1. Introduction
The South China Sea (SCS) has a deep, central basin surrounded by a steep continental slope and links to the western Pacific Ocean via Luzon Strait, to the Java Sea via Karimata Strait, to the East China Sea via Taiwan Strait, and to the Sulu Sea via Mindoro Strait (Fig. 1).
Simulated transport streamfunctions (Sv) in the upper (<750 m), middle (mid) (750–1500 m), and deep layers (>1500 m) in (left) winter and (right) summer in the South China Sea. The circles represent the mean circulation in each layer. The thick contour line in the upper layer refers to the edge of the basin along the 100-m isobath. A thinner circular line in the schematic circulation represents a relatively weak circulation.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Simulated transport streamfunctions (Sv) in the upper (<750 m), middle (mid) (750–1500 m), and deep layers (>1500 m) in (left) winter and (right) summer in the South China Sea. The circles represent the mean circulation in each layer. The thick contour line in the upper layer refers to the edge of the basin along the 100-m isobath. A thinner circular line in the schematic circulation represents a relatively weak circulation.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Simulated transport streamfunctions (Sv) in the upper (<750 m), middle (mid) (750–1500 m), and deep layers (>1500 m) in (left) winter and (right) summer in the South China Sea. The circles represent the mean circulation in each layer. The thick contour line in the upper layer refers to the edge of the basin along the 100-m isobath. A thinner circular line in the schematic circulation represents a relatively weak circulation.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
The East Asia monsoonal winds, and lateral inflow/outflow through the straits around the periphery of the SCS, jointly force the seasonal circulation pattern in the SCS basin (Qu 2000). During winter, the northeasterly monsoonal winds have a dominant positive wind stress curl in the deep basin. The winds turn southwesterly with a much weaker positive wind stress curl during summer. An annual-mean westward transport of ~4–10 Sv (1 Sv ≡ 106 m3 s−1) from the Pacific to the SCS through Luzon Strait has been widely reported in previous studies (e.g., Wyrtki 1961; Chen and Huang 1996; Qu et al. 2000, 2004; Xue et al. 2004; Tian et al. 2006). Based on these studies, a sandwichlike inflow–outflow–inflow pattern in the upper, middle, and deep layers of Luzon Strait, respectively, has been identified, though the depth ranges of the three layers are different from different studies. The intrusive transport through Luzon Strait is balanced by the extrusive transport through all other straits in the upper layer and through Luzon Strait in the middle layer. As a result of both wind and lateral forcing, the basin circulation in the upper layer of the SCS is cyclonic in winter, and, in summer, it is cyclonic in the northern half of the basin and anticyclonic in the southern half of the basin (Qu 2000). In the deep layer, there is a cyclonic circulation (Wang et al. 2011; Lan et al. 2013).
However, we still do not understand the overall vertical structure of circulation and its seasonality, especially regarding the dynamic coherence of the circulation in the water column in the SCS. We also have little knowledge of the relative importance of the wind forcing, lateral inflow/outflow, and intrinsic dynamics that arise from the flow–topography interaction on the SCS circulation. Filling this knowledge gap will provide insight into the basin circulation physics in the SCS and other oceans that are driven by multiforcing processes.
2. Materials and methods
a. Ocean model
We use the Regional Ocean Modeling System (ROMS; Shchepetkin and McWilliams 2005) to simulate three-dimensional, time-dependent circulation. The model domain covers all the China seas (SCS, East China Sea, Yellow Sea, and Bohai Sea) from ~0° to ~49°N and extends to the central Pacific Ocean. We adopt a curvilinear grid with ~10-km horizontal grid size and a 30-level stretched generalized terrain-following vertical coordinate.
We force the model with climatological (averaged from 1981 to 2010) wind stress calculated from monthly reanalysis of blended sea winds (0.25° × 0.25°) at 10 m released from the National Oceanic and Atmospheric Administration (NOAA; https://www.ncdc.noaa.gov/oa/rsad/air-sea/seawinds.html), using the bulk formulation by Large and Pond (1981). We calculate the atmospheric heat and salt fluxes from climatological-mean monthly National Centers for Environmental Prediction (NCEP) Reanalysis-1 meteorological variables. The reanalysis has ~1.875° resolution.
