait, a southward lower finally it flows out of the SS via the Sibutu Passage. Our layer current is rather strong; its depth mean velocity at result supports the evidence that the flow in the Mindoro section A is about 8 cm sÀ1; during its flowing southward, it Strait is stronger in the lower layer compared to the upper gets stronger with a mean velocity larger than 30 cm sÀ1 layer [e.g., Quadfasel et al., 1990; Chen et al., 2006]. There (which is more larger than that of the upper layer; e.g., see is also a weak eastern boundary current flowing southwest- Figure 5a) at about 12°N where the lower channel is the ward from the Dipolog Strait to the Sibutu Passage with a narrowest; from then on, it gets weaker when flowing depth mean velocity of about 3 cm sÀ1. The reason why further southward with the channel gets wider, and its there exists a stronger lower layer western boundary current velocity reduces to 8 cm sÀ1 at about 12°N again; then, than the upper layer one near the Mindoro Strait is mainly this current keeps flowing southward along the Palawan that the joint effect of bottom topography and b effect is Island, i.e., along the western boundary of the SS, it also important; when compared to the upper layer, the channel in

Figure 13. Same as Figure 6 but for experiment E11.

13 of 14 C03026 CAI ET AL.: CIRCULATION IN THE SULU SEA C03026 the lower layer here suddenly gets much narrower, while a that there exists a stronger lower layer current than the large inflow via the Mindoro Strait has to be induced to upper layer one near the Mindoro Strait. compensate most of the outflow via the Sibutu Passage. [31] Acknowledgments. The authors are indebted to the anonymous referees for helpful comments. This work is jointly supported by National 5. Conclusion Basic Research Program (2007CB816003), National Special Project 2006BAB19B01, grant 40520140074 from the Chinese National Science [28] In the paper, on the basis of the EOF analysis of Foundation, and China 908-Project under grant 908-02-01-03. The altim- 8 years of ADT satellite altimetry data and the numerical eter products were produced by Ssalto/Duacs and distributed by Aviso, with experiments of a connected single-layer and two-layer support from Cnes. QuikScat data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team. model, the seasonal variability of the surface circulation in Data are available at http://www.remss.com. the SS and its dynamic mechanism are investigated. Some conclusions can be drawn as follows: References [29] First seasonal EOF mode shows a basin-scale anti- Cai, S., et al. (2005), Application of LICOM model to the numerical study cyclonic circulation in summer and a cyclonic circulation in of the water exchange between the South China Sea and its adjacent winter, and second seasonal EOF mode shows a weak oceans, Acta Oceanol. Sin., 24(4), 10–19. Cai, S., X. Long, and S. Wang (2007), A model study of the summer basin-scale anticyclonic meander flow from the Sibutu Southeast Vietnam Offshore Current in the southern South China Sea, Passage to the Mindoro Strait in the first half year and a Cont. Shelf Res., 27(18), 2357–2372, doi:10.1016/j.csr.2007.06.002. cyclonic meander flow in an opposite direction in the second Cai, S., et al. (2008), Model study of the upper circulation in the Sulu Sea and its relation to the South China Sea circulation, Acta Oceanol. Sin., 27, half year. The typical surface circulation in the SS, as shown 119–128, suppl. by the combined first EOF and second EOF modes in Chen, C. T. A., et al. (2006), Carbonate-related parameters of subsurface August and December, is a basin-scale anticyclonic/cyclonic waters in the West Philippine, South China and Sulu , Mar. Chem., circulation (or meander) centered at about 120.8°E, 8.6°N. 99(1–4), 151–161, doi:10.1016/j.marchem.2005.05.008. Fang, G., et al. (2003), Interbasin freshwater, heat and salt transport through [30] According to the numerical experiments, it is shown the boundaries of the East and South China Seas from a variable-grid that the upper circulation in the SS is closely related to the