We apply active open boundary conditions (OBCs) to integrate external tidal and subtidal forces along the open boundaries of the computational domain. The OBCs are similar to Gan and Allen (2005). The external forcing of velocities and thermohaline variables along the boundaries are provided by the climatological monthly solutions of the Ocean General Circulation Model For the Earth Simulator (OFES) global model (Sasaki et al. 2008) and by tidal forcing using eight harmonic constituents (M2, S2, N2, K2, K1, O1, P1, and Q1) from the Inverse Tide Model (ITM; Egbert et al. 1994). To account for the self-attraction and loading effect of the ocean, we add tidal potential to the pressure term following Ray (1998). We initialize the model with winter climatological World Ocean Atlas 2013 (Locarnini et al. 2013; Zweng et al. 2013) potential temperature and salinity and spun up the model for 50 yr. We use the last 5-yr-averaged values of the model’s output variables for the analyses in this study.
Our circulation model is part of the China Sea Multi-Scale Ocean Modeling System (CMOMS) that integrates coupled physical–biogeochemical processes in estuary/shelf/basin/open oceans within different subscales in the China Seas. CMOMS has been extensively validated by measurements and physics (Gan et al. 2016, manuscript submitted to J. Geophys. Res.).
b. Hydrographic data and geostrophic currents
We use monthly averaged hydrographic data to calculate the geostrophic velocity in the water column based on the thermal wind relation. The data are from the U.S Navy Generalized Digital Environment Model (GDEM, version 3.0), which is gridded ocean temperature and salinity (T–S) data generated by 4.5-million observed T–S profiles dating back to 1920 (Carnes 2009). The GDEM data have a horizontal resolution of 0.25° × 0.25° and a vertical resolution that ranges from 2 m in the upper 1600 m to 200 m from 1600 to 6600 m. We use 500 m (Qu et al. 2000) and 2400 m (Wang et al. 2011) as the levels of no motion to derive the geostrophic currents in the upper 1500 m and below 1500 m. These reference depths appear to be reasonable according to the three-layer circulation shown in this study.
3. Three-layer basin circulation
a. Three-layer basin circulation
Domain-averaged annual- and seasonal-mean vorticity as a function of depth from (a) model results and (b) annual-mean geostrophic current velocities derived from GDEM hydrographic data. Green (negative) and purple (positive) bars are the averaged vorticity in the specific layers.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Domain-averaged annual- and seasonal-mean vorticity as a function of depth from (a) model results and (b) annual-mean geostrophic current velocities derived from GDEM hydrographic data. Green (negative) and purple (positive) bars are the averaged vorticity in the specific layers.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Domain-averaged annual- and seasonal-mean vorticity as a function of depth from (a) model results and (b) annual-mean geostrophic current velocities derived from GDEM hydrographic data. Green (negative) and purple (positive) bars are the averaged vorticity in the specific layers.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
In the upper layer, a basinwide cyclonic circulation characterizes the SCS circulation in winter. The cyclonic circulation is stronger in the northern half of the basin. The circulation pattern is the result of forcing by the northeasterly monsoon wind stress, positive wind stress curl, and a stronger winter Kuroshio intrusion along the continental slope around the SCS (e.g., Qu 2000; Gan et al. 2006). However, in summer, a southwestward current along the northern slope and an eastward returning current at ~13°N form a cyclonic circulation in the northern half of the basin and a weak anticyclonic circulation in the southern part of the basin. The cyclonic and anticyclonic circulations are separated by the ψ = 0 contour line (Fig. 1) associated with the zero wind stress curl line in the dipole wind stress curl and with the recirculation in a coastal jet separation to the east of Vietnam (Gan and Qu 2008).
Unlike the flow pattern in the upper layer, the circulation in the middle layer of the SCS is distinctly characterized by an anticyclonic circulation during winter and summer, and the circulation is more intense in summer.
In the deep layer, the SCS is dominated by a cyclonic circulation in both winter and summer, as was also observed by Wang et al. (2011) and Lan et al. (2013). Similar to what occurs in the middle layer, the intensity of the cyclonic circulation in the deep layer is stronger in summer than in winter, opposite to what occurs in the upper layer.