global ocean circulation model, Sci. China, Ser. D, 46, 149–161. outflow via the Sibutu Passage and seasonal local wind Gill, A. E. (1982), Atmosphere-Ocean Dynamics, Academic, San Diego, Calif. stress. Either an outflow via the Sibutu Passage or the Hurlburt, H. E., and J. D. Thompson (1980), A numerical study of loop summer monsoon may cause an anticyclonic circulation in current intrusions and eddy shedding in Gulf of Mexico, J. Phys. Oceanogr., the SS, while the winter monsoon may cause a cyclonic 10, 1611–1631, doi:10.1175/1520-0485(1980)010<1611:ANSOLC> 2.0.CO;2. circulation. Either an outflow via the Sibutu Passage or Metzger, E. J., and H. E. Hurlburt (1996), Coupled dynamics of the South the winter monsoon would push the water out of the SS via China Sea, the Sulu Sea, and the Pacific Ocean, J. Geophys. Res., the Balabac Strait but bring the water into the SS via the 101(C5), 12,331–12,352, doi:10.1029/95JC03861. Mindoro Strait, while the summer monsoon would bring Metzger, E. J., H. E. Hurlburt, and X. B. Xu (2008), Global HYCOM in the Philippines Seas, paper presented at PhilEx Meeting, Naval Res the water into the SS via the Balabac Strait but push the Lab., Monterey, Calif. (Available at http://mseas.mit.edu/Research/ water out of the SS via the Mindoro Strait. Thus, once both Straits/Monterey_Meeting/Tuesday/PhilEx_Metzger.pdf) the outflow via the Sibutu Passage and the local wind stress Nitani, H. (1972), Beginning of the Kuroshio, in Kuroshio: Its Physical Aspects, edited by H. Stommel and K. Yoshida, pp. 129–163, Univ. of are considered, the couple linear effect of an outflow via the Washington Press, Seattle. Sibutu Passage and the southwest monsoon in summer O¨ zgo¨kmen, T. M., E. P. Chassigent, and A. M. Paiva (1997), Impact of would induce a stronger anticyclonic circulation with neg- wind forcing, bottom topography, and inertia on midlatitude jet separa- tion in a quasigeostrophic model, J. Phys. Oceanogr., 27, 2460–2476, ative relative vorticity but less water transports via the doi:10.1175/1520-0485(1997)027<2460:IOWFBT>2.0.CO;2. Mindoro and the Balabac straits, while the couple linear Quadfasel, D., H. Kudrass, and A. Frische (1990), Deep-water renewal by effect of an outflow via the Sibutu Passage and the northeast turbidity currents in the Sulu Sea, Nature, 348(6299), 320–322, monsoon in winter would induce a weaker cyclonic circu- doi:10.1038/348320a0. Wajsowicz, R. C. (1999), Models of the southeast Asian seas, J. Phys. lation with the mainly positive relative vorticity but larger Oceanogr., 29(5), 986–1018, doi:10.1175/1520-0485(1999)029<0986: water transports via the Mindoro and the Balabac straits. MOTSAS>2.0.CO;2. The inflow via the Mindoro Strait also has a significant Wyrtki, K. (1961), Scientific Results of Marine Investigation of the South China Sea and the Gulf of Thailand, vol. 2, NAGA Report, 195 pp., impact on the upper circulation in the SS, since the outflow Scripps Inst. of Oceanogr., La Jolla, Calif. via the Sibutu Passage is mainly compensated by the inflow Yang, Y., L. Li, and Z. Wang (1988), Characteristics of average T-S, S-Z via the Mindoro Strait. Because of the b effect, the and T-Z in the South China Sea (in Chinese with English abstract), J. Tropic Oceanol., 7(3), 54–59. anticyclonic/cylconic circulation is asymmetric and Yaremchuk, M., and T. Qu (2004), Seasonal variability of the large-scale strengthened along the Palawan Island. Generally speaking, currents near the coast of the Philippines, J. Phys. Oceanogr., 34(4), the transport via the Mindoro Strait is much larger than that 844–855, doi:10.1175/1520-0485(2004)034<0844:SVOTLC>2.0.CO;2. via the Balabac Strait. An inflow into the SS via the Dipolog ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ Strait has an unimportant effect on the circulation in the SS S. Cai, Y. He, X. Long, and S. Wang, LED, South China Sea Institute of except for the current field near this strait. It is also shown Oceanology, Chinese Academy of Sciences, Guangzhou, 510301, China. ([email protected])

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