Similar to what ψ illustrates in Fig. 1, Γ exhibits a clear three-layer alternating cyclonic–anticyclonic–cyclonic (CAC) circulation in the upper–middle–deep layers of the water column (Fig. 2a). The data-derived geostrophic currents also show the existence of the CAC circulation (Fig. 2b).
b. Lateral transports in the SCS basin
The three-layer CAC circulation in the SCS has a coherent link with the similar sandwichlike inflow–outflow–inflow in Luzon Strait (Fig. 3a). Meanwhile, the outflows through Mindoro Strait (Fig. 3b) and crossing the 100-m isobath (Fig. 3c) provide the necessary mass balance in the basin. The sandwichlike vertical structure of the inward–outward–inward transport through Luzon Strait has annual-mean, layer–integrated transports of ~−5.9, ~1.4, and ~−0.9 Sv in the upper, middle, and deep layers, respectively. Our net transport of −5.4 Sv is similar to those derived from observations by Chu and Li (2000), Su (2004), and Tian et al. (2006). The mean transport in the each of the three layers varied greatly among different measurements, which ranged from −10 to −3.5, 5 to 0.22, and −2 to −0.22 Sv in the upper, middle, and deep layers (e.g., Tian et al. 2006; Yuan et al. 2008), respectively. Our deep inward transport of −0.9 is smaller than ~−1.5 Sv derived from a short-term observation by Zhao et al. (2014), who attributed the formation of the deep inflow to a baroclinic pressure gradient across Luzon Strait. The intrusive transport from Luzon Strait is balanced by the outward transports of ~3.2 Sv through Mindoro Strait and of ~2.3 Sv crossing the 100-m isobath (Figs. 3b,c). The transport of Mindoro Strait is similar to the finding of Qu and Song (2009), and the transport crossing the 100-m isobath equals the sum of the transports in Taiwan Strait (1.2 Sv) and Karimata Strait (1.1 Sv).
Simulated volume flux as a function of depth normal to (a) Luzon Strait, (b) Mindoro Strait, and (c) the 100-m isobath, respectively.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Simulated volume flux as a function of depth normal to (a) Luzon Strait, (b) Mindoro Strait, and (c) the 100-m isobath, respectively.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Simulated volume flux as a function of depth normal to (a) Luzon Strait, (b) Mindoro Strait, and (c) the 100-m isobath, respectively.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Strong seasonality also exists in these lateral transports, similar to CAC circulation. The weakened Kuroshio in winter is favorable to the upper-layer intrusion in Luzon Strait (Qu et al. 2000), and the net westward transport in Luzon Strait strengthens from summer to winter (Fig. 3a). As a result, the outward transports through Mindoro Strait and crossing the 100-m isobath are stronger in winter (Figs. 3b,c). The transport crossing the 100-m isobath has an opposite phase between winter and summer and between the upper and lower layer.
c. Vorticity dynamics of the CAC circulation
The terms Lu and Lb are the depths of the top and bottom for each specific layer, respectively; η is sea level and D = Lu − Lb; u and υ are the zonal (x) and meridional (y) velocities, respectively; f is the Coriolis parameter; HNL is the horizontal nonlinear advection plus horizontal viscous term (hvisc); the horizontal viscous term is generally very small; VNL is the vertical advection term; Kυ is the vertical viscous coefficient; PLu and PLb are pressure at the top and bottom of each layer, respectively; and J is a Jacobean operator. The overbar refers to the layer average and the suffix represents derivation.
The terms in Eq. (2), from the left to right, are acceleration Ωacce, divergence Ωcor (vortex stretching), horizontal advection Ωhadv, tilting Ωvadv, pressure curl Ωpgf, and vertical viscosity Ωvvis. The Qvvis can be expressed as Qvvis = Ωsstr (wind stress curl) − Ωbstr (bottom stress curl) in the upper layer. The tilting of horizontally oriented components of vorticity into the vertical by a nonuniform vertical velocity field generates the tilting term Ωvadv. The Ωpgf, or joint effect of baroclinicity and relief (JEBAR), is generated by a pressure torque that is formed by the interaction between baroclinically induced pressure and the variable slope topography (Mertz and Wright 1992). Figure 4 shows the balance of Eq. (2) in the each of the three layers.
Terms (m3 s−2) of domain and layer-integrated vorticity equation in the upper, middle (mid), and deep layers during (left) winter and (right) summer: Ωacce is acceleration, Ωcor is divergence or vortex stretching, Ωhadv is horizontal advection, Ωvadv is tilting, Ωpgf is the pressure curl, Ωvvis is vertical viscosity (middle and deep layers), Ωsstr is wind stress curl, and Ωbstr is bottom stress curl in the upper layer.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Terms (m3 s−2) of domain and layer-integrated vorticity equation in the upper, middle (mid), and deep layers during (left) winter and (right) summer: Ωacce is acceleration, Ωcor is divergence or vortex stretching, Ωhadv is horizontal advection, Ωvadv is tilting, Ωpgf is the pressure curl, Ωvvis is vertical viscosity (middle and deep layers), Ωsstr is wind stress curl, and Ωbstr is bottom stress curl in the upper layer.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Terms (m3 s−2) of domain and layer-integrated vorticity equation in the upper, middle (mid), and deep layers during (left) winter and (right) summer: Ωacce is acceleration, Ωcor is divergence or vortex stretching, Ωhadv is horizontal advection, Ωvadv is tilting, Ωpgf is the pressure curl, Ωvvis is vertical viscosity (middle and deep layers), Ωsstr is wind stress curl, and Ωbstr is bottom stress curl in the upper layer.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
In the upper layer, in winter, Ωcor and Ωsstr are the major sources of the cyclonic circulation. The two terms are mainly balanced by JEBAR and Ωbstr. In summer, the main source for the relatively weak cyclonic circulation becomes Ωhadv and Ωcor, while Ωsstr has a very small influence. The summer source terms are also balanced mainly by JEBAR. In both winter and summer, Ωacce is negative, which suggests that the cyclonic circulation weakens following the autumn and spring accelerations.
In the middle and deep layers, Ωcor is also the main source term for the respective anticyclonic and cyclonic circulation. It balances primarily the JEBAR and secondarily other relatively small terms. The signs for almost all terms in the middle layer are the opposite those of the upper and deep layers. The magnitudes of all the terms and their seasonality are much smaller in middle and lower layers than in the upper layer.
Mean values of Ωcor term (m3 s−2) in the upper, middle (mid), and deep layers and the respective contributions by planetary vorticity flux through Luzon Strait ΩLZ, Mindoro Strait ΩMS, and crossing the 100-m isobath Ωisobath in winter and summer.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Mean values of Ωcor term (m3 s−2) in the upper, middle (mid), and deep layers and the respective contributions by planetary vorticity flux through Luzon Strait ΩLZ, Mindoro Strait ΩMS, and crossing the 100-m isobath Ωisobath in winter and summer.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
Mean values of Ωcor term (m3 s−2) in the upper, middle (mid), and deep layers and the respective contributions by planetary vorticity flux through Luzon Strait ΩLZ, Mindoro Strait ΩMS, and crossing the 100-m isobath Ωisobath in winter and summer.
Citation: Journal of Physical Oceanography 46, 8; 10.1175/JPO-D-16-0044.1
The planetary vorticity flux through Luzon Strait ΩLZ primarily contributes to Ωcor in all the layers during winter and summer (Fig. 5). The vertical structure of ΩLZ has alternating negative–positive–negative signs in the upper–middle–deep layers that correspond to the sandwichlike structure of the volume flux in Luzon Strait (Fig. 3a). The seasonal variation of ΩLZ is also well-correlated with the seasonal variation of the volume flux in Luzon Strait. The inputs of the planetary vorticity flux in the upper and deep layers from Luzon Strait are offset by the outputs of the fluxes crossing the isobath and through Mindoro Strait. The cross-isobath flux transports Ωcor toward the shallower shelf waters, and most of it will exit the SCS through the Taiwan Strait and Karimata Strait. The contribution from Mindoro Strait ΩMS is relatively small as compared with that from ΩLZ, particularly in winter.
4. Summary
Using observations and results from a numerical model, we identify a new CAC circulation pattern in the SCS. We attribute the CAC circulation to vortex stretching from the planetary vorticity flux that is extrinsically induced by the inflow–outflow–inflow in the upper–middle–deep layers through Luzon Strait. We also attribute the cyclonic circulation in the upper layer to wind stress curl in winter and nonlinear advection in summer. The inflow–outflow–inflow in Luzon Strait interacts with the interior circulation of the SCS, and it varies coherently with the overall CAC circulation. JEBAR, related to bottom pressure torque, forms in each specific layer as a result of interaction between the CAC flow and bottom topography. Vortex stretching is predominantly balanced by the JEBAR. The planetary vorticity outfluxes that cross the edge of the basin and the Mindoro Strait offset the influx from Luzon Strait. These outfluxes around the SCS, together with the influx from Luzon Strait, are necessary in sustaining the CAC circulation.
Acknowledgments
This research was supported by the National (China) Key Basic Research Development Program (2015CB954004), the Key Project of the National Science Foundation of China (91228202 and 91328202), and Hong Kong Research Grant Council (GRF16202514).